study on embedded length of piles for slope reinforced ... · study on embedded length of piles for...

Journal of Rock Mechanics and Geotechnical Engineering. 2011, 3 (2): 167–178

Study on embedded length of piles for slope reinforced with one row of piles Shikou Yang, Xuhua Ren, Jixun Zhang College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098, China

Received 26 January 2011; received in revised form 7 May 2011; accepted 9 May 2011

Abstract: The embedded length of anti-slide piles for slope is analyzed by three-dimensional elastoplastic shear strength reduction method. The effect of embedded pile length on the factor of safety and pile behavior is analyzed. Furthermore, the effects of pile spacing, pile head conditions, pile bending stiffness and soil properties on length and behavior of pile are also analyzed. The results show that the pile spacing and the pile head conditions have significant influences on the critical embedded length of pile. It is found that the critical embedded length of pile, beyond which the factor of safety does not increase, increases with the decrease in pile spacing. The smaller the pile spacing is, the larger the integrity of the reinforced slope will be. A theoretical analysis of the slip surface is also conducted, and the slip surface determined by the pressure on piles, considering the influences of both soil and piles for slope, is in agreement with the ones in previous studies. Key words: slope stability; factor of safety; embedded pile length; strength reduction method; slip surface

1 Introduction

Anti-slide piles have been widely utilized in the treatment of unstable slopes, and have been proved to be an effective reinforcement method. The piles are considered as passive piles in the upper unstable soil layer but active piles in the lower stable soil layer. For slopes with great depth between ground surface and stiff stratum, the solution is impractical to embed piles into bedrock or a stable layer (infinite pile length assumption). Thus, the embedded length of piles is an issue and attracts great attention.

Some studies have analyzed the embedded length of piles due to lateral soil movements. Ito et al. [1] investigated the design method for a row of piles in a slope with a fixed slip surface, and analyzed the effect of pile length above the slip surface on the factor of safety of pile-slope system. Poulos [2] presented an approach for the design of piles to reinforce slopes by a computer program ERCAP, and analyzed the effect of embedded length on resistance of piles. Chen [3] and Poulos et al. [4] described a series of laboratory tests on single-instrumented model piles embedded in

Doi: 10.3724/SP.J.1235.2011.00167 Corresponding author. Tel: +86-15105194996; E-mail: [email protected]

calcareous sand undergoing lateral soil movements, and analyzed the effect of the ratio of embedded pile length in the upper “unstable” sand layer to that in the lower “stable” sand layer on the maximum bending moment. Griffiths et al. [5] studied the influence of pile reinforcement on stability of slopes by numerical analysis, and presented the influences of pile length on stability and factor of safety of slope. But the analysis was carried out under two-dimensional plane strain, which could not reflect the actual pile-slope interaction. Qin and Guo [6] conducted some model tests on vertically loaded single piles in sand subjected to either a uniform or a triangular profile of soil movement, and studied the effect of depth of soil movement on pile behavior. Yoon et al. [7] introduced a simple chart for laterally loaded short piles in cohesionless soils to account for the effect of “finite slope”, and expressed the required pile length in a slope as a dimensionless ratio. Guo and Qin [8] developed an experimental apparatus to investigate the behavior of vertically loaded free-head piles in sand receiving lateral soil movements, and analyzed the effect of sliding depth on the maximum bending moment.

The embedded length of piles subjected to lateral soil movements or in unstable slopes has been referred to either by model tests or by numerical simulations. However, only two parameters, the embedded pile

168 Shikou Yang et al. / J Rock Mech Geotech Eng. 2011, 3 (2): 167–178

length above the slip surface and the ratio of embedded pile length in the upper “unstable” layer to that in the lower “stable” layer, have been analyzed. More systematic studies are needed to better interpret the pile-slope interaction. In the present study, the embedded pile length of slope reinforced with one row of piles is analyzed, and the effects of the pile spacing, pile head conditions, pile bending stiffness and soil properties on the embedded piles length in a slope are also analyzed with three-dimensional elastoplastic shear strength reduction method.

