Deu Mai Khola HEP Text - Feasibility

Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies

1. INTRODUCTION

This report deals with regional geology, geological and engineering geological

condition as well as the geotechnical condition, tectonics and seismicity, construction

material and muck disposal survey of the Deumai Hydropower Project. This study has

been carried out as per feasibility study requirement of the guideline of DoED.

The proposed Deumai Hydropower Project is located between downstream from

Gajurmukhi village and upstream from the junction between the Mai Khola and

Deumai Khola, Ilam District, Mechi Zone, Eastern Development Region of Nepal. The

intake area is located about 15 km east of Ilam Bazaar.

The Deumai Khola is a rain fed river and a main tributary of the Mai River and finally

drains out to the Mechi River. The proposed Deumai HEP is a storage or reservoir

scheme project. The powerhouse is located on the left bank of the Deumai Khola

about 0.5 km southeast from the confluence between the Mai Khola and Deumai

Khola along the Deumai Khola. The water will be diverted into the headrace tunnel to

the powerhouse to generate power. Related structures are located at the left bank of

the Deumai Khola on bedrocks as well as alluvial and colluvial deposits.

1.1 Objectives

The main objectives of the present geological study are as follows:

To obtain information on regional geology and geomorphology of the project area

To study detail geological and engineering geological condition of the locations of

proposed project structures

To prepare detailed engineering geological map (1:2,000), geological cross-sections

and profiles of the locations of major project structures like reservoir, tunnel

alignment, intake and weir axis, surge tank and penstock alignment and

powerhouse and tailrace areas

To carry out construction material survey and locate the disposal area for the

project area

To assess the slope stability of the project area including especially the tunnel

alignment

To evaluate the geotechnical condition of the project area

Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar


1.2 Scope of Work

The present study comprises of the following works:

Collect and review available literatures, topographical and geological maps,

photographs and landsat images

Collection and study of geological and geomorphologic information of previous

studies

Conduct field survey to collect and verify geological information prior to general and

detail geological mapping, engineering geological mapping of project components

and particular structures

Identify geological and seismic hazards such as faults, thrusts and landslides

Measurement of discontinuities to analyze slope stability and collect geotechnical

information of the rock mass to identify the rock mass classification

Prepare maps (engineering geological map) at the scale mentioned in DoED’s

guideline.

1.3 Methodology

To accomplish the objectives and scope of work, desk study, field visit and field data

analysis were carried out. During the desk study, available geological information and

geological maps of the Deumai Khola section relevant to the project area was

thoroughly studied. After the desk study, the field visit to the project was conducted.

During the field visit, discontinuity survey and geological as well as engineering

geological mapping of the project area including intake and weir area, and reservoir

area, tunnel alignment, surge tank, penstock alignment, powerhouse and tailrace

area was done. The instability and mass wasting area and necessary geological data

were also collected. After field observation, the detail analysis of geological data was

carried out which includes graphical analysis, slope stability analysis and rock mass.

The 2D-Electrical Resistivity Survey has been conducted and proposed the location

of the core drilling point in the headworks as well as the powerhouse area.

Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar2


Figure 1: Location Map of Project Area


Project Area


1.4 Background Information

Eastern Region of Nepal within varied geomorphic scenario and with complex

geological set up offers immense scope for utilization of water resources. The water

resources of the Deumai Khola which drains into the Kankai Khola at Mahaguna

village and finally drains out into the Mechi River still remains unutilized. These rivers

are perennial and carry considerable quantity of water. Since the rainfall of the

catchment area is high, these exists a steady discharge of water in these rivers

throughout the year making them ideal for hydropower development in tandem. In

view of above, number of hydropower projects were identified and awarded to Private

Developers by the Government of Nepal to harness these vast natural resources for

the hydropower generation. The proposed Deumai Hydro Electric Project is a self

identified project which has been awarded to Dolakha Nirman Company (P) Ltd.,

Nepal by the Department of Electricity Development (DoED) under the Ministry of

Energy, Government of Nepal, development of hydropower in Mai Khola basin,

through construction of a 20 MW hydropower station utilizing the water resources of

the Deumai Khola in Ilam District, Mechi Zone, Eastern Development Region of

Nepal. Installed capacity has been worked out as a storage scheme.

1.5 Project Layout and Salient Features

The proposed Deumai Hydro Electric Power envisage construction of a 50 m high

and 50 m long concrete dam with central spillway across Deumai Khola, two 9.5 m

diameter and 511 m and 610 m long diversion tunnels, two 8 m diameter intake

tunnels, two 8 m diameter and 298 m long underground pressure shaft, a dam toe

powerhouse and two 10.4 m diameter and 4.1 km long HRT for generation of 30 MW

of hydropower utilizing a gross head of about 70.77 m. It is a reservoir project and all

the appurtenant structures are located along the left bank of the Deumai Khola.

Salient features of the Deumai HEP are given in Table 1.

Table 1: Salient Features of Deumai HEP



To be inserted by Consultant

1.6 Present Investigation



In order to fulfil the objectives and scope of work, the present studies were focused

mainly on general and detailed geological mapping and subsurface explorations. The

main activities performed during the present investigation include the following:

Geological Mapping

-General geological mapping of the project area in 1:4,000 scale

-Detailed geological mapping of the headworks/powerhouse in 1:1,000 scale

-Geological section of the headrace tunnel in 1:4,000 scale

-Mapping of the mucking area and source of the construction materials in 1:50, 000

scale.

Geotechnical Investigation

-Rock mass classification (Q and RMR Values) of the rock mass of headwork site,

tunnel alignment and powerhouse area for suggesting support system,

-Construction material survey and laboratory testing,

-Stereographic projection of major discontinuities of the rock of the project area.

1.7 Sub-surface Exploration

In order to study the sub-surface geology of the area eight lines of the ERT covers

about 1,500 m in the reservoir and intake area. Bedrocks are found at shallow depth

in each line.

2. HIMALAYA IN GENERAL



The Himalaya is the largest mountain range of the world, which extends for a total

length of about 2,400 km. This lengthy mountain chain is geologically divided into five

sections from west to east (Figure 2; Gansser, 1964). The brief descriptions are as

follow:

2.1 Punjab Himalaya

The Punjab Himalaya (about 550 km) lies between the Indus River in the west and

Sutlej River in the east.

Figure 2: Physiographic Subdivision of the Himalayan Arc (after Gansser, 1964)

2.2 Kumaon Himalaya

It borders the Sutlej River in the west and the Mahakali River in the east and extends

about 320 km.

2.3 Nepal Himalaya

The Nepal Himalaya (800 km) lies between the Mahakali River in the west and the

Mechi River in the east.



2.4 Sikkim-Bhutan Himalaya

It starts from the Mechi River and extends along Sikkim and Bhutan for a length of

400 km.

2.5 NEFA (North East Frontier Agency) Himalaya

It stretches for 440 km from eastern boundary of Bhutan to the Tsangpo River in the

east.

3. GEOLOGY OF THE NEPAL HIMALAYA

The Nepal Himalaya is situated in the central part of the Himalayan arc and has

covered about one third part. The Nepal Himalaya is located between Kumaon

Himalaya in the west and the Sikkim-Bhutan Himalaya in the east. The Nepal



Himalaya is subdivided into the following five major tectonic zones from south to north

(Upreti and Le Fort, 1999; Figure 3).

Indo-Gangetic Plain (Terai)

---- Himalayan Frontal Thrust (HFT) ----

Sub-Himalaya (Siwalik or Churia Group)

---- Main Boundary Thrust (MBT) ----

Lesser Himalaya

---- Main Central Thrust (MCT) ----

Higher Himalaya

---- South Tibetan Detachment System (STDS) ---

Tibetan-Tethys Himalaya

3.1 Indo-Gangetic Plain (Terai)

This zone represents the northern edge of the Indo-Gangetic Plain and forms the

southernmost tectonic division, represents Pleistocene to Recent in age and has an

average thickness of about 1,500 m. This zone lies in the southern part of the

Himalaya, basically composed of the boulder to clay. The uppermost part of the Indo-

Gangetic Plain is the Bhabar zone and it comprises of boulder to pebble. The Middle

part (Marshy zone) is composed of sands whereas the clays are dominant in the

southern Terai.

3.2 Sub-Himalaya (Siwaliks or Churia Group)

The Sub-Himalaya (Siwaliks or Churia Group) is developed in the southern part of the

country and is represented by low hills of the Churia Range. The Siwalik Group of

Nepal is composed of 5-6 km thick fluvial sediments of the middle Miocene to early

Pleistocene age. The sediments are generally layers of mudstone, sandstone and

conglomerate. The Siwalik Group is divided into the Lower, Middle and Upper

Siwaliks in ascending order based on the lithology and increasing grain size. The

Lower Siwalik is comprised of mudstone and sandstone, whereas the Middle Siwalik

represented by thick-bedded, coarse-grained, "pepper and salt" appearance

sandstone. The Upper Siwalik is identified with the presence of conglomerate.



3.3 Lesser Himalaya

The Lesser Himalaya lies in between the Sub-Himalaya (Siwalik Group) in the south

and Higher Himalaya in the north. Both the southern and northern limits of this zone

are represented by thrusts, the Main Boundary Thrust (MBT) and the Main Central

Thrust (MCT), respectively. Tectonically, the entire Lesser Himalaya consists of

allochthonous and para-autochthonous rocks. Rock sequences have developed with

nappes, klippes and tectonic windows, which have complicated the geology. The

Lesser Himalaya is made up of mostly the unfossiliferous sedimentary and

metasedimentary rocks, consisting of quartzite, phyllite, slate and limestone ranging

in age from Pre-Cambrian to Miocene.

3.4 Higher Himalaya

This zone is geologically as well as morphologically well defined, and consists of a

huge pile of highly metamorphosed rocks. It is situated between the fossiliferous

sedimentary zone (the Tibetan-Tethys Himalaya} in the north, separated by STDS

and the Lesser Himalaya, separated by the MCT in the south. Paradoxically it is

made up of the oldest rocks of Pre-Cambrian metamorphic and granitic gneiss. The

north-south width of the unit varies from place to place. This zone consists of almost

10 km thick succession of the crystalline rocks also known as the Tibetan Slab (Le

Fort, 1975). This sequence can be divided into four main units. From bottom to top

these units are: Kyanite-Sillimanite gneiss (Formation I), Pyroxenes marble and

gneiss banded gneiss (Formation II) and Augen gneiss (Formation III).

3.5 Tibetan-Tethys Himalaya

Rocks of the Tibetan-Tethys Himalayan zone are made up of thick pile of richly

fossiliferous sediments and their age ranges from early Paleozoic to Cretaceous. This

zone is about 40 km wide and composed of sedimentary rocks such as shale,

limestone and sandstone. In Nepal, these fossiliferous rocks of the Tibetan-Tethys

Himalaya are well developed in the Thak Khola (Mustang), Manang and Dolpa.


