Geosynthetic-reinforced Slopes Research Articles

Stability Analysis of Geosynthetic-Reinforced Slopes Considering Multiple Potential Failure Mechanisms Based on the Upper Bound Theorem

Stability Analysis of Geosynthetic-Reinforced Slopes Considering Multiple Potential Failure Mechanisms Based on the Upper Bound Theorem

Loss of stability in geosynthetic-reinforced slopes

Geosynthetics are commonly considered to provide restoring forces against sliding during overall slope stability analyses. Where a slipping surface intersects a geosynthetic layer, the geosynthetic layer produces a reaction force either in the opposite direction of sliding, in the direction of the geosynthetic alignment, or in some intermediate direction. The provision of geosynthetic reinforcements typically increases the factor of safety in limit equilibrium against overall sliding in the design of a mechanically stabilized earth (MSE) wall, and for this reason has become popular among practitioners. However, geosynthetics are typically installed in contiguous layers. These layers are potential interfaces for sliding which need to be checked with respect to slope stability. In other words, it is possible for a slope to become unstable via partial sliding along the interface of a geosynthetic. In this paper, a comprehensive method of analysis is demonstrated via an example which evaluates the stability of a slope reinforced by geosynthetics. All the cases of failure with respect to slope stability are considered via the dual treatment of the geosynthetic elements as weak layers and supporting elements in a limit equilibrium analysis in software.

Reliability analysis of geosynthetic-reinforced slopes under rainfall infiltration

The rainfall-induced instability of geosynthetic-reinforced is a time-dependent phenomenon owing to the infiltration process, and is influenced by rainfall patterns. Catering to the inherent uncertainty in soil properties, this study conducted a reliability analysis of three-dimensional (3D) vertical geosynthetic-reinforced slopes, in order to explore how the probabilistic stability of slope evolves over time under different rainfall patterns. A 3D horn-like mechanism incorporating the Conte-Troncone (CT) model is adopted as a framework for deterministic analysis. Through a Fourier transform-based theoretical reasoning, the CT model assesses the time-variable pore-water pressure of soils in response to any continuously varying rainfall intensity over time. Subsequently, the pore water pressure-driven changes in soil unsaturated strength and the corresponding extend power are integrated into the three-dimensional mechanism, enabling a rapid determination of the instantaneous safety factor at discrete time instants. To avoid the tedious computation generated by Monte Carlo simulation, a simplified Hasofer-Lind-Rackwitz-Fiessler (HLRF) algorithm is used to calculate the time-varying reliability indices. Using the implantation of the proposed method, the effects of rainfall pattern, slope width, and reinforcement tensile strength are investigated by parametric analysis.

Reliability and parameter sensitivity analysis on geosynthetic-reinforced slope with considering spatially variability of soil properties

The Bishop method is often adopted to evaluate the stability of slope, but this method can’t effectively consider the inherent variation characteristics of soil. This study focuses on the reliability of geosynthetic-reinforced slope considering the spatial variability of soil properties. The improved Bishop method is adopted to evaluated stability of slope. The Karhunen-Loève expansion is adopted to generated 2-D random fields of spatial variability of soil properties. The Monte Carlo simulation combined with Latin hypercube sampling is adopted to analyze the reliability of geosynthetic-reinforced slope considering the spatial variability of soil properties. As a result, the spatial variability of soil shear strength has an important impact on the reliability of reinforced slope. The reliability of reinforced slope is more sensitive for variation of cohesion (COVc) compared with coefficient of variation of internal friction angle (COVφ). Considering slope’s spatial variability of cohesion (c) and internal friction angle (φ), the reinforced slope’s reliability is greater than that considering the spatial variability of φ or c. The reliability is sensitive for the autocorrelation distance especially the vertical direction. The correlation coefficient (r (c, φ)) has also a great influence on the reinforced slope’s reliability. The study may help provide more scientific help for the design of reinforced slope and make the construction of the project running safely and smoothly.

Efficient Probabilistic Stability Analysis of Geosynthetic Reinforced Slopes Using Collocation-Based Stochastic Response Surface

Abstract This paper presents an efficient and robust probabilistic approach to analyze the geosynthetic reinforced slopes (GRSs). The deterministic analysis will be performed that employs the rigor...

