Reinforced Soil Walls Research Articles

Upper bound seismic limit analysis of geosynthetic-reinforced unsaturated soil walls

The assessment of the internal stability of geosynthetic-reinforced earth retaining walls has historically been investigated in previous studies assuming dry backfills. However, the majority of the failures of these structures are caused by the water presence. The studies including the water presence in the backfill are scarce and often consider saturated backfills. In reality, most soils are unsaturated in nature and the matric suction plays an important role in the wall's stability. This paper investigates the internal seismic stability of geosynthetic-reinforced unsaturated earth retaining walls. The groundwater level can be located at any reinforced backfill depth. Several nonlinear equations relating the unsaturated soil shear strength to the matric suction and different backfill type of soils are considered in this study. The log-spiral failure mechanism generated by the point-to-point method is considered. The upper-bound theorem of the limit analysis is used to evaluate the strength required to maintain the reinforced soil walls stability and the seismic loading are represented by the pseudo-dynamic approach. A parametric study showed that the required reinforcement strength is influenced by several parameters such as the soil friction angle, the horizontal seismic coefficient, the water table level, the matric suction distribution as well as the soil types and the unsaturated soils shear strength.

Deformation Behaviour of Geogrid Reinforced Soil Walls under Strip Loading

Deformation Behaviour of Geogrid Reinforced Soil Walls under Strip Loading

Analyses and design of steep slope with GeoBarrier system (GBS) under heavy rainfall

A GeoBarrier system (GBS) is a combination system of reinforced soil walls to stabilize near-vertical cut slopes and capillary barrier principles to protect the wall from the effect of rainfall infiltration. Singapore requires construction materials that are cost-effective to support sustainable urban development. Therefore, recycled materials are utilized as GBS materials to avoid the use of high-cost materials, such as steel or concrete. GBS consists of planting geobags with unique geosynthetic pockets for sustainable plant growth as a facing layer of GBS. The negative pore-water pressure (suction) within the reinforced soil behind GBS was assured to be constant during rainfall since GBS is designed specially to minimize the rainfall infiltration into the reinforced soil. This paper presents the practical design and stability analysis of the GBS, considering the presence of suction within the reinforced soil body. The monitoring of GBS performance in the field was carried out via field instrumentation. Finite element analyses of the GBS under extreme rainfalls were also performed for evaluation of the GBS performance. The field instrumentations and numerical analysis results showed that GBS was able to protect the slope from rainfall infiltration; therefore, the stability of the slope retained by GBS was not affected by the rainfall. Results from the analytical calculation showed that the most critical mode of failure is sliding along the base, followed by the global and local slope stability. The GBS is not susceptible to local instability.

Geogrid Reinforced Soil Walls with Marginal Backfills Subjected to Rainfall: Numerical Study

Altering natural landforms without impacting stability has become quintessential due to unprecedented infrastructure development. Geosynthetic reinforced soil wall (GRSW) as a retaining structure is a congenial option addressing land scarcity and right of way challenges. However, with dearth of good-quality backfill soil and prohibitive transportation costs, engineers are being forced to use native low-permeable marginal soils for constructing geosynthetic reinforced soil walls. Adverse climatic changes leading to erratic rainfall have triggered numerous failures in such soils due to their inefficiency to dissipate excess pore water pressure. The objective of the present paper is to evaluate applicability of different backfill soils for GRSW. Seepage and stability analyses were conducted to appraise the behaviour of a geogrid reinforced soil wall with rigid facing of height 9.6 m having 6° batter during rainfall. The effects of backfill soil types with fines content of 0%, 20% and 40% on seepage behaviour and stability were assimilated under moderate rainfall. It was ascertained that failure occurred faster when the backfill soil with 20% fines was considered. For backfill soil with 40% fines, a significant portion of rainfall was observed to simply disappear as runoff. In the section with sand backfill, failure did not occur as water drained off faster than rainfall infiltration, restraining mounting of water table.

