Design of A-Walls for Stabilization of Slopes and Embankments in Soft Soils

The mechanics of reinforced embankments on soft foundation soils are examined to try to establish appropriate methods for their analysis and design. The focus is the bearing capacity of the foundation under the combined vertical and shear loading derived from the embankment fill. Lower bound plasticity solutions exist for this case, and indicate the magnitude of the reduction in bearing capacity due to outward shear stresses, compared to the 'smooth' case, and the improvement due to inward shear stresses. The improvement is greatest for soil with strength increasing with depth, or relatively thin layers of soft soil.

A limit equilibrium approach is proposed to evaluate the seismic stability of embankment slopes stabilized with a wall such as a sheet pile wall. Different slip surface and earthquake coefficient can be assumed respectively in the upslope and downslope soil masses separated by the wall. A target value of the factor of safety is prescribed by the designer, contrary to the conventional stability analysis method where the factor of safety is usually unknown. The design for wall itself is then conducted to ensure that the target factor of safety is really reached, though the design method for wall itself is not discussed in the present paper. A practical method is developed to evaluate forces which the wall must sustain and to search for a pair of critical slip surfaces which are the most dangerous for the wall.

Geosciences, 2021

The presence of weak layers in geotechnical systems, including soil or rock masses, both natural and man-made, is more frequent than is normally believed. Weak layers can affect both failure mechanisms, in drained and in undrained conditions, as well as in static and seismic conditions, and the safety factor. In the present study, conducted numerically using the finite-element method (FEM) Plaxis 2D code, the influence of a horizontal thin weak layer on stress and strain distribution, on failure mechanisms and on the overall stability of an embankment was evaluated. The results obtained prove that when the weak layer is located at a significant depth from the foundation plane, the failure mechanisms are normally mixtilinear in shape because the shear strains largely develop on the weak layer. As a result, the safety factor highly decreases compared to the same case without a weak layer. Then, in the presence of weak layers, even embankments that, if founded on homogeneous soils, wou...

Engineering Geology, 2013

The reinforcement of soils using rigid inclusions is a technique used to reduce settlements and to ensure the stability of an embankment built over soft soils. This technique reduces construction delays and is an economical and reliable solution, which has led to its widespread use. Thus, many design methods have been developed to assess the performance of these reinforced structures. These methods are mainly based on results from small scale models and numerical analyses. The reliability of these methods must be validated under in-situ conditions. This paper presents an analytical and numerical study of full-size experiments at the Chelles test site (France). The work presented in this paper is part of the ASIRI French National Research Project. The experiment consisted of a 5-m-high embankment built over soft alluvial ground improved by rigid vertical piles. The embankment is divided into four zones that illustrate the influence of the piles and the geosynthetic reinforcements on the soil's behavior. The performance of the embankment support system is assessed by monitoring data (total stresses, horizontal and vertical displacements). Several in-situ and laboratory soil investigations were performed using two axially loaded test piles. These tests verified the geotechnical hypothesis used for the numerical model and defined the soil-pile interaction parameters. Several analytical methods and numerical models were tested to assess the arching effect. Comparisons between the experimental data and these design methods are presented in terms of stress and the settlement efficacy of the improved system. The results show that these methods overestimate the stress efficacy but that the settlement efficacy is a reliable parameter to assess the overall performance of the rigid inclusion technique.

The Limit Equilibrium method of analysis has a variety of advantages. However, this method requires a large amount of computation and an arbitrary iteration process for obtaining the overall minimum factor of safety. Within the iteration process, locating the critical slip surface is of the greatest essence. The proposed method of embankment stability analysis provides a simplified approach of directly locating the critical slip surface on the basis of which the minimum Factor of Safety (FS) can be computed. General solutions are developed for the rotational stability analysis of reinforced embankments with and without ground improvement constructed on soft ground that has an undrained shear strength varying with depth. The general cases of an embankment having a berm, dry or wet tensile cracks, and multiple layers of reinforcement are considered. The additional friction caused by the reinforcement force normal to the slip plane is incorporated into the solution. The solutions are presented in the form of simple equations. The relationship between the critical slip circles through the embankment with and without geosynthetics reinforcement is expressed explicitly. The simplified analysis procedures and supporting computational examples allow the user to obtain the solutions using hand computations. A simple computer program in the form of excel worksheets can be developed to quickly obtain the final results. Applications and illustrative examples of embankments reinforced with geosynthetics and steel strips, are given. In order to validate the effectiveness of the proposed method, comparative analyses are made with results obtained using other approaches. The results also compare very well to the solutions obtained from the method of slices using the computer programs SB-SLOPE and PC-STABL6 for the geosynthetics reinforced embankment.

