Experimental and Numerical Study on Pile Behaviour Under Lateral Load in Clayey Slope

Indian Geotech J (January–March 2013) 43(1):105–114 DOI 10.1007/s40098-012-0037-z

ORIGINAL PAPER

Experimental and Numerical Study on Pile Behaviour Under Lateral Load in Clayey Slope S. V. Sivapriya • S. R. Gandhi

Received: 25 August 2012 / Accepted: 8 December 2012 / Published online: 25 December 2012 Ó Indian Geotechnical Society 2012

Abstract Structures such as jetties, transmission towers, elevated highways and various industrial units are often supported by pile foundation on natural or man-made soil slope. The piles on slopes are subjected to lateral load from the super structure, earth pressure from the unstable soil, wave and current actions in case of marine structures, etc. The behaviour of a pile on sloping ground under lateral load is different from that on a horizontal ground. The aim of this paper is to experimentally and numerically evaluate the behaviour of a single pile in sloping clay layer subjected to lateral load. 1 g model tests are performed in laboratory test tank on instrumented pile embedded in clayey bed with varying slopes and shear strength. The behaviour of a single pile placed either at crest or at different distances from crest on slope is evaluated. Static lateral load was applied in a direction towards the downward slope. Numerical study comprise of 3-D finite element analysis using PLAXIS code. The input parameters used for the analysis were validated by comparing the PLAXIS results with the experimental result. A detailed parametric study was then carried out by varying the clay shear strength and the slope angle. Based on the analysis, non-dimensional design charts are prepared for lateral capacity in piles on sloping ground. A worked example is included demonstrating the use of the design charts for pile on clay slope subjected to lateral loading.

S. V. Sivapriya  S. R. Gandhi (&) Geotechnical Division, Department of Civil Engineering, IIT-M, Chennai 600036, India e-mail: [email protected] S. V. Sivapriya e-mail: [email protected]

Keywords Pile foundation  Lateral load  Sloping ground  Laboratory experiment  Numerical analysis  Design charts

Introduction Structures are often required to be supported on slopes which are natural or manmade. Typical examples are hill slopes, rail embankment, road embankments, river training bunds, dredged slope in harbour, etc. These structures on slopes are frequently heavy and subjected to large lateral loads due to wind, earthquake, waves, etc. which require pile foundation. The lateral capacity of pile in the direction of downward slope is expected to be less compared to pile on horizontal ground and it further decreases with increase in slope steepness. The lateral capacity also depends on the relative position of the pile with respect to crest of the slope. The behavior of soil slope with a pile foundation passing through it is a complex soilstructure interaction problem. This paper reports experimental and numerical study to evaluate the behaviour of piles and to estimate it’s capacity in a sloping clay layer, when subjected to lateral loads. Pile passing through slope can be classified into two categories; active piles and passive piles as shown in Fig. 1. The active pile passes through a stable slope and subjected to lateral load which is transmitted to the stable soil with slope through shear and moment in the pile. In a passive pile, the unstable soil slope induces a force and the bending moment to the pile in addition to the externally applied forces which is transferred to the soil to a deeper level, below the probable soil slope failure surface or unstable soil zone [6]. Passive piles can be considered as reinforcements for preventing further sliding of an unstable slope [4, 13, 14, 17].

123

106 Fig. 1 Active/passive piles in slope. a Active pile, b Passive pile

Indian Geotech J (January–March 2013) 43(1):105–114

(a)

Load

Firm Soil 2R ϕ

45–ϕ

Passive wedge

Active Wedge Stable Slope, F.O.S > 1.4

(b)

