Settlement of Pile Groups Exposed to Excavation Induced Soil Movement

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Settlement of Pile Groups Exposed to

Excavation Induced Soil Movement

Rajesh Prasad Shukla

Research Scholar

IIT Roorkee, Roorkee

e-mail: [email protected]

Nihar Ranjan Patra

Associate Professor

IIT Kanpur, Kanpur, India

e-mail: [email protected]

ABSTRACT Nowadays, it is very often to see new structures being constructed in the vicinity of existingones. Excavation work during a new construction can affect the existing soil conditions,

foundation behaviour and the structure as well. In the study presented below, settlement of

model pile group subjected to unsupported soil movement is studied. Various pile groups(single pile, 2x1, 3x1, and 2x2) are studied with the consideration of factors such as spacing,

embedment depth, the distance between pile group and excavation surface and the number of

piles. A specially designed wooden shutter is installed in the mild steel tank to simulate

excavation induced soil movement. Experimental study reveals that the excavation induced

soil movement leads to lateral displacement as well as settlement of pile group. The effect ofnumber of piles in a group and spacing is dependent on the embedment depth of pile.

Settlement of a single pile and pile groups increases with increase in the depth ratio of

excavation. It also increases with the reduction in distance between pile group and excavationsurface. Embedment depth has a significant influence on the behaviour of pile group.

KEYWORDS: Piles; excavation; sand; settlement; depth ratio; spacing.

INTRODUCTIONPile foundation are deep foundation used to transfer the load of heavy infrastructures to the

strong strata or to bypass the loose soil located at shallow depth [1]. Piles can be subjected to

vertical loading, lateral loading, moment loads or combination of loads. Based on the loadtransferring mechanism, pile under laterally loading are classified into two categories; active piles

or passive piles [2]. In case of active piles, piles are principally loaded at the top and loads are

transferred from piles to the soil whereas in case of passive piles loads are transferred from soils

to piles. Passive piles are subjected to lateral thrust arising from slope movement, landslides,

excavation of soil, landslide, tunneling and underground construction.

Piles subjected to excavation induced soil movement are a case of passive piles as they are

subjected to lateral thrust arising due to horizontal movement of surrounding soil along the pile.

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Vol. 20 [2015], Bund. 10 4294

Excavation for new construction can cause a reduction of confining pressure, drawdown of water

table on the side of excavation and relief in the vertical pressure. Piles can experience a

settlement due to reduction in confinement pressure, shaft friction and relief of vertical pressure

and it may also induce a considerable amount of deflections and additional bending moments. In

an extreme case excavation may leads to failure of structure [3]. Prediction of passive pileresponse under unsupported soil movement is a complex issue, because of difficulties involve in

the simulation of free soil movement. Excavation activity below the water table can leads to

various problems and serious threat to the nearby existing structures supported on pile

foundations due to seepage-induced consolidation and drawdown in the areas having relatively

high permeability [4].

Several studies are listed in literature where performance of structures have been severely

affected as a result of reduction in confining pressure, decrease in density of soil and additional

bending moment due to excavation-induced lateral soil movement [5-6]. Many experimental,

theoretical and analytical approaches have been developed to predict the response of pile

subjected to excavation induced soil movement but most of studies have considered the retaining

wall supported excavation [7-22]. It is possible that the excavations can be carried out withoutany support, for that case only a few studies have been conducted in earlier. All the studies, either

supported excavation or unsupported excavation have considered the lateral deflection and

bending of piles, but most of them have considered the settlement of pile under excavation

induced soil movement [7-22]. Design charts have also been established by [23-24] to estimate

the pile responses near an un-strutted and a strutted excavation. Most of studies are uneconomical

and need high technology to perform test. It is very costly to conduct full scale instrumented tests

on piles to find out the response of pile group subjected to lateral soil movement. Other available

theoretical and numerical methods are not reliable and they consume lots of time and resources.

Small scale model testing is an efficient alternative way to determine the performance of pile

group and then correlate to the full scale piles. Though model testing does not reflex the true

behaviour of pile but under a well-controlled environment, it provides the flexibility and

repeatability that cannot be achieved through full scale tests. In this article, an attempt has been

made to determine the settlement of pile group without using the sophisticated instruments and

huge money. Total expenditure on this study is negligible as compared to other studies.