Pseudo-static Analysis of Reinforced Soil Wall Based on Pasternak Model

One of the major reasons for the popularity of reinforced soil structures is their ability to perform better under seismic loading conditions. This could be attributed to the conservatism associated with the design procedure that considers only the axial direction of the pullout force rather than an appropriate seismic design. The kinematics of failure, however, suggests that the direction of the pullout force is not axial, and it is inclined to the horizontal direction. This paper presents the pseudo-static stability analysis of a reinforced soil wall considering the obliquity of the pullout force, where the backfill soil is idealized as a two-parameter Pasternak model. The effect of various controlling parameters such as horizontal seismic coefficient, material properties of the backfill and the wall geometry on the pullout response is also studied under the seismic loading conditions. The results are compared with the conventional methods of analysis. The comparisons show that the present analysis removes the extra degree of conservatism prevailing in the conventional methods and thus gives a better estimation of the pullout capacity. A typical design of reinforced soil wall is also shown to demonstrate that the present analysis can be easily integrated with the existing methods.

Reinforced Soil Wall Analysis under Working Stress Conditions Using a Two Phase Model with the Introduction of a New Design Parameter

A previously introduced two phase model was used to assess its capability in predicting the behavior of reinforced soil walls under working stress conditions. The two phase model is a homogenization method based on the virtual work theorem. The reinforced soil medium is considered as the superposition of two continuous phases, the reinforcement and matrix phases that interact within the medium. Application of the two phase model simplifies changes in the arrangement and properties of the inclusions and decreases the computation time considerably. This approach can be used to reduce the time needed for optimization in practical applications. The introduced approach was first validated by comparison with the filed measurements. Therefore, a full-scale reinforced soil wall that has been constructed and tested under working stress condition at the Public Works Research Institute in Japan was simulated using the two phase model. A finite difference code was used to implement the two phase model and simulate the model. A nonlinear elasto-plastic law and a linearly elastic, perfectly plastic constitutive law were employed for the matrix and reinforcement phases, respectively. Then an extensive parametric study including 125 reinforced soil wall models was conducted to show the capability and strength of the introduced approach for simulation of the reinforced soil walls under working stress condition. The effect of inclusion length and stiffness, inclusion spacing, and wall height on maximum lateral displacement of the models was investigated. Eventually, a new dimensionless design parameter was introduced to achieve a simple criterion for evaluating lateral displacements.

Numerical study of the effects of bed resistance and groundwater conditions on the behavior of geosynthetic reinforced soil walls

Due to the difference in the behavior of the reinforced soil wall on a compressible or loose bed, relative to rigid bed, investigating the behavior of this system on a compressible bed is considered necessary. In this research, the behavior of geogrid reinforced soil seated on a bed with different resistance parameters and different groundwater levels, using finite element method for the analysis of deformation and forces, and limits equilibrium method for checking wall stability, has been evaluated. Also, the effect of the wall height in two categories with a height of 4 and 8 m is considered and evaluated. The result showed that reducing the resistance parameter bed of reinforced soil wall would lead to increase lateral displacement of the wall and tensile load of the geogrids, and to reduce the wall’s stability. Also, as the surface of groundwater level closer to the below of the reinforced soil wall, the lateral displacement of the wall will be more and the overall stability of the wall will be less, and this effect of groundwater on the wall’s behavior is significant at a distance of 0 to 1 m below the wall.

Study on seismic stability and performance of reinforced soil walls using shaking table tests

The paper reports a 1 g shaking table test that was carried out on a reinforced soil wall with an objective to study the acceleration amplification in the backfill, and phase differences between dynamic responses of the reinforced and retained zones. Results of the study show that including the observed larger acceleration amplification in the upper half of the wall, and the phase difference between maximum lateral earth pressure and inertial load in the backfill in the analysis would lead to more accurate predictions of: (1) the wall response relative to predicted reinforcement load, (2) elevations of line of action for both the inertial and lateral earth forces in the backfill, and (3) wall deformations, as compared to pseudo-static methods of analysis.

Centrifuge model studies on the performance of soil walls reinforced with sand-cushioned geogrid layers