Load

Unstable

2R

Passive wedge Active Wedge 45 +ϕ/2

The theoretical studies [5, 7] reported a separation between the pile and the surrounding soil in case of an active pile at small load levels. The ultimate lateral resistance reduction factor was found to be a function of soil property and slope angle [12]. From experimental and theoretical study, it was found that the lateral capacity increases with increase in spacing between the piles in the embankment when it is loaded either towards the slope or against the slope. The other way of increasing the lateral capacity is by increasing the pile length (in case of short rigid pile) and the relative density of soil [1]. The behaviour of a pile in a slope is studied [16] and the authors proposed that the effectiveness of pile lateral resistance can be increased by providing restrain at the pile top, in addition to increasing the soil stiffness and strength. The bending moment of the pile however, depends on the slope and the soil profile. The lateral resistance of the pile decreases as the ground profile changes from horizontal to a slope and

123

Slope F.O.S < 1.4

45 – ϕ/2

this results in larger displacement [1, 3]. The bending moment is higher for slope with lower relative density compared to slope with higher relative density. With increase in surcharge load, the bending moment increases along the length of the pile irrespective of relative density. However for safe design of pile in slope, maximum displacement, moment and shear acting on the pile should be checked [9]. The pile closer to the crest of the slope shows higher bending moment compared to piles away from crest of slope on horizontal ground and when it is at crest, the bending moment is 1.15 times of that in horizontal ground. The effect of bending moment in pile becomes negligible for piles placed beyond 10–45 times diameters away from the crest slope [2]. The optimum location of pile group founded in slope is away from the crest of the slope [18]. Due to the existence of slope, the lateral resistance reduces and it could be fully exerted when pile penetration below the slope is five or six times that of pile diameter [10].

Indian Geotech J (January–March 2013) 43(1):105–114 Fig. 2 Schematic diagram of the experimental set-up

107

Data Acquisition System Computer Read-Out

Unit

LVDT 0.3 m Pulley 0.1l 0.15l

0.45 m

Variable 0.4l

Dead weight

1 Instrumented Pile

0.2 l

Clay

Variable

1m

As existing analytical methods do not consider the slope effects on lateral capacity of piles, a new p-y criterion is proposed [8] through finite element analysis with the effect of slope and it is found to overestimate the lateral capacity of the pile on the crest of slope when compared to pile in horizontal ground. Although there have been many research on the lateral capacity of piles in a horizontal ground, there is limited guidelines or codal procedures for estimating the lateral capacity of piles on a slope. This study therefore attempts a detailed study on the lateral behaviour of a single pile in sloping clay layer. The behaviour is evaluated both by experiments on 1-g model and by numerical method using 3D-PLAXIS.

Experimental Work A typical test set-up is shown in Fig. 2. The dimension of the steel tank chosen is 1 m 9 0.6 m 9 0.5 m deep. A minimum width of 0.6 m is chosen to eliminate the side wall effects. One side of the wall of the tank was made of glass to enable the observation of slope during bed preparation and testing. Clayey soil collected from Siruseri, Chennai (India) was used for the experiments and its properties are given in Table 1. The tank was filled with the clayey soil with required consistency for two different shear strengths of 30 and 50 kPa. The pile was positioned at required locations and the soil was placed such that

Table 1 Index properties of the soil S. no

Properties

Value

1

Liquid limit

66 %

2

Plastic limit

27 %

3

Plasticity index

39 %

4

Specific gravity

2.68

5

Grain size distribution

6

Sand

0.5 %

Silt size

23.0 %

Clay size

76.5 %

IS-classification

CH

desired slope was obtained. The model pile was subjected to lateral loads in the direction of downward slope at horizontal surface level without restrain to evaluate its pile capacity under free head condition. The bending moment along the length of the pile was measured with electrical resistance type strain gauges bonded to the pile surface. A series of 20 tests were performed on a free headed long flexible pile with different locations of the pile and slopes as shown in Table 2. The pile was connected to the loading frame by a steel rope for applying lateral load through dead weights and the displacement at pile head was measured using LVDT. The displacement and bending strain readings were noted after 15 minutes of each load application.