HHSS Paleozoic

HHSS Mesozoic

LH Paleozoic

HH leucogranite

LH granite

Thrust

Terai

Duns & recent filling

Siwaliks

Lesser Himalayan zone

LH crystalline nappe

Higher Himalaya cryst.and crystalline nappe

TIBET

INDIA

Langtang Ri

Janakpur

Biratnagar

Everest Kanchenjunga

AnnapurnaDhaulagiri

Manaslu

Kanjiroba

Api

Pokhra

Bhairawa

Piuthan

Nepalganj

JajarkotDhangadhi

Baitadi

Jumla

Simikot

Okhaldunga

SindhuliDhankuta

Taplejung

30° 30°

28°

26°

28°

26°

80° 84° 88°

88°

25 25 50 75 100 km0

MCT

MCT

MBT

MBT

MT

Gosainkund

Kathmandu

STDS

MT

MCT

Tila

MBT

Geological map of Nepal (after Upreti and Le Fort, 1999).

Deumai HEP

Figure 3: Geological Map of the Nepal Himalaya (after Upreti and Le Fort, 1999)

7


The proposed Deumai Hydropower Project belongs to the rocks of the Lesser

Himalaya, Eastern Nepal Himalaya.

4. REGIONAL GEOLOGY OF THE PROJECCT AREA

The area around Deumai Khola area lies in the Lesser Himalaya and equivalent to

the rocks of the Kathmandu Group (DMG, 1987) of Eastern Nepal and comprised of

gneiss, schist and schistose gneiss (Figure 4). Structurally, the area lies north of the



Main Central Thrust (MCT) and Main Boundary Thrust (MBT). The lithostratigraphy of

the Lesser Himalaya of the eastern Nepal has been given in Figure 3 and Table 2.

Table 2: Lithostratigraphy of Eastern Nepal Himalaya

Group Formation Lithology Age

HIMALAYAN FRONTAL THRUST (HFT)

Siwalik

Upper Conglomerate/sands/muds

NeogeneMiddle Sandstone/mudstone

Lower Mudstone/sandstone

MAIN BOUNDARY THRUST (MBT)

Midland

Lakharpata Limestone, limestone, shale

Upper Pre-Cambrian

-Late Paleozoic

Syangja Quartzite, limestone, shale

Sangram Shale, limestone, quartzite

Galyang Slate, sandstones, calcareous slate

Seti* Gritty phyllite, phyllite, quartzite

Ulleri Augen gneiss

Takure* Shale, schist, conglomerates

Kathmandu

Sarung Khola* Schist, gneiss, schistose gneiss

Upper Pre-Cambrian

-Late Paleozoic

Tawa Khola Schist, quartzite

Maksana Schist

Shiprin Khola* Schist, quartzite, metabasic rocks

Udaipur Schist, marble, quartzite

MAIN CENTRAL THRUST (MCT)

HIGHER HIMALAYA

* Rock exposed in the project area

4.1 Lesser Himalaya

This Himalaya consists of high-grade metamorphic rocks like gneiss, schist as well as

schistose gneiss. The rocks of the area are subdivided into the Kathmandu and

Midland groups. The rocks of the Kathmandu Group are also comparable with the

rocks of the Higher Himalaya.

4.1.1 Midland Group

This group comprises five formations, which consist of phyllite, dolomite and

metasedimentary rocks



4.1.1.1 Seti Formation is comprised of alteration of greenish-grey, crenulated phyllite

and gritty phyllite and grey to greenish grey, fine-grained quartzite. This formation

attains more than 3 km thickness. This unit is exposed in the tunnel alignment.

Around the area only the 500 m rocks are exposed.

4.1.1.2 Naudanda Quartzite is represented by presence of thick-bedded, white,

coarse-grained quartzite with frequently developed rippled marks. Total thickness of

the lithounit is about 400 m.

4.1.1.3 Galyang Formation is characterized by presence of dark grey to black

phyllite and spotted white, fine-grained quartzite. Total thickness of the lithounit is

about 1,000 m.

4.1.1.4 Syangja Formation has grey metasandstone intercalates with dark grey

phyllite and dolomite. Total thickness of the lithounit is about 800 m.

4.1.1.5 Lakharpata Formation is represented by presence of bluish-grey dolomite.

This formation attains 500 to 1,000 m thickness.

4.1.1.6 Takure Formation is represented by presence of black shale and schist with

layers of the conglomerates. Rocks unit is exposed in the tunnel alignment. The area

has attained about 650 m thick bedrock.

4.1.2 Kathmandu Group

This group has high grade metamorphic rocks like schist, gneiss. The group has been

subdivided into more than five lithounits.

4.1.1 Sarung Khola Formation is the youngest rock unit of the Kathmandu Group of

the Lesser Himalaya and consists of grey, coarse-grained, garnetiferous schist

intercalated with grey gneiss and schistose gneiss. The unit attains about 4,000 m

thick but proposed are has more than 2,300 m in thickness.



4.1.2 Tawa Khola Formation is represented by presence of grey schist intercalate

with quartzite. It has about 1,200 m thickness.

4.1.3 Maksana Formation is characterized by presence of grey schist. The unit is

about 800 m thick.

4.1.4 Shiprin Khola Formation is comprised of dark grey, garnetiferous schist and

quartzite and meta basic rocks. The area has about 1250 m thick bedrock exposed.

4.1.5 Udaipur Formation is represented by presence of schist, marble and quartzite.

4.2 Siwalik Group

The Siwalik Group is composed of sedimentary rock contains of mudstone,

sandstone and conglomerates. Based on the rock types as well as grain size Siwalik

is divisible into Lower, Middle and Upper Siwaliks.

The Lower Siwalik is comprised of thick mudstone and sandstone. Mudstone is

slightly greater than the sandstone.

The Middle Siwalik is represented by tick beeded sandstone with mudstone and

pebbly sandstone.

The Upper Siwalik is characterised by presence of the conglomerates and lenses of

muds and sands.

The project area is geologically located partly in the rocks of the Sarung Khola

Formation and partly in the rocks of the Shiprin Khola Formation of the Kathmandu

Group and partly in the Seti and Takure formations of the Midland Group. The surge

tanka and powerhouse lies in the rocks of the Lower Siwalik. Most part of the project

area falls in the rocks of the Sarung Khola and Shiprin Khola formations which units

are composed of mainly garnetiferous schist, gneiss and schistose gneiss.



4.2 Geolgical Structures

In proposed area, the Main Boundary Thrust (MBT) and the Main Central Thrust

(MCT) represents the large-scale geological structure (Figure 3). The foliation and

fold area the example of small-scale structure.

4.2.1 Main Central Thrust (MCT)

The MCT strikes in E-W direction and separates the high-grade metamorphic rocks of

the Higher Himalaya to the north and low-grade metamorphic rocks of the Lesser

Himalaya to the south. This thrust is located less than 0.1 km (aerial distance) north

of the proposed project area. This thrust separates the rocks of the Seti Formation in

south and Shiprin Khola Formation in north.

4.2.1 Main Boundary Thrust (MBT)

The MBT strikes in E-W direction and separates the low-grade metamorphic rocks of

the Lesser Himalaya to the north and sedimentary rocks of the Siwalik Group to the

south. This thrust is located 0.1 km (aerial distance) south of the proposed project

area. The MBT separates the rocks of the Lower Siwalik in south and the Takure

Formation of the Lesser Himalaya in the north.

Thrust

The thrust developed between the Takure Formation in the south and the Seti

Formation in the north.

4.2.2 Foliation

The trends of the foliation plane of the project area are northeast to southwest dipping

towards north. The dip directions of rocks range from 0300 to 0400 and dipping

towards south (500 to 750).



LS=Lower Siwalik, MS-Middle Siwalik, MCT-Main Central Thrust, MBT-Main Boundary Thrust


MCT

MBT

10 km

13


Figure 3: Geological Map of Ilam Area, Eastern Nepal (after DMG, 1987)

4.2.3 Fold

A regional syncline fold is developed within the rocks of the Sarung Khola Formation

and the fold can be seen south from the project area (Figure 3). The project area is

located northern limb of the syncline fold.

Proposed intake and reservoir area lies about 5 km north from the MCT and MBT

area but the powerhouse and tailrace area is located in between the MCT and MBT



zones. The tunnel alignment crosses the fold. The activities of the MBT and MCT and

fold seem to be nominal.

Figure 4: Geological map of the Project area

5. DETAIL GEOLOGICAL STUDIES OF THE PROJECT AREA


Sarung Khola Formation

Shiprin Khola Formation

Seti Formation

Takure Formation Formation

Lower Siwalik

Middle Siwalik

15


The project area lies partly in the rocks of the Sarung Khola Formation and Shiprin

Khola Formation, Kathmandu Group and partly lies in the rocks of the Midland Group

(e.g., the Seti and Takure formations) and powerhouse component is located

geologically the rocks of the Siwalik Group. The units of the Lesser Hilamaya are

comprised of gneiss, schistose gneiss and quartzite as well as metabasic rocks, slate

and conglomerates whereas the units of the Siwalik is copmposed of sandstone and

mudstone. The dip directions of rocks range from 0300 to 0500 and dips (600 to 750).

The powerhouse and tailrace are located on the alluvial deposits and bedrocks of the

Lower Siwalik Formation. Thickness of the alluvial deposit is expected to be about 5

m, and high possibility to meet the bedrocks at shallow depth at the powerhouse as

well as in the weir axis area. The tunnel alignment passes through rocks of the

Shiprin and Sarung Khola formations of the Kathmandu Group and rocks of the

Takure and Seti formations whereas the weir axis and intake are located in the rocks

of the Sarung Khola Formation. The surge tank, portal outlet and powerhouse as well

as tailrace and penstock alignment lies in the rocks of the Lower Siwalik Formation.

Colluvial deposits are sparsely found in the project area. Geologically, the area of

powerhouse lies south of the MBT. The tunnel alignment crosses the MBT and MCT.

The detailed geological map of the project area is shown in Figure 4 (1:50,000 scale).

5.1 Reservoir Area

The proposed reservoir area is extended from weir axis to Gajurmukhi village.

Geologically, the proposed basin area also lies in the Sarung Khola Formation (Figure

5). Around the proposed area, thick bedded gneiss and schistose gneiss can be seen

on uphill side. Superficially, the proposed project is covered with shallow depth

alluvial deposits on the bedrocks. The bedrocks are exposed along the both banks of

the Deumai Khola around the reservoir area. Individual thickness of gneiss is more

than 2 m. The proposed area at uphill side covered with bushes as well as dry and

wet cultivated area and has gentle topography. The cross-sections of the reservoir

are shown in Figure 6. The sections are taken 500 m interval to show the distribution

of the soil and rocks.

