PERFORMANCE OF STRIP FOOTINGS ON SLOPE REINFORCED WITH INCLINED PILE

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 7, Issue 3, May–June 2016, pp. 211–222, Article ID: IJCIET_07_03_021

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PERFORMANCE OF STRIP FOOTINGS ON

SLOPE REINFORCED WITH INCLINED

PILE

Dr. S. S. Pusadkar

Associate Professor, Department of Civil Engineering, Government College of

Engineering, Amravati, M.S, India

A. U. Mankar

P.G. Scholar, Department of Civil Engineering, Government College of Engineering,

Amravati, M.S, India

ABSTRACT

The footing that placed on slope surface results in decreasing the bearing

capacity of soil. The pile reinforcing the slope may affect the bearing capacity

of footing and factor of safety of slope. The piles installed within the slope

mechanically provide a resistance to slope system along the failure surface.

The present work was focused on the analysis of strip footing behavior on

slope with and without pile using a finite element software PLAXIS 2D. The

various parameters considered for present work includes location of pile from

slope crest, inclination of pile, effect of pile length and effect of width of load.

The results indicated that stabilizing pile has a significant effect in improving

the bearing capacity and factor of safety of slope. The bearing capacity of

footing was found to be maximum when slope was reinforced by pile at crest.

For smaller width of strip, the factor of safety of slope was maximum when

pile is placed at crest while for larger width the pile location at 0.5 horizontal

widths of slope gave maximum factor of safety.

Key words: Strip Footing, pile, PLAXIS 2D, Bearing Capacity, Factor of safety.

Cite this Article: Dr. S. S. Pusadkar and A. U. Mankar, Performance of Strip

Footings on Slope Reinforced with Inclined Pile, International Journal of

Civil Engineering and Technology, 7(3), 2016, pp. 211–222.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType

Dr. S. S. Pusadkar and A. U. Mankar


INTRODUCTION

Many situations necessitates the placement of footings on sloping surfaces or adjacent

to a slope crest, e.g. footings for bridge abutments on sloping embankments, small to

medium size building adjacent to slope crest. When a footing is located on a sloping

ground, the bearing capacity of the footing may be significantly reduced, depending

on the location of the footing with respect to the slope. Over years, the subject of

stabilizing the earth slope has become one of the most interesting areas for scientific

research and attracted a great deal of attention. Slope stability can be increased in

different ways, such as modifying the slope surface geometry, using soil

reinforcement, or installing continuous or discrete retaining structures such as walls or

piles. The use of stabilizing piles to support an active earth slope has been considered

to be one of the important slope reinforcement techniques in the last few decades.

These piles, which can be driven at the crest or within the slope itself, act as resisting

members and are usually subjected to lateral forces by the horizontal movements of

the surrounding soil. Here the study will be carried out using PLAXIS 2D for slope

subjected to strip footing and reinforced using pile at different location.

LITERATURE REVIEW

The slopes naturally made or manmade are being used in construction of structures on

or nearby them. These structures are located at various places on slopes or on

embankment side. The presence of structure on or nearby slope may disturb the

stability of slopes or lead to sustain low bearing load. The slopes safety or bearing

load can be improved by providing suitable measures. The various researchers took

effort to suggest alternative methods. The present work is related to improvement in

slope stability by reinforcing the slopes with pile. The work carried by various authors

is illustrated in brief to understand the extent of work carried out.

The slope reinforced with piles using the finite element method was analyzed by

Cai and Ugai (2000). It was concluded that the maximum safety factor for the slope

can be achieved when the piles are located in the middle of the slope and the pile head

restrained. The numerical comparison of predictions by limit equilibrium analysis and

3D numerical analysis for a slope–pile system was carried out by Won et al. (2005). It

was observed that for stability to be improved optimally, the piles should be installed

in the middle of the slope and with restrained pile head.

Yang et al. (2011) studied the effect of embedded length of piles for slope

reinforced with piles by varying parameters such as pile head condition viz. free,

fixed, hinged and non-rotated head, embedded pile length. It was concluded that a

restrained pile head was recommended and free head should be avoided to stabilize

the slope.

Gullu (2013) studied the effect of pile on slope stability by PLAXIS 2D by

changing the location of pile at different position within slope. It was concluded that

factor of safety of slope-pile system increases with the increased the row of pile and

suggests using either the single row pile at the toe or two-row piles at the toe and

middle of slope in practice.