123

108

Indian Geotech J (January–March 2013) 43(1):105–114

Table 2 Load (N) corresponding to 5 mm displacement Shear strength (kPa)

Slope

50

Dial Gauge

Distance from the crest Crest

30

1R

2R Test Pile

Horizontal

340

1V:3H

330

265

165

1V:2.5H

300

215

155

1V:2H

255

175

100

Horizontal

485

1V:3H

435

310

1V:2.5H

405

300

225

1V:2H

330

265

200

450 mm Stand

240 Fig. 3 Schematic diagram of beam bending test

6

Model Pile and Instrumentation A hollow aluminum pipe of 16 mm outer diameter, 14 mm inner diameter and 450 mm long with a bottom plug was used as pile. The pile was placed with its tip resisting on bottom of the tank. Strain gauges were fixed along the pile at different elevations as shown in Fig. 2. The strain gauges used are electrical resistance type with a gauge length of 5 mm to measure the bending moment at fixed depth. The flexural stiffness of the pile was determined by conducting simply supported beam test as shown in Fig. 3. From the central deflection under a known load at centre, the flexural stiffness of the beam was calculated from the Eq. 1. ð1Þ

where d is the deflection at centre (mm), P is the load at centre (N), l is the centre to centre span (450 mm), EI is the flexural stiffness (N mm2).

123

Bending Moment,Nm

The soil was mixed with a predetermined quantity of water to obtain the desired shear strengths. The required quantity of clay was air dried and required amount of water was added to achieve moisture content of 40 and 31 % for shear strengths of 30 and 50 kPa respectively. Appropriate care was taken to maintain the water content throughout the test in all cases. The conditioned soil was mixed thoroughly and filled in the tank in layers by the kneading compaction technique manually (Rao et al. [15]). The pile is considered as non-displacement type which was initially placed in position, followed by filling the tank with clayey soil around the pile. The verticality of the pile was ensured throughout process of filling. The slope was then prepared and extra clay was removed. The strength achieved after complete placement was checked by taking cylindrical undisturbed sample of 38 mm diameter and 76 mm height with a sampler tube and conducting unconfined compression test.

Pl3 ; 48EI

Strain Gauge Dead Weight

Preparation of the Test Bed

d¼

225 mm

225 mm

5 4 3 2 1 0

0

50

100

150

200

Strain, µm/m Fig. 4 Strain gauge response

The relationship between applied load and deflection was found to be linear. For P = 73.5 N and observed deflection d = 1.5 mm, the flexural stiffness of the pile is worked out as 93.1 9 106 N mm2. Figure 4 shows the relation between strain gauge reading and actual bending moment, which is almost linear and the same relationship was used to find the same relationship was used to find the bending moment from strain readings. Testing Program The tests were conducted for three different slopes, three positioning of piles and two soil strength. The position of pile is fixed in-terms of R which is the relative stiffness factor obtained from the lateral load test conducted in horizontal ground using Eq. 2 by Matlock and Reese [11] Au HR3 EI rffiffiffiffiffiffiffiffi EI 4 R¼ ; Dkh

u¼

ð2Þ ð3Þ

where u (mm) is the displacement of a single free head pile, Au is the displacement constant for a free head pile at ground level, H(N) is the lateral load applied, kh is the soil modulus

Indian Geotech J (January–March 2013) 43(1):105–114

(kN/m3) and D is the breath of the pile (mm). The soil modulus was worked back from the test using Eq. 3. The relative stiffness factor for soil with cohesive strength of 30 and 50 kPa soil was worked out to be 94 and 76 mm respectively.