SUMMARY The design against failure of an embankment resting upon a soft soil improved by a group of columns is investigated with the help of the yield design homogenization approach. Assuming that both constituents of the reinforced ground are purely cohesive materials ('lime column' technique), an upper bound estimate for the macroscopic strength condition of the reinforced soil as a homogenized medium is first obtained, providing definite evidence of a shear strength anisotropy associated with the reinforcement preferential orientation. The kinematic method of yield design is then performed on the basis of such a criterion, making use of rotational failure mechanisms involving slip circles in the reinforced ground. Upper bound estimates are finally obtained for the embankment stability factor, as functions of the degree of reinforcement and relative thickness of the soil layer. These results are compared with those derived from a simplified analysis, where the reinforced soil is assumed to exhibit an averaged isotropic cohesion. This comparison clearly indicates that the latter simplified analysis may produce quite unsafe estimates for the embankment stability, which can be attributed to the fact that it fails to capture the inherent strength anisotropy of the reinforced soil.

As a technical and economical alternative to foundations on piles, but also to shallow foundations on improved soil, in recent decades a high number of soil improvement methods have been developed and established. Many of these methods use non-reinforced, cylindrical load bearing elements. A very common application of stabilizing columns is the improvement of a few meters thick soft soils below dams and embankments. But especially for this application, many failure cases are documented worldwide. In the contribution the substantial content and results are presented for investigation, testing and further development of methods for evaluating the slope stability. After a description of the problem and consequential tasks the contribution contains main results of the investigations of international sources with the stepwise development of analytical solutions. Next to the in practice well-known approaches for gravel columns, less common approaches from Scandinavia are explained. The contribution is completed with a presentation and discussion of an illustrative example, taking into account a number of different failure modes of the columns and the surrounding soil. The example was compared and validated with a 3D Model using the Finite Element Method.

Journal of Geotechnical and Geoenvironmental Engineering, 2010

This paper evaluates the significance of basal reinforcement and the presence of the surface sand layer in the stability. This evaluation is carried out by means of field measurements and stability analyses of three test embankments on soft clay taken to failure. Two of the test embankments were reinforced and one was unreinforced. Stability analyses were carried out taking into account measured values of reinforcement tension forces during construction. The set of analyses have shown that the top sand layer was more important to the stability of the embankments than the basal reinforcement. The cases studied have also shown that the conventional design practice that assumes for the reinforcement a fixed tension contribution may lead to unrealistic higher factor of safety.

General solutions are developed for the rotational stability analysis of an embankment with and without reinforcement constructed on soft ground that has an undrained shear strength varying with depth. The general cases of an embankment having a berm, dry or wet tensile cracks, and multiple layers of reinforcement are considered. The additional friction caused by the reinforcement force normal to the slip plane is incorporated into the solution. The solutions are presented in the form of simple equations. The relationship between the critical slip circles through the embankment with and without reinforcement is expressed explicitly. The analysis procedures and supporting graphs allow the user to obtain the solutions using hand computations. A simple computer program in the form of a worksheet was developed to quickly obtain the final results. Applications and illustrative examples of embankments with one-and two-step berms, and distributed loads are given. The results obtained using the approach presented in this paper compare very well to the solutions obtained from the method of slices using the computer programs SB-SLOPE and PC-STABL6. Telephone: 1/612-222-2508, Telefax: 1/612-222-8215. Geosynthetics International is registered under ISSN 1072-6349.

1989

Volume II was essentially prepared as an Appendix of supporting information for Volume I. This volume contains much of the supporting theory and a summary of the research used to verify the design approach contained in Volume I, as well as general information concerning proprietary reinforced soil systems. The information provided in this volume is not required for design evaluation and as such Volume I can stand alone. The basis for this volume was the NCHRP Report 290, "Reinforcement of Earth Slopes and Embankments" (Mitchell and Ville!, 1987) and the research program performed as part of the contract to develop the design guidelines contained in Volume I. A summary of that research program is contained in the Introduction section of this volume.