5.2 Intake and Weir Axis Area



The proposed weir axis area is located about 3.0 km downstream from the

Gajurmukhidham. The intake area is lies about 100 m upstream from the proposed

area of the weir axis area. Geologically, the area is located in the Sarung Khola

Formation (Figure 7). Around the proposed intake area, thick bedded gneiss and and

schistose are exposed on the both banks of the Deumai Khola. Superficially, thick

alluvial deposits on the bedrocks of the Sarung Khola Formation can be seen along

the riverbed. Individual thickness of gneiss is more than 2 m whereas schistose

gneiss can be seen thickness range of 0.5 to 1 m. Presently, the sparsely forest and

cultivated land at uphill can be found around the proposed structure.

5.3 Diversion Tunnel Alignment

The proposed Diversion tunnel passes though the rocks of the Salung Formation.

This unit has thick gneiss and schistose gneiss (Figure 7). Thickness of the beds

range from 2 to 5 m. The proposed diversion tunnel follows the left bank of the

Deumai Khola.

5.4 Tunnel Alignment Area

The proposed tunnel alignment follows on the left bank of the Deumai Khola, also

belongs to the rocks of the Sarung Khola Formation and rocks of the Shiprin Khola

Formation (Figure 8). Intercalation of gneiss and schist as well as schistose gneiss

and quartzite can be found along proposed tunnel alignment. Last portion of the

tunnel alignment passes through the rocks of the Takure and Seti Formations. Thin

layers (< 1 m thick) of colluvial deposits are found on the hill slope. Individual

thickness of schistose gneiss is more than 3 m. The proposed alignment area is

covered by bush and dry cultivated and the forest can be found around the proposed

structure. The geological cross-section shows the detailed geological condition of the

tunnel alignment (Figure 9).

5.5 Surge Tank and Penstock Alignment Area

The proposed surge tank and penstock alignment area is located geologically in the

rocks of the Takure Formation (Figures 10 and 11). Around the proposed surge tank

area, thick gneiss can be observed. The beds of the schistose gneiss range from 1 to



3 m. The proposed area has covered barren to forest. Geological cross-section is

shown in Figure 11.

5.6 Powerhouse and Tailrace Area

The powerhouse and tailrace lies on the left bank of the Deumai Khola about 1 km

upstream from the confluence between the Deumai Khola and Mai Khola.

Geologically, proposed area belongs to the Shirin Khola Formation (Figure 9). The

rocks of the Shiprin Khola Formation at the powerhouse and tailrace area are

exposed on hillside. The beds of schist and quartzite range from 1 to 3 m and 0.5 to 1

m, respectively. At the proposed location, the alluvial terrace is wide and deposited

by the Deumai Khola. The ground surface of recent alluvial deposits of the Deumai

Khola is almost flat. Uphill side has gentle topography and bedrocks are well

exposed. Geologica cross-section is given in Figure 11.

5.7 Adit Area

The proposed adit area is located in the rocks of the Shiprin Khola Formation. Around

the proposed area thick schist can be observed. The tentative length of the proposed

adit shall be less than 500 m.

5.8 Geomorphology

The Deumai Khola is one of the major tributaries of the Mai Khola. The Deumai Khola

originates from the northern of the Ilam within the Lesser Himalayan zone. The

catchment area of the river is characterised by very rugged and steep to mild

topography, which was resulted by the upliftment of the Himalayan Range. It is mainly

composed of sharp crested ridges, steep to very steep slopes and very little spaces

are left for gently sloping lowlands in the valley. The river valley has steep slope and

rocky area. The slope of the both banks of the river valley range from 50 to 60

degrees and somewhere vertical topography can be seen. The proposed weir axis

area has gentle topography. The tunnel alignment has gentle slope whereas the

penstock alignment has also gentle slope. The Sawa Khola and Fawa Khola are two

major tributaries of the Deumai Khola between the reservoir and intake area.



6. ENGINEERING GEOLOGICAL STUDIES OF THE PROJECT AREA

The project layout map also represents engineering geological map of the project

area, has been prepared in 1:1,000 scale for the main project structure sites such as

weir axis site, tunnel alignment (1:4,000 scale), surge tank and penstock alignment,

powerhouse and tailrace area.

The dip directions of rocks range from 0300 to 0400 and dips (300 to 750). The

powerhouse and tailrace located superficially in the alluvial deposits which are

expected to be less than 5 m in thickness. Colluvial deposits are sparsely found in hill

slope in the project area. The exposed rocks on the riverbed as well as on the hill

slope has two sets of the joints, are predominant in the rock mass. They are rough

and stepped. The filling materials in the joints are silty sand. Details of discontinuities

present in the rock mass were measured for the analysis of slope stability of the

project area including stability of the tunnel alignment.

6.1 Quaternary Deposits

The quaternary deposits are found around the project area. The deposits include the

recent river and colluvial deposits.

6.1.1 Recent River Deposits

The alluvial deposits are unconsolidated low-level recent flood plain deposits. These

deposits consist of younger river deposits along the actual riverbed. Low-level

terrace deposits consist of mainly pebble (40-50%) to boulder sized (50-60%), sub

rounded gravels of quartzite and gneiss (90%), and other (10%) filled with loose to

semi-consolidated coarse sand and very fine clay deposits. Thickness of deposits at

powerhouse site is more than 5 m. Remarkable recent river deposits are found along

the river valley of the Deumai Khola.

6.1.2 Colluvial Deposits

Colluvial deposits consist of washed out debris from slope areas and landslide

materials as well as disintegrated and weathered rock fragments. The accumulations

of colluvial deposits are on the hill slope just above the alluvial deposits of the Deumai

Khola.



6.2 Description of the Rock Types

6.2.1 Gneiss

The rock of the Sarung Khola Formation consists of gneiss as well as associated with

schistose gneiss. They are massive in thickness, grey colour. Gneiss is exposed at

powerhouse and tailrace area of the project area along the Deumai Khola project.

6.2.2 Schistose gneiss

The schistose gneiss is intercalated with gneiss and exposed concordantly with

gneiss. The proportion of schistose gneiss is less than and that of gneiss. Schist and

quartzite bands are well exposed in the tunnel alignment area. These rocks are

generally found in the Sarung Khola and Shiprin Khola Formation.

6.2.4 Slate

Grey slate of the Seti Formation is well exposed on the tunnel alignment area.

Thickness of the beds are range from 0.2 to 0.5 m.

6.2.5 Sandstone/mudstone

Thick beaded, fine- to medium-grained sandstone and mudstone with calcareous

sandstone can be seen in the Lower Siwalik Group around the surge tank and

powerhouse area.

6.3 Description Structure wise

6.3.1 Reservoir Area

The reservoir area is extended from weir axis to Gajurmukhi village. Geologically, the

area is located in the Sarung Khola Formation. Around the proposed area, thick

bedded gneiss and schistose gneiss is exposed on the both banks of the Deumai

Khola very rare. Most of the area is covered by the alluvial deposits. The exposed

rocks are fresh to moderately weathered in nature. Generally, two sets of the joints

are observed in rock mass. Individual thickness of the bed varies from 1 to 4 m. The

spacing along the foliation and joints are large (more than 2 m). Superficially, thick

alluvial deposits can be seen along the riverbed. The Fawa Khola and the Sawa

Khola are the main tributaries of the Deumai Khola between the location of the weir

axis and the Gajurmukhi village. The area is covered with thick alluvial deposits



because the Fawa Khola has brought thick debris and deposited on the riverbed

along the Deumai Khola riverbed. The alluvial deposits are composed of boulders and

gravels of the recent alluvial deposits are generally subangular to subrounded in

shape exposed up and downstream from the proposed weir axis area. Majority of

clasts along the riverbed is composed of gneiss and schistose gneiss. Thickness of

deposits is expected more than 4 m along the reservoir area. Maximum diameter of

the boulder is more than 2 m. Both banks, the topography are steep slope gentle

slope but comparatively the left bank has steeper slope than the right bank. Detailed

geological condition of the reservoir area is shown in Figures 5 and 6.

6.3.1.1 Findings

1. The proposed reservoir area is covered thick alluvial deposits and

intermittently bedrock at riverbed and the bedrocks can be seen on both

banks. So, the survey of the ERT has been recommended only in the Fawa

Khola to know the structures.

2. The proposed area has no unstable rock mass as well as the landslides and

shows good stability.

3. The irregular pattern of the ERT result shows some big boulders are along the

riverbed.

6.3.1.2 Recommendations

1. To know the condition geotechnical properties of the rock mass exposed along

the reservoir area, two boreholes have been recommended even the bedrocks

area visually can be seen along the river section at proposed area.

2. Seismic refraction survey is required for determining of the overburden as well

geological structures like the lineaments, faults. It helps to make safe from the

reservoir leakage.



Plate 1: Right bank of the reservoir area Plate 2: Left bank of the reservoir

area

Plate 3: Thick alluvial deposits along the Deumai Plate 4: Bedrock at reservoir area

Khola (reservoir area)

6.3.2 Intake and Weir Axis Area

The weir axis area is located about 3 km downstream from the Gajurmukhi village.

Geologically, the area is located in the Sarung Khola Formation. Around the proposed

weir axis area, thick bedded gneiss schistose gneiss is exposed on the both banks of

the Deumai Khola. The exposed rocks are fresh to moderately weathered in nature.

Generally, two sets of the joints are observed in rock mass. Individual thickness of the

bed varies from 1 to 4 m. The spacing along the foliation and joints are large (more

than 2 m). Superficially, thick alluvial deposits can be seen along the riverbed. The

alluvial deposits are composed of boulders and gravels of the recent alluvial deposits

are generally sub angular to subrounded in shape exposed up and downstream from

the proposed weir axis area. Majority of clasts along the riverbed is composed of

gneiss and schistose gneiss. Thickness of deposits is expected less than 4 m at

headworks site. Both banks, the topography is steep slope but comparatively the

slope is more flat than the right bank. Maximum diameter of the boulder is more than

2 m.

6.3.2.1 Findings



1. The proposed intake and weir axis area is covered bedrock at riverbed and

the bedrocks can be seen on both banks. So, the survey of the ERT has

been recommended.

2. The proposed area shows good stability in rock.


1. To know the condition geotechnical properties of the rock mass exposed at

the weir axis area of the subsurface bedrock, 9 boreholes have been

recommended even the bedrocks area visually can be seen along the river

section at proposed area.

Plate 5: Right bank of the weir axis area

Plate 6: Bedrock at right bank of the Deumai

at weir axis area

Plate 7: Bedrocks of left bank of Deumai

Plate 8: Downstream from weir axis area

Khola at weir axis area

2. Two holes are

recommended in the intake

area to know the subsurface

geological condition

6.3.3 Tunnel Alignment Area



The proposed tunnel alignment follows on the left bank of the Deumai Khola, also lies

initially in the Sarung Khola Formation. Schist of the Sarung Khola Formation and

schistose gneiss, quartzite intercalation can be found. Then, passes through the

gneiss of the Shiprin Khola Formation. Last portion of the alignment also passes

through the rocks of the Seti and Takure formations. Thin to thick layers of colluvial

deposits area found on the bedrocks. Thickness of the beds range from 2 to 4 m

generally found in the gneiss and schist whereas the thickness of schistose gneiss

varies from 0.5 to 1 m. Slates and schist of the of the Seti and Takure formations can

be seen at last section of the tunnel. The orientation of the foliation plane of the

exposed rock is directed toward northeast, which is nearly perpendicular to the tunnel

alignment. Other characters of the exposed rock along the alignment are thick-

bedded, fresh in nature, containing one to two joint sets. The spacing of the

discontinuities is large. Land use pattern at proposed area is barren to forest. The

topography at proposed area has moderate steep slope.

