Stability of Slopes Reinforced with Truncated Piles

Sun, Shu-Wei; Zhao, Fu; Zhang, Kui

doi:https://doi.org/10.1155/2016/1570983

Advances in Materials Science and Engineering

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Abstract Introduction Results and Discussion Conclusions Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2016 | Article ID 1570983 | https://doi.org/10.1155/2016/1570983

Stability of Slopes Reinforced with Truncated Piles

Shu-Wei Sun,^1,2Fu Zhao,¹and Kui Zhang¹

Academic Editor: Hossein Moayedi

Received10 Aug 2016

Revised05 Nov 2016

Accepted16 Nov 2016

Published19 Dec 2016

Abstract

Piles are extensively used as a means of slope stabilization. A novel engineering technique of truncated piles that are unlike traditional piles is introduced in this paper. A simplified numerical method is proposed to analyze the stability of slopes stabilized with truncated piles based on the shear strength reduction method. The influential factors, which include pile diameter, pile spacing, depth of truncation, and existence of a weak layer, are systematically investigated from a practical point of view. The results show that an optimum ratio exists between the depth of truncation and the pile length above a slip surface, below which truncating behavior has no influence on the piled slope stability. This optimum ratio is bigger for slopes stabilized with more flexible piles and piles with larger spacing. Besides, truncated piles are more suitable for slopes with a thin weak layer than homogenous slopes. In practical engineering, the piles could be truncated reasonably while ensuring the reinforcement effect. The truncated part of piles can be filled with the surrounding soil and compacted to reduce costs by using fewer materials.

1. Introduction

The use of piles for stabilizing active landslides and as a preventive measure in the stable slopes has been considered a reliable and effective technique and has been implemented successfully in a number of applications over the last few decades, for example, [1–7]. A traditional stabilizing pile generally has a full length and pile head extrudes outside the slope surface whose cross section ranges from circular to rectangular [8].

The stabilization of unstable slopes with piles is complicated by factors affecting pile performance under the loading conditions of slope reinforcement and factors controlling the effect of piles on global slope stability. The current design practices for pile-reinforced slopes often use the limit equilibrium methods, where the soil-pile interaction is not considered, and the piles are assumed to only supply an additional sliding resistance. Ito and Matsui [3] have analyzed the development of lateral forces on the stabilizing piles when the soil is forced to squeeze between piles. Ito et al. [9, 10] subsequently developed a design method for a pile-reinforced slope based on this approach. Poulos [5] presented an approach for evaluating the pressure on single piles where a modified boundary element method is employed to study the response of the passive piles, in which the solution incorporating nonlinear soil-pile interface elements can represent a hardening or softening response prior to reaching an ultimate state. Yamagami et al. [11] reported another limit equilibrium design method for slopes with one row of piles. The method allows two different critical slip surfaces for the two slides of the pile, and the forces acting on the stabilizing piles are estimated based on a given critical slip surface where the safety factor is prescribed to ensure slope stability. Recently, Liang et al. [7] proposed a design procedure for stabilizing an unstable slope with a row of equally spaced drilled shafts, in which the limiting equilibrium-based slope stability analysis method was modified to incorporate the drilled-shaft-induced arching effect through a semiempirical load-transfer factor. In general, the ability of the limit equilibrium methods to determine the stability of slopes reinforced with piles may be in doubt because the soil-pile interaction is not clearly considered.

