Indian Geotechnical Conference 2010, GEOtrendz

December 1618, 2010

IGS Mumbai Chapter & IIT Bombay

Instrumented (Cyclic & Lateral) Pile Load Tests for the Proposed Air

Traffic Control Tower in Mumbai

Ghan, Sandeep M. Mishra, Niraj K. 1 Shankar, C.P. Mahesh2

Engineering Manager Asst. Engineering Manager Asst. Engineering Manager

e-mail: sandeepghan@lntecc.com e-mail: nirajmishra@Intecc.com e-mail: cpmahesh@Intecc.com

1(Geotech), EDRC ECC, Larsen and Toubro Ltd., Mumbai

ABSTRACT

The Air Traffic Control Tower (ATC Tower) of Chhatrapati Shivaji International Airport, Mumbai is a 84m tall

structure which is proposed for regulating air traffic movement and will replace the old air traffic control tower.

This structure will be constructed as a part of the airport expansion and renovation programme. The entire

building is supported on pile foundations socketed in Breccia rock. The present paper describes the details of the

pile load test exercise which was carried out on test piles to ascertain the capacities (vertical, pull out and lateral)

of piles. Out of the four tests carried out on test piles, two tests were carried out in both traditional and instrumented

ways simultaneously. The strain gauges and extensometers (embedded in pile mass) were used in cyclic load test

to record strains and settlements in the test pile though data logger. In lateral load test, an inclinometer was used

to record the lateral deflection of the test pile. The vertical settlements and lateral deflections were also measured

with dial gauges in both the tests and the results were compared. The results were also further compared with the

design parameters.

1. INTRODUCTION

Pile foundations are used to transfer the load to deeper

strata of high bearing capacity or to rock avoiding

shallow layers of soil of lower bearing capacities. Pile

foundations socketed in rock strata is transmitting the

load to the deeper layers through end bearing and

socket friction with adjacent weathered rock layers.

The proposed Air Traffic Control (ATC) Tower of

the Chhatrapati Shivaji International Airport, Mumbai

is approximately a 84m tall structure founded on 108

piles of 800mm diameter socketed into Breccia rock.

The rock socketed piles derive a major portion of their

capacity by mobilizing the socket friction. The socket

length (ranging from 5.5m to 7.0m) was provided in

the present case. Cyclic instrumented pile load test was

carr ied out on test pile to ascertain the design

capacities and also to separate out the end bearing and

skin friction components. The instrumented lateral load

test was conducted to assess the lateral capacity of the

pile for the test load with corresponding deflection.

This paper describes the details of the instrumented

pile load tests and the interpretation of the results thus

obtained.

2. ENGINEERING GEOLOGY OF PROJECT

AREA

The project site is located in the Deccan Trap region.

Hence the usual types of rocks commonly occurring in the

Deccan Trap region, such as different types of basalts,

volcanic Breccias, Tuff Breccias with intercalation of black

shale bands etc are occurring. However, most of the rocks,

on which the structure is to be founded, are concealed

below the overburden. The project site is explored by

taking four boreholes within the building footprint.

Detailed core logging of all the four drill holes was carried

out as per IS 13365: 2006 (Part I).

The rock encountered at the site i.e. Tuff Breccias

are formed by compaction of volcanic tuffaceous

material. Some coarser fragments were blown up

during volcanicity and are caught up in Tuffaceous

matrix giving rise to Tuff Breccia. These rocks are soft,

weak and become weaker and more softer in contact

with water. Due to weathering, these rocks become so

weak and soft that, when tested in soaked condition,

these pieces crumble down. Therefore while deciding

foundation level of pile and socketing length, utmost

care was taken.

1000 M. Sandeep Ghan, K. Niraj Mishra and C.P. Mahesh Shankar

3. GEOTECHNICAL INVESTIGATION

The important information from foundation point of

view was obtained by sub-surface exploration. The

drilling was undertaken to ascertain the type of rocks

occurring at the site, their engineering characters, and

physical properties, etc. The geotechnical investigation

program comprised of 4 (four) boreholes. All the four

boreholes falling within the footprint of proposed

building were extended upto sound rock strata.