Wang and Zhang (2013) conducted centrifuge testing and observed the behavior

of pile in cohesive soil slopes with varying inclination of slope and pile head

conditions. It was observed that the stability level of the slope was increased by

compression and shear effect provided by pile.

From the above discussion it was observed that the previous studies on pile-

stabilized slope were done for improvement of slope only. The little work was carried

Performance of Strip Footings on Slope Reinforced with Inclined Pile


out on the improvement of load-carrying characteristics of shallow footings supported

on pile-stabilized slope. The main objectives of the present study was to study the

effect of various parameters of footing, location of pile and load inclination on factor

of safety of slope and bearing capacity of footing.

NUMERICAL ANALYSIS

The geometry of the finite element soil model adopted in the PLAXIS 2D analysis

was 5H X 7H with the width of strip footing as 1.5m. The analysis was carried out for

different pile locations and L/B ratio with different pile inclinations. The details of the

parameter studied are as shown in Table 1.

Table 1 Details of Parametric Study

SS.N PARAMETER VALUE

11 Diameter of Pile (D) 20 cm

22 Width of footing (B) 1.5 m

33 Slope Inclination 1:2

44 d/B ratio 0

55 L/B ratio 3, 4, 5, 8, 10

66 Location of pile from crest (d) 0X, 0.2X, 0.4X, 0.6X, 0.8X

77 Inclination of pile 0º, 10

º, 20

º, 30

º

88 Width of load 2m, 4m, 8m, 16m

The slope model along with pile and footing used for analysis is as shown in

Figure.1. The slope modeled in PLAXIS 2D for one condition is shown in Figure. 2.

Figure 1 Pile-Stabilized slope



Figure 2 Model Geometry in Plaxis 2D

MATERIAL PROPERTIES

The properties of soil used in slope, strip footing and pile are given in Table 2, Table

3 and Table 4 respectively. These properties are used for PLAXIS 2D analysis.

Table 2 Soil Properties

Sr. No Parameter Value

1

Type of material Sand

2 Material Model

Mohr - Coulomb

3

4

Young’s modulus of sand(kN/m2) 50000

4 Cohesion (kN/m2) 0.5

5 Poisson’s ratio 0.30

6 Friction angle(Φ) 34°

7 Interface reduction factor Rinter 0.67

8 Dry density(kN/m3) 18



Table 3 Strip footing properties


1 Type of material Elastic

2 Normal stiffness, EA (kN/m) 30000000

3 Flexural rigidity, EI (kN/m2/m) 25000


Table 4 Pile Properties


1 Type of material Elastic

2 Normal stiffness, EA (kN/m) 2.826E06

3 Flexural rigidity, EI (kN/m2/m) 7065


REULTS AND DISSCUSSION

The analysis was carried out using PLAXIS 2D for slope with strip footing and piles.

The numbers of piles per meter length were considered for the analysis and equivalent

normal stiffness (EA) and bending stiffness (EI) were calculated accordingly. The pile

was modeled as a plate element of equivalent stiffness to consider it as plain strain

problem for analysis. The results of analysis for different parameters affecting the

performance of reinforced slope using piles are discussed with respect to bearing

capacity, factor of safety and width of strip load.

Factor of Safety of Slope

The effect of pile length, pile inclination and pile location on factor of safety (FOS) of

slope was studied for a load which results in failure of load without reinforcement i.e.

for FOS =1. The ultimate load obtained was divided by factor of safety 3 to determine

the safe load to be applied. For all analysis, this safe load was applied on slope and

the piles were used to improve factor of safety. The FOS value of unreinforced slope

was used to compare the results with pile-reinforced slope. The variation of FOS for

different L/B ratios with pile location and pile inclinations are shown in Fig. 3. It can

be seen that the factor of safety was maximum when the pile was located at crest of

slope for all L/B ratios and the pile reinforcement was observed to be effective up to

0.5X distance from crest i.e. at middle of slope.