6.3.3.1 Findings

1. The whole alignment of the tunnel alignment lies in the bedrocks except some

locations but at depth there shall be found the bedrocks.

2. The orientation of the foliation plane of the rock is favorable for the tunnel

alignment direction, the angle between foliation plane and direction of tunnel

alignment is more than 60 degrees.

3. The alignment passes through the rocks of gneiss and schistose gneiss of the

Sarung Khola and Shiprin Khola formations. The exposed rock mass is slightly

in weathering condition.

4. The slope stability is good due to characters of the rock mass with long

spacing joints.

5. Last portion tunnel alignment lies in the rocks of the Takure Foramtion

6. The tunnel alignment crosses the MBT and MCT at the last portion.


1. Almost all area covered with bedrocks, It should be better to do the 2D-

Electrical Resistivity Survey in some crossing area to know the groundwater

potentiality.



2. First option-It is better to determine the seismic activities of the thrust in the

DPR stage if the tunnel alignment passes through the MCT and MBT.

3. Second option- It is better to align the pipe on the surface to avoid the area of

the thrust.

4. Between These options which one is better can be cleared during the DPR

phase of the project.

6.3.4 Surge Tank and Penstock Alignment Area

The proposed surge tank and penstock alignment area belongs to geologically in the

rocks of the Lower Siwalik Formation. Around the proposed surge tank area, thick

bedded sandstone and mudstone can be observed. Thickness of the beds varies

from 1 to 2 m generally found in the sandstone and mudstone. The rocks around the

proposed structure area are slightly weathered. Two sets of the joints are visible in

the rocks exposed. The spacing of the rock mass is moderate. The land use pattern is

forest.

6.3.4.1 Findings

1. The bedrocks are clearly observed at the proposed area.

2. The proposed penstock alignment shall follow gentle slope.

3. Moderate space joints are seen in slightly weathered rock mass.

4. No any colluvial deposits on the hill slope along the proposed penstock

alignment.


1. At least two line of the 2D-Electrical Resistivity survey around the surge tank

area has recommended.

2. Based on the ERT report at least a core hole can be proposed.

3. At least two pitholes are dug up along the penstock alignment to determine the

bearing capacity.



6.3.5 Powerhouse and Tailrace Area

The proposed powerhouse and tailrace lies on the left bank of the Deumai Khola.

Geologically, proposed area belongs to the Lower Siwalik Formation. The rocks of the

Lower Siwalik Formation at the powerhouse and tailrace area are exposed on hillside.

But, superficially, thick river deposits have covered the proposed area. At the

proposed location, the alluvial terrace is wide. The ground surface of recent alluvial

deposit of the Mai Khola is almost flat. It is assumed that thickness of alluvial deposits

more than 5 m thick at the proposed powerhouse. The tailrace also lies on thick

alluvial deposits and is more than 10 m from the riverbed. The riverbed of the Deumai

Khola is composed of boulder (> 70%) and fine materials (< 30%). More than 70% of

the boulders of gneiss as well as sandstone are found. The material found at the

proposed sites is fine- to medium-grained, grey sand with rounded to sub rounded

gravels. Thick-bedded, fresh to slightly weathered, greenish-grey to grey sandstones

are exposed uphill side of the proposed powerhouse area. Two sets of the joints are

prominent in the sandstone. The persistency of the rock mass of the discontinuities is

moderate.

6.3.5.1 Findings

1. Thickness of alluvial deposits at proposed powerhouse and tailrace area is

more than 5 m can be observed along the left bank of the Deumai Khola.

These thick deposits need to excavate for the foundation of the

powerhouse.

2. Uphill side topography has gentle slope.


1. At least two lines of the ERT are required to know the basement rocks

depth.

2. At least two core drilling for determination of the geochemical properties of

the rocks at powerhouse is proposed.

3. The river deposits can be used as construction materials.

6.3.6 Adit Area



The adit area is located on the left bank of the Deumai Khola and shall be about 200

m in length. The area is geologically located in the rocks of the Sarung Khola

Formation and almost on the central portion of the tunnel alignment. The topography

of the area is gentle and the exposed rocks are composed of thick schist and

quartzite.



7. GEOTECHNICAL ASPECTS OF PROJECT AREA

Rock mass classification was carried out based on the NGI “Q” and CSIR “RMR”

system. Based on the computed “Q” and “RMR” values the rock mass could be

classified into very good to excellent, good, good to fair, poor, very poor rock,

extremely poor and exceptionally poor rock. Classified rock masses are given in

Table 3. The calculated values can be used for rock support in the headrace tunnel

alignment as well as the underground structures.

Table 3: Rock Mass Classification

(Bieniawaski, 1989); (Barton, 1995)

Descriptions Range of Q-values Range of RMR-values

Rock Class Quality descriptions Minimum Maximum Minimum Maximum

Class 1 Very good to excellent 100 1000 85 100

Class 2 Good 10 100 65 85

Class 3 Fair to good 4 10 56 65

Class 4 Poor 1 4 44 56

Class 5 Very poor 0.1 1 35 44

Class 6 Extremely poor 0.01 0.1 20 35

Class 7 Exceptionally poor 0.001 0.01 5 20

7.1 Reservoir Area

The hill slope along the proposed section starts abruptly with an average slope of 30

degrees and increasing of up to 70 degrees in the high uphill slope at right bank and

also in the left bank the natural slope is upto 40 degrees. The bedrock exposed at the

site has foliation attitude (Dip Direction/Dip Amount) of 025/36. One major (330/66)

and other three minor joint systems (162/29 and 256/75) are observed at the area

in Table 4.

7.1.1 Rock Classification

Geotechnical classification for jointed rock mass of the reservoir using CSIR

classification was carried out based on the detailed surface discontinuity (Table 5).

Most of the Rock Mass Rating (RMR) of the headwork area falls in the range 60 to 70

and it indicates that the rock mass of headwork site is categorized as a Class II-III

type, which is defined as the fair to good rock (Table 5).



Table 4: Attitudes of Rock Mass (Dip Direction/Dip Amount) of Project Area

Location Natural Hill Slope Foliation Joint (J1) Joint (J2) Joint (J3)

Reservoir Area

Left bank 092/81 025/36 330/66 256/75 162/29

Right bank 247/78 048/67 332/65 247/78 148/67

Weir Axis Area

Left bank 135/30 056/78 321/78 235/67

Right bank 280/69 077/77 342/65 224/52 150/38

Diversion Tunnel Area

Left Bank 280/69 070/70 322/59 245/80 165/32

Tunnel Alignment

CH. 0+000 - CH. 0+725 066/60 318/72 144/35 248/78

CH. 0+725 - CH. 2+250 084/60 330/68 136/36 158/60

CH. 2+250 - CH. 4+125 080/50 310/64 120/22 141/78

CH. 4+125 - CH. 4+850 045/62 314/72 140/40 224/74

CH. 4+850 - CH. 5+178 054/62 330/60 235/60 110/50

Surge Tank/Penstock Alignment 075/34 305/80 220/80 115/75

Powerhouse Area 075/30 300/80 202/60 072/48

Source: Field data

Table 5: Geotechnical Parameters of the Rock Mass

7.1.2 Weathering and Strength

Rock mass in the reservoir area is fresh to slightly weathered (Table 5). Generally,

the rocks along riverbank are fresh rock and slightly weathered at higher hill slope.

Gneiss is strong rock. The compressive strength of the augen gneiss range from 200

to 250 MPa. Because of presence of the high percentage of the feldspar, the

weathering shall be quite quick when it reacts with water.



7.1.3 Slope Stability

Slope stability assessment analysis of the both banks hill slope was carried out on the

basis of aerial photos interpretation and geological observations. An analysis of

foliations to determine the stability of the rock mass due to the presence and

orientations of the foliations in the rock mass at the reservoir was done using Lower

Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The

wedges formed by the foliation planes and joints were then analyzed with respect to

the hill slope surface. The dipping of the foliation plane is favorable to the natural hill

slope and the relation between them is opposite to the hill slope so less possibility to

occur failure. The wedge formed by the intersection of the joints (J1, J2 and J3) may

occur stable because these wedges are developed opposite to the natural hill slope.

The slope stability condition of the rock mass is presented in Figures 12 and 13.

Similarly, the stability condition is more or less similar to the left bank (Figures 14 and

15).

Figure 12: Contour Density Map of the Rock Mass on Left Bank of the Reservoir Area



Figure 13: Stereographic Projection of the Rock Mass on Left Bank of the Reservoir Area

Figure 14: Stereographic Projection of the Rock Mass on Right Bank of Reservoir Area

Figure 15: Contour Density Map of the Rock Mass on Right Bank of the Reservoir Area



7.2 Intake and Weir Axis Area

The intake and weir axis area along the Deumai is HEP is located on the left bank

and in the bedrocks. The hill slope starts abruptly with an average slope of 52

degrees and increasing of up to 80 degrees in the high uphill slope at left bank. The

natural slope is comparatively gentler than right bank. The bedrock exposed at the

site has foliation attitude (Dip Direction/Dip Amount) of 056/78. One major (321/78)

and other three minor joint systems (235/69) are observed at the area in Table 4.

Table 6: Rock Mass Rating of the Project Area


Geotechnical classification for jointed rock mass of the headwork using CSIR

classification was carried out based on the detailed surface discontinuity. Most of the

Rock Mass Rating (RMR) of the headwork area falls in the range 59 to 64 and it

indicates that the rock mass of headwork site is categorized as a Class III type, which

is defined as the fair to good rock (Table 6).


Rock mass in the headwork area is fresh to slightly weathered (Table 5). Generally,


Gneiss is fairly strong rock.


Slope stability assessment analysis of the left bank hill slope was carried out on the




1

1

2

2

3

3

4

4

N

S

EW

Orientations

ID Dip / Direction

1 30 / 135

2 69 / 235

3 78 / 321

4 78 / 056

Equal AngleLower Hemisphere

54 Poles54 Entries

1

1

2

2

3

3

4

4

5

5

N

S

EW

Orientations

ID Dip / Direction

1 38 / 150

2 77 / 077

3 69 / 280

4 65 / 342

5 52 / 224

Equal AngleLower Hemisphere

11 Poles11 Entries


orientations of the foliations in the rock mass at the intake site was done using Lower

Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The

wedges formed by the planes were then analyzed with respect to the hill slope

surface. The dipping of the foliation plane is favorable to the natural hill slope and the

relation between them is opposite to oblique so very less possibility to occur failure.

The wedge formed by the intersection of the joints (J1 and J2) may occur failure. The

slope stability condition of rock mass is presented in Figures 16 and 17.