Numerical Analyses A finite element program PLAXIS-3D 2010 version 2 is used for numerical analysis. The soil-bed used in the experiments was modelled in the software with slope. Lateral load was applied at the pile head similar to the experimental condition. The bending moment values obtained from these analyses were compared with the experimental results. Subsequently, analyses were carried out considering concrete pile of 1 m diameter concrete pile to develop design charts for shear strengths of 30, 50, 100 and 150 kPa. Soil and Pile Modeling The linear elastic and perfectly-plastic Mohr–Coulomb failure criterion was used. The properties of the soil used in the model are given in Table 3. Discretization of the soil element is a triangular element with 10 node tetrahedral soil element and 3 node line element for beam. Fine mesh is generated with element dimension being 0.075 m. An inbuilt model Embedded pile was used as pile element similar to the beam element modeled as linear elastic with interface element. The boundary conditions imposed are of general fixity; the vertical boundary in normal x-direction, parallel to yz plane where Ux = 0 and free in yz direction. Similarly for other two directions and the bottom boundary is taken as fixed in all directions (Ux = Uy = Uz = 0). Lateral load was applied as point load at the head of the pile at soil level in the direction of downward slope as applied in the experiments. Due to the long elastic flexible behaviour of the pile, the displacement at the bottom of the pile is small and hence the toe of the pile is hinged.

Table 3 Input parameter–numerical study Parameter

Name

Clay

Pile

Material model

Model

Mohr–Coulomb

Linear elastic

Drainage type

Type

Undrained C

–

Young’s modulus E(kPa)

8025,16640, 35000 70E6 and 27.4 E6 and 52500

Unit weight

c(kN/ m3)

17.9, 18.4,18.9 and 19.8

27 and 25

Poisson’s ratio

l

0.495

0.21

Cohesion

c(kPa)

30,50,100 and 150 –

Ko determination –

Automatic

Automatic

109

Analysis of the Model In total stress analysis, three modules viz. input parameter, calculation and output phases were involved. In input phase, the geometry of the experimental model was modeled. The drainage condition adopted is Undrained (C), in which total stress analysis is done with undrained parameters assuming the load applied on pile is sudden. The stiffness is modelled as undrained Young’s modulus as Eu, undrained poisson’s ratio lu and soil strength modelled as cu and uu = 0. In calculation phase, the water level and staged constructions are involved. In staged construction, the parts of the geometry model can be activated/deactivated and the properties can also be modified. The analysis is divided into two phases; in the first phase of analysis, generation of initial stresses in equilibrium with the self-weight of the soil is carried. The second phase of analysis is to carry out elastic–plastic deformation analysis to a small deformation theory where undrained analysis is considered. As embedded pile option is used in the analysis, a replica layer is generated to simulate a layer above the slope. The generation of initial stress is done in the phase-I analysis, where ko procedure is adopted. In phase II- calculation module, plastic analysis is opted where the replica layer is deactivated. The results were viewed in graphical form in the output phase.

Results and Discussion From the experimental study carried, comparison is made for load and bending moment for different shear strength, slope angle and position of pile. Using PLAXIS numerical analysis is made and it is then compared with the experimental results to validate the model. Further parametric study is carried out for wider range of parameters. Experimental Investigation The design lateral load of long flexible pile is governed by lateral deflection rather than the ultimate lateral capacity. For initial loads, the soil near the pile at surface carries the load by mobilising passive pressure and transfers the load to a greater depth as load increases. Initial test was performed by keeping the pile in a horizontal ground and subjected to a lateral load. This was used as base parameter to find percentage reduction in pile capacity for different slope. Thereafter the experiments were repeated with varying slope angle. Figure 5 shows the typical load displacement curve for a slope of 2H: 1 V (30 and 50 kPa) and for different positions of the pile. The load carrying capacity for the pile corresponding to 5 mm deformation in various slopes for different location of pile is given in Table 2.