Probabilistic stability analysis of geosynthetic-reinforced slopes under pseudo-static and modified pseudo-dynamic conditions

An efficient and accurate reliability-based design of the geosynthetic-reinforced slopes (GRS) using the pseudo-static and modified pseudo-dynamic framework is proposed in the present study. Deterministic formulation used in the present study is made robust with the help of nonlinear constrained optimization. The collocation based stochastic response surface method (CSRSM) is used to probabilistically analyze the GRS. The critical modes of failure pertaining to the internal and external stability of the GRS are considered in the formation of the performance functions. The horizontal seismic acceleration coefficient (kh), internal friction angle of soil (φ), soil unit weight (γ), shear wave velocity (Vs), and friction angle at the interface between soil and reinforcement (φb) are chosen as the random variables, owing to their high influence on the stability of the GRS. The influence of correlation on the stability of the reinforced slope is illustrated considering the internal and external stability. System reliability analysis considering the internal and external modes of failure is also performed. An illustrative example is presented showing the steps to design a GRS using the proposed formulation. The results confirm the necessity of performing the system reliability analysis to estimate an accurate value of probability of failure of GRS.

Stochastic stability analysis of geosynthetic reinforced slopes subjected to harmonic base shaking

Geosynthetic reinforcement is a lucrative solution for stabilizing the recurring failure of slopes. This paper presents an efficient probabilistic approach to analyze the internal stability of geosynthetic reinforced slopes (GRSs) resting on a soil deposit subjected to harmonic base shaking. Collocation based Stochastic Response Surface (SRS) method is used for the same. The deterministic analysis is performed employing a rigorous formulation of Horizontal Slice Method (HSM) in a modified pseudo-dynamic framework. The formulation satisfies the horizontal, vertical, and the moment equilibrium equations of each slice. The algorithm is made efficient using the non-linear constrained optimization. The effect of exciting frequency of soil on the stability of GRS is examined in detail. The accuracy of the proposed probabilistic formulation is also compared with the results of the FORM. Four input parameters are considered to be random in nature namely internal soil friction angle, unit weight, ultimate tensile strength of reinforcement, and shear wave velocity of reinforced soil (GRS). The Gaussian copula is used to correlate the input random variables. The results depict that the proposed formulation is effective and precise in predicting the probability of failure (Pf) of GRSs. The performance function is evaluated only 376 times in comparison to the direct Monte-Carlo Simulation where it is evaluated at least 105 times, reducing the computation time from approximately 2 days to 25 min. To the best of author’s knowledge, the proposed method is original.

Seismic internal stability of bilinear geosynthetic-reinforced slopes with cohesive backfills

The bilinear geosynthetic-reinforced slope (BGRS) is a new two-tier reinforced earth structure without an offset. Analyzing its seismic internal stability is aimed to determine the resultant reinforcement force. In this paper, using a top-down log spiral mechanism, the moment limit equilibrium equations are established to formulate the resultant reinforcement forces for upper and lower tiers of the BGRS, considering influence of angle of lower tier, depth of tension crack, cohesion of backfill and pseudo-static seismic loads. Results show that ignoring cohesion of backfill and/or vertical upward seismic load is beneficial to conservative stability analysis, and the existence of tension cracks changes the distribution of reinforcement forces in upper and lower tiers. Additionally, the critical slip surface is also studied, which is found that the volume of the failure body decreases under the global stability with increase of angle of lower tier, cohesion of backfill or depth of tension crack, or with the decrease of seismic coefficients.

Behaviour of geosynthetic in anchored geosynthetic-reinforced slopes subjected to seepage

The anchored geosynthetic system (AGS) is a technique to improve the stability of unstable or quasi-stable slopes. In this study, the behaviour of geosynthetic in slopes reinforced with AGS subjected to seepage is investigated. Centrifuge model tests were carried out at 50g centrifuge acceleration on silty sand slopes with an inclination of 63·4° (2V:1H) and reinforced with AGS. Seepage was generated by gradually raising the water table within the reinforced slope models. By means of a digital image analysis technique, the behaviour of the surface geosynthetic during the seepage was studied and the compressive stresses produced beneath the geosynthetic were determined. Limit equilibrium stability analyses were conducted with the application of AGS loads measured by digital image analyses. The results indicate that the maximum tension mobilised in the geosynthetic during seepage depends on the tension induced in the geosynthetic due to the anchorage prior to introducing seepage. Furthermore, including the compressive stresses produced by the tensioned geosynthetic in the analyses gives a more realistic value for the safety factors.

Discretization-Based Kinematic Analysis Method to Seismic Stability of Geosynthetic-Reinforced Slopes Involving Differing Earthquake Approaches

Abstract For soil slopes susceptible to a potential failure, some preemptive measures in the form of reinforced structures are commonly adopted to improve stability. In this study, the seismic stab...