To better simulate the actual mechanism of failure, numerical methods including the finite element method and the finite difference method have been developed quickly and are becoming increasingly popular for piled slope analysis based on the shear strength reduction method. Chow [12] presented a numerical model where the piles are modeled using beam elements and the soil is modeled using a hybrid method of elasticity theory analysis. Cai and Ugai [13] have considered the effects of stabilizing piles on the stability of a slope by a three-dimensional (3D) finite element analysis (FEA) using the shear strength reduction method. The effects of pile spacing, pile head conditions, bending stiffness, and pile positions on the safety factor were analyzed and the location of the critical slip surface was determined by the maximum shear force of piles. Wei and Cheng [14] showed that the location of the maximum shear force is not necessarily the location of the critical slip surface for a piled slope, and the critical slip surface for a piled slope is determined by the shear strain rate. One of the main advantages of the shear strength reduction method is that the critical slide surface is found automatically and it is not necessary to specify the shape of the slide surface (e.g., circular, log spiral, and piecewise linear) in advance. Most importantly, numerical methods automatically satisfy translational and rotational equilibrium, and pile behavior characteristics can be obtained simultaneously.

Complicating the issues of pile-stabilized slopes including prediction of load distribution along piles were effects of soil type, pile size and spacing, pile orientation, truncating of piles, and so on [15, 16]. Most previous research has focused on the response of slopes stabilized with common full length piles as well as the influence factors. However, from the perspective of the rational loading of a pile, if the moment-arm on which the load acts is appropriately shortened while the pile location is unchanged, the sliding thrust acting on the stabilizing pile will be reduced. Therefore, a stabilizing pile with a pile head truncated at a certain depth underneath the slope surface is more economical and rational than a conventional full length pile. To the present author’s knowledge, the effect of truncating on the stability of pile-reinforced slopes has never been considered systematically. In this study, the stability of slopes reinforced with truncated piles is analyzed systematically. A simplified numerical method is proposed to analyze the stability of slopes stabilized with truncated piles based on the shear strength reduction method. Besides, the influential factors including pile diameter, pile spacing, depth of truncation, and existence of a weak layer are also systematically investigated from a practical point of view.

2. Problem Definition and Methodology

2.1. Problem Definition

The problem definition is given in Figure 1. It can be seen from Figure 1(a) that a conventional stabilizing pile generally has a full length and pile head extrudes outside the slope surface. For truncated piles (see Figure 1(b)), the pile heads are truncated at a certain depth below the surface of a slope, which is unlike traditional stabilizing piles. The key to slope stabilization using piles is the control of the sliding mass. Therefore, once the deformation of the slip surface is suitably restricted, the deformation of the sliding mass that lies on and moves along the slip surface may be restricted to a great extent. A truncated stabilizing pile is proposed for this purpose. The deformation of the sliding mass around the pile head is partially restricted, allowing the load of the sliding mass above the pile head to be released to a certain degree. Thus, the corresponding sliding force is not acting entirely on the piles, and it is possible to reduce the load on the piles. Furthermore, the truncated stabilizing pile has a reduced cost due to its shortened length and smaller load.

(a) Conventional stabilizing piles (full length)

(b) Truncated piles

The depth of truncation is given by and the depth of the critical slip surface is signified by in Figure 1(b). To maximize the benefits of the truncated piles, the depth of truncation is a key parameter. If is too small, the rational capacity of piles cannot be fully achieved; if it is too large, the overpass sliding might occur over pile head and thereby decrease the stability of the reinforced slope. In the interest of generality, the depth of truncation will be expressed in dimensionless form , where .

2.2. Model of Materials

In this study, the soil material of the slope is simulated with an elastic, perfectly plastic model. The yielding is described by a composite Mohr–Coulomb criterion with a tension cutoff as shown in Figure 2 [17].

The failure envelope from points to is defined by the Mohr–Coulomb criterion withwhere and are the major and minor principal effective stresses, respectively; is the effective cohesion of soil; is the internal friction angle of soil; and is a function of

The failure envelope from points to is represented by the tension failure criterion of the form withwhere is the effective tensile strength, the maximum value of which, , is given by

The truncated stabilizing piles are treated as a linear elastic solid material. Routines are developed to calculate the required parameters including the pile shear force and bending moment for the analyses. The shear force in each horizontal pile section (see Figure 3) was calculated by multiplying the -direction horizontal shear stress of each element by its plan surface area. The plan area of each element was calculated by dividing its volume by its thickness in the -direction. The bending moment developed in each pile section was obtained by summation of the product of the vertical stress at each element, the plan area of that element, and the -distance from the center of the pile to the centroid of the element.