123

110

Indian Geotech J (January–March 2013) 43(1):105–114

(a)

(a) 350

400 Horizontal Ground Crest 1R 2R

Crest

Load, N

Load, N

300

200

1R

2R

250

150

100 50 15

0 0

1

2

3

4

5

6

20

25

30

Slope Angle, Degree

Deformation, mm

(b) 450

(b) 600

Crest 1R 2R

Load, N

Load, N

500 400 300

350

250

200 Horizontal Crest 1R 2R

100 0

0

1

2

3

4

5

150 15

6

20

25

30

Slope Angle, Degree

Deformation, mm Fig. 5 Load–deformation of single pile for a slope of 2H:1V. a Soil strength-30 kPa, b Soil strength-50 kPa

The capacity increases about 30 % when the strength increases from 30 to 50 kPa in case of pile loaded in horizontal ground condition. Subsequently the reduction in capacity increases as the pile moves away from the crest towards the slope. Figure 6 shows the lateral load capacity corresponding to 5 mm for all the three slopes at different positions of the pile for two different shear strengths. It is observed that as the slope increases (3H: 1 V to 2H :1 V), the capacity reduces by 10–30 % when the pile is placed at crest position. The reduction is almost linear and consistent. The reduction in capacity increases as the pile distance from crest, towards the slope increases. The capacity reduces by 10–40 % when the pile is placed at crest compared to pile placed in horizontal ground condition. The reduction in capacity increases as the pile is moved towards the slope as shown in Fig. 7. The reduction in capacity is due to reduction in passive pressure acting in front of the pile. The maximum moment in a free-head pile subjected to lateral load occurs at a depth of 0.5–2.4 times the relative stiffness factor. It depends on the relative stiffness of the pilesoil, loading condition, slope and soil profile. The bending

123

Fig. 6 Load at 5 mm deformation for various position of pile. a Soil strength-30 kPa, b Soil strength-50 kPa

moment increases with the increase in loading. Figure 8 shows the bending moment diagram for the slope 3H:1 V with the pile position at crest, 1R and 2R; where the load is applied towards the slope for 50 kPa soils for 51 N. Numerical Investigation The model generated using PLAXIS-3D is shown in Fig. 9. In lateral loaded pile, the parameter influencing the loaddeformation character of the pile is the interface element and it posses zero thickness positioned along the shaft and tip of the pile. These interface element is used when there is a possibility of occurrence of gap in the back of the pile during loading. However in the analysis the beam element used as embedded pile has in-built interface element. Figure 10 shows the bending moment of the pile under various load conditions for a slope of 2H:1 V (30 and 50 kPa soil) and it is inferred that the bending moment increases with increase in load but the pattern is similar for both the soil condition. In Fig. 11a, a comparative graph between experimental and numerical values of loaddeformation for a slope of 2H:1V with different positioning of piles with shear strength of 30 kPa is shown. The bending moment comparison graph is shown in Fig. 11b,

Indian Geotech J (January–March 2013) 43(1):105–114

(a)

111

350

0.6 m

1V:3H 1V:2.5H 1V:2H Horizontal

300

Application of load

Pile with Interface

Load, N

250

Element 0.45 m

200 150 100

Crest

1R

2R

50

1m

0

50

100

150

200

Distance from crest, mm Fig. 9 Model generated using PLAXIS-3D for a slope of 3H:1V

550

1V:3H 1V:2.5H 1V:2H Horizontal

Load, N

450

(a)

Bending Moment, Nm 0

350

Crest

1R

Depth, m

250

2R

150

5

50

100

150

200

Distance from crest, mm Fig. 7 Comparison of lateral capacity of a pile when moved away from the crest. a Soil strength-30 kPa, b Soil Strength-50 kPa

(b)

Bending Moment, Nm

-5

0

0

0

0.05

0.05

0.1

0.1

0.15

0.15

0.2 0.25 0.3

0

10

51N 111N 167N

0.35

225N

Depth, m

(b)

5

10

15

0.2 0.25

50N 93.63N

0.3 138.23N

0.35

0.4

0.4

0.45

0.45

227N 330N

Fig. 10 Bending moment of single pile at crest in a slope of 2H:1VNumerical. a Soil strength-30 kPa, b Soil strength-50 kPa

Bending Moment, Nm

Depth, m

0

0

2

0

3

0

0

0

0.05

0.05

0.05

0.1

0.1

0.1

0.15

0.15

0.15

0.2

0.2

0.2

0.25

0.25

0.25

0.3

0.3

0.3

0.35

0.35

0.35

0.4

0.4

0.4

0.45

0.45

Pile @ crest

5

Application of load

2 1

cu =50 kPa

From the comparative study, it is observed that the numerical values are higher than the experimental values. The variation in load—deformation compared to the experimental result is about 12–17 % and the percentage variation in bending moment is about 10 %. As the interface between the pile and soil is bonded and there is no allowance of gap formation is the reason for the variation.