Subsurface profile at this site generally consists of

existing road crust and fill (as it was existing car

parking) overlying residual soil underlain by Completely

Weathered Rock followed by weathered bedrock.

4. FOUNDATION RECOMMENDATIONS

Bedrock was encountered at depths between 6.0m to

7.50m below ground surface.The pattern of irregular

weathering of the rock was observed in the borelogs

especially in the upper rock stratum. Termination

depths were so suggested that the pile rests on a sound

stratum (Fair as indicated by the RMR classification).

The building was proposed to be supported on bored

cast-in-situ piles socketed in Tuff Breccia bedrock and

were suggested for termination at depths ranging from

12.25m to 13.5m below ground level (socket length

varies from 5.5m to 7.0m). While deciding the

termination depth for each borehole area, it was ensured

that the pile lengths should not vary drastically.

However, for identification of start of socket and

termination of piles at fixed depth, the strata was

compared with geological identification of rock and the

termination criteria defined in terms of penetration rate.

The comparison was carr ied out by Engineering

Geologist using physical properties such as colour, grain

size and mineralogy.

The pile capacities were worked out by the various

methodologies like pile capacity as per IS 14593- 1998

for rock socketed piles, pile design as given by

Foundation design handbook, allowable pile stress as per

IS 2911-1979, and compared with the recommended pile

stress of 550t/m2 (as recommended for rock socketed

piles in Bombay region). Finally, the vertical pile

capacities are finalized based on the allowable pile stress

of 550t/m2 which is followed in Mumbai region.

5. PILE LOAD TESTS

Considering the importance of the structure, it was

decided to verify these pile capacities by carrying out

test pile exercise. Three tests were initially planned

(vertical, lateral and pull out). Cyclic load test was later

scheduled to identify end bearing and socket friction

components separately. Four test piles were cast in

the vicinity of borehole BH-2(which was identified for

weakest rock profile) namely ITP-1, ITP-2, ITP-3 and

ITP-4. Vertical and uplift load tests were performed on

ITP-1 and ITP-4 respectively. Instrumented cyclic load

test and lateral load tests were conducted on ITP-2 and

on ITP-3 respectively. The instrumentation in form of

load cells, strain gauges, extensometers and inclinometer

was also proposed in addition to conventional measuring

instruments like dial gauges and hydraulic jacks to

differentiate between skin fr iction, end bearing

component as also to accurately measure settlement and

lateral deflection of the pile.

In cyclic pile load test, the instrumentation was

done in test pile by installing strain gauges and

extensometers at different elevations along the pile

length. A total of 16 strain gauges (4 at each level)

were installed at depths 3.5m, 7.0m, 9.5m and 12.0m

from ground level. Similarly extensometers were placed

at depths 6.5m, 9.0m and 12.0m.

In case of lateral load test, an inclinometer was

used apart from dial gauges for assessing the lateral

deflection of the pile along the length. The horizontal

modulus of subgrade reaction was also determined from

the test.

6. TEST DETAILS

The tests were carried out as per the procedure given in

IS 2911-2006. The cyclic test load (1.50 times of

working load) and lateral test load (2.50 times of

working load) were applied on the test pile in

incremental way as also decreased in the same way. The

strain gauge readings and extensometer readings were

recorded through data logger every 15 minutes

automatically. Manual readings were taken every 30

minutes.

The inclinometer readings were recorded after each

load increment as well as decrement.

In the case of cyclic instrumented load test, the net

differences in strain gauge readings (at one elevation)

for each load cycle were averaged out and were

multiplied by modulus of concrete to obtain stress at

that point. The axial force in the pile mass at that

point is obtained by multiplying with the pile cross

section area. In the later exercise, the axial force at

two subsequent elevations is divided by the pile

perimeter in between to obtain skin friction component.

The skin friction component is considered to be acting

at -4.50m where CWR is encountered and friction zone

is assumed to be started.

