Investigation and Design for Restoration of Hill slope in ... · Soil nailing, RE-wall, Drainage measures, jute geotextiles for vegetation growth to prevent erosion etc are the set

Indian Geotechnical Journal, 41(4), 2011, 215-225

Investigation and Design for Restoration of Hill slope in Mizoram

R.K.Panigrahi1, U.K.Guruvittal2, P.S.Prasad3, Sudhir Mathur4 and Pankaj Gupta5

Key words Landslide, slope stability, Rock Mass Rating, soil nailing, drainage, RE wall

Abstract: A landslide problem was investigated at Khawruhlian village located in the northeastern side of Aizawl city at km.45.00 on NH-150. The landslide hampered many developmental activities and caused immense hardship to the people depending on this road. Due to the enlargement of the slide area in this location more houses were shifted and temporary bridge was constructed in the place of road. The bridge was seated without a strong foundation, so this factor also contributed adding to the instability. The predominate rock types in the study area are sandstone, shale and siltstone. Slope stability Analysis was carried out along with other geological and geotechnical studies to evolve a set of suitable remedial measures for implementation to attain a long term remediation of this slope. The paper highlights the outcome of field and laboratory investigations of landslide. Soil nailing, RE-wall, Drainage measures, jute geotextiles for vegetation growth to prevent erosion etc are the set of remedial measures designed and suggested to restore the road.

1 Scientist, Geotechnical Engineering Division, Central Road Research Institute,New Delhi-110025, Email : [email protected]. Scientist, Geotechnical Engineering Division, Central Road Research Institute,New Delhi-110025. Email : [email protected]. Scientist, Geotechnical Engineering Division, Central Road Research Institute,New Delhi-110025. Email : [email protected]. Head , Geotechnical Engineering Division, Central Road Research Institute,New Delhi-110025. Email : [email protected]. Scientist, Geotechnical Engineering Division, Central Road Research Institute,New Delhi-110025. Email : [email protected]

Introduction The state of Mizoram as a whole because of its

geographical location, experiences the impact of the high intensity rainfall especially during the monsoon, which generally lasts for about five months in a year. As a result of heavy rainfall and weak rock strata, many landslides occur in Mizoram, disrupting road connectivity and damaging buildings. One such location at Khawruhlian village located in the northeastern side of Aizawl city is at Km. 45 on NH 150, where landslide occurrence is observed every year. It was observed in this location that a bailey bridge is seated without a strong foundation. When heavy vehicles move over the bridge, they do not slow down; as a result the entire bridge structure vibrates, transmitting the vibrations to the sub-soil, which is in contact with the bridge truss. Physico-mechanical properties of Sandstone, Shale and Siltstone present in play a very important role on stability of slope in this location.

Detailed investigations were carried out in the close vicinity of the landslide affected area to assess the type of soil and rock and their in-situ condition. To assess the stability, alope stability analysis was carried and to evolve a set of suitable remedial measures for implementation. This paper presents the outcome of field and laboratory investigations carried out, result of

stabits analysis and the remedial measures suggested to restore the slope.

Description of Site Condition The site is located in a village about 87 kilometers

away from Aizawl city. The National Highway No.150 passes through this village. This road links Mizoram and Manipur states. The slide is located at the centre of the village and it is on a ridge. A bailey bridge has been constructed to keep the traffic moving over the landslide portion. The length of the bridge is 45.8 m and height of the bridge truss is 3.2 m. Clear width between trusses is 4.8 m and the roadway width is 4.3 m. On one side of the bridge, presently the length of the ground supporting the trusses is only 8.5 m. An additional length of 1.1 m of ground is seen below the bailey bridge at this end. However this 1.1 m length of the ground has suffered settlement and is not providing any support to the bridge structure. Towards another side, the gorge is steep and length of the ground supporting the bridge is about 12.3 m. The bridge has been seated without a strong foundation. When heavy vehicles move over the bridge, they do not slow down; as a result entire bridge structure vibrates, transmitting the vibrations to the sub-soil, which is in contact with the truss. Figure 1, 2, 3 and 4 show of the landslide affected area. Figure 5 shows cross section of geological profile.