Figure 16: Stereographic Projection of the Rock Mass on Left Bank of the Proposed Weir Axis

Area

Figure 17: Stereographic Projection of the Rock Mass on Right Bank of the Proposed Weir Axis

Area

7.3 Diversion Tunnel Alignment Area

The exposed rock beds are competent and are favourably dipping against slope face

direction. The attitude of the rock bands are more or less same as the geotechnical

characters of the rock exposed at proposed intake and weir axis area i.e., 070/70,

322/59, 245/80, 165/32 foliation, joint 1, joint 2 and joint 3, repetitively. The exposed



rock is fresh to slightly weathered with average joint spacing 2 to 3 m. The joint

surfaces are rough and have some silty sand fillings in the exposed areas.


Geotechnical classification for rock mass tunnel alignment using CSIR classification

was carried out based on the detailed surface discontinuity measurements on

exposed rock outcrops in the tunnel alignment. The results of the geotechnical

classification are presented in Table 5.

NGI Tunneling Index ‘Q’ was also carried out based on the detailed surface

discontinuity measurements on exposed rock outcrops in the tunnel alignment in

some location. The results of the geotechnical classification at the tunnel alignments

are presented Table 6 which is more or less same as the initial section of the tunnel

alignment from the intake area.


Rock mass in the headwork area is fresh to slightly weathered (Table 5). Generally,


Gneiss is fairly strong rock.


Slope stability assessment analysis of the left bank hill slope was carried out on the



orientations of the foliations in the rock mass at the intake site was done using Lower

Hemisphere Projection of the foliation planes in Schmidt’s equal area net (Figures 18,

19). The internal friction angle has been adopted as 30 degrees for the stability

calculation. The dipping of the foliation plane is favorable to the natural hill slope and

the relation between them is opposite to oblique so very less possibility to occur

failure. The wedge formed by the intersection of the joints (J1 and J2) may occur

failure.



Figure 18: Contour Density Map of the Rock Mass along the Diversion Tunnel Alignment Area

Figure 19: Stereographic Projection of the Rock Mass along Diversion Tunnel Alignment Area

7. 4 Tunnel Alignment Area

The conveyance of water is proposed to be done by tunnel. This has been proposed

considering the following aspects:

a) Morphological conditions of the alignment route,

b) Surfacial slope stability conditions,

c) Rock mass property observations and

d) Economical aspects



The slope along the proposed tunnel is generally favorable and stable. The

foundation of the tunnel is on the bedrock. This most of the alignment is on gneiss

and schistose gneiss bedrock of the Sarung, Shiprin Khola formations and is over

moderate steep-to-steep hill slope. The hill slope starts abruptly with an average

slope of 45 degrees. The attitude of foliation of the rock mass in the area is shown in

chaingae wise in the sections (Table 10). The exposed has average joint spacing of 1

to more than 4 m (Table 5). The joint surfaces are rough and steeped; and have

some silty clay fillings in the exposed areas. Measured discontinuities are given in

Table 4.


Geotechnical Classification for rock mass tunnel alignment using CSIR classification


exposed rock outcrops in the tunnel alignment. The results of the rock mass

classification are presented in Table 6. Most of the alignment is covered with the good

to good to fair rock are seen at the initial and last part of the tunnel but poor and very

poor rock mass can be expected at the last section of the tunnel alignment because

of the presence of some geological structures as well as the lithological variation.

NGI Tunneling Index ‘Q’ was also carried out based on the detailed surface

discontinuity measurements on exposed rock outcrops in the tunnel alignment in

some location. The results of the geotechnical classification at the tunnel alignments

are presented Table 7. Most of the alignment is covered by the good rock and some

of the locations are good rock are seen at the initial and middle part of the tunnel but

poor and very poor rock mass can be expected at the last section of the tunnel

alignment because of the presence of some geological structures as well as the

lithology.




Rock mass in most of the tunnel alignment is fresh to slightly weathered, with some

moderately weathered rock exposed. The rock at initial portion is competent and hard

rock but last portion of the rocks are weak in nature due to presence of low grade

metamorphic rock and sedimentary rocks.

Table 12: Rock Mass Classification Using NGI Tunneling Index Q Classification

Note: Q = RQD/Jn x Jr/Ja x Jw/SRF


An analysis of foliations to determine the stability of the rock mass due to the

presence and orientations of the foliations in the rock mass along the tunnel was

done using Lower Hemisphere Projection of the foliation planes in Schmidt’s equal

area net. The internal friction angle has been adopted as 30 for the stability

calculation. Analysis of slope stability has been given in Figures 20-27.

Between the CH. 0+000 and 2+250

More or less stable in condition but wedge formed by intersection of F and J2 is some

critical. But wedge formed by intersection of the F and J1 is more or less stable

because of low angle wedge. But the size of the wedges is blocky. Three sets of the

joints are seen and very tightly joint can be found. Mainly the rock of the gneiss and

schist can be found with large persistency.

Between the CH. 2+250 and 4+125

Stability is good but the intersection of the F and J1, J2 as well as J3 are danger and

may occur failure. The section is partly covered with gneiss and schist. Large

persistency is generally found in the rock mass.

Between 4+125 and 4+850



More or less the stability condition is same as the chainage between 4+125 and

4+850. The section is covered with slate and schist. moderate persistency is found in

the rock mass in general.

Between 4+850 and 5+178

Stable in condition very less possibility to occur failure but the presence of the highly

jointed rocks may create the problem. The area is influenced by the thrust and

sedimentary as well as soft rocks.

Figure 20: Contour Density along the Tunnel Alignment Area CH. 0+000-0+725

Figure 21: Stereographic Projection of the Rock Mass along the Tunnel Alignment Area CH.

0+000-0+725



Figure 22: Contour density Map Mass along the Tunnel Alignment Area CH. 0+725-2+250


0+725-2+250



Figure 24: Contour Density Map along the Tunnel Alignment Area CH. 2+250-4+125


2+250-4+125



Figure 26: Contour Density along the Tunnel Alignment Area CH. 4+125-5+178

Figure 27: Stereographic Projection of the Rock Mass along the Tunnel Alignment Area CH. -

4+125-5+178

Assessment of Initial Support Requirement For Tunnel

Empirical Method

Empirical assessment of rock enforcement requirement for the tunnel has been

empirically assessed based on the rock mass classification and stability analysis

carried out based on assumed/extrapolated data. Once the excavation begins, the

parameters used to determine the rock mass quality must be re-evaluated

continuously.

These parameters include:



1. Number of joints per unit volume and their orientations

2. Joint conditions such as tightness, loose openings and in-fill materials

3. Continuity of joints

4. Joint surface conditions such as roughness, degree of weathering and

coatings.

5. Joint water conditions

6. Presence and orientation of shear zones, clay seams or loose open joints

crossing the tunnel excavation or the presence of squeezing of swelling rock

7. The rock strength with ratio to the major principal rock stress expected at the

tunnel periphery.

Support requirement based on the Q value: A Chart of equivalent dimension De,

plotted against the Q value, is used to define a number of support categories (Barton

et al., 1974).

De = Excavation span, diameter of height (m) / ESR (excavation support ratio)

For headrace tunnels of Hydropower, ESR is taken to be 1.6, hence for an excavation

of span of maximum 4m, De = 1.63.

About 93 % of the whole tunnel length is expected to be in fair to good rock with Q= 5

to 11 and only about 07 % in poor to very poor rocks with some highly fracture zones.

From the Chart, the 93% of the tunnel section falls in Category 1 showing

unsupported span. However, such conditions will not be anticipated in real practice.

For tunnel section with poor rock Q < 4, De = nearly 2.0, support category 4 is

required by the chart.

The Length of Rock bolt can be estimated from excavation width B and Excavation

Support Ratio ESR, as follows

L = (2 + 0.15B)/ESR

Substituting, B = 4.0 m and ESR = 1.6, the minimum length of rock bolt comes to be

1.625 m (~ 2.0 m).



7.5 Estimated Rock Support

Rock support in the tunnel and underground cavern is provided to improve the stability

and to safeguard the opening with respect to safety of the working crew. The guiding

principle of rock support design is that it is capable to response the actual ground

conditions that is encountered in the tunnel and the safety requirement at the tunnel face

is met. This requires provision of flexible rock support methods that can be quickly

adjusted to meet continuously changing heterogeneous rock mass (Table 8). The best

way to achieve such flexibility is the use of rock bolts, steel fiber shotcrete, pre-injection

grouting, and the use of steel ribs.

Table 8: Designed Tunnel Rock Support Class and Respective Rock Support

Rock Mass Quality

Description

Rock

Support

(RS) Class

Assigned Tunnel Rock Support

Good Rock RS II Fully grouted 25 mm diameter and 2 m long rock bolts spaced at

4.5 m circumferentially and 4.5 mnlongitudinally plus 100 mm thick

shotcrete with single layer of wire mesh reinforcement (4 mm dia

welded in mesh size 100 mm x 100 mm) or steel fibercrete.

Fair to good rock mass RS III 25 mm diameter 2 m long systematic grouted rock bolts at a spacing

of 1.5 m x 1.5 m and 10cm thick steel fiber shot crete in all tunnels

and at settling basin cavern 25 mm diameter 4 m long systematic

grouted rock bolts at a spacing of 1.5 m x 1.5 m 15 cm thick steel

fiber shotcrete at the settling basin cavern.

Poor rock mass RS IV 25 mm diameter 2 m long systematic grouted rock bolts at a spacing

of 1.3 m x 1.5 m and 15 cm thick steel fiber shot crete.

Very poor rock mass RS V 25 mm diameter 2 m long systematic grouted rock bolts at a spacing

of 1.3 m x 1.3 m and 15 cm thick steel fiber shot crete.

Extremely poor rock

mass

RS VI 25mm diameter 2 m long systematic grouted rock bolts at a spacing

of 1.2 m x 1.2 m and 20 cm thick steel fiber shotcrete. Steel ribs at a

spacing of 1 meter to control plastic deformation. Advance pre-

injection grouting is provisioned to control water inflow into the

tunnel.

Exceptionally poor

rock mass

RS VII 25 mm diameter 2 m long systematic grouted rock bolts at a spacing

of 1.1 m x 1.1 m and 20 cm thick steel fiber shot crete. Steel ribs at a

spacing of 1 meter to control plastic deformation.



The headrace tunnel will be in hydrostatic condition during its operation. Since the

designed rock support in the table is not water tight, the concept of pre-injection

grouting should be applied at the required length of headrace tunnel to control

possible water leakage during operation. The BOQ and respective drawings

illustrates the given rock support for all underground works. In Table 9 the rock

support assigned for headrace tunnel has been presented since the ground condition

changes at different tunnel segment.