The lateral load test was carried out as per IS

2911-2006. The deflections were measured with

inclinometer which moved through the PVC conduit

Instrumented (Cyclic & Lateral) Pile Load Tests for the Proposed... 1001

laid in the test pile. The lateral deflection was measured

at every 0.50m through the length of the pipe. The

same was also measured with the help of dial gauges.

6. ANALYSIS AND DISCUSSION

Cyclic Load Test

The interpretation of the test results is done in two

ways:-

(i) Calculations as per IS 2911- 2006 (by dial

gauge readings and corresponding loadings)

(ii) Calculations as per strain gauge readings and

extensometer readings with corresponding

loadings

(i) As per IS 2911-2006, the socket friction and

end bearing stresses are worked out with dial

gauges for each cycle and the results are

tabulated below.

Table1: Values Based on Dial Gauge Readings

(as per IS: 2911-2006)

Test Load (T) Socket friction

(T/M2)

End bearing (T/M

2)

56.74 3.05 25.18

113.49 5.90 56.13

170.23 8.47 95.27

226.98 11.76 113.46

283.72 14.48 147.97

340.46 17.31 179.61

397.21 21.16 181.96

415.65 22.14 189.09

Table 2: Percentage Based on Dial Gauge Readings

(as per IS: 2911-2006)

Test

Load

(T)

Total

Load

Taken by

Socket

Friction

(T)

Total

Load

Taken by

End

Bearing

(T)

% of

Socket

Friction

% of

End

Bearing

56.74 44.08 12.66 77.69 23.21

113.49 85.27 28.21 75.13 24.86

170.23 122.33 47.89 71.86 28.14

226.98 169.93 57.03 74.86 25.14

283.72 209.32 74.38 73.77 26.22

340.46 250.16 90.28 73.47 26.52

397.21 305.74 91.47 76.96 23.03

415.65 319.95 95.05 76.98 23.02

(ii) The analysis is done for all load ranges for readingsobtained through instrumentation also and the

range of the values is as given below.

Table 3: Values Based on Strain Gauge Reading

Test Load

(T)

Socket Friction

(T/M2)

End Bearing

(T/M2)

56.74 1.48 48.89

113.49 4.14 49.72

170.23 6.50 62.33

226.98 9.72 38.57

283.72 12.60 29.06

340.46 17.62 71.59

397.21 18.26 14.02

415.65 18.84 26.28

Table 4: Percentage Based on Strain Gauge Reading

Test

Load

(T)

Total

Load

Taken by

Socket

Friction

(T)

Total

Load

Taken by

End

Bearing

(T)

% of

Socket

Friction

% of

End

Bearing

56.74 31.66 25.08 55.80 44.20

113.49 88.50 24.99 77.98 22.02

170.23 138.90 31.33 81.59 18.41

226.98 207.59 19.39 91.46 8.54

283.72 269.11 14.61 94.85 5.15

340.46 328.37 12.08 96.45 3.55

397.21 390.16 7.05 98.22 1.78

415.65 402.44 13.21 96.82 3.18

The variation of skin frictional resistance with depthis shown in the figure.

Fig. 1: Variation of Skin Friction with Depth for Each Load

Lateral Load Test

The test was carried out as per the procedure elaborated in

IS 2911-2006. The lateral deflection obtained in the test

was 4.19mm at the top of the pile after the maximum test

load of 22 T was retained for 24 hours. The dial gauges

readings were suitably extrapolated to the cut-off level. The

cut- off level is 2.875m below existing ground level, which

was the test level. The inclinometer readings were also

taken at cut-off level and other elevations. The deflection

obtained with inclinometer at existing ground level was

3.58mm. The values obtained with dial gauge and

instruments are presented in Table 5 placed below.

1002 M. Sandeep Ghan, K. Niraj Mishra and C.P. Mahesh Shankar

Table 5: Comparison of Lateral Deflections

Instrument of

Measurement

Deflection at Top

(mm)

Deflection at Cut-off

Level (mm)

Dial gauge 4.19 3.16 (extrapolated)

Inclinometer (for

A) (In the direction of

loading)

3.58 0.94 (actual)

Inclinometer (for

A) (In the direction of

loading)

3.10 1.10 (actual)

Cut-off level: 2.875m below EGL.