EXPERIMENTAL SET UP AND TESTING PROGRAMThe complete experimental setup for testing consist of model piles, wooden shutter attached

to the mild steel tank to simulate soil excavation, measuring devices like dial gauge, sand

pouring device, loading arrangement and other ancillary equipment to prepare the setup for

testing.

Soil

Indian standard sand of grade-II “Ennore sand” bought from Chennai, India is used asfoundation material. Its behavior is considered free from time effect. Results of sieve analysis isshown in figure 1. The laboratory testing results are presented in the tabular form and presented

in Table 1.

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Vol. 20 [2015], Bund. 10 4295

Figure 1: Grain size distribution curve of soil

Table 1: Properties of sand used in study

Property Value

Maximum void ratio (e max) 0.78

Minimum void ratio (e min) 0.58

Maximum 16.8 kN/m3

Minimum 14.4 kN/m3

Uniformity coefficient 1.70

Coefficient of curvature 0.97

Relative density 54.3

Unit weigh 15.61 kN/m3

Angle of internal friction 34.210

Pile soil friction (δ) 20.50

Tank and Model PilesModel tank is built up of mild steel plates of 6mm thickness and having size is 980mm x 980

mm x 980 mm. Tank is having high weight and stiffened at different levels to avoid any

distortion, effect of filling of sand and loading during the testing. This size is more than enough

to consider the pile and pile group influence zone and boundary condition. A schematic view of

experimental setup is shown in Figure 2.

0

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20

30

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50

60

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80

90

100

0.01 0.1 1 10

P e r c e n t a g e f i n e r ( % )

Particle size (mm)




Vol. 20 [2015], Bund. 10 4296

Figure 2: Setup of model testing

The model piles used in present study are fabricated from a circular hollow aluminium tube

with an outer dimension of 32.0 mm and thickness of 1.0 mm. Length of piles are varied for

different embedment depth (L/d ratio). The aluminium piles can be spitted longitudinally into

two pieces to place the strain gauge into model pile. Different embedment lengths of 320 mm

and 640 mm are adopted for all the tests. The cross section of model piles was kept constant

throughout the investigation. The top portion of the piles are threaded to connect it to pile cap.

Piles cap is placed around 5 mm above the level of sand tank to avoid any contribution of pile cap

in the load carrying capacity and soil-pile cap interaction. All tests have conducted by placing the

pile and piles group at a distance of 6.2d and 9.7d from excavation surface or wooden shutter.

AccessoriesTo measure the displacement of the piles four dial gauges have been used. Two dial gauges of

0.01mm sensitivity and 50.mm capacity and another two of 25 mm capacity and 0.01mm

sensitivity are used. Dial gauges measuring needles are placed on the top surface of aluminium

strips and aluminium strip is attached to pile caps using four nut-bolts. It has been ensured that all

dial gauges are equidistant from the centre of the pile cap. They are placed diagonally so that pile

rotation can be also recorded. Other fastenings are mainly used to connect the piles to the piles

cap, and place dial gauges and piles in the tank at the desired location. Nut and bolts system are

used to assemble and detach the piles and pile cap. This system also enable the fast detachment of

pile cap from piles and pile groups. C-clamps are used to fasten the pile groups to the model tank

during sand filling to avoid any disturbance to the piles and foundation media near the pile

influence zone. There are plenty of other ancillary equipment used for proper functioning of

experimental program.

EXPERIMENTAL PROCEDUREPouring of sand in tank is a most important part of experimental study as it affects the density

of sand and performance of piles. Rain-fall technique has been used for pouring of sand and this

technique gives a desired density of soil in the tank. This technique is a verified technique to

achieve the desirable density. [25-28] have used this technique to for sand to get required density.

Dial Gauge

Pile cap

Model pile

Tank

Aluminum strip

Loadings

Wooden shutter




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Pile cap is attached to pile and pile group at the outside of tank. Tank is filled up to a fixed

depth, below the level of pile tip and then pile group has placed in tank without disturbing the

sand filled in the tank. Pile and the piles group are fixed in the tank with the help of two flat mild

steel plates and four C clamps. Various precautions have been has been taken during this

process so it will not disturb the density of sand in tank.

Level of piles cap is examined by sprit leveler to avoid tilting of piles cap. C clamps are

tightened to maintain the pile cap at fixed position. After placing of piles at desired location, sand

pouring is started again and continued till the sand has covered the two-third length of pile.