2 Analytical method 2.1 Shear strength reduction method [9]

The stability of slopes reinforced with one row of piles is commonly estimated using pressure- or displacement-based methods, applying an assumed or measured pressure or displacement distribution on piles according to experiences or in-situ measured values caused by lateral soil movements. But it cannot describe the actual coupled pile-soil interaction, and may cause some inaccuracies. Alternatively, the shear strength reduction finite element/finite difference methods can be a good choice, because it cannot only simulate the coupled pile-slope interaction but also calculate the global factor of safety conveniently.

The definition of global factor of safety of slopes reinforced with one row of piles in shear strength reduction method is identical to the one in limit equilibrium method. The reduced shear strength parameters, fc and f , are defined as

f s/c c F (1)

f sarctan(tan / )F (2) where c and are the actual shear strength parameters; and sF is the shear strength reduction factor, which is considered as the global factor of safety of slopes. 2.2 The depth of slip surface of slopes reinforced with piles

For uncoupling analyses, the force that the sliding mass exerts on a row of piles can be expressed as a function of soil strength, pile diameter and pile spacing [10], and the factor of safety of slopes can be calculated based on the limit equilibrium method [1, 11–13]. Thus, the slip surface can be determined. However, the assumption of rigid piles and linear distribution of lateral force per unit thickness may not reflect the actual activities. For coupling analyses, Cai et al. [14, 15] determined the slip surface by the maximum shear force, as shown in Fig.1. Wei and Cheng [16] determined the

(a) Model of Cai and Ugai [14].

(b) Model of Won et al. [15].

(c) Model of Wei and Cheng [16] and shear stress obtained.

Fig.1 Slip surfaces obtained by the maximum shear force, shear stress and Bishop’s simplified method, respectively.

slip surface of a slope reinforced with piles by shear stress or shear strain rate and the failure mode with respect to different pile spacings. Although those approaches seemed to be helpful, Cai et al. [14, 15] did not consider the shear stress along the slip surface and

D1/D = 2.0

Extreme point of shear force

Critical

slip surface

Bishop’s simplified method

(slipping depth: 3.6 m)

FLAC3D (6.33 m):extreme point of shear force

Detailed pile element

1 000 800 600 400 200 0

Shear stress (kPa)

Hei

ght (

m)

0

2

4

6

8

10

12

16

14

Maximum point of shear force

Shikou Yang et al. / J Rock Mech Geotech Eng. 2011, 3 (2): 167–178 169

the pressure acting on the pile, which could make the shear stress of piles downward even larger. Wei and Cheng [16] found that the differences between the strains within the regions, where stress was highly concentrated, were so small that the construction of a precise slip surface appeared to be not meaningful. In the present study, the slip surface will be determined by the pressure on piles considering the influences of both soil and piles.

In Fig.2, the critical slip surface is DEF, i.e. slip surface with the maximum factor of safety of a slope reinforced with one row of piles. It divides the slope into two parts: the upper unstable soil layer and the lower stable soil layer. It is also assumed that the slip surface DEF is perpendicular to the pile. As shown in

Fig.2, the surfaces ABC and MNP, both parallel to the surface DEF, are assumed to be very close to the critical slip surface DEF, with BE = EN, and the pressure on each part (BE and EN) of the pile is assumed to be uniform. The pressure herein signifies an earth pressure difference acting on both sides of a pile per unit thickness divided by the diameter of the piles. The pressure is positive when its direction is same as the sliding direction of the slope. The negative pressure acting on the piles implies that the parts of the piles cannot supply resistant force to the slope and it decreases the factor of safety of the slope.

Fig.2 Critical slip surface for the slope reinforced with one row

of piles. For the mass ABCFED, the equilibrium of forces

acting on it will be considered in the direction along the

slip surface:

1 2 upd sin d d 0ABC ABCFED DEF

s f S s P BE

(3)

where 1 and 2 are the shear stresses along the surfaces ABC and DEF, respectively; f is the body force

in the vertical direction; is the angle between the normal direction and the vertical direction of the slip surface; and upP is the pressure acting on BE.