7. CONCLUSIONS

The exercise was conducted to verify the design parameters

of end bearing and socket friction. The assessment of

separate values of end bearing and socket friction was also

an aim to conduct the test exercise. Although, end bearing

values are exceeding the safe bearing capacities (worked

out with RMR) of the stratum in case of higher loads, this

is expected because of ignorance of friction component in

soil region which is definitely sharing some load.

The results obtained by conventional instrumentation

and digital instrumentation are analysed. The load tests

are conducted in the region influenced by BH-2 as the RMR

analysis has shown minimum values for this borehole.

Following are the observations

(i) The settlements by both ways is found to be less

than 12mm and hence permissible as per IS 2911-

2006.

(ii) Skin friction as per strain gauge readings is

between 0.05 T/M2 and 28.70 T/M2. The

minimum value of skin friction was observed

between depth of 9.5m and 12.0m and the

maximum value was observed between -4.5m and

-7.0m. In design, the skin friction component is

taken as 10 T/M2 for entire length of rock

socketing. However, it is a general pattern seen in

the test results that the skin friction reduces with

depth and found minimum at the base of the pile.

The average skin friction for design load of the

pile (276 T) is found to be 8.92 T/M2 which is in

tune of 10 T/M2 as per instrument readings.

However the conventional method of analysis

shows the skin friction as 14 T/m2.

(iii) End bearing as per strain gauge readings is

between 10.11 T (20.12 T/M2) and 151.83 T

(302.21 T/M2). The end bearing is observed to be

much less than the skin friction component and

the major portion of the load gets dissipated with

depth. The end bearing at design load is observed

to be 230.15T/M2. The safe bearing capacity by

RMR value is worked out as 165 T/M2 at this

depth. The probable explanation for the same is

given in point no. iv.

(iv) The end bearing value for the pile is observed to

be more than safe bearing capacity of the rock

stratum. This is due to the fact that the socket

friction in the soil portion is neglected in this

analysis. As this value is not accounted for in the

design (but actually existing in the field), end

bearing stress is observed to be more than

allowable Safe Bearing Capacity.

(v) A lateral load i.e. test load of 22T was used

considering the case of test pile as a free head pile

for lateral load test (ITP 3). The lateral deflection

was obtained as 3.16mm from dial gauges and

0.94mm from inclinometer at cut off level. The

lateral deflection was estimated as 0.40mm at 18T

(design load at fixed head condition) in the pile

design. However, if the comparison is done with

the actual observed values, the field deflection is

obtained as 0.0535mm for the test pile at design

load (8.75T for free head condition) at cut-off level

which is less as compared to design value. The

value of Kh obtained from the test as 484.35kg/

cm2 which is higher as compared to the design

value of 48.80 Kg/cm2.

(vi) After comparing the results obtained with various

criterion given in IS 2911 and IS 14593, the design

load values are recommended as acceptable.

ACKNOWLEDGEMENTS

We are thankful to MIAL led by GVK group and their

Program Managers CH2MHill for granting us permission

to present the paper and to use the experimental data to

reach to the conclusions.

REFERENCES

IS 2911 Part IV, Code of Practice for the Design and

Construction of Pile Foundations Load Test on Piles.

New Delhi.

IS 14593, Design and Construction of Bored Cast In-Situ

Piles founded on Rock-Guidelines. New Delhi.

Premchitt J., Gray I., and Ho K. K. S.; Skin Friction on

Piles at the New Public Works Central Laboratory;

Geo Report No. 38.

Machan George and Bennett Victoria G ; Use of

Inclinometers for Geotechnical Instrumentation on

Transportation Projects.(TRB Circular Number E-C

129; October 2008)

IS 13365 Part I, Quantitave Classification Systems of Rock

Mass - Guidelines. New Delhi.