The shear force and bending moment are therefore given bywhere and are the horizontal shear stress and vertical normal stress of element in Figure 3, respectively; is the plan area of the element; is the centroid distance of the element from the center of the pile in the -direction; and is the total number of elements in a horizontal pile section.

2.3. Simulation of Pile-Soil Interface

In this study, the interfaces between piles and soils are characterized by a linear Coulomb shear strength model. Interfaces have the properties of friction, cohesion, dilation, normal and shear stiffness, and tensile and shear bond strength. Figure 4 illustrates the components of the constitutive model acting at an interface node ().

The normal and shear forces that describe the elastic interface response are determined by the following equations:where and are the normal and shear force at time (), respectively; is the absolute normal penetration of the interface node into target face; is the incremental relative shear displacement vector; and are the normal and shear stiffness, respectively; is the additional normal stress added due to interface stress initialization; is the additional shear stress vector due to interface stress initialization; and is the representative area associated with an interface node.

In this study, where the use of interface element covers the pile-soil separation and nonlinear analysis is carried out, the value for the interface stiffness should be high enough to minimize the contribution of those elements to the accumulated displacements. A good rule-of-thumb is that and be set to at least ten times the equivalent stiffness of the stiffest neighboring zone [18]. To satisfy this requirement, and for the interface elements are both taken equal to 1 × 10⁷ kPa/m in the current analyses. These values were also adopted by Comodromos and Papadopoulo [19] in their benchmark test on interface elements, which were validated to represent a rough pile interface.

2.4. Determination of the Factor of Safety ()

For slopes, is often defined as the ratio of the actual shear strength to the minimum shear strength required to prevent failure. The conventional way to compute the with a finite element or finite difference program is to reduce the shear strength until collapse occurs; is the ratio of the soil’s actual strength to the reduced shear strength at failure. This shear strength reduction method was used first by Zienkiewicz et al. [20] to compute the of a slope composed of multiple materials.

The shear strength reduction method has two main advantages over limit equilibrium slope stability analyses. First, the critical slide surface is found automatically, and it is not necessary to specify the shape of the slide surface in advance. In general, the failure surface geometry for slopes is more complex than simple circles or segmented surfaces. Second, numerical methods automatically satisfy translational and rotational equilibrium, whereas not all limit equilibrium methods do satisfy equilibrium. Consequently, the shear strength reduction method usually determines the equal to or slightly less than limit equilibrium methods.

To perform a slope stability analysis with the shear strength reduction method, simulations are run for a series of strength reduction factors with shear strength parameters and adjusted according to the equations:where (cohesion) and (friction angle) are real shear strength parameters and is a strength reduction factor.

Figure 5 shows the proposed numerical method to calculate the of slopes reinforced with truncated piles. The tensile strength of soil, , in Figure 5 could be set to zero generally. The value of at which collapse occurs can be found efficiently using bracketing and bisection. First, the value of is set to be 1.0 initially in order to judge the stability of the piled slope. Then, upper and lower brackets are established. For an initial stable piled slope, the initial lower bracket is any value of for which a simulation converges, and the initial upper bracket is any value of for which the simulation does not converge. Conversely, for an unstable piled slope, the initial upper bracket is any value of for which a simulation converges and the initial lower bracket is any value of for which the simulation does not converge. Next, a point midway between the upper and lower brackets is tested. For an initial stable slope, the lower bracket is replaced by this new value if the simulation converges, and the upper bracket is replaced if the simulation does not converge. Conversely, for an unstable piled slope, the upper bracket is replaced if the simulation converges, and the lower bracket is replaced if the simulation does not converge. The process is repeated until the difference between the upper and lower brackets is less than a specified tolerance . When this critical value has been found, the factor of safety of the slope is equal to the strength reduction factor and .