0.45 1R

2R

Design charts

Fig. 8 Bending moment in pile at different location

when the pile is kept at crest for the slope 2H:1V. Figure 12 show the increase in maximum bending moment with increase in load, the variation is almost linear. The maximum bending moment occurred at half-depth for pile on horizontal ground; for pile at crest, 1R and 2R, the depth of occurrence maximum moment increases.

Non-dimensional design charts are developed for the different cohesive strength of the clay, slope angle and position of pile with respect to the crest. The load is converted in terms of a non-dimensional parameter H5mm/cuDR and plotted against different slope angles in Fig. 13a–c. The charts are prepared with the results of parametric study obtained from experimental work and on PLAXIS analysis. As the experimental and numerical values show a similar trend, both the values are incorporated in the chart. The

123

Indian Geotech J (January–March 2013) 43(1):105–114

(a) 400

Load, N

300

200

100

0 0

1

2

3

4

5

Maximum Bending Moment, Nm

112

(a)

15.00

Crest 1R 2R

10.00

Crest + FEM 1R+FEM 2R+FEM

5.00

0.00

0

100

Deformation, mm Crest 2R Crest - FEM 2R - FEM

Maximum Bending Moment, Nm

Horizontal Ground 1R Horizontal - FEM 1R - FEM

Bending Moment, Nm

(b) -5

0

5

10

15

0 0.05

Depth, m

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

51N-FEM 111N-FEM 167N-FEM 255N-FEM 51N 111N 167N 255N

400

25

(b)

Crest 1R

20

2R Crest + FEM

15

1R+FEM 2R+FEM

10 5 0

0

100

200

300

400

Fig. 12 Comparison of maximum bending moment between experimental and numerical values for single pile in a slope of 2H:1V. a Soil strength-30 kPa, b Soil strength-50 kPa

relationship in the non-dimensional chart generated is represented in the form of best fit straight line. With known shear strength value, diameter and calculated R value the lateral capacity of the pile corresponding to 5 mm displacement is calculated for the position of pile related to R value and slopes angles. Inorder to calculate the lateral capacity of the pile in field condition corresponding to 5 mm displacement founded in sloping ground, non-dimensional charts are also developed. Numerical parametric study is considers pile of 1 m dia, 25 m long, M30 grade concrete, with four different shear strengths (30, 50, 100 and 150 kPa), varying slopes (3H:1 V, 2.5H:1 V and 2H:1 V) and positions of the pile (at crest, 1R from crest and 2R from crest towards the slope). The Young’s Modulus (Es) of the soil and pile (Ep) is obtained using the formulae 4 and 5. The chart developed as explained above is shown in Fig. 14.

123

300

Load, N

Fig. 11 Experimental and numerical comparison of pile in 2H:1V slope with soil strength of 30 kPa. a Load-deformation, b Bending moment for pile at crest

Es ¼ 350 cu ðkPaÞ pffiffiffiffiffi Ep ¼ 5000 fck ðN=mm2 Þ;

200

Load, N

ð4Þ ð5Þ

pffiffiffiffiffi where, fck is the compressive strength of the concrete. The properties of the soil and pile are given in the Table 3.