Carefully, all C-claps are loosen and flat steel plates are removed from testing setup. Pouring of

sand is started again to fill the sand in the tank. Pouring filling is stopped when sand has reachedthe level just below the cap to avoid the interaction of pile cap and soil. The sand surface is

levelled carefully.

Four aluminum strips are attached on the pile cap by means of small size nuts and bolts. Tight

the nut and bolt very carefully without disturbing the pile arrangement. Four dial gauges are

fixed on the tank with the help of two flat steel plates in such a way that dial gauge needle reach

up to the middle of aluminum strips. Aluminum strips have been used to support the dial gaugeneedle.

Apply the working safe load on the piles cap and leave the whole assembly for two hours to

provide the time to piles for settlement due to application of loading. Note down the readings of

the dial gauges.

Release the first shutters to simulate the excavation and note down the reading of all four dial

gauges after ten minute. Release the other wooden shutter and note down the readings of all dial

gauges. Repeat this process until the desirable level of excavation is achieved. One factor “depth

ratio of excavation” is defined as the ratio of depth of excavation to the length of pile. For piles of

L/d=10 and L/d=20, the excavation is performed up to depth ratio of 0.68 and 0.53. The density

of sand is checked at the end of experiments through a dynamic penetrometer. Calculation of

depth ratio for different depth of excavation and length of pile is shown in table 2.

Table 2: Depth ratio of excavation

Length of pile

(cm)

No. of a wooden

shutter loosen

Depth of

excavation

Depth ratio of excavation

(length of pile/depth of excavation)

32 0 0 0

1 10 0.312

2 22 0.687

64 0 0 0

1 10 0.156

2 22 0.344

3 34 0.532




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Figure 3: Pile group configuration used in experiments

RESULTS AND DISCUSSIONSettlement of piles subjected to excavation induced soil movement are observed in the

laboratory. Various parameters such as number of pile, spacing of pile groups, working load of

piles, depth ratio of excavation and, embedment ratio and distance between pile group and

excavation surface have been considered for analysis. All tests are performed in the mediumdense sand. Spacing values are expressed in the term of diameter of footing. Spacing values of

3d, 4d and 6d are used in study. All the test have been carried out for embedment ratio of 10 and

20. Distance between pile group and excavation surface is kept 6.25d and 9.7d throw-out the

study. Number of piles are varied from one to four. Depth ratios of excavation are varied with

embedment ratio of pile.

Effect of embedment ratio

Embedment ratio is an important factor which affects the performance of pile. More the

embedment depth, more the frictional resistance. End bearing also depends on embedment of pile

and pile group. The tests on single piles are conducted for L/d ratio of 10 and 20 in mediumdense sand. Settlement decreases with increase in the embedment depth of pile and pile group.

For same depth of excavation, piles having more embedment depth show less settlement

comparison to small embedment ratio. In case of higher embedment ratio (L/d=20), increase in

spacing causes a decrease in settlement of pile and piles group but in case of small embedment

depth increase in spacing leads to increase in settlement.

Effect of distance between excavation surface and pilesgroup

Effect of distance between excavation surface and piles group can be predicted very easily bychanging the distance between pile group and excavation surface. Effect of distance between

excavation surface and pile group is shown in figure 4. An increase in the distance betweenexcavation surface and piles group leads to decrease in the settlement of single pile and pile

group. This is due to increase in the confining pressure and pile friction. If distance between

excavation surface and piles group is more than the threshold distance then it will not affect the

confining pressure and at same time only less area of the pile influencing zone will be influenced

by the excavation. Pressure bulb of pile group cannot be affected or less affected if distance

between piles and excavation surface is more.

2x23x1

2x1

Single pile




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Figure 4: Effect of distance between excavation surface and pile group

Effect of spacing between piles within pile group

Spacing is a most important factor in pile design and it affects the pile behaviour by a great

amount. After a threshold value of spacing piles behave like a single pile and group interaction

effects become negligible. Effect of spacing on settlement of piles groups are shown in figure 5.

In case of higher embedment depth, settlement of piles group is decreased with increase in

spacing between piles for a given configuration. But for a pile group having small embedment

depth (L/d=10), increase in spacing causes an increase in the pile settlement. So behaviour of passive piles and piles group are completely different from the behaviour of active piles. In caseof active pile increase in spacing leads to decrease in the settlement of piles and after threshold

spacing piles with a group behave like a single pile.