Figure 3: Effect of Pile Location for Different L/B Ratios

(a) For L/b = 3

(b) For L/b = 5

(c) For L/b = 8



(d) For L/b = 10

Ultimate Bearing Capacity

The strip footing behavior in terms of ultimate bearing capacity (UBC) for different

parameters was studied. The maximum load just before failure was observed and a

load settlement curve was drawn for each case to determine the ultimate bearing

capacity of strip footing under vertical load. The ultimate bearing capacity of

unreinforced slope was used to compare the effectiveness of pile reinforcement. The

variation of UBC for different L/B ratios with pile location and pile inclinations is

shown in Fig. 4. The results indicate that the pile reinforcement had significance

effect on ultimate bearing capacity of the strip footing. The UBC was observed to

maximum when the pile was located at crest of slope for all the L/B ratios. For L/B of

3 to 5, 10⁰ inclined piles gives maximum value of UBC as compared to other piles and for L/B ratio more than 8; vertical pile gives maximum value of UBC as

compared to all inclined piles.

Figure 4 Variation of UBC for Different L/B Ratios

(a) For L/b = 3



(b) For L/b = 4

(c) For L/b = 5

(d) For L/b = 8



(e) For L/b = 10

Effect of Load Width

The strip load width considered for the analysis was 2m, 4m, 8m and 16m. The strip

load was applied as uniformly distributed load on embankment. The effect of strip

load width was determined in terms of factor of safety of slope.

The variation of FOS for different length of piles with pile location and pile

inclinations for strip load width of 2m and 16m are shown in Fig. 5 and Fig. 6

respectively. The results indicate that for small load width of 2 m and 4 m, the

optimum location of pile at slope crest gave maximum factor of safety. There is

marginal increase in factor of safety of slope as length of pile increases. For strip

width more than 8m, the optimum location of pile which gave maximum factor of

safety was observed at the middle of the slope. The inclination of pile had marginal

effect on the factor of safety of slope.

Figure 5 Variation of FOS for Load Width 2 M

(a) For Lp = 3m

5



(b) For Lp = 6m

(c) For Lp = 10m

(d) For Lp = 15m



Figure 6: Variation of FOS for Load Width 16 M

(a) For Lp = 3m

(b) For Lp = 6m (c) For Lp = 3m

(d) For Lp = 6m



CONCLUSIONS

The analysis of pile reinforced slope had been carried using PLAXIS 2D to evaluate

the effect of strip footing on performance of pile reinforced slope in terms of factor of

safety and / or ultimate bearing capacity. The piles installed within the slope,

mechanically provides a resistance to slope system along the failure surface. The main

aim of this study was to investigate the effect of pile location from crest, pile length,

pile inclination and width of strip load. From this study the following conclusions

were drawn:

Pile reinforcement had significant effect on the performance of slope with strip

footing on embankment.

The maximum UBC was observed when the pile was located at crest of slope for all

the L/B ratios.

For smaller L/B ratios up to 5, 10 degree pile inclination gave maximum bearing

capacity and for higher L/B ratio vertical pile gives maximum bearing capacity as

compared to inclined piles.

The optimum pile location which gave maximum value of factor of safety was

observed to be at crest of slope for all L/B ratios.

As strip load width increases, the optimum pile location was observed to be at middle

of slope for higher values of L/B as compared to small strip load width.

REFERENCES

[1] Cai F. and Ugai K. (2000), Numerical Analysis of the Stability of a Slope

Reinforced with Piles, Soils and Foundation, 40, pp.73–84.

[2] Gullu H. (2013), A Numerical Study on Pile Application for Slope Stability, 2nd

International Balkans Conference on Challenges of Civil Engineering, BCCCE,

Epoka University, Tirana, Albania, pp. 810–816.

[3] Won J., You K., Jeong S. and Kim S. (2005), Coupled Effects in Stability

Analysis of Pile–Slope Systems, Computers and Geotechnics, 32, pp. 304–315.

[4] Wang L. and Zhang G. (2013), Pile-Reinforcement Behaviour of Cohesive Soil

Slopes: Numerical Modeling and Centrifuge Testing, Journal of Applied

Mathematics.

[5] Yang S., Ren X. and Zhang J. (2011), Study on Embedded Length of Piles for

Slope Reinforced with one Row of Piles, Journal of Rock Mechanics and

Geotechnical Engineering, 3(2), pp. 167–178.

[6] Anuj Chandiwala, FEM Modeling for Piled Raft Foundation in Sand,

International Journal of Civil Engineering and Technology, 4(6), 2014, pp. 239–

251.

[7] Sunil S. Pusadkar and Sachin N. Ghormode, Uplift Capacity of Piles in Two

Layered Soil, International Journal of Civil Engineering and Technology, 6 (3),

2015, pp. 132–138.