Required strength of geosynthetics for reinforced 3D slopes in cohesive backfills with tensile strength cut-off

In extant studies, most of the stability analyses of geosynthetic-reinforced slopes focused on two-dimensional conditions using the Mohr-Coulomb (M-C) failure criterion to describe the strength of backfills. However, in reality, all failures of slopes indicate a somewhat three-dimensional (3D) feature, and the M-C criterion is observed to overestimate the tensile strength of cohesive soils. To partially remedy this shortcoming, the concept of tensile strength cut-off is adopted to include the actual tensile strength of backfills in the yield envelope, and a kinematic approach is presented to evaluate the required strength of geosynthetics for 3D reinforced slopes in cohesive backfills. A 3D rotational mechanism of collapse that is associated with the strength envelope with tension cut-off is developed. The amount of required reinforcement is evaluated and listed as a dimensionless coefficient. The results indicate that the inclusion of the 3D effect and soil cohesion can lead to substantial savings in terms of the reinforcement to be made. In addition, a higher amount of reinforcement is required when the effect of tension cut-off is considered; this effect is more distinct for backfill with a higher amount of cohesion.

Kinematic analysis of geosynthetics-reinforced steep slopes with curved sloping surfaces and under earthquake regions

A procedure of kinematic analysis is presented in this study to assess the reinforcement force of geosynthetics required under seismic loadings, particularly for steep slopes which are hardly able to maintain its stability. Note that curved sloping surfaces widely exist in natural slopes, but existing literatures were mainly focusing on a planar surface in theoretical derivation, due to complicated calculations. Moreover, the non-uniform soil properties cannot be accounted for in conventional upper bound analysis. Pseudo-dynamic approach is used to represent horizontal and vertical accelerations which vary with time and space. In an effort to resolve the above problems, the discretization technique is developed to generate a discretized failure mechanism, decomposing the whole failure block into various components. An elementary analysis permits calculations of rates of work done by external and internal forces. Finally, the upper bound solution of the required reinforcement force is formulated based on the work rate-based balance equation. A parametric study is carried out to give insights on the implication of influential factors on the performance of geosynthetic-reinforced steep slopes.

Energy analysis of geosynthetic-reinforced slope in unsaturated soils subjected to steady flow

Soils are actually unsaturated in nature. In the present study, a stability analysis of a geosynthetic-reinforced slope in unsaturated soils subjected to various steady flow conditions is conducted based on limit analysis. Work rate by apparent cohesion due to matric suction is calculated based on the effective stress-based equation. Analytical expression of the required cohesion/stability number of slope is derived from the energy balance equation. An optimization code is programmed to capture the optimized solution of the stability number. Comparison is made to verify the present work and a parametric analysis is conducted to investigate the effects of soil type, infilitration rate, reinforcement strength and soil suction on slope stability afterwards. A set of numerical solutions is presented at the end of the paper for preliminary design purposes.

Effect of cohesion on internal stability of bilinear geosynthetic-reinforced slopes

This paper presents a limit equilibrium (LE) approach to analyze the internal stability of the bilinear geosynthetic-reinforced slopes comprised of cohesive reinforced fill. This LE analysis uses a top-down log spiral mechanism to formulate the resultant reinforcement force in the lower tier required for internal design of such reinorced slopes. The presented formulation considers both cohesion and depth of crack, and then the required unfactored reinforcement strength is defined and used for subsequent analysis. Results are presented in the form of stability charts, enabling quick assessment of reinforcement strength required for internal stability. In addition, the effect of cohesion and depth of crack on the critical slip surface is discussed, respectively.

Factor of Safety of Geosynthetic-Reinforced Slope in Unsaturated Soils

Factor of Safety of Geosynthetic-Reinforced Slope in Unsaturated Soils

Required strength of geosynthetics in a reinforced slope with tensile strength cut-off subjected to seepage effects

The Mohr-Coulomb (M-C) yield criterion is found to overestimate the tensile strength of cohesive soils. By introducing the concept of tensile strength cut-off, the M-C criterion is modified to reduce or eliminate the tensile strength from the criterion. In this study, a new approach is proposed to investigate the stability of geosynthetic-reinforced slopes in cohesive soils subjected to seepage effects by means of the kinematic approach of limit analysis. The distribution of pore-water pressure is obtained using the numerical modeling software package, FLAC3D. A kinematically admissible failure mechanism is discretized to incorporate the results from the numerical simulation. The strength of geosynthetics required for maintaining the slope stability is evaluated from the work-energy balance equation. An optimization routine is used to seek out the maximum value among all possible results. Design charts providing the normalized required reinforcement under different parameters are plotted for a parametric study and convenient use in engineering. The obtained results show that less reinforcement is required in the presence of soil cohesion, and that the inclusion of the effect of tensile strength cut-off leads to a more conservative solution, which is more obvious in the presence of seepage effects.