It should be noted that there are several possible definitions of failure, for example, some test of bulging of the slope profile, limiting of the shear stresses on the potential failure surface, or nonconvergence of the solution [21]. In the present analysis, the nonconvergence option is taken as being a suitable indicator of failure. The model is normally assumed to be in equilibrium when the maximum unbalanced force ratio is below 1 × 10⁻⁵.

2.5. Method of Execution

The proposed numerical method is executed with FLAC [22], which is appropriate to simulate the behavior of geomaterials that may undergo plastic flow. In FLAC, the explicit calculation scheme and the mixed-discretization zoning technique facilitate accurate modeling of plastic collapse and flow; and the explicit solution scheme involving a large number of calculation steps progressively redistributes the unbalanced force resulting from changes of the stresses or boundary displacements through the mesh.

The model is considered to be in equilibrium when the maximum unbalanced force for the whole grid is small in comparison with the total of the applied forces associated with boundary displacement or stress changes. Denoting as the ratio of maximum unbalanced force to the representative internal force, expressed as a percentage, a value of 1% or 0.1% may be acceptable as denoting equilibrium, depending on the degree of precision required. If the unbalanced forces approach a constant nonzero value, this usually indicates that failure and plastic flow are occurring within the model.

3. Validation of the Proposed Method

3.1. Model Slope

Idealized slopes with a height of 10 m, gradient of : , and ground thickness of 10 m are analyzed using a 3D finite difference mesh, as shown in Figures 6(a) and 6(b). Model is a homogenous soil slope while Model has a thin weak intercalation to simulate the effect of a thin weak layer. Both models have the same physical dimensions. The boundary conditions are given as “smooth-smooth” for the 3D analysis [23]. Thus, the bottom () of slopes is fully fixed, while the left () and right side ( = 35) of the slope are constrained by vertical rollers, and the front ( = 0) and far side ( = ; is the pile spacing as stated later) also are constrained by vertical rollers.

(a) Model (a homogenous slope)

(b) Model (with a thin weak intercalation)

Truncated stabilizing piles with outer diameters = 0.4 m, 0.8 m, and 1.6 m are investigated parametrically in the present study. Piles are installed in the middle of the slope with a distance of slope toe 7.5 m as shown in Figure 7. The depth of truncation in the analysis varies in the range 0–6.0 m and 0–4.0 m for Models and , respectively; thus the relative depth of truncation of ranges approximately from 0 to 1 for both slopes. The pile spacing, , varies from 2 to D. Two symmetric boundaries are used so that the problem to be analyzed really consists of a row of truncated piles with planes of symmetry through the pile center line and through the soil midway between the piles. Pile heads are free while bottoms are fixed.

(a) Model

(b) Model

3.2. Validation of the Proposed Numerical Method

It is well documented that the of slopes assuming elastic-perfectly plastic constitutive models are insensitive to the construction sequence. In the current study, model slopes in Figure 6 are equilibrated under gravity initially. The purpose of this analysis is to calibrate the numerical model to present the in situ soil conditions. Then, the simulations are run to obtain the of slopes following the proposed method. The Mohr-Coulomb model is used to simulate the nonlinear soil behavior as stated above. The Mohr-Coulomb model requires soil parameters including the unit weight, friction angle, cohesion, Young’s modulus, and Poisson’s ratio. Truncated piles are modeled as a linear elastic material described by parameters of Young’s modulus and Poisson’s ratio. Material properties in the current analysis are presented in Table 1.

When the slopes are not reinforced with truncated piles, the values for Models and are 1.17 and 0.82, respectively. Compared with the result of 1.14 obtained by Cai and Ugai [13] using the shear strength reduction finite element method for the same slope as Model , the proposed method shows a slightly higher value for the predicted . This is because the calculated by FLAC depends strongly on the size of the element, unlike finite element methods in which a shape function can be used within the elements. In general, the finer element sizes give more precise results. As a check on the result, we performed a 3D Spencer’s analysis for Model and obtained an of 1.18. It is demonstrated that the calculated values by the proposed method are acceptable.