(a) (b)

Figure 5: Continues on the next page…

0

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S e t t l e m e n t ( m m )

Distance of excavation/diameter of pile

2x1, S=3d, L/d=20

2x1, S=4d, L/d=20

2x1, S=6d, L/d=20

3x1, S=3d, L/d=20

3x1, S=4d, L/d=10

3x1, S=6d, L/d=20

2x2, S=3d, L/d=20

2x2, S=4d, L/d=20

2x2, S=6d, L/d=10

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2x1

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2x2




Vol. 20 [2015], Bund. 10 4300

(c)

Figure 5: Effect of spacing on settlement of piles; (a) L/d=20 and distance between excavation

surface and pile group is 9.7d (b) L/d=20 and distance between excavation surface and pile group

is 6.25d, (c) L/d=10 and distance between excavation surface and pile group is 9.7d

Effect of number of pile

The effect of number of piles is different for different pile group; it is mainly depends on the

embedment depth of pile. Effect of number of piles are shown in figure 6.In case of higher

embedment depth, settlement of piles group is decreased with increase in number of piles for a

given configuration. But for a pile group having small embedment depth (L/d=10), increase in

number of piles causes an increase in pile settlement.

(a) (b)


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0 2 4 6 8


S/d

2x1

3x1

2x2

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Number of piles

S=3d S=4d S=6d

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Number of pile

S=3d S=4d S=6d




Vol. 20 [2015], Bund. 10 4301

(c)

Figure 6: Effect of number of piles on settlement of pile group; (a) L/d=20 and distance between

excavation surface and pile group is 9.7d (b) L/d=20 and distance between excavation surface and

pile group is 6.25d, (c) L/d=10 and distance between excavation surface and pile group is 9.7d

Effect of depth ratio of excavation

It is observed from experimental results that due to excavation induced soil movement,

settlement of single piles and group of piles under working load, increases with increase in depth

ratio of excavation irrespective of other factors. The effect of depth ratio is different for different

pile group; it is depend on the number of piles, depth ratio, embedment depth and spacing as well.

Effect of depth ratio is more prominent for piles having small embedment depth (L/d=10) as

compared to piles of higher embedment depth (L/d=20). Effect of depth of excavation is more

prominent when distance between pile group and excavation surface is less. Effect of depth ratio

of excavation on piles settlement are shown in figure 7.

(a) (b)


0

0.1

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0.4

0.5

0.6

0 1 2 3 4 5


Numer of piles

S=3d S=4d S=6d

0

0.05

0.1

0.15

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0.25

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0 0 . 2 0 . 4 0 . 6

S e t t l e m e t ( m m )


S=3d, L/d=20

S=4d, L/d=20

S=6d, L/d=20

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S=3d, L/d=20

S=4d, L/d=20

S=6d, L/d=20




Vol. 20 [2015], Bund. 10 4302

(c) (d)

Figure 7: Effect of depth ratio of excavation; (a) 3x1 pile group, L/d=20 and distance between

pile and excavation surface 6.25d (b) ) 3x1 pile group, L/d=20 and distance between pile and

excavation surface 9.7d, (c) 3x1 pile group, L/d=10 and distance between pile and excavation

surface 6.25d, (d) 3x1 pile group, L/d=10 and distance between pile and excavation surface 9.7d

CONCLUSIONSettlement of passive pile groups under excavation induced soil movement depends on

various factors such as embedment depth, depth ratio of excavation, spacing between piles within

a group and the distance between a pile group and excavation surface. Embedment ratio and

depth ratio of excavation are two most influencing factors that affect the settlement of pile group.

Settlement of a single pile and group of piles under working load, increases with increase in the

depth ratio of excavation irrespective of other factors. Behaviour of pile groups having

embedment ratio 10 is completely different from those having an embedment ratio 20. In case of

piles having a higher embedment ratio, settlement decreases with an increase in the number of

piles as well as with increase in spacing between piles within a pile group. However, the behaviour of pile groups having lower embedment ratio are showing contrary behaviour under

similar condition. Effect of spacing is more prominent in case of 2x1 pile group as compared to

3x1 and 2x2 pile group. It means that effect of spacing on settlement is reducing with increase in

number of piles in the group. An increase in the distance between excavation surface and pile

group causes reduction in the settlement of single pile and pile group. This is caused due to

increase in the confining pressure and pile friction. In a pile group, the effect of distance between

the group and excavation surface reduces with increase in number of piles in the group.

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0

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0 0.2 0.4 0.6 0.8

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Settlement of Pile Groups Exposed to Excavation Induced Soil Movement

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