216Investigation and Design for Restoration of Hill slope in Mizoram

R.K. Panigrahi, U.K. Guruvittal, P.S. Prasad , Sudhir Mathur and Pankaj Gupta

Fig 1. View of Bailey bridge Fig 2. Erosion and Slide below the bridge

Fig 3. Formation of vertical slope due to erosion of soil below the bridge

Fig 4. Gorge below the bridge

Fig. 5 Cross sections considered for geological profile

217Indian Geotechnical Journal, 41(4), 2011

On the left flank at about 250 m distance from the slide, a high school is situated on the hill top. The residents/school authorities have cut the slope at many locations and several trees been grown by them. Rainwater from the hilltop is allowed to travel down kuccha drains and gullies, and hence the running water is seeping inside subsoil. Rill /gullies have been formed due to rainwater. Tension cracks are not visible on the high school side ground since plenty of wild grass has grown. There is also a pond on the high school hill top. This pond was being used as a pool by the school authorities. They constructed this pool after further excavating a natural pond. Overburden soil in the high school hilltop is highly permeable and rainwater percolates into subsoil rapidly. Observing the enormous quantity of seepage taking place from this pond, the pond has now been backfilled and water no longer collects into it. However seepage of run-off water still takes place. The percolated water travels down the overburden material and enters weak rocks. Four seepage points were seen on left flank in grayish/blackish soil and disintegrated rock (carbonaceous shale or compacted clay). One prominent seepage point was seen on the face of the landslide.

Investigation at the location of slide area indicated that geological feature i.e. the direction of dipping of exposed rock is against the slide direction. This can be considered favourable to withstand the slope failure. However, the rocks are soft broken sandstone overlain by carbonaceous shale, which are found to be highly jointed, fissured and highly weathered in nature. Just below the bridge, rock due to continuous weathering is converted into soil mass maintaining the rock structure. Due to the action of water, rock gets saturated, loses its strength and becomes more vulnerable to weathering. Once rock gets altered into soil, it is further subjected to rill erosion by the surface water. These rills become wider and deeper with time and finally lead to slope failure. Figure 5 shows the photographs of the types of geological strata encountered during the period of investigation. The landslide first occurred in this area in the year 1995 and since then the landslide activity is unabated. When the rains stop during the months of Sept-Oct, landslides occur The Bailey bridge was constructed in the year 2002. At that time, bridge was shorter. The bridge was extended in 2004 to its present length of 45.8 m.

Geology of area The main lithology of the area comprises of thick

overburden top soil; silty shale; silty sandstone and sandstone. The average thickness of overburden topsoil is about 9.70 m. These overburden soils comprise mainly fine grained sand intercalated with silty shale. They are unconsolidated in nature and friable. The silty shale underlying the topsoil is relatively hard, greyish in colour and occasionally brownish streak at some intervening point. The exposed bed rock is very vulnerable to weathering, prone to disintegration and found to be splintery in nature. This silty shale bed is more prominent at the northern side of the ridge and is more than 20 m in thickness. However the continuity of this bedrock can be seen at the southern side of the ridge also but the thickness is much lesser than the northern side. Silty sandstone bed of about 2.5 m thickness underlies the silty shale bed. This silty sandstone bed is relatively hard and compact. Its continuity is absent at the northern side of ridge. Even though this bed is relatively compact, it is intercalated with silty shale bed of about 10-15 cm thickness at about 1.0 to 1.25 m interval. About 2 m thick sandstone bed is exposed below silty sandstone. However, it is not noticeable in the north side of the ridge. This sandstone bed is massive and compact, fine to medium grained, greyish to light brown in colour. This massive sandstone bed is relatively devoid of joints and fractures compared to silty shale and silty sandstone bed. This bedrock seems to be the main rock formation controlling the general feature of the landform. Figure 6 and 7 show the geological details.

Representative pieces of exposed rock were collected during field work and they were tested to determine compressive strength in CRRI laboratory. Based on the results of these tests and available literature (Bieniawski, 1976,1978 and 1980) the physical and strength properties of local rocks were derived and adopted in the analysis. Similarly, the value of bulk density for sandstone was found to be 19.1 kN/m3 and for shale it was 20 kN/m3. Based on different properties of rocks of study area from borehole data and data collected during field investigation, Rock mass ratings were determined for sandstone and shale, which are shown in Table 1.

Table 1 Rock mass classes determined from total ratings

RockRating for

compressive strength

Rating for RQD

Discontinuity condition and

spacing

Ground water rating

Rating for orientation of

strike/Dip

total Rock mass rating

Category

Sandstone 12 17 20 15 -25 39 PoorShale 7 13 10 10 -25 15 V. Poor



Subsoil investigationsThe Geology and Mining wing, Directorate of

Industries, Government of Mizoram carried out subsoil investigation by making five boreholes in the study area during September 2004. Out of five bore holes, three bore holes were drilled at the road level on both the sides of the bridge whereas two bore holes were drilled below

the bridge. Figure 8 shows the bore holes BH – 1 to BH – 5 along the slide.

CRRI team carried out in-situ density tests to determine the bulk density of topsoil and field moisture content. These tests were conducted at 14 locations spread over the slide area and its adjoining flanks. The results are presented in Table 2.

Table 2 Results of field and laboratory test conducted on soil samples

Sample No Sample Location

Field Bulk Density (gm/cc)

Natural Moisture Content

(%)

Grain Size AnalysisLiquid Limit (%)

Plastic Limit (%)

Direct shear test

Gravel (%)

Sand (%)

Silt (%)

Clay (%)

C(kg/cm2)

φ

1 On the right flank, adjacent to old demolished church

2.24 9.18

2 Same spot, next sample taken at a depth of 50 cm

1.98 9.73

3 Right flank, on the sunken portion near a small tree

2.06 19.44 2 5 84.5 8.5 40.50

4 Taken on the face of land slide below the bridge

2.03 22.35 10.3 29.3 49.4 11.0 32.50 22.50 0 38

5 - do - 2.08 22.40 3.5 45 34 17.5 0.07 35

6 - do - 1.87 17.51 0 62.3 33.7 4.0 30.75 24.00

7 - do - 2.34 10.33 0 26.2 69.3 4.5 28.50 18.28

8 Taken on the other side of the ridge (Backside of the landslide area)

1.77 27.96 0 25.2

9 - do - 1.91 23.30 37.00 24.71 0 35

10 - do - 1.82 32.86 23 14 42.00 29.50

11 - do - 1.85 24.64 0 37

12 - do - 1.84 34.68 18 13.5 0 31

13 Sample taken from the debris collected at the toe of landslide

2.20 15.39

14 - do - 1.98 13.56 44 18

Stability Analysis of the Slope The selection of potential failure surface essentially

depends on the geology of the site as revealed from site investigation. The GEO-4 stability analysis software was used in the analysis and stability of the slope was determined as per Bishop Method and Patterson method. The parameters considered in the stability analysis are given in Table 3.

Table 3 Parameters considered for stability analysis

Problematic slope Cohesion (kPa)

Angle of internal friction

Density (kN/m3)

Soil layer (1 Moorum type silty sand)

15 300 22

Soil layer (2 Sandy silt – Lower layer)

15 300 20

Rock layer - - 20


The entire hill slope as shown in Figures 1, 2, 3 and 4, i.e. adjoining and close to the bailey bridge was also analysed for possible deep seated failure. Based on the data available from the bore logs, it was observed that the rock layer exists below the overburden at depths varying form 1.5 m to 9.0 m.

The top layer is essentially loose overburden which rests on weathered rock formation. The analysis of the data revealed that the global failure is unlikely, since the failure surface at various cross-sections would cut across the heterogeneous matrix of rock mass and fragmented rock, which would invariably impart adequate cohesion leading to stable condition.

The stability of slide area was analysed on the face of the landslide (slope under the bridge), on the right flank slope and on the left flank slope and on the back side of the bridge. The factors of safety estimated were less than or close to 1.0, clearly indicating that slope under the bridge and left flank slope on high school side

are unstable and prone to failure particularly with the incidence of rainfall. The failure type would be surfacial. The right side slope and slope on the other side of the ridge are stable but if the erosion continues, it may also become unstable in due course of time. The results of stability analysis for different slopes are given in Table 4.

Table 4 Results of the stability analysis

Problematic slope Method of analysis Factor of safety

Slope under the bridge(Section 1) Bishop method 0.88

Patterson method 0.86

Slope under the bridge(Section 2) Bishop method 0.52


Right flank slope Bishop method 1.30


Left flank slope Bishop method 0.97


Slope on the other side of the ridge (Under dry condition)

Bishop method 1.25


Slope on the other side of the ridge (Under saturated condition)

Bishop method 1.20


Remedial Measures for hill slope resto-ration

The detailed analysis of entire hill slope revealed the present study area is experiencing surface erosion and slope failure due to seepage of rain water but there is no possibility of deep seated movement of slope leading to catastrophic failure. Presently the Bailey bridge has been constructed over a land, which is prone to failure due to erosion caused by seepage from the adjoining area. By preventing erosion and regulating surface runoff in a proper way and also through provision of suitable drainage measures, slope failure can be effectively checked. So the main focus of the remedial measures is to stabilise the affected slopes. The remedial measures for preventing slope failure are separately given below for each face of the landslide and its two flanks (BIS code of practice SP: 36(Part 1) 1987 and BS 8006, 1995).




Face of the landslide (under the bridge) Under the bridge, there is a near vertical soil

slope of about 10 m depth. Below this soil slope, highly weathered and jointed shale and sandstone stratum of about 3 m thickness can be seen. Debris from the landslide has collected below this layer up to a depth about 50 m below the bridge. The stability analysis of the slope area has indicated that vertical soil face is prone to sliding. Infiltration of water from rainfall as well as household waste water was observed to be seeping through the weathered rock strata below overburden soil. Based on above findings suitable remedial measures are proposed to be adopted to prevent shallow movement and to control percolation of water into the slope. The remedial measures suggested have been categorized into two groups as Alternative One and Alternative Two.

Alternative One – Soil nailing coupled with Reinforced Earth wall

The shear strength characteristics of overburden material can be improved by adopting soil nailing technique. This in turn would lead to enhanced stability of the slope. Soil nailing is one of the methods developed in recent times for in-situ stabilisation for natural slope. The fundamental concept of soil nailing consists of driving closely spaced inclusions (which are called soil nails) into the ground so that overall shear strength of the in-situ soil increases. Since the width of ridge in the top 2 m portion of the vertical soil slope is very much limited (an average of about 2 m thickness) this section is proposed to be treated with Steel Fibre Reinforced Shotcrete (SFRS). Remaining 8 m height of the soil slope is to be treated with soil nailing technique Figure 9. The conventional Soil Nailing system has three components:

Fig.9. Remedial measures proposed below the bridge

(1) Soil nails,

(2) Facing elements and

(3) Arrangements for drainage

Soil nails: In a conventional soil nailing system, an ordinary tor-steel rod is used as a soil nail. For installing the soil nails, holes of appropriate diameter are drilled. Casing pipes are used, if necessary. The nails are installed into the holes and the annular space between the nail and bore hole is grouted with suitable material. Anchorage at rear end of the soil nail can also be designed and provided. Instead of tor steel rod, galvanized iron (GI) pipes of suitable diameter can also be used as nails. The annular space between the pipe and borehole can be filled with graded material comprising coarse and fine aggregates; afterwards cement grout can be pumped inside the GI pipe. In this project it is proposed to use cement mortar grout.

Facing elements: The purpose of facing element is to protect the soil nailed surface from damage due to direct rain falling on it and subsequent erosion due to run-off. Use of pre-cast concrete panels, wire mesh with shotcrete, fibre reinforced shotcrete are common for constructing facing element. In this project, the surface of the slope is proposed to be protected by a layer of cement mortar applied on a wire mesh on to the slope.

Arrangement for drainage: The seepage points on the face of the landslide need to be identified and suitable drainage measures using perforated pipes are to be installed to drain out the subsurface water. The percolation of rain water from the bridge into the ridge should be checked by paving the top surface of the ridge using wire mesh and cement mortar.

Design of soil nailsThe properties of soil, nails and the concrete facia

considered for the design are given below in Table 5. For designing the soil nail structure both internal stability and external stability conditions were checked. The layout of the soil nails and RE wall are shown in Figure 10a. The cross section of the proposed remedial measures are shown in Figure 10b and special are 10c. The top 2 m of the vertical soil slope has very less width and it would not be feasible to carry out soil nailing at this location. This portion as well as shale and sandstone strata of 3 m thickness below the soil nailed portion (at about 10 m depth) are proposed to be treated with steel fibre reinforced shotcrete.



Table 5. Parameters considered for design of soil nails

Parameter Value

Internal angle of friction of overburden soil 300

Cohesion (C) of soil 10.0 kPa

Density of soil 20.00 kN/m3

Shot crete thickness 0.1 m

Grade of concrete M 20

Vertical spacing of nails 1 m centre to centre

Horizontal spacing of nails 1.5 m centre to centre

Length of nails 6 m

Diameter of nail 30 mm

Bearing capacity of single nail 20 kN/m

Surcharge 60 kPa

Fig. 10(a) Layout Plan of the remedial measures in landslide area (Option-I)


Fig.10 (ab) Cross-section of RE walls (6m) proposed in Landslide Area(option-I)

Fig.10(c)Cross-section of RE walls (6m) proposed in Landslide Area(option-II)



Design of reinforced earth wallThere is debris accumulation at the toe of the

vertical soil slope below the bridge. The debris is in loose condition. The debris portion has already cracked and is highly unstable and has a tendency to undergo slow movement along shallow surface of sliding. Therefore to retain the slope mass in place and to retard its movement, it is considered necessary to construct suitable restraining structure. Since the movements are small and shallow, reinforced earth type retaining structure has been considered. Reinforced earth is fundamentally different from the conventional earth retaining systems, which are externally stabilized. An externally stabilized system (e.g. retaining wall) uses an external structural wall against which stabilizing forces are mobilized. On the other hand, internally stabilized system like reinforced earth, involves reinforcement installed within and extending beyond the potential failure mass to ensure stability of the system. Like reinforced concrete, reinforced earth is also a composite material consisting of soil and reinforcement. Since soil is weak in tension, reinforcements are provided to take up tensile stresses. The soil reinforcement provides an internal restraint against lateral deformation. The composite mass of soil and internal reinforcement acts as a coherent unit in resisting external loads. Using the Computer software the reinforced earth (RE) wall was designed. Polymeric geogrids of ultimate strength 70 to 200 kN/m were considered in the design. The maximum height of the RE wall was considered to be 6.0 m. Hence a series of RE walls of height 6 m as shown in Figure 10b are proposed. The length of the reinforcement required is 4.2 m and geogrid reinforcement is required to be provided at every 0.4 m interval. Other types of polymeric reinforcement can also be used but in such a case the design has to be modified to take into account the strength properties of such reinforcement material.

Alternative Two - Construction of RE wall

Instead of adopting a combination of RE wall, soil nailing and shotcreting, RE wall itself can be used as a retaining structure from the toe of the embankment up to the bridge level. This has been depicted in Figure 10c. The stability analysis of the RE wall was carried out using GEO 4 computer software. It is suggested that Mizoram PWD may consider both the alternatives given above. The selection of the alternative for implementation would depend upon relative costs of the both alternatives.

Remedial measures for the right flank (demolished church side) of landslide

Towards the demolished church side the exposed rock surface had not suffered weathering to the extent noted on the other sides. As a result this side of the slope was relatively stable. However if erosion and water seepage problems are checked, the slope may suffer instability in future. The dumping of household waste by the villagers was preventing growth of vegetation. To reduce surface erosion, growth of vegetation was required to be promoted. It was therefore proposed that dumping of household waste be immediately discontinued and the waste collected should be removed. Afterwards the exposed soil surface on the right flank should be treated with jute geotextiles/ geogrids and mulching is to be undertaken.

Remedial measures for the left flank (High school side) of landslide

Towards the high school side, the end portion of the bridge, which was resting on concrete footing, is touching the ground surface. The vibrations created due to vehicular traffic are transferred from bridge to the soil slope. During rainy season the soil slope gets saturated. The saturated slope fails due to vibration created by vehicular traffic. The bridge should be properly rested on concrete footing and to minimize vibrations, Neoprene bearings should be provided in between bridge and concrete footing. It is proposed to construct a stone masonry retaining wall to support the soil mass below the bridge and shortcrete the exposed rock surface on the left flank as shown in Figure 11.

Fig. 11 Remedial measures proposed on left flank

Other remedial measures To arrest the damage to the slope, certain other

remedial measures are required to be taken up on the back side of the slope (along the existing houses and behind the houses) and the surrounding area. These measures are given below:


• Distressed/ damaged surface drains such as catch water drains, chutes, etc should be repaired and choking of these drains, if observed, should be rectified.

• The residential houses presently located on the ridge adjacent to landslide should be relocated farther away from landslide prone area.

• Household discharge of water towards the landslide area and on the other side of the ridge should be discontinued.

• The farming activity on the other side of the ridge should not be undertaken at least for a length of 150 m from the bridge.

• Growth of vegetation especially bamboo should be encouraged on the slope.

• The runoff water coming towards the landslide along the road surface from both side of the bridge should be properly diverted. The existing pipe culvert has a smaller diameter and needs to be replaced. Concrete lined chutes of at least 200 m length should be constructed to safely carry the water coming out from this culvert. The natural depression (pool) in the high school premises should be filled up and toilets are to be located away from the slide area.

• All precautions should be taken to prevent seepage of rain water from the high school campus by providing system of lined drains and diverting water away from the slide area.

• Slide prone area should be inspected periodically for presence of tension cracks and same should be sealed.

ConclusionsThe paper highlights the detailed investigations

carried out to assess the causes of failure and suggested remedial measures to prevent the slope from further

degradation. The treatment consists of providing; soil nailing for improving the strength characteristics of underlying slope; jute geotextile for erosion control, retaining wall for retaining steep slopes etc. It was suggested to the state government officials that efficacy of remedial measures be monitored for at least two monsoon seasons. The feedback received from such studies can be useful in refining the process of controlling landslides measures and give further directions to provide a long term stability of road.

AcknowledgementThe authors are thankful to Engineer-in- Chief and

other staffs of Public Works Department; Mizoram for giving an opportunity to work on the R&D project entitled ‘ Investigation and Remedial Measures for Slope Failure at km 45 on NH 150, Khawruhlian, Mizoram’. Authors are also thankful to Director, CRRI, New Delhi for his kind permission to publish this paper.

ReferencesBieniawski, Z.T., and Orr, C.M. (1976) ‘Rapid site appraisal

for large dam foundations by the Geomechanics Classification’. Proc. 12th Cong. Large Dams, ICOLD, Mexico City, pp. 483 - 501.

Bieniwaski, Z.T., (1978) ‘Determining rock mass deformability: Experience from case histories’. Int. J. Rock.Mechanics Min. Sci. & Geomech. Abstract, Vol. 15, pp. 237.

Bieniawski, Z.T, (1980) ‘Rock classifications: State of the Art and Prospects for Standardization’, Transportation Research Record 783, Rock Classifications and Horizontal Drilling and Drainage, TRB, National Academy of Science, Washington D.C, USA, , pp. 2-9.

BIS code of practice SP: 36(Part 1) 1987 ‘Compendium of Indian standards on soil engineering’.BS 8006 (1995), ‘Code of practice for Strengthened / reinforced soils and other fills’.

Documents

Investigation and Design for Restoration of Hill slope in ... · Soil nailing, RE-wall, Drainage measures, jute geotextiles for vegetation growth to prevent erosion etc are the set