The critical slip surfaces for the two models are shown in Figure 8, and it can be seen that slope failures occur along a circular surface, which is similar to the results of Cai and Ugai [13]. The depth of the critical slip surface is about 4.25 and 4.02 m for Models and , respectively.

(a) Model (FS = 1.17)

(b) Model (FS = 0.82)

4. Results and Discussion

4.1. Stability of Slopes Reinforced with Truncated Piles

The of slopes with piles = 0.4 m, 0.8 m, and 1.6 m and = 2 to 8 for Model is listed in Table 2. It can be seen that the values mainly decrease as increases. This is because the reinforcement function is gradually lost due to the piles being continuous truncated. When is up to or above 1, the is 1.17, which is the same as an unreinforced slope.

The of piled slopes for increasing relative truncation length, , shown in Table 2 for Model are portrayed in Figure 9. For = 1.6 m, it can be seen in Figure 9(a) that decreases rapidly with the increasing of when = 2. However, when increases from 3 to 7, decrease moderately with increasing; furthermore, the rate of decline in the decreases gradually with increasing. For = 8, of piled slope remains unchangeable as increases from 0 to 0.18 and then decreases slowly with the increasing of . For piles with = 0.8 m which are more flexible, and when = 2, it can be seen from Figure 9(b) that value of of piled slope remains unchangeable as increases from 0 to 0.18, then decreases slightly as increases from 0.18 to 0.35, and then decreases rapidly as is above 0.35. For S = 3D to 8D, of piled slopes remain unchangeable as increases from 0 to 0.18 and then decrease gradually with increasing. It is also noted that the rate of decline in FS slows down as pile spacing increases. In Figure 9(c), piles with = 0.4 m are much more flexible, and when S = 2D, it can be seen that FS of piled slopes remains unchangeable as increases from 0 to 0.35 and then decreases rapidly with increasing. For S = 3D to 4D, the FS of piled slopes remain unchangeable as increases from 0 to 0.35, then decrease slightly as increases from 0.35 to 0.53, and then decrease rapidly as is above 0.53. For S = 5D to 8D, FS of piled slopes remain unchangeable as increases from 0 to 0.53 and then decrease rapidly as increases above 0.53.

(a) = 1.6 m

(b) = 0.8 m

(c) D = 0.4 m

It can be seen from Figure 19 that an optimum value of the relative depth of truncation, C, exists for piles in Model slopes. The largest truncation length of pile, beyond which overhead sliding may occur, is very meaningful and important for the detailed design of a pile-reinforced slope. The optimal truncation length of pile, beyond which overhead sliding may occur, is investigated with a dimensionless ratio of as follows:(1)For D = 1.6 m, the value of can be taken as 1/6 when S = 8D, and values could be zero when S = 2D to 7D.(2)For D = 0.8 m, the value of can be taken as 1/6 when S = 2D and 1/3 when S = 3D to 8D.(3)For D = 0.4 m, the value of can be taken as 1/3 when S = 2D to 4D and almost 1/2 when S = 5D to 8D.

It is demonstrated that the optimum value of the relative depth of truncation, varies with the pile diameter and spacing. It is bigger when piles are more flexible and pile spacing is larger as portrayed in Figure 10. For the actual ratio is below , the value of the of piled slope hardly changes with pile truncation, indicating that truncation in this range has no influence on the reinforcement function of piles. Conversely, once the actual ratio is above , decreases rapidly. Truncation in this range will have a serious influence on piled slope stability and the reinforcement function of piles will be much weakened. In practical engineering, piles could be truncated appropriately on the premise of ensuring the reinforcement function; thus resources could be saved and costs could be reduced. It is advised that boreholes could be filled with surrounding soil and compacted in the standard procedure for truncated parts of piles.