Illustrative Example To illustrate the use of the design charts developed, lateral capacity of 0.5 m diameter concrete pile placed in a slope of 2H:1 V with cu = 50 kPa and placed at the crest, 1R and 2R towards slope as shown in Fig. 15 has been evaluated using the following steps. (i)

Find the value of Relative stiffness using the Eq. 3. For Ep = 27.4 9 106 kPa (for M30 grade concrete), I = 3.0625 9 10-3 m4 and kh = 2515.2 kN/m3 (worked back based on lab test); the value of R works out to 2.4 m. (ii) The lateral capacity of the pile corresponding to 5 mm is calculated using Fig. 14a–c. For a slope angle of 2H:1 V, the non-dimensional value of H5mm/ cu DR is found as 0.82, 0.5 and 0.19 for corresponding pile location at crest, 1R and 2R from crest respectively.

Indian Geotech J (January–March 2013) 43(1):105–114 10

(a)

Exp-30 kPa

9

H5mm/cuDR

FEM-30 kPa

8

1.4 1.3

Exp-50 kPa

Hu/CuDR

(a)

113

FEM-50 kPa

1.2 1.1 1 0.9 0.8

7

0.7 16

18

6

18

22

24

26

28

Slope Angle (Degree)

Pile @ crest

5 16

20

20

22

24

26

Pile @ crest

28

FEM-30 kPa

FEM-50 kPa

FEM-100 kPa

FEM-150 kPa

Slope Angle, Degree

(b)

Exp-30 kPa Exp-50 kPa FEM-30 kPa FEM-50 kPa

7

0.8

6

H5mm/cuDR

1 0.9

Hu/CuDR

(b)

5

0.7 0.6 0.5 0.4 0.3

4

3 16

0.2 16

Pile @ 1R

18

20

18

20

22

24

26

28

Slope Angle (Degree) 22

24

26

Pile @ 1R

28

FEM-30 kPa

FEM-50 kPa

FEM-100 kPa

FEM-150 kPa

Slope Angle, Degree

(c)

5

H5mm/cuDR

4

0.6 0.5 0.4 0.3

3

0.2

Exp-30 kPa

0.1 16

Exp-50 kPa

2

1 16

18

20

18

20

22

24

26

28

Slope Angle (Degree)

FEM-30 kPa Pile @ 2R

FEM-50 kPa

22

24

Pile @ 2R

26

FEM-30 kPa

FEM-50 kPa

FEM-100 kPa

FEM-150 kPa

28

Slope Angle, Degree Fig. 13 Non-dimensional curves for different slopes with different soil strength- laboratory model. a Pile at crest, b Pile at 1R towards the slope, c Pile at 2R towards the slope

(iii)

0.8 0.7

Hu/CuDR

(c)

The horizontal capacity of the pile when placed at crest, 1R and 2R corresponding to 5 mm correspondingly works out to be 49, 26 and 11 kN respectively.

Fig. 14 Non-dimensional curves for different slopes with different soil strength-real time model. a Pile at crest, b Pile at 1R towards the slope, c Pile at 2R towards the slope

Similarly, the lateral capacity of single free headed pile in sloping ground in different slope can be found using the developed charts. These values are applicable typically for a berthing structure where the shear force is applied at cut-off level close to the horizontal ground at top. For applications

123

114

Indian Geotech J (January–March 2013) 43(1):105–114

6.

The finite element predicts the bending moment which is higher compared to experimental results.

25 m

500 mm ϕ bored pile Clay Soil cu = 50 kPa

Fig. 15 Schematic diagram of the illustrative example

where shear is applied at lower level on slope, there will be increase in pile capacity.

Summary and Conclusion Based on the detailed literature review it has been noted that no detailed guidelines are available for estimating the lateral capacity of piles placed on slope. In the present study the capacity of piles on slope have been determined based on 1 g model tests in laboratory test tank and numerically by PLAXIS- 3D. The results are presented in non-dimensional form for different slope angles and for different cohesive strength of soil. Use of design chart is demonstrated by an illustrative example for the field condition. The design chart developed can be a useful tool for estimating the lateral capacity. Based on the study carried out, following conclusions have been drawn. 1.

2.

3.

4.

5.