Table 9: Assigned Rock Support in Respect with Rock Mass and Rock Support Class

Location

Rock

Mass

Class

Rock

Support

Class

Assigned rock support measures

Ch. 0+000-Ch. 0+725 Class III RS III

25 mm diameter 2 m long systematic grouted rock bolts at a

spacing of 1.5 m x 1.5 m and 10cm thick steel fiber shot crete

in all tunnels and at settling basin cavern 25 mm diameter 4 m

long systematic grouted rock bolts at a spacing of 1.5 m x 1.5

m 15 cm thick steel fiber shotcrete at the settling basin cavern.







Ch. 2+250-Ch. 4+125 Class II RS II

Fully grouted 25 mm diameter and 2 m long rock bolts spaced

at 4.5 m circumferentially and 4.5 mnlongitudinally plus 100

mm thick shotcrete with single layer of wire mesh reinforcement

(4 mm dia welded in mesh size 100 mm x 100 mm) or steel

fibercrete.







Ch. 4+850-Ch. 5+178 Class IV RS IV


spacing of 1.3 m x 1.5 m and 15 cm thick steel fiber shot crete.

Steel ribs at a spacing of 1 meter to control plastic deformation.

Advance pre-injection grouting is provisioned to control water

inflow into the tunnel due to the very weak and fractured zone.



The rock mass (Table 9) from Ch. 0+000 to Ch. 2+250 requires the III support type

whereas of tunnel from Ch. 2+250 to Ch. 4+125 needs of II support types. Similarly,

from Ch. 4 + 125 to Ch. 4+850 there is required of support III. From Ch. 4+850 to Ch.

5+178 (0.50 km) requires the IV support system. Details about the tunnel and

supports are given in Table 10.

Out of 5178 m of the length, the alignment covers 4,850 m of the tunnel length which

is equal to 93.86% of the total length of the tunnel alignment needs of the support II

and III and remain of 6.14% (e.g., 328 m of the tunnel length) needs of support IV to

V.





Table 10: Assigned Rock Support in Respect with Rock Mass and Rock Support Class

Description/Structures Tunnel AlignmentChainage CH. 0+000- CH. 0+725 CH. 0+725-CH. 2+250 CH. 2+250-CH. 4+125 CH. 4+125-CH.4+850 CH. 4+850-CH.5+178

Rock typeGneiss/Schistose

gneissGneiss/Schistose

gneiss Gneiss/Schist Slate/Schist Schist/sandstone/mudstoneWeathering Fresh-slightly Fresh-slightly Fresh-slightly Fresh-slightly Fresh-moderatelyStrike and Dip 242-062/38S 234-054/35S 226-046/36S 030-210/22S 050-230/40SDip Direction/Dip Amount 066/60 084/60 080/50 045/62 054/62

RMR Value 61 58 65 62 40-37Rock Classification III III II III IVDefinition Good-Fair Good-Fair Good Good-Fair Poor to very poor rockQ values 6.5 7.46 10.88 9.64 3.29Rock Classification III III II III IVDefinition Good-Fair Good-Fair Good Good-Fair Poor to very poor rockSupport system III III II III IV-V



7. 6 Surge Tank/Penstock Alignment Area

The hill slope of surge tank face is moderate. The exposed rock beds are less

competent and are favourably dipping against slope face direction. The attitude of the

bedrock is 075/34 (dip direction/dip). One major (345/70) and two other minor joint

sets (228/70 and 172/79) are observed in the exposed area. The surge tank as well

as the penstock alignment passes though sandstone of the Lower Siwalik Formation.

The joint surfaces are slightly to altered with average joint spacing of 1 to 2 m. The

joint surfaces are rough and have silty sand fillings in the exposed areas. The

measured discontinuities are given in Table 4.


Geotechnical classification for rock mass of the alignment using CSIR classification


exposed rock outcrops in the alignment. The results of the geotechnical classification

are presented in Tables 5 and 6.


Rock mass is fresh to slightly weathered, and has less competent rock.


The stability of surge tank/penstock has been analyzed on the basis of geotechnical

and geological observations on the surface of the hill slopes. Analysis of all the

observed bedding plane attitudes and their conditions at different locations has been

presented in Figures 28 and 29. In general, the stability condition is good and some

of the wedges formed by joints may unstable.

Figure 28: Stereographic Projection of Rock Mass at Surge Tank Area



Figure 29: Stereographic Projection of Rock Mass at Surge Tank Area

7.7 Powerhouse and Tailrace Area

The powerhouse and tailrace area is situated in the rocks of the Lower Siwalik, is

situated at the low land depressed valley. The powerhouse area is composed of

superficially alluvial deposits and the bedrock is encountered at uphill side.


Geotechnical classification for rock mass powerhouse using CSIR classification was

carried out based on the detailed surface discontinuity measurements on exposed

rock outcrops around the powerhouse area. The results of the geotechnical

classification are presented in Table 4 was also carried out based on the detailed

surface discontinuity measurements on exposed rock outcrops. Generally poor rocks

are exposed around the powerhouse area.


An analysis of foliations to determine the stability of the rock mass due to the

presence and orientations of the foliations in the rock mass is done using Lower

Hemisphere Projection of the bedding planes in Schmidt’s equal area net. Analysis of

all the observed foliation plane attitudes and their conditions at different locations has

been presented in Figures 30 and 31.



Figure 30: Contour Density map of the Powerhouse area

Figure 31: Stereographic Projection of Rock Mass at Powerhouse Area

8. SEISMICITY

This chapter deals with the preliminary investigation of maximum credible earthquake

and peak ground acceleration for an assessment of the proposed Deumai Khola

Hydroelectric Project (Figures 36, 37 and 38). The analysis is basically made by

deterministic evaluation of earthquake sources in the vicinity with the state of art

consideration of attenuation for the Himalayan terrain. It should be acknowledged that

the problems of seismo-tectonic events of Himalaya are not fully understood and the

knowledge is increasing with more and more accumulation of research results and

data analysis. The study has considered the latest results of seismo-tectonic study of



the Himalaya and the vicinity. For comparison purpose, both deterministic and

probabilistic assessments of seismic hazards have been considered.

8.1 Seismo-tectonic Model

The Himalaya seismicity, in general, owes its origin to the continued northward

movement of Indian plate after the continental collision between Indian plate and

Eurasian plate. The magnitude, recurrence and the mechanism of continental

collision depend upon the geometry and plate velocity of Indian plate in relation to

southern Tibet (Eurasian Plate). Recent results suggest that the convergence rate is

about 20 mm/year and the Indian plate is sub-horizontal below the Sub-Himalaya and

the Lesser Himalaya.

The result of micro seismic investigation, geodetic monitoring and morphotectonic

study of the Central Nepal has depicted that the more frequent medium sized

earthquakes of 6 to 7 magnitude are confined either to flat decollment beneath the

Lesser Himalayas or the upper part of the middle crustal ramp. The ramp is occurring

at about 15 km depth below the foothills of the Higher Himalaya in the south of MCT

surface exposures. Big events of magnitude greater than eight are nucleated near the

ramp flat transition and rupture the whole ramp-flat system up to the blind thrust

(MBT) of the Sub-Himalaya (Pandey et. al. 1995).

This general model worked out for the Western Nepal can be applied to other parts of

the Himalaya with the evaluation of further subsequent ramping towards more south

in the Lesser Himalaya and the associated seismicity. This structural variation along

Himalayan arc is responsible for the segmentation of potential ruptures along the arc

i.e. along the longitudinal direction. For deterministic assessment of seismogenic

sources, the local structural environment magnifying the general model near the

project site is considered.

8.2 Deterministic Assessment

Considering the above interpretation, the deterministic design earthquake can be

taken as a sub- horizontal thrust of rupture extent of about 30 km occurring at a depth

of 15 km within a plan distance of a few km, (e.g., 5 km) from the site. The width of



the rupture is proposed to be about 25 km. A magnitude of 7.0 is estimated from

rupture area of 750 km2 with Ms = 4.15 + Log A (Wyss, 1979). Actually there has

been an earthquake of M = 7 at a distance of about 15 km from the site in 27 May

1936. However the epicentre may be closed to the site considering the uncertainty of

location. It should be noted that a similar environment exists in the Uttarkashi area of

Garhawal Himalaya where an event of magnitude Mb 6.5, Ms 7.1 occurred in 19

October 1991. Its moment magnitude was Mw 6.8 with moment equal to (0.8- 1.8)* 10

E 19 N.m. and the mechanism was a low angle thrust. The rupture length is reported

to be about 25 km with maximum slip of 2.5 m. The deterministic assessment of

maximum credible earthquake can be considered to be the big earthquake rupturing

the entire detachment of the Indian plate as discussed in the model and therefore

considered to be of magnitude 8.3 -8.6 like other great earthquakes of the Himalaya.

8.3 Horizontal Acceleration

Deterministic Approach

Evaluation of peak ground acceleration is carried out by applying the mostly used

formula of McGuire (1968), Katayama (1975), Oliveira (1984) and Kawashima (1984)

for the above earthquakes concluded deterministically from seismo-tectonic models.

Log A = 3.090 + 0.347 M -2 lag (R + 25) (C. Oliveira)

Log A = 2.308 + 0.411 M- 1.637lag-(R + 30) (T. Katayama)

Log A = 2.674 +0.278 M – 1.30 1 log (R + 25) (R. K. McGuire)

A = 1.006* 10E (0.216*M)*(R+30) E-l.218 (Kawashima)

R = hypo central distance in kilometre

The recorded peak acceleration data in Uttarkashi earthquake of Ms = 7.1 of 1991 is

0.21 9 at a distance of about 28 km. from the epicentre. Katayama's relation gives an

estimate of 0.20g; Kawashima's estimate is 0.23g while McGuire's relation estimates

as 0.24g. Oliveira's relation underestimates the acceleration.

8.4 Probabilistic Approach

Preliminary seismic hazard assessment of the country using Gumbel's third

asymptotic extremes with the instrumental seismicity database of ISC is carried out

by Bajracharya (1994) for different return periods 50, l00, 200 and 300 years,



Attenuation model with mean value of McGuire and Oliveira ". (see above) is used for

horizontal acceleration.

Return period (years) Peak horizontal acceleration (g)

50 0.10

100 0.15

200 0.20

300 0.25

8.5 Recurrence Period

The best estimate of b value for the project area is 0.84 as shown by the analysis of

micro seismic events. The 1991 event of Uttarkashi is considered by many

investigators to be the repetition of 1833 event, which gives a basis for a recurrence

of 158 years for 7.1 magnitude event in similar geological setting. Moreover the

observed slip of about 2.5m in Uttarkashi earthquake also is consistent with 178

years of recurrence considering 70% contribution of 20 mm/year plate convergence

rate to seismic strain.

8.6 Historical Seismic Activity

The Nepal Himalaya has experienced several large earthquakes over the past

centuries. The earthquakes of larger magnitudes that have occurred in Nepal

Himalaya are summarized below in Table 11.