In regard to the mass DEFPNM, similar equations

can be obtained:

2 3d sin d dDEF DEFPNM MNP

s f S s

down 0P EN (4) where 3 is the shear stress along the surface MNP, and

downP is the pressure acting on EN. Then, combining Eqs.(3) and (4), it can be gotten

that

1 2

3 2

d d sin d

sin d d d

ABC DEF ABCFED

DEFPNM DEF MNP

s s f S

f S s s

up downP BE P EN (5)

Considering the assumptions that the thickness of the masses ABCFED and DEFPNM is infinitesimal and BE = EN, we can obtain

ABC DEF MNP (6)

sin d sin dABCFED DEFPNM

f S f S (7)

According to the definition and feature of critical slip

surface DEF, it can be obtained that

1 2 2, (8) where the equal sign is selected when the element fails.

Using Eq.(6) and integrating Eq.(8), it is obtained

that

1 2d d ,ABC DEF

s s 3 2d dMNP DEF

s s (9)

Substituting Eqs.(7) and (9) into Eq.(5), it can be

obtained as follows:

up downP P (10)

Thus, Eq.(10) is a requirement for slip surface of the slope reinforced with one row of piles, regardless of the pile head conditions. Therefore, the range of slip surface of slope reinforced with one row of piles can be obtained by the pressure distribution of pile, and the maximum depth of the slip surface is determined by the position of the maximum pressure on the upper pile. Due to the simplicity of Eq.(10), it can be conveniently applied in practice. According to the above analysis, however, it can only be used to determine the slip surface near the pile. For large pile spacing, it may cause great errors when determining the slip surface near the section through the soil midway between the piles. 2.3 Validation and application of the model slope

Slip surface

C F P

Unstable layer

Stable layer

3τ 2τ

1τ

N

B A D

M E


The embedded length of piles in slopes is analyzed by three-dimensional elastoplastic shear strength reduction finite difference method, with software of FLAC3D [17]. The validation of FLAC3D has been conducted by many previous researchers [15, 16]. The slope model considered by Cai et al. [14–16] is used in the present paper, except some changes in the size of model and the boundary conditions of embedded pile, such as the length of the pile and the restraints of the pile bottom, as shown in Fig.3.

Fig.3 Model slope and finite difference mesh.

The uniform slope has a height of 10 m and a gradient

of 1: 1.5 (vertical : horizontal). Considering the effect of embedded length of piles, a ground thickness of 25 m is selected, and two symmetric boundaries are used so that the slope herein consists of a row of piles with planes of symmetry through the centerlines of two adjacent piles. A steel tube pile with an outer diameter (D) of 0.8 m is used in the study. The piles are treated as a linear elastic solid material and installed in the middle of the slope and center-to-center pile spacing (s) varies from 2D to 6D. The parameters of the soil, the pile-soil interface, and the piles are shown in Table 1 (unless

Table 1 Material parameters [14].

MaterialYoung’s modulus, E (MPa)

Poisson’sratio,

Unit weight, (kN/m3)

Cohesion, c (kPa)

Frictionangle,

(°)

Dilation angle,

(°)

Soil 200 0.25 20 10 20 0

Interface 200 0.25 — 10 20 0

Pile 60 000 0.20 — — — —

otherwise stated). The soil is modeled by elastic perfectly plastic model with the Mohr-Coulomb failure criterion. However, the assumption of an elastic pile means that the yielding of the pile itself cannot be identified, and it is a limitation of the present results.

3 Results and discussion

3.1 Effect of embedded length of piles

When piles with an equivalent Young’s modulus, Ep = 60 GPa, are installed in the middle of the slope with Lx = 7.5 m. The effect of embedded pile length on the factor of safety of the slope reinforced with piles is shown in Fig.4 (where Ep = 200 GPa is given to study the effect of pile bending stiffness). As expected, the factor of safety of the slope reinforced with one row of piles tends to increase with increasing length of piles. When the embedded pile length exceeds a critical value, namely, the critical embedded pile length, which may be different for various pile spacings, the factor of safety of the slope will gradually approach to be a constant. For a free head pile, as the pile length increases, there is not any alteration in the depth of the maximum pressure in the upper pile, as shown in Fig.5(a). However, for hinged head piles, the depth of the maximum pressure in the upper pile fluctuates apparently, and increases with increasing pile length (Fig.5(b)).

(a) Free head. (b) Fixed head.