The reduction in pile capacity is observed to be about 10–50 % depending on cohesive strength and slope angle when the pile is kept on the slope compared to that on a horizontal ground. With increase in shear strength from 30 to 50 kPa, the lateral load capacity of the pile increases about 25 % (pile at crest), 30 % (pile at 1R) and 30–50 % when the pile is at 2R towards the slope. The capacity reduction compared to the capacity in horizontal ground at crest is about 3–25 % for 30 kPa soil and 10–32 % for 50 kPa soil as the slope increases from 3H:1 V to 2H:1 V. When the pile is placed at 1R position towards the slope, the capacity reduction is about 20–50 % for 30 kPa soil and 35–45 % for 50 kPa soil with increase in slope angle. Similarly, pile at 2R position the reduction capacity 50–70 % as the slope increases. The elevation of maximum bending moment differs depending upon the position of piles from 0.5 to 2.4R.

123

References 1. Almas Begum N, Muthukkumaran K (2008) Numerical modelling for laterally loaded piles on sloping ground. In: Proceedings of the 12th International conference of International Association for Computer Methods and Advances in Geomechanics, pp 1–6 2. Boufia A, Bouguerra A (1995) Odelisation en centrifugese du comportement d’un pieu flexible charge horizontalement a proximite d’un talus. Can Geotech J 32(2):324–335 3. Chae KS, Ugai K, Wakai A (2004) Lateral resistance of short rigid piles and pile groups located near slopes. Int J Geomech 4(1):93–103 4. Chen LT, Poulos HG (1996) The behaviour of piles subjected to lateral soil movements. Res. Rep. No. 731. University of Sydney, Sydney Australia 5. Davidson HL (1982) Laterally loaded drilled pier research Final report prepared for Electrical Power Research Institute 6. De Beer EE (1977) Piles subjected to static lateral loads State of the Art Report Proceedings of the 9th ICSMFE Speciality Session-10 Tokoyo, pp 1–14 7. Gabr MA, Borden RH (1990) Lateral resistance analysis of piers constructed on slopes. J Geotech Eng ASCE 116(2):1831–1850 8. Georgiadis K, Georgiadis M (2010) Undrained lateral pile response in sloping ground. J Geotech Geoenvironmental Eng 136:1489–1501 9. Hassiotis S, Chameau LJ, Gunaratne M (1997) Design methods for stabilization of slopes with piles. J Geotech Geoenvironmental Eng 123(4):314–323 10. Liu JH, Zhao MH, Yang MH (2011) Study on mechanics behaviour and model tests of bridge piles foundation in slope. J Geotech Geoenvironmental Eng GSP 219:97–104 11. Matlock H, Reese LC (1960) Generalised solutions for laterally loaded piles. J Soil Mech Found ASCE 86(5):63–91 12. Muthukkumaran K (2004) Non-linear Soil structure interaction of piles on sloping ground, Ph D Thesis, Indian Institute of Technology, Madras 13. Poulos HG (1995) Design of reinforcing piles to increase slope stability. Can Geotech J 32:808–818 14. Poulos HG, Chen LT (1997) Pile response due to excavation– induced lateral soil movement. J Geotech Eng ASCE 123(2):94–99 15. Rao SN, Ramakrishna VGST, Rao MB (1998) Influence of rigidity on laterally loaded pile groups in marine clay. J Geotech Geoenvironmental Eng ASCE 124(6):542–549 16. Rowe RK and Poulos HG (1979) A method for predicting the effect of piles on slope behaviour. 3rd International Conference on Numerical methods in Geomechanics, Aachen, pp 1073–1085 17. Stewart DP, Jewell RJ and Randolph MF (1992) Piles bridge abutments on soft clay experimental data and simple design methods. In: Proceedings 6th Australia–New Zealand Conference Geomech, vol 1, pp 199–204 18. Wei WB, Cheng YM (2009) Strength reduction analysis for slope reinforced with one row of piles. Comput Geotech 36:1176–1185