Table 11: Larger Magnitude of Earthquake Occurred in Nepal Himalaya

S.N Location of Earthquake Year Magnitudes

1 Udayapur, Eastern Nepal 1988 6.6

2 Chainpur, Eastern Nepal 1934 8.3

3 Dolakha, Central Nepal 1934 8.0

4 Sindhupalchowk, Central Nepal 1833 8.0

5 Kaski, Western Nepal 1954 6.4

6 Myagdi, Western Nepal 1936 7.0

7 Bajhang, Far Western Nepal 1980 6.5

8 Dharchula, Far Western Nepal 1966 6.1





8.7 Earthquake Catalogue

The National Building Code Development Project (BCDP, 1994) has developed an

earthquake catalogue using earthquake data catalogues of the US Geological

Survey, The National Earthquake Information Centre (NEIC), National Oceanic and

Atmospheric Administration and National Geological Data Centre (NGDC). The

complete earthquake catalogue for the magnitudes M 4.5 and greater is given in

Table 12.

Table 12: Instrumentally Recorded Earthquake

S.N Magnitudes Catalogue Year

1 M 6.0 and grater than M 6.0 Catalogue complete for the period 1911 to 1992

2 M 5.5 and greater than M 5.5 Catalogue complete for the period 1925 to 1992

3 M 5.9 and greater than M 5.9 Catalogue complete for the period early 1960 to 1992

4 M 4.5 and greater than M 4.5 Catalogue complete for the period late 1970 to 1980s

The largest event reported in the catalogue is the magnitude 8.3 Bihar–Nepal

earthquake (Chainpur), which appears to have occurred in1934.

Figure 36: Epicenter of the Earthquake in the Nepal Himalaya


Project Area

Project Area

87


Figure 37: Probabilistic Seismic Hazard Assessment Map of the Nepal Himalaya

Figure 38: Seismic Risk Zone of the Nepal Himalaya

For the minimum acceleration of 150 gal (Figure 37), reduction factor of 0.5 the

calculated effective design seismic coefficient is approximately 0.09.


Project Area

88


For the maximum acceleration of 200 gal (Figure 37), reduction factor of 0.65 the

calculated effective design seismic coefficient is approximately 0.13.

Hence, the design horizontal seismic coefficient ranges from 0.09 to 0.13 (calculated

values).

Based on above results the design seismic coefficient for the project can be taken

range of 0.09 to 0.13 which is more or less same value represent from the return

period of the earthquake. The project area lies seismically in zone 2.

8.8 Seismic Zoning

The Seismic Hazard Mapping and Risk Assessment component of the NBCDP

carried out detailed analysis of the earth activity and the tectonic structure of Nepal,

and had identified groups of earthquakes with major tectonic features leading to the

identification of seismic zones of assumed uniform seismicity. The three seismic

zones thus identified in Nepal are shown Figure 38.

8.9 Seismic Design Acceleration Coefficient

On the basis of the MHSP studies on Seismic Hazard Assessment and the

derivation of the basic design earthquake accelerations, and on the basis of the

earthquake design coefficient used in other major hydroelectric projects in Nepal,

e.g. the Kali Gandaki ”A” Hydroelectric Project and the Arun-III Hydroelectric Project,

the following earthquake coefficients were recommended and used in the design of

major and minor structures of the LIVHEP are as given in Table 13.

Table 13: Design Earthquake Acceleration Coefficients

S.N StructureBasic Horizontal Acceleration

Coefficient (αH)

Vertical Acceleration Coefficient

(αv)

1 Major Structure 0.25 67% of(αH)

2 Minor Structure 0.20 67% of(αH)

9. CONSTRUCTION MATERIALS SURVEY



A construction material investigation was conducted in the vicinity of the headworks

and powerhouse site as well as along the Deumai HEP area. The investigation is

focused on locating prospective borrow areas of non-cohesive and cohesive

materials, which are to be used mainly as an ingredient of concrete as well as clay.

The prospective borrow sites were identified as sources of coarse aggregates from

either boulders of gneiss or bedrocks. So, the materials should be collected either

from the Kankai Khola for the powerhouse area and the Deumai Khola for the intake

and weir axis area. The area for the materials for the project is located entire the

project area.

The requisite quantities of construction material like boulders, cobble, gravel and

sand are generally available in and around the project. Point bar and braided bar

deposits of the Deumai Khola and Kankai Khola and excavated materials are the

main source of construction material. These deposits predominantly consist of gneiss

boulder, cobble and gravel including some gneiss and quartzite. The boulder, gravel

and sand deposits in the point bars in and around the powerhouse site along the

Kankai Khola and Deumai Khola can be used as construction materials.

9.1 Borrow Area

At the intake site, construction material can be extracted from the alluvial bar deposit

on the both banks as well as upstream of the river from the Kankai Khola. Material

excavated during the construction period from Deumai Khola between

Gajurmukhidham and Weir axis area. Similarly the red clay can be extracted from the

area around Gajurmukhidham area and surrounding area whereas around the

powerhouse area the construction materials can be extracted from either the Kankai

Khola or the Deumai Khola. The location of the materials extraction is shown in the

Figure 39. The excavated materials can be used as for the construction materials.

Four samples of construction material including aggregates and sands from the

project area have been collected and tested in the laboratory to determine the

physico-mechanical properties.

The location as well as the expected volume and composition of the materials are



presented in Table 13 and Figure 33. The materials for the core and filter for the dam

is more than sufficient quantity.

Table 13: Location and Tentative Volume of the Construction Materials

Location Volume Stability Hydrology Distance Materials

S1 Weir axis and Gajurmukhidam

Intake Area

Aggregate and sands

Unlimited Stable Dry Maximum 2 km from

Intake area

Aggregate of

gneiss, quartzite

S2 Upstream from the Intake Area

Sands and aggregates

Unlimited Stable Dry 2 km km from intake

area

Riverbed

materials

S3 Kankai Khola just down from

tailrace

Sands and aggregate

Unlimited Stable Dry 0.3 km from intake Riverbed

materials

S4 Iban and Jitpur as well as

Dhuseni area

Red soil clay

Unlimited Stable Wet-dry 5km from the

headworks area

Hill slope

Total Volume Unlimited

Three rock samples from the quarry sites to measure the point load test as well as the

petrographic analysis. The following physic-mechnical properties of the collected

samples shall be done:

9.2 Sampling of Construction Material

Sampling of aggregate for concrete including packing and marking of samples shall be undertaken

following IS: 2430 – 1986.

Coarse Aggregate

The following tests shall be carried out to test the material for use as coarse aggregate:

S.No. Name of Test Reference Standard to be Followed

1 Specific Gravity IS: 2386 (Part 3)-1963

2 Water Absorption IS: 2386 (Part 3)-1963

3 Aggregate Abrasion Value (Los-Angeles) IS: 2386 (Part 4)-1963

4 Aggregate Crushing Value IS: 2386 (Part 4)-1963

5 Aggregate Impact Value IS: 2386 (Part 4)-1963

6 Soundness ( 5 cycles) (Sodium Sulphate) IS: 2386 (Part 5)-1963

7 Flakiness Index IS: 2386 (Part 1)-1963

8 Elongation Index IS: 2386 (Part 1)-1963




9 Petrographic Examination IS: 2386 (Part 8)-1963

10Potential Alkali Reactivity of Aggregate (Chemical Method)

IS: 2386 (Part 7)-1963

11Alkali Aggregate Reactivity by Mortar Bar Method (Accelerated Technique)

ASTM Designation: C 1260-01

Fine Aggregate

The following tests shall be carried out to test the material for use as fine aggregate:


1 Gradation and Fineness Modulus IS: 2386 (Part 1)-1963

2 Silt and Clay Content IS: 2386 (Part 2)-1963

3 Specific Gravity IS: 2386 (Part 3)-1963

4 Organic Impurities IS: 2386 (Part 2)-1963

5 Mortar making properties (7 & 28 days) IS: 2386 (Part 6)-1963

6 Soundness (5 cycles) (Sodium Sulphate) IS: 2386 (Part 5)-1963

7Petrographic Examination (for natural river sand samples only)

IS: 2386 (Part 8)-1963

8Potential Alkali Reactivity (Chemical Method)

IS: 2386 (Part 7)-1963

9Alkali Aggregate Reactivity by Mortar Bar Method (Accelerated Technique)

ASTM Designation: C 1260-01

10 Water absorption IS 2386 ( Part 3) – 1963

11 Clay lump (%) IS 2386 (Part 2) – 1963

12 Soundness ( 5 cycles) IS: 2386 (Part 5) – 1963

Table 14: Laboratory tests on the soil, aggregate/rock samples

Sample Location Rock Type Test

Aggregate/ sands

S1 Weir axis and

Gajurmukhidam

Intake Area

Aggregate/Sands ACV, PTG, LAA, SSS, AIV, AAR,

SPG, UWT

S2 Intake Area

Sands

Aggregate/Sands GAC, SPG/ABS, ABL, SST,

NMC, OIM, DDT, CT

S3 Kankai Khola just down from

tailrace

Aggregate/Sands ACV, PTG, LAA, SSS, AIV, AAR,

SPG, UWT

S4 Hill slope around the Red clay GAC, SPG/ABS, ABL, SST,



Reservoir area NMC, OIM, DDT, CT

Rock Samples

RX-1 Intake Gneiss Point Load Test (PLT)

RX-2 Tunnel, Schist Point Load Test (PLT)

RX-3 Powerhouse Sandstone Point Load Test (PLT)

GAC-Grain Size Analysis, SPG/ABS-Specific Gravity/Absorption, ABL-Atterberg Limit Test, NMC-Natural Moisture

Content, OIM-Organic Impurities, DDT-Dry Density Test, CT-Compact Test, ACV-Aggregate Crushing Value, PTG-

Petrographic Test, LAA-Los Angeles Abrasion Test, SSS-Sulphate Soundness Test, AIV-Aggregate Impact Value, AAR,

SPG-Specific Gravity, UWT-Unit Weight Test, PLT-Point Load Test, ART-Alkali Reactivity Test

9.3 Laboratory Test

All laboratory tests were carried out at well-equipped Vishwa drilling Geotechnical

laboratory in Kathmandu.

Sieve Analysis

The grain size analysis (gradation test) was carried out according to AASHTO T 27 –

82 standard procedures by receiving the sample through a stack of sieve from 75mm

to 0.075mm in diameter. The mass of material retained in each individual sieve was

determined and the cumulative percentage was calculated. The grain size curve was

plotted on the basis of obtained data.

Atterberg Limits

The Atterberg Limits namely, the liquid limit and plastic limit were determined

according to the standard procedure outlined in BS 1377: 1975, Test 2(A) and Test 3.

Plasticity Index was obtained after the test. However, depending the nature of the

materials Indian Standard (IS) was also adopted for the test.

Specific Gravity and Absorption Test

The specific gravity and absorption test were carried out for fine and coarse

aggregate in accordance to BS 812: Part 2: 1975 standard. The absorption value is

also in the range of 1%. Usually aggregate with absorption value of greater than 2%

are considered as unsuitable for construction material. Specific gravity of the

materials has range of 2.64 and 2.69.