Seismic stability of 3D soil slope reinforced by geosynthetic with nonlinear failure criterion

This work conducts a seismic stability of a three-dimensional (3D) geosynthetic-reinforced slope in soils following the linear and nonlinear Mohr-Coulomb failure criterion. Three categories of reinforcement distribution patterns and four kinds of clays are considered. Within the framework of limit analysis and the generalized tangent technique, expressions of required reinforcement strength and the stability factor under different distribution patterns are derived from the energy balance equations. Comparisons are conducted to verify the validity of new expressions. Parametric analysis is conducted to explore the impacts of seismic force, soil strength nonlinearity, 3D character of slope and reinforcement strength on slope stability. It is found from the results that the downwardly-strong triangular (DTD) distribution pattern is the most effective choice for slope reinforcement, while the upwardly-strong triangular (UTD) pattern is the worst. One should intensify the density of reinforcement layers at the bottom of the slope to achieve a better slope stability condition. Besides, soil strength nonlinearity and seismic forces both have non-negligible negative effects on slope stability.

Design of geosynthetic-reinforced slopes in cohesive backfills

Currently, geosynthetic reinforcements for slopes are calculated assuming the ground strength to be purely frictional, i.e. without any cohesion. However, accounting for the presence of even a modest amount of cohesion could allow using locally available cohesive soils as backfills to a greater extent and less overall reinforcement. But cohesive soils are subject to the formation of cracks that tend to reduce slope stability so their presence has to be accounted for in the design of the slope reinforcement. In the paper, limit analysis was employed to derive a semi-analytical method for uniform c−ϕ slopes that provides the amount of reinforcement needed as a function of ground cohesion, tensile strength, angle of shearing resistance and of the slope inclination. Both climate induced cracks as well as cracks that form as part of the slope collapse mechanism are accounted for. Design charts providing the value of the required reinforcement strength and embedment length are plotted for both uniform and linearly increasing reinforcement distributions.From the results, it emerges that accounting for the presence of cohesion allows significant savings on the reinforcement to be made, and that cracks are often significantly detrimental to slope stability so they cannot be overlooked in the design calculations.

Seismic design of bilinear geosynthetic-reinforced slopes

The scenario of two tiered geosynthetic-reinforced slopes, where the upper tier is vertical and the lower tier is inclined at an angle, is termed as a bilinear geosynthetic-reinforced slope (BGRS) in this note. This note presents a pseudo-static limit equilibrium approach employing a top-down log spiral mechanism to determine the resultant reinforcement force in the lower tier required for global seismic stability. An example was presented to illustrate steps for achieving the resultant reinforcement force required for internal seismic design of reinforcement rupture and show how much the maximum reinforcement force at each layer is in line with its distribution function. The reinforcement force in the BGRS is subsequently compared with that in the equivalent geosynthetic-reinforced slope under different case. In addition, it is found that the resultant reinforcement force in the lower tier increases first and then decreases with an increase of height ratio of the upper tier to the BGRS.

Failure mechanisms of steep-faced geosynthetic-reinforced retaining walls subjected to toe scouring

ABSTRACTWaterfront structures such as seawalls, dikes, and levees are frequently subjected to scouring at the toe of the slope, leading to deteriorated performance and increased failure potential. To this end, some model reinforced steep-faced slopes consisting of a two-dimensional backfill were brought to failure to explore the failure mechanisms of some geosynthetic-reinforced slopes subjected to simulated toe scouring. Results of model tests indicate that in the case of shallow scouring, a reinforcement length (L) increase from 0.4 to 1.0 Ht (Ht, total height of reinforced walls) significantly increases the tolerance against toe scouring-induced failures. In this case, a local bearing capacity failure of facing is the dominant failure mode. In the case of deep scouring, an increase in L beyond 0.7 Ht provides no additional tolerance against toe scouring because the ultimate state is always associated with a global circular sliding in the unreinforced zone. Experimental values of the lateral pressure coefficient (Kt) converted from the measured reinforcement forces indicate that reinforcement forces consistently increase in response to toe scouring up to the final collapsing state and that the reinforcement forces for L = 1.0 Ht mobilize more effectively than those for L = 0.7 Ht.