4.2. Truncated Stabilizing Pile Behaviors

The bending moment and shear force of truncated stabilizing piles with different pile spacing and relative depths of truncation, , are shown in Figure 11, and it is noted that the positive direction shear force is identical to the -direction. In this section, the truncated stabilizing piles with = 0.8 m for Model slopes are mainly discussed. The results for = 1.6 and 0.4 m are not presented because they are consistent with those for = 0.8 m.

(a) C = 0

(b) C = 0.18

(c) C = 0.35

(d) C = 0.53

(e) C = 0.71

(f) C = 0.88

It can be seen from Figure 11 that lateral shear force along -direction in the pile reaches the first extreme point at a critical depth. This critical depth could be regarded as a level of the potential slip surface because the shear strength reduction method cannot predict a clear slip surface like the limit equilibrium method. Thus depth of potential slip surfaces at the position of piles is shown in Table 3. It can be seen that smaller spacing of piles gives deeper levels of potential slip surface when is below 0.35. However, for above 0.35, the depth of potential slip surface remains constant regardless of pile spacing. For the same pile spacing, the depth of potential slip surface remains constant when is below 0.35 and then becomes shallower with increasing, indicating that overhead sliding may occur.

The values of the first extreme point of shear force and the maximum bending moment occurring below the slip surface in piles for various pile spacings are discussed. It can be seen from Figure 12(a), when = 2D, that at potential slip surface remains unchangeable as the value of is below 0.18, then decreases slightly as increases from 0.18 to 0.35, and finally decreases rapidly with above 0.35. For S = 3D to 5D, at potential slip surface increases slightly as increases from 0 to 0.35 and then decreases rapidly with above 0.35. For S = 6 to 8D, at potential slip surface increases slightly from C = 0 to 0.35, then decreases slowly as increases from 0.35 to 0.53, and finally decreases rapidly with above 0.53. Shear force in piles at the potential slip surface is generally identical to the resisting reaction force to the sliding mass. Therefore, a larger shear force in piles at potential slip surface corresponds to a larger reaction force to the sliding mass by piles and accordingly will cause a higher factor of safety of the reinforced slope. These results reveal the differences in the values of slopes reinforced with different truncated piles.

(a) in piles for different pile spacing and truncating length

(b) in piles for different pile spacing and truncating length

It can also be seen from Figure 12(b) that maximum bending moment occurs below potential slip surface of piles. For = 2D to 5D, decreases slightly as increases from 0 to 0.35. For S = 6D to 8D, decreases slightly or moderately as increases from 0 to 0.53. These results demonstrate that truncation of a pile facilitates the achievement of the rational stress of a pile to a certain extent. These benefits will be particularly important for piled slopes, because piles are more likely to yield by the bending moment rather than by shear force in practical engineering.

4.3. Failure Process of Piled Slope due to Truncation

Failure mode of slopes stabilized with truncated piles ( = 2D, D = 0.8 m) is shown in Figures 13–19. When is below 0.18, it can be seen from Figures 13 and 14 that potential slip surfaces are clearly divided into two parts because of the presence of piles. With increasing from 0.18 to 0.35, the potential slip surface in Figure 15 is still two parts at the section near the pile but becomes connected at the section through soil midway between piles. The explanation is that lateral soil movement between piles is less resisted at = 0.35, and accordingly, the of reinforced slope decreases. When increases from 0.35 to 0.53, it can be seen from Figure 16 that potential slip surface at the section through soil midway becomes deeper and shear strain mobilizes along the vertical direction at the interface between soil and pile. With increasing from 0.53 m to 0.71, potential slip surface in Figure 17 clearly becomes a single potential slip surface at the section near pile, indicating that this part of pile cannot supply a resisting force to the slope, and overhead sliding occurs over pile head. When continuously increases, potential slip surface of overhead sliding becomes deeper and deeper (see Figure 18). When is equal to or above 1, it can be seen in Figure 19 that potential slip surface becomes the same as an unreinforced slope. These results systematically reveal the failure process of piled slope due to truncation.