Los Angeles Abrasion Test

The Los Angeles Abrasion test was carried out according to the standard procedures

outlined by AASHTO T 96 – 77 (1982). The percentage of abrasion was calculated on

the basis of the tests. Samples collected for construction material were subjected to

the Los Angeles Abrasion Test. The Los Angeles Abrasion tests were performed in

the samples for coarse aggregates S1 and S2 the abrasion values are 31.0% and

35.0% respectively. Usually for ordinary concrete, the LAA value of aggregate should

not exceed 45%.

Sulphate Soundness Test

The soundness test was carried out on the construction material to determine the

durability of aggregates against physical weathering. The test was done as per the

standard procedures of determining the sulphate soundness of aggregates as

recommended by AASHTO T 104 – 77 (1982). The sulphate soundness tests were

performed on the samples S1 and S2 and the sulphate soundness values are 2.1%

and 2.7%, respectively. The test results indicate that the value obtained does not

exceed the limiting value of 10%.

Loose Density Determination

The loose density determination test was carried out on foundation materials as per

the standard procedures outlined by the AASHTO T 19 – 80.

Compaction Test

The Compaction test namely, the moisture/density relationship were determined

according to the standard procedure outlined in IS 2720 (part VIII 1983).

Point Load Test

The point load test was carried out on the core sample collected from the bore hole

according to the suggested method for determining load strength by point load tester

model PIL – 5 of rock test.

Aggregate Crushing Value Test



The aggregate crushing test was carried out according to British Standard of BS 812:

Part 110: 1990. The test was performed on the samples S1 and S2 and the

aggregate crushing values are 14.9% and 16.9%, respectively.

Table 14 Summary of the Pithole

Test Pit Location Material Depth Remarks

S1 Deumai Khola Aggregate/

Sand

> 2 m Construction

material

S2 Deumai Khola Aggregate/

Sand

> 2 m Construction

material

S3 Kankai Khola Aggregate/

Sand

> 2 m Construction

material

S4 Headwork area Red clay > 2 m Cohesive material

The test pit S1 and S2 were excavated from Deumai Khola near Gajurmukhidham

whereas the S3 and S4 were also excavated from the Kankai Khola near the

powerhouse area of the proposed Deumai HEP and Iban village area. The samples

are collected for both fine and coarse aggregates as well as the red soil for laboratory

testing.

9.4 Results of the Laboratory Tests

The results laboratory test and summary of the results of the construction materials

are given in Table 15. Distribution of the construction materials are shown in Figure

15. Details results of the laboratory is shown in Annex A.



Table 15: Summary of Results of the Construction Materials

Sam

ple

Specific

gravity

Absor

ption

Crushing

value

Sulphate

Soundness

Los

Angeles

Aggregate

Reactivity

Unit

weight

Density Elongation

Index

Moisture

content

Grain

size

Organic

matter

Point

Load

Flakiness

Index

Plastic

Limit

Petrograp

hy

S1 2.69 18.54

0.74

34.2

38.43

1.75 1.86 10.59 0.74 Mediu

m

sand

0.21 20.12

S2 2.67 0.41 20.210.34

36.639.37

8.97 19.27

S3 2.59 1.98 19.60

0.45

30.74

34.67

1.54 1.87 8.83 0.52 Mediu

m

sand

0.26 17.56

S4 1.64 17.5 34.36

RX1 6.99 Gneiss

RX2 7.95

RX3 4.35 Sandstone


S1/S2


Figure 39: Location Map of the

Construction Materials

10. DISPOSAL AREA


S1/S2

S3

S4


The excavated materials from the tunnel cannot be disposed on the both banks of the

Deumai Khola and Kankai Khola because of the wide valley and available wide

space. The proposed disposal area at Mahaguna and Dumre villages. The proposed

area is presently used as the barren land. The tentative area available for the

disposal of the excavated materials is shown in Table 15 and Figure 40. The

proposed area for mucking has sufficient space and easy to connect access road to

proposed area.

Table 15: Location of the Disposal Area

Location Area (m2) Geomorphology Land use Stability

MD1 Just

downstream from the

weir axis area

600x100 Flat, River valley cultivated/barren Seems to be stable

MD2 Below Besi

village along the

Deumai Khola

500x30 Flat, River valley cultivated/barren Seems to be stable

MD3 Kankai Khola

near Dumre village

1000x100 Flat River valley Barren Stable

Total Area 175000



Figure 40: Location Map of the Muck Disposal area


MD1

MD2

MD3

83


10. GEOPHYSICAL SURVEY

10.1 2D-Electrical Resistivity

Eight lines of the 2D-Electrical Resistivity Tomogram covers 1,500 m has been

conducted in the reservoir and the intake as well as the weir axis area. Details of the

data is given in Annex B.

Lines Length (m) Summary of Results

L1, Weir axis-2 300 Thin layer of overburden, irregular pattern of the

resistivity below the overburden

L2, Weir axis-2 240 Right bank of the Deumai Khola, bedrocks can be found

at shallow depth

L3, weir axis-2 135 Left bank of the Deumai Khola, bedrocks are met at

shallow depth

L4, Weir axis-1 165 Left bank of the Deumai Khola, 90% fresh rocks at

shallow depth

L5, Weir axis-1 165 Thin layer of overburden with slightly weathered rocks,

weak zones are not seen

L6, Weir axis-1 120 Right bank, thin layers of overburden and fresh rocks

L7, Reservoir area

Phawa Khola

135 Left bank of Phawa Khola irregular pattern of distribution

of the rocks

L8, Reservoir area

Phawa Khola

225 Right bank of Phawa Khola, thick overburden materials

Total 1485

The results of the 2D-Electrical resistivity Survey shows that geologically the best

option for the weir axis is suitable is weir -1 (lines 4, 5 and 6). Fresh and thick

bedrocks can be met at shallow depth but the option weir-2 has the irregular pattern of

the overburden distribution. However, the option weir-1 shall be better than the option

weir-2. The area for the diversion tunnel alignment, bedrocks exposed on both banks

and narrow width are the best way to choose the location for the weir axis.

The reservoir area where only two ERT lines have been conducted at the both banks

of the Phawa Khola. The results show that irregular distribution of the overburden. The

main causes for finding the irregular pattern is presence of the big boulders along with

the alluvial deposits.



10.2 Core Drillings

Based on the ERT report the core drilling at the weir axis shall be conducted.

Location Depth (m) Required testsWeir axisLeft bank of the weir axis at proposed location 20 m

VerticalLugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test

Left bank of weir axis 50 m upstream from the proposed weir axis

25 mVertical

Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test

Left bank of weir axis 50 m downstream from the proposed weir axis

25 mVertical


Right bank of the weir axis at proposed location 20 mVertical


Right bank of weir axis 50 m upstream from the proposed weir axis

25 mVertical


Right bank of weir axis 50 m downstream from the proposed weir axis

25 mVertical


Central part of the weir axis at proposed location 20 mVertical


Central part of weir axis 50 m upstream from the proposed weir axis

30 m Vertical


Central part of weir axis 50 m downstream from the proposed weir axis

30 mVertical


Intake areaCentral part of intake area 30 m

45 degree inclined


Surge Tank and Powerhouse areaSurge tank area 30 m

VerticalLugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test

Powerhouse area 30 mVertical


Powerhouse area 30 mVertical


Total depth 340 mSeismic RefractionReservoir area left bank 1 kmReservoir area, right bank 1 km

2 kmDriftingOn right bank of the weir axis 20 mOn left bank of the weir axis 20 mOn the intake 30 mSurge tank 30 mTotal 100 m



11. CONCLUSIONS AND RECOMMENNDATIONS

11. 1 Conclusions

1. Geologically, the project lies in partly in the gneiss and schistose gneiss of the

Sarung Khola Formation and Shiprin Formation, Kathmandu Group Seti

Formation and Takure Formation of the Midland Group (93%), of the Lesser

Himalaya, Eastern Nepal and 7% of lies in the rocks of the Siwailk Group.

2. Headworks is located the rocks of Sarung Khola Formation, tunnel alignment

passes on the rocks of the Sarung Khola, Shiprin Khola, Seti and Takure

formations and powerhouse component is located in the rocks of the Lower

Siwalik. Headwoksarea comprises gneiss and tunnel alignment composes the

gneiss and slate, schist. The powerhouse is comprised of sandstone and

mudstone.

3. Structurally, the project is located south of the MCT and MBT. The foliation and

bedding plane of the rocks extends northeast. The tunnel alignment crosses

the MBT and MCT zones.

4. The alluvial deposits are found around the powerhouse and weir area.

Thickness of the alluvial deposits is more than 5 m. Thin layers of colluvial

deposits are found along the tunnel alignment.

5. The slope stability of the project area is good. The dipping of the foliation and

bedding plane is opposite to oblique to the natural hill slope and thick bedded

so favorable condition for the tunneling. Stability condition of the reservoir area

and powerhouse are more or less fair

6. The RMR and Q values of the rock mass exposed along the tunnel alignment

range from 37 to 65 whereas the Q values 3 to 10.

7. The tunnel alignment has fair to good rock mass and last portion of the tunnel

has poor to very poor rock mass. The support system of the tunnel varies from

III to V types.



8. Only one adit is recommended which has the length about 500 m and situated

at central part of the tunnel alignment.

9. The exposed rocks mainly gneiss and schist are fresh to slightly weathered

condition and the spacing of the discontinuities range from 1 to 3 m whereas

the sandstone has low to moderate spacing in the rock mass.

10.Sufficient quantity of the construction materials are found along the Deumai

Khola and Kankai Khola riverbed and the excavated materials can be used as

materials.

11.The disposal areas for the excavated materials from the tunnel are located

along the both banks of the Deumai Khola as well as in the Kankai Khola.

11.2 Recommendations

1. Detailed geotechnical studies of the underground structures should be

carried out during the preparation of the DPR.

2. Stability condition of the reservoir area is more or less fair but there should

be controlled the influx of the sediments from the Phawa Khola and Sawa

Khola.

3. Based on the results of the ERT along the Deumai Khola at the weir axis

area 9 core drill holes recommended and two holes in the intake area. At

least one hole in the surge tank and two holes in the powerhouse area.

Total 340 m core drill is recommended.

4. At least two pitholes and sampling for the calculation of the bearing capacity

of the soil along the penstock alignment is suggested.

5. At least two km of the Seismic Refraction survey is proposed for along the

reservoir area to determine the some geological disturbance like fault and

lineaments.



6. At least 6 holes in weir axis and 2 holes in 2 holes in the powerhouse area

should be done to clarify the geotechnical properties of the rocks.

7. The following parameter should be done in the core samples: Point Load

Test, Uniaxial Compression Strength, Petrography, Bulk Density, Porosity,

Slack Durability, Specific gravity, Direct Shear Strength Test, Modulus of

Elasticity and Poisson Ratio, Void Index, Water content saturation

8. Drifting is proposed at the weir axis as well as surge tank area.


Documents

Deu Mai Khola HEP Text - Feasibility