1.2

1.3

1.4

1.5

1.6

1.7

1.8

5 10 15 20 25

Embedded pile length (m)

Fac

tor

of s

afet

y

s = 2D, Ep = 60 GPa s = 3D, Ep = 60 GPa s = 4D, Ep = 60 GPa s = 6D, Ep = 60 GPa

s = 3D, Ep = 200 GPas = 3D, cs = 20 kPa, s = 10°

30 1.4

1.5

1.6

1.7

1.8

5 10 15 20Embedded pile length (m)

Fac

tor

of s

afet

y

1.9

s = 2D, Ep = 60 GPa s = 3D, Ep = 60 GPa s = 4D, Ep = 60 GPa

s = 6D, Ep = 60 GPa



(c) Hinged head. (d) Non-rotated head.

Fig.4 Factors of safety for various pile lengths, pile spacings, pile head conditions, pile bending stiffness and soil properties (cs and s are

the cohesion and friction angle of soil, respectively).

(a) Free head.

1.2

1.3

1.4

1.5

1.6

1.7

1.8

5 10 20

Fac

tor

of s

afet

y

1.9

15 Embedded pile length (m)

s = 2D, Ep = 60 GPa s = 3D, Ep = 60 GPa s = 4D, Ep = 60 GPa s = 6D, Ep = 60 GPa


Embedded pile length (m)

0

4

8

12

16

20

24

28

2 2 6 10 14 18

Deflection (cm)

Dep

th (

m)

10 m

14 m

6 m

8 m

12 m

16 m

18 m

20 m22 m

24 m26 m

0

4

8

12

16

20

24

28

0.8 0.6 0.4 0.2 0.0 0.2

Bending moment (MNm) D

epth

(m

)

6 m

10 m

14 m

8 m

12 m

16 m

18 m 20 m 22 m 24 m 26 m

0

4

8

12

16

20

24

28

2.0 1.0 0.0 1.0 2.0

Shear stress (MPa)

Dep

th (

m)

6 m

10 m

14 m

8 m

12 m

16 m

18 m

20 m22 m

24 m

26 m

0

4

8

12

16

20

24

28

0.2 0.1 0.0 0.1 0.2Pressure (MPa)

Dep

th (

m)

6 m

10 m

14 m

8 m

12 m

16 m

18 m

20 m 22 m

24 m

26 m

Fac

tor

of s

afet

y

1.2

1.3

1.4

1.5

1.6

1.7

1.8

5 10 15 20 25 30

s = 2D, Ep = 60 GPa

s = 3D, Ep = 60 GPa

s = 4D, Ep = 60 GPa s = 6D, Ep = 60 GPa

s = 3D, Ep = 200 GPa

s = 3D, cs = 20 kPa,

s = 10°


(b) Hinged head.

Fig.5 Pile behaviors for various pile lengths.

For free head piles (Fig.5(a)), the pile deflection at

collapsing increases as the pile length increases, and comes to a steady distribution when the pile length is greater than the critical length. The bending moment of the piles increases with the increase in pile length, and the point of the maximum bending moment moves downward until the bending moment tends to be a steady distribution. The shear stress distribution moves forward as the pile length increases, and approaches to be steady with the maximum shear stress slightly reduced. According to the horizontal force equilibrium, the summary of positive pressure and negative pressure tends to zero for free head piles. Longer pile tends to produce larger positive pressure on the top and decreases the maximum pressure on the bottom, transmitting the pressure downward and increasing the total resistance, which improves the stability of the slope reinforced with piles.

For hinged head piles (Fig.5(b)), when the embedded length of piles exceeds the critical value, the deflection of the piles is very small compared with that of free

head piles, and hardly varies while the pile length increases. However, when the pile length is smaller than the critical value, the deflection increases almost linearly with the depth. It stems from less anti-slide force provided by the bottom of the pile. The positive pressure on the top of the pile increases with increasing pile length and also tends to a constant value while the pile is longer than the critical value. 3.2 Effect of pile spacing (s/D)