This paper is to investigate the effectiveness of encapsulating geogrid layers within thin sand layers, for enhancing the deformation behavior of vertical reinforced soil walls constructed with marginal backfills. Centrifuge model tests were performed on vertical soil walls, reinforced with geogrid layers, using a 4.5 m radius large beam centrifuge available at IIT Bombay at 40 gravities. The backfill conditions, height of soil wall, reinforcement length, and reinforcement spacing, were kept constant in all the tests. A wrap-around technique was used to represent flexible facing. Three different geogrid types with varying stiffness were used in the present study. The walls were instrumented with vertical linear variable differential transformers to monitor surface settlements during the tests. Marker-based digital image analysis technique was used to determine face movements and distribution of geogrid strain along the wall height. The deformation behavior of soil walls, reinforced with geogrid layers encapsulated in thin layers of sand, were compared against a base model having no sand-cushioned geogrid layers. Provision of sand-cushioned geogrid layers and increase in geogrid stiffness were found to limit normalized face movements (Sf/H), normalized crest settlements (Sc/H), and change in maximum peak reinforcement strain (dεpmax). Sand-cushioned geogrid layers were also found to limit the development of tension cracks behind and within the reinforced zone. Significant reduction in rate of maximum face movement (dSfmax/dt) and rate of maximum peak reinforcement strain (dεpmax/dt) was observed, with an increase in value of normalized reinforcement stiffness (Jg/γH2) of geogrid layers. The analysis and interpretation of centrifuge model tests on soil walls, constructed with marginal backfills and reinforced with sand-cushioned geogrid layers, indicate that their performance is superior to the walls without sand-cushioned geogrid layers.

Centrifuge model studies on geocomposite reinforced soil walls subjected to seepage

This paper investigates the effect of geocomposite layers (as an internal drainage system) and a chimney sand drain (as an external drainage system) on the performance of reinforced soil walls with panel facing and marginal backfill. Four models subjected to seepage simulating a rising groundwater surface were tested at 40 gravities using a 4.5 m radius large beam centrifuge. In this study, geocomposite layers played a dual function of drainage and reinforcement. The behavior of wall models was monitored using displacement and pore water pressure transducers during centrifuge tests. An image analysis technique was also used to measure displacement and strain fields. A geogrid reinforced soil wall with no drainage system experienced catastrophic failure at the onset of seepage. Provision of geocomposite layers at the bottom portion of the wall improved the wall behavior. Further, including geocomposite layers up to the mid-height of the wall resulted in superior performance compared to geogrid-reinforced soil walls. The model with the chimney sand drain experienced piping failure at the toe of the wall. The performance of reinforced soil walls with geocomposite layers was found to be superior to the geogrid reinforced soil walls with the chimney drain.

Effect of reinforcing technique on strain-dependent dynamic properties of reinforced earth walls

Due to lack of sufficient understanding of the strain-dependent dynamic properties of reinforced mass, existing soil and reinforcement models are not capable of accurately representing the seismic response of reinforced soil structures in numerical and analytical analyses. In the present study, to evaluate the effect of reinforcing technique on strain-dependent dynamic properties of reinforced earth walls, the values of equivalent shear modulus (Ge) and damping ratio (D) were estimated for soil-nailed wall (SNW) and steel-strip reinforced-soil wall (SSW) as a function of shear strain level using 1-g shaking table tests. The results obtained showed that the variation trend of Ge and D versus γ is strongly influenced by the type of reinforcing technique and confining pressure, so that the variation trend of these two parameters versus shear strain can be well expressed as an exponential equation with a high correlation coefficient for each type of reinforced earth wall.

COMPACTION–INDUCED STRESS IN GEOSYNTHETIC REINFORCED SOIL WALLS

Compaction equipment worked behind the retaining walls causes additional lateral earth pressures acting on the wall. The effect of compaction-induced stress (CIS) usually neglected when designing the retaining walls. Geosynthetic-Reinforced Soil walls (GRS walls) have increasing popularity in Egypt in the last years. The earth pressure at the facing of a Geosynthetic Reinforced Soil wall (GRS wall) is different from that in the natural soil. The internal lateral pressure in the GRS soil is governed by compaction-induced stresses (CIS) and additional confining effects that the reinforcement provides to the soil. The compaction-induced stress for GRS wall is difficult to be predicted during the design stage because it depends on the characteristics of compaction equipment and other factors. The objectives of the research are:1) Studying the effect of compact the backfill of GRS wall by vibratory plates on the internal stability, the external stability, and the foundation soil for GRS wall. 2) Evaluating the performance of the various types of vibratory plates which used to compact the backfill for GRS wall. So, a finite element analysis using GEO5 program was used to achieve these objectives. Finally, the recommendations for the design and construction of GRS walls were highlighted.

Centrifuge and numerical model studies on the behaviour of geogrid reinforced soil walls with marginal backfills with and without geocomposite layers

The aim of this paper is to study the effect of geocomposite layers as internal drainage system on the behaviour of geogrid reinforced soil walls with marginal backfills using centrifuge and numerical modelling. