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Pile Termination Criteria for Rock Socketed Piles in

Mumbai A New Approach

Khare, Makarand G. Mhaiskar, S.Y. 1

Engineering Manager Principal

e-mail makarandkhare@yahoo.com e-mail sharad_55@yahoo.co.uk

Larsen and Toubro ECC Division, Chennai1Sardar Patel College of Engineering, Mumbai

ABSTRACT

The availability of hydraulic rotary rigs resulted in faster piling even in hard rocks, but engineers could no longeruse the chisel penetration resistance to decide the pile termination. The study presents a pile termination criteria

based on rock classification which includes geological identification of rock and rock strength assessment using

breakability, penetration and scratch tests. This paper discusses the results of nine vertical load tests conducted

on piles of 600 mm, 800 mm and 1000 mm diameters, and constructed using hydraulic rotary rigs. The piles were

subjected to compressive stress of 18.75 MPa. The study shows that testing of piles with higher test load may

demonstrate higher safe capacities in rock socketed piles and would benefit the project.

Indian Geotechnical Conference 2010, GEOtrendz

December 1618, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

Pile construction using hydraulic rotary rigs has become a

popular choice across the country. The rotary rigs offer

faster construction, control on the alignment of pile and

easy installation and removal of casing to greater depths.Anirudhan (2008) discussed the problems such as absence

of energy measuring tools and underperformance of piles

drilled with rotary rigs when compared to DMC (direct

mud circulation) chisel method. The DMC chisel technique

offers an indirect assessment of rock strength by way of

chisel penetration resistance. In Mumbai, the chisel

penetration resistance was being used as a guide to decide

the pile termination and the end bearing resistance in pile

was usually restricted to 5 MPa. In soft rocks such as

breccia a target pile length may be recommended based on

detailed geotechnical investigation to determine rock

properties such as core recovery (CR), rock quality

designation (RQD), unconfined compressive strength(UCS) and rock mass rating (RMR). However if the site

has random occurrence of soft rocks and very hard rocks

(such as basalt) then a predetermined pile length can be

uneconomical and even difficult to implement in site. The

study presented here discusses a pile termination criteria

based on rock classification.

2. PROJECT DETAILS

A common user terminal building with an approximate

footprint of 5 million square feet is under construction as a

part of modernization work of Mumbai International Airport

(MIAL). A detailed geotechnical investigation was carried

out which included 65 numbers of boreholes (ranging from

12 m to 40 m deep), 74 in-situ pressuremeter tests in soiland rock, 2 cross hole tests (up to 40 m deep) and laboratory

determination of UCS and modulus of elasticity of rock

core samples. The geotechnical investigation revealed the

depth of rock varying from 1 m to 9 m. More than 45

boreholes (out of 65) showed the presence of highly to

moderately weathered volcanic breccia and in remaining

areas, hard rocks such as basalt and trachyte were

encountered. In view of the above observations it was

necessary to adopt a foundation scheme consisting of spread

footings/raft and bored cast-in-situ pile foundation. The

settlement at working load was restricted to less than 12

mm in the case of footings/raft as well as pile foundation.

This paper discusses design of pile foundation based onnine static vertical load tests performed in breccia and

trachyte rock.

3. ENGINEERING PROPERTIES OF ROCKS

The engineering properties of rock were assessed

through in-situ tests such as pressuremeter and

laboratory tests such as unconfined compression test and

point load index test. The summary of these test results

is given in Table 1.

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1044 G. Makarand Khare and S.Y. Mhaiskar

4. TEST PILE DETAILS

All test piles were constructed using hydraulic rotary rigs.

The top soil was supported with a casing and no bentonite

was used. The preliminary pile capacities were estimated

based on Cole and Stroud (1977) and piling practice in

Mumbai reported by Datye (1990). The preliminary pile

capacities for 500 mm, 600 mm, 800 mm, 1000 mm and

1200 mm diameter piles were estimated to be 970 kN, 1410

kN, 2500 kN, 3930 kN and 5660 kN respectively. Nine

static vertical load tests were carried out to determine the

safe pile capacity and pile termination criteria.

The pile testing program included small (600 mm),

medium (800 mm) and large (1000 mm) diameter piles as

envisaged in the project. The rock samples recovered during

construction of test piles were classified by an experienced

geologist using physical properties such as colour, grain

size and mineralogy as described in Table 2. The rock

samples were further classified as suggested by Cole and

Stroud (1977) and explained in Table 3. The Table 4 shows

classification of rock samples recovered at different depths

during construction of test pile.

Table 2: Geological Identification of Rock Samples

Physical PropertiesRock

TypeColour

Grain

SizeMineralogy

VolcanicBreccia

Yellowishbrown to

purple

Coarse Feldspar, siltyclay with somequartz content

Trachyte Light pinkish

grey

Fine -

medium

Feldspar more

than 75%Basalt Dark greenish

blackFine Hornblende and

Feldspar

5. LOAD TEST RESULTS & PILE TERMINATION

All piles were subjected to maximum compressive stress

of 18.75 MPa(which is 2.5 times the safe structural capacity

of pile constructed with M30 grade concrete). The test

load was applied by taking reaction from rock anchors

installed around the test pile. The load-settlement curves

for 600 mm, 800mm, and 1000mm diameter piles are

shown in Figures 1, 2 and 3 respectively. The test results

are interpreted as per guidelines of IS 14593 (1998) and IS

2911 (2000) and summarised in Table 5. For TP-1, 4, 6, 7

and 9 the ultimate load is considered equal to the maximum

test load. From Table 5, it is observed that, 600 mm

diameter piles with 3D socketing in Highly to moderately

weathered breccia of Grade C can carry a safe load of

1570 kN as shown by TP-2. In case of 800 mm diameter

piles viz. TP-4, 5 and 6 the socketing depths are 4D, 3D

and 7D respectively. The test pile TP-5 which is socketed

3D in volcanic breccia shows pile capacity of 1890 kN as

per IS: 14593 against preliminary design load of 2500 kN.

Fig.1:. Load-Settlement Response: 600 mm Dia Piles

Fig.2: Load-Settlement Response: 800 mm Dia Piles

Fig. 3: Load-Settlement Response: 1000 mm Dia Piles

Table 1: Summary Rock Properties

Modulus of Elasticity of rock (MPa)Rock Description

From In situ

pressuremeter tests

From Laboratory

rock core tests

UCS of soaked rock

samples (MPa)

Slightly - Moderately Weathered Basalt 7717 (5 tests) 21893 (4 tests) 61.5 (24 tests)Fresh - Moderately Weathered Trachyte 6466 (18 tests) 16180 (5 tests) 46.3 (77 tests)Fresh - Highly Weathered Breccia 4182 (32 tests) - 6.9 (4 tests)Fresh - Highly Weathered Volcanic Breccia 2771 (14 tests) - 11.4 (160 tests)

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Pile Termination Criteria for Rock Socketed Piles in Mumbai... 1045

However, TP-4 with 4D socketing in volcanic breccia

shows a capacity of 3180 kN as per IS: 14593. Therefore,

for 800 mm diameter piles, 4D socketing in Highly to

moderately weathered breccia of Grade C was

recommended for 3180 kN safe capacity. For 1000 mm

diameter piles a safe capacity of 4620 kN was recommended

based on the results of TP-8. For 1000 mm diameter piles

2D socketing in Highly to moderately weathered breccia

of Grade C (as in TP-7, 8) and 1D socketing in hard rock

like trachyte (as in TP-9) was recommended to achieve safe

capacity of 4620 kN. Interestingly, if the TP-1, 4, 6, 7 and

9 were subjected to higher test load; higher safe pile

capacities could have been observed.

Though no load test was proposed for piles socketed

in basalt, theoretical pile capacities were calculated using

Cole and Stroud (1977) approach and by considering the

Grade Breakability Penetration Scratch

A Difficult to break against solidobject with hammer

- Cannot be scratched with knife

B Broken against solid objectwith hammer - Can just be scratched with knife

C Broken in hand by hitting withhammer

- Scratched with knife. Can just bescratched with thumb-nail

D Broken by leaning on samplewith hammer

No penetration with knife Scratched with thumb-nail

E Broken by hand Penetration to about 2mm with knife -

F Easily broken by hand Penetration to about 5mm with knife -

Pile diameter TP NoDepth of rock

sample (m)Grade of rock Depth of socket and socket material

TP-1 4.0 7.1 C-B 5 D in Volcanic Breccia ( Grade C)5.5 6.5 E - DTP-26.58.5 C

3.3 D in Volcanic Breccia (Grade C)

4.05.0 D - C

600 mm

TP-35.06.5 C - B

2.5 D in Volcanic Breccia (Grade C)

5.06.9 C7.17.5 C

TP-4

8.0 C

3.7 D in Volcanic Breccia (Grade D-C)

TP-5 4.06.5 C 3.2 D in Volcanic Breccia (Grade C)

5.0 E7.6 E

800 mm

TP-6

10.6 E

7 D in Volcanic Breccia (Grade E)

8.5-9.5 D-CTP-7

9.510.7 C

2.2 D in Volcanic Breccia (Grade C)

TP-8 5.4 C 2.1 D in Volcanic Breccia (Grade C)

1000 mm

TP-9 5.0 6.0 B 1.1 D in Trachyte

IS: 14593 IS: 2911

Pile

No.(1/2) of load at whichsettlement is 12 mm

(1/3) ofultimate

load

(2/3) of load at whichsettlement is 12 mm

(1/2) of load at which piledisplacement is equal to 10 % of

pile diameter

TP-1 Pile settlement < 12 mm 1770 kN Pile settlement < 12 mm Pile settlement < 60 mmTP-2 2225 kN 1570 kN 2930 kN Pile settlement < 60 mm

TP-3 2530 kN 1770 kN 3360 kN Pile settlement < 60 mmTP-4 Pile settlement < 12 mm 3180 kN Pile settlement < 12 mm Pile settlement < 80 mmTP-5 2330 kN 1890 kN 3100 kN 2830 kNTP-6 Pile settlement < 12 mm 3180 kN Pile settlement < 12 mm Pile settlement < 80 mmTP-7 Pile settlement < 12 mm 4950 kN Pile settlement < 12 mm Pile settlement < 100mm

TP-8 5050 kN 4620 kN 6730 kN Pile settlement < 100 mmTP-9 Pile settlement < 12 mm 4950 kN Pile settlement < 12 mm Pile settlement < 100 mm

Table 3: Rock Grade Classification Based on Breakability, Penetration and Scratch Test (After Cole and Stroud, 1977)

Table 4 : Classification of Rock Samples Recovered During Piling Based on Geology, Breakability, Penetration, Scratch Test

Table 5: Safe Pile Capacties Based on IS 14593 (clause 9.2.2) and IS 2911 Part 4 (Clause 6.1.5)

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1046 G. Makarand Khare and S.Y. Mhaiskar

unconfined compressive strength (UCS) of rock cores (UCS

= 25 MPa to 114 MPa). The analysis showed that 0.5 D

socketing is sufficient to meet design capacities.

For 500 mm diameter piles, socketing identical to 600

mm diameter and for 1200 mm diameter piles, socketingidentical to 1000 mm diameter was proposed (though no

pile load test was carried out in 500 mm and 1200 mm

diameter piles). This recommendation was based on the

assumption that the effect of size between test piles and the

corresponding working piles is small and acceptable.

However for 500 mm diameter piles a minimum socketing

of 0.3 m in basalt was proposed (against 0.5 D

recommended for other pile diameters) to ensure a proper

seating on rock. The recommended socketing depth

corresponding to each type of rock and the safe pile capacity

is summarized in Table 6.

The pile termination criterion presented here was

implemented successfully in 3000 piles. All the routineload tests carried out with test load equal to 1.5 safe

capacities showed the pile settlements less than 8 mm.

Table 6: Recommendations for Pile Socketing

Depth of socket (m)Pile

Dia.

(mm) Volcanic

Breccia of

Grade B or

C

Highly to

Moderately

Weathered,

Fresh Trachyte

Basalt

Safe

capacit

(kN)

500 1.5 0.5 0.3 970

600 1.8 0.6 0.3 1570

800 3.2 0.8 0.4 3180

1000 2.0 1.0 0.5 4620

1200 2.4 1.2 0.6 5750

6. FRICTIONAL RESISTANCE OF ROCK FROM

LOAD TEST

TP-1, 4, 6, 7 and 9 show settlements less than 1% of

respective pile diameters. In case of TP-2, 5 and 8 very

large settlement values indicate that full shaft friction is

mobilized with little or no end bearing resistance. Based

on these observations it is postulated that, the maximum

test load observed in the TP-1, 2, 4, 5, 6, 7, 8 and 9 may be

equated to the mobilized frictional resistance of rock socket

alone. The average mobilized socket frictional resistance

under test load is estimated based on above stated hypothesis

and given in Table 7.

Table 7: Estimated Mobilized Friction and Rock Types

TP No. Rock TypeEstimated Average

Mobilized socketfriction (MPa)

1,2,4,5,7,8 Volcanic Breccia(Grade C)

2.0

6 Volcanic Breccia(Grade E)

0.9

9 Trachyte(Grade B)

7.16

7. CONCLUSIONS

The study presents the results of nine static vertical load

tests. A pile termination criteria and safe pile capacities

based on geological classification and strength of rock is

presented. The estimated mobilized friction for breccia of

grade C, breccia of grade E and trachyte of grade B is found

to be 2, 0.9 and 7.16 MPa respectively. The study also

shows that testing of piles with higher test load may

demonstrate higher safe capacities particularly in rock

socketed piles and would benefit the project.

ACKNOWLEDGEMENTS

Authors are thankful to MIAL led by GVK group and their

Program Managers CH2MHill for granting the permission

to present the paper and to use the experimental data to

reach the conclusions. Authors also thank to all the staff

of Larsen and Toubro, ECC Division and Foundation and

Geotechnical Engineering Cell (FGEC) in particular for

their support in pile testing program.

REFERENCESAnirudhan, I.V. (2008). Construction of Bored Cast-in-situ

Piles in Chennai using Rotary Type Hydraulic Piling

Rigs - Some Quality Control Aspects. The Master

Builder, Dec. 2008, pp. 58-66.

Cole, K.W. and Stroud, M.A. (1977). Rock Socketed Piles

at Coventry Point, Market Way, Coventry. Published in

Piles in Weak Rock, by Thomas Telford Ltd for

Institution of Civil Engineers London.

Datye, K.R. (1990). Bored Piling in Bombay Region.

Advance in Geotechnical Engineering, IGC 1990,

Mumbai, India, pp. 571-587.

IS 14593 (1998). Indian standard code of practice forDesign and Construction of Bored Cast In-Situ Piles

founded on Rock-Guidelines.BIS, New Delhi.

IS 2911 Part 4 (2000). Indian standard code of practice for

Design and Construction of Pile Foundations Load Test

on Piles.BIS, New Delhi.