The effect of pile spacing on the factor of safety and critical pile length is shown in Fig.4. As the pile spacing decreases, the piles become more like a continuous pile wall and the integrity of the soil and piles becomes larger, so the lateral bearing capacity of the reinforced slope has been greatly improved and the affected area, reflected by the critical pile length, has been expanded. This can be interpreted by the pile behaviors for various pile spacings, as shown in Fig.6. Moreover, as the pile spacing increases, the bearing capacity of single pile increases. This can be explained by the fact that the loading zone of single pile, influenced

0

4

8

12

16

20

0 4 8

Deflection (cm) D

epth

(m

)

6 m

8 m

10 m

12 m

14 m

16 m

18 m

20 m

0

4

8

12

16

20

0.5 0.0 0.5

Bending moment (MNm)

Dep

th (

m)

6 m

8 m

10 m

12 m

14 m

16 m

18 m

20 m

0

4

8

12

16

20

1.0 0.0 1.0

Shear stress (MPa)

Dep

th (

m) 6 m

8 m

10 m

12 m

14 m

16 m

18 m

20 m

0

4

8

12

16

20

0.1 0.0 0.1

Pressure (MPa)

Dep

th (

m)

6 m

8 m 10 m

12 m

14 m

16 m 18 m

20 m


(a) Free head.

0

4

8

12

16

20

24

28

4 0 4 8 12 16 20

Deflection (cm) D

epth

(m

)

2D

3D

4D

6D

0

4

8

12

16

20

24

28

1.0 0.5 0.0 0.5

Bending moment (MN·m)

Dep

th (

m)

2D

3D

4D

6D

0

4

8

12

16

20

24

28

2.0 1.0 0.0 1.0 2.0

Shear stress (MPa)

Dep

th (

m)

2D

3D

4D

6D

0

4

8

12

16

20

24

28

0.2 0.1 0.0 0.1

Pressure (MPa)

Dep

th (

m)

2D

3D

4D

6D

0

4

8

12

16

20

0.0 0.5 1.0 1.5

Deflection (cm)

Dep

th (

m)

2D

3D

4D

6D

0

4

8

12

16

20

0.1 0.0 0.1 0.2


Dep

th (

m)

2D

3D

4D

6D


(b) Hinged head.

Fig.6 Pile behaviors for various pile spacings under critical pile length. by lateral soil movements, is expanded as the spacing becomes larger. Since the factor of safety, pile deflection, bending moment, shear stress and pressure are affected by the mesh size, slope model (various pile lengths and spacings), and numerical iteration and termination criterion, some differences are noted in the results. For a hinged head pile, the effect of pile spacing is not particularly evident. The regularity of the maximum pressure on the piles with various pile spacings is different for free head piles and hinged head piles. This means that the slip surface is different. For free head piles, the depth of the deepest slip surface increases with increasing pile spacing, while it decreases for hinged head piles. It is in a good agreement with the results obtained by Wei and Cheng [16] (Fig.7) and by Bishop’s simplified method (Cai and Ugai [14]). In Fig.7, the solid line indicates the critical slip surface when the pile spacing is large, and the dotted line depicts the critical slip surface when the pile spacing is small. However, it does not agree with the results based on the shear strength reduction finite element method (FEM) by Cai and Ugai [14], because the slip surface determined by Cai and Ugai [14] was only through the extreme point of shear force, regardless of the effect of pile spacing. 3.3 Effect of pile head conditions and pile bending stiffness

Based on boundary conditions at the top of the piles, the following four possible pile head conditions are investigated: (1) free head (displacement and rotation); (2) fixed head (neither displacement nor rotation); (3) hinged head (rotation without displacement); and (4) non-rotated head (horizontal displacement without

Fig.7 Slip surfaces at the section of soil midway between piles by Wei and Cheng [16].

vertical displacement and rotation). In the simulations, the non-rotated pile head condition can be obtained by restraining the vertical displacement of the pile head. The effect of pile bending stiffness is studied by changing only the equivalent Young’s modulus, Ep, of the piles. The piles are installed with Lx = 7.5 m, the center-to-center spacing of 3D and various pile lengths. The numerical results show that pile head conditions have great influences on the factor of safety of reinforced slope and reflect a completely different relationship between factor of safety and pile length, as shown in Fig.4. The simulated results of a slope reinforced with piles for different bending stiffnesses show that the factor of safety of the slope is almost the

0

4

8

12

16

20

1.0 0.5 0.0 0.5 1.0

Shear stress (MPa) D

epth

(m

)

2D

3D

4D

6D

2D

3D

4D

6D

0

4

8

12

16

20

0.05 0.00 0.05 0.10

Pressure (MPa)

Dep

th (

m)

(a) s = 2D. (b) s = 3D.

(d) s = 5D.(c) s = 4D.

(e) s = 6D. (f) s = 8D.


same, regardless of the pile head conditions when the pile length is greater than the critical pile length. It is different from the results obtained by Cai and Ugai [14] and Won et al. [15]. This can be explained by the numerical iteration algorithm and the termination

criterion selected for the numerical calculation. However, some considerable differences are noted by the pile deflection, bending moment, shear stress and pressure, as shown in Fig.8. For piles with larger bending stiffness, with the same calculation step,

(a) Ep = 60 GPa.

(b) Ep = 200 GPa.

Fig.8 Pile behaviors for various pile head conditions under critical pile length.

0

4

8

12

16

20

24

28

4 0 4 8 12 16

Deflection (cm)

Dep

th (

m)

Free head

Fixed head

Hinged head

Non-rotated head

0

4

8

12

16

20

24

28

1.0 0.5 0.0 0.5 1.0


Dep

th (

m)

Free head

Fixed head

Hinged head

Non-rotated head

Free head

Fixed head

Hinged head

Non-rotated head

0

4

8

12

16

20

24

28

2.0 1.0 0.0 1.0 2.0

Shear stress (MPa)

Dep

th (

m)

Free head

Fixed head

Hinged head

Non-rotated head

0

4

8

12

16

20

24

28

0.2 0.1 0.0 0.1 Pressure (MPa)

Dep

th (

m)

0

4

8

12

16

20

24

28

4 0 4 8

Deflection (cm)

Dep

th (

m)

Free head

Fixed head

Hinged head

Non-rotated head

0

4

8

12

16

20

24

28

1.0 0.5 0.0 0.5 1.0


Dep

th (

m)

Free head

Fixed head

Hinged head

Non-rotated head

Free head

Fixed head

Hinged head

Non-rotated head

0

4

8

12

16

20

24

28

0.1 0.0 0.1

Pressure (MPa)

Dep

th (

m)

Free head

Fixed head

Hinged head

Non-rotated head

0

4

8

12

16

20

24

28

2.0 1.0 0.0 1.0 2.0

Shear stress (MPa)

Dep

th (

m)


the maximum deflection is smaller than that of more flexible piles, which means that the critical pile length increases as the pile stiffness increases. From the bending moment of piles with different pile head conditions, the free head pile has the largest bending moment, followed by the non-rotated, fixed and hinged head piles. In the context, the pile stiffness is taken as a constant by assuming that the pile is elastic. However, the piles may be yielded during loading and they are more possible to be yielded by the bending moment than by the shear stress [11]. So a restrained (fixed or hinged) pile head is recommended and a free head should be avoided to reinforce the slope. For a restrained pile head, the pressure on piles is considerably larger than that on unrestrained head piles, which means that the negative pressure decreases and the reaction force is transferred by piles from upward to downward, so a restrained pile head can also result in a larger factor of safety for slopes reinforced with flexible piles. It can also be obtained from Fig.8 that the fixed head pile has the deepest slip surface, followed by the hinged, non-rotated and free head piles, and larger bending stiffness leads to deeper slip surface, regardless

of pile head conditions.

3.4 Effect of soil properties

In order to get more insight into this issue, another slope model with different soil properties is analyzed. The soil cohesion and friction angle are 20 kPa and 10, respectively, and the piles are installed in the middle of slope with a pile spacing of 3D. The pile behaviors for different pile lengths with free head and hinged head conditions are shown in Fig.9, and the factors of safety of slope with various pile lengths are shown in Fig.4. For slopes with clayey soil (soil cohesion is large but friction angle is small), the pile deflection increases as the pile length increases firstly, and then it decreases and tends to be a constant, which is different from that of slopes with sandy soil (soil cohesion is small but friction angle is large). For slope with sandy soil, the pile deflection increases continually with increasing pile length when the embedded pile length surpasses the critical pile length of about 18 m, decided by the factor of safety of reinforced slope, which is different from that decided by the bending moment of the pile (about 22 m). The distributions of bending moment,

(a) Free head.

0

4

8

12

16

20

24

28

0.1 0.0 0.1 0.2 Pressure (MPa)

Dep

th (

m)

6 m 8 m 10 m 12 m 14 m 16 m 18 m 20 m 22 m 24 m 26 m

0

4

8

12

16

20

24

28

2 2 6 10 14 18

Deflection (cm)

Dep

th (

m)

6 m

8 m

10 m

12 m

14 m

16 m

18 m

20 m

22 m

24 m

26 m

0

4

8

12

16

20

24

28

0.5 0.4 0.3 0.2 0.1 0.0 0.1 0.2


Dep

th (

m)

6 m8 m10 m12 m14 m16 m18 m20 m22 m24 m26 m

6 m8 m10 m12 m14 m16 m18 m20 m22 m24 m26 m

0

4

8

12

16

20

24

28

1.0 0.0 1.0 2.0Shear stress (MPa)

Dep

th (

m)


(b) Hinged head.

Fig.9 Pile behaviors for various pile lengths (cs = 20 kPa, s = 10, s = 3D).

shear stress and pressure of piles are similar to those of slopes with sandy soil, as shown in Fig.5. For hinged head pile, the critical pile length of slopes with clayey soil is greater than that of sandy soil, and the ranges of pile deflection, bending moment, shear stress and pressure for various pile lengths are comparatively larger. The factor of safety for slopes with clayey soil is much smaller than that of slopes with sandy soil for free head pile and non-rotated head pile, but larger for restrained head pile. The critical pile length for slopes with clayey soil is smaller than that of slopes with sandy soil free head pile and non-rotated head pile, but longer for restrained head pile. 4 Conclusions

In the study, the embedded length of piles for slopes reinforced with one row of piles is analyzed by three-dimensional elastoplastic shear strength reduction method. The effects of embedded pile length on factor

of safety and pile behavior, and effects of pile spacing, pile head conditions, bending stiffness and soil properties on the critical pile length and pile behavior are also analyzed. The following conclusions can be obtained:

(1) According to the requirement for slip surface of slope reinforced with one row of piles, the slip surface can be obtained by the pile pressure distribution. The maximum depth of the slip surface is determined by the position of the maximum pressure on the upper pile. However, it can only be used to determine the slip surface near the pile. For large pile spacing, it may cause great errors when determining the slip surface near the section through the soil midway between the piles. The slip surface determined by the pressure on piles, considering the influences of both soil and piles in a slope, is in agreement with the results obtained by Wei and Cheng [16] (Fig.7) and by Bishop’s simplified method [14]. However, it does not agree with the results based on shear strength reduction FEM by Cai and Ugai

Deflection (cm)

Dep

th (

m)

6 m

8 m 10 m

12 m

14 m 16 m

18 m 20 m

4 0 4 8 12 0

4

8

12

16

20

6 m

8 m 10 m

12 m

14 m16 m

18 m20 m

0

4

8

12

16

20

0.1 0.1 0.3 0.5


Dep

th (

m)

0

4

8

12

16

20

1.5 0.5 0.5 1.5

Shear stress (MPa)

Dep

th (

m)

6 m

8 m 10 m

12 m

14 m16 m

18 m20 m

0

4

8

12

16

20

0.1 0.0 0.1

Pressure (MPa)

Dep

th (

m)

6 m

8 m 10 m

12 m

14 m16 m

18 m20 m


[14]. (2) The stability of a slope can be improved with piles,

and as expected, the factor of safety increases with increasing pile length and tends to be a constant when the pile length exceeds the critical length. The critical length increases with decreasing pile spacing, and smaller pile spacing tends to increase the integrity of reinforced slopes.

(3) Pile head conditions and bending stiffness influence the factor of safety of slopes. However, the factor of safety herein is almost the same with the Young’s moduli of 60 and 200 GPa, partly due to the numerical iteration and the termination criterion for the reinforced slope. For a restrained pile head, the negative pressure decreases and the reaction force is transferred by piles from upward to downward, so a restrained (fixed or hinged) pile head is recommended and free head should be avoided to stabilize the slope. However, in practice, with a single row of piles, a restrained head condition would be difficult to be accomplished unless the pile heads are strongly anchored.

(4) The soil properties also have significant influences on embedded length of piles. The critical pile length for slopes with clayey soil is smaller than that for slopes with sandy soil for free head pile and non-rotated head pile, but is longer for restrained head pile. For free head pile, the pile deflection increases with increasing pile length for slopes with clayey soil, and then decreases when the pile length surpasses the critical pile length, which is different from that for slopes with sandy soil.

Acknowledgement

The authors are extremely grateful to Prof. H. G.

Poulos for his helpful advice to improve the original

manuscript substantially.

References

[1] Ito T, Matsui T, Hong W P. Design method for stabilizing piles against

landslide—one row of piles. Soils and Foundations, 1981, 21 (1): 21–

37. [2] Poulos H G. Design of reinforcing piles to increase slope stability.

Canadian Geotechnical Journal, 1995, 32 (5): 808–818. [3] Chen L. The effect of lateral soil movements on pile foundations. PhD

Thesis. Sydney: University of Sydney, 1994. [4] Poulos H G, Chen L T, Hull T S. Model tests on single piles subjected to

lateral soil movement. Soils and Foundations, 1995, 35 (4): 85–92. [5] Griffiths D V, Lin H, Cao P. A comparison of numerical algorithms in

the analysis of pile reinforced slopes. In: Proceedings of the

GeoFlorida 2010: Advances in Analysis, Modeling, and Design. [S.l.]:

[s.n.], 2010: 175–183.

[6] Qin H Y, Guo W D. Pile responses due to lateral soil movement of

uniform and triangular profiles. In: Proceedings of the GeoFlorida 2010:

Advances in Analysis, Modeling, and Design. [S.l.]: [s.n.], 2010:

1 515–1 522. [7] Yoon Y H, Heung W, Mun B. Simplified design approach of laterally

loaded short piles on finite slope in cohesionless soil. In: Proceedings

of the GeoFlorida 2010: Advances in Analysis, Modeling, and Design.

[S.l.]: [s.n.], 2010: 1 767–1 776. [8] Guo W D, Qin H Y. Thrust and bending moment of rigid piles subjected

to moving soil. Canadian Geotechnical Journal, 2010, 47 (2): 180–196. [9] Zheng Yingren, Tang Xiaosong, Zhao Shangyi, et al. Strength

reduction and step-loading finite element approaches in geotechnical

engineering. Journal of Rock Mechanics and Geotechnical Engineering,

2009, 1 (1): 21–30. [10] Ito T, Matsui T. Methods to estimate lateral force acting on stabilizing

piles. Soils and Foundations, 1975, 15 (4): 43–59. [11] Ito T, Matsui T, Hong W P. Design method for the stability analysis of

the slope with landing pier. Soils and Foundations, 1979, 19 (4): 43–

57.

[12] Ito T, Matsui T, Hong W P. Extended design method for multi-row

stabilizing piles against landslide. Soils and Foundations, 1982, 22 (1):

1–13. [13] Hassiotis S, Chameau J L, Gunaratne M. Design method for

stabilization of slopes with piles. Journal of Geotechnical and

Geoenvironmental Engineering, 1997, 123 (4): 314–323. [14] Cai F, Ugai K. Numerical analysis of the stability of a slope reinforced

with piles. Soils and Foundations, 2000, 40 (1): 73–84. [15] Won J, You K, Jeong S, et al. Coupled effects in stability analysis of

pile-slope systems. Computers and Geotechnics, 2005, 32 (4): 304–

315. [16] Wei W B, Cheng Y M. Strength reduction analysis for slope reinforced

with one row of piles. Computers and Geotechnics, 2009, 36 (7):

1 176–1 185. [17] Itasca Consulting Group Inc.. FLAC3D (fast Lagrangian analysis of

continua in 3 dimensions) user’s manual (version 2.1). Minneapolis,

USA: Itasca Consulting Group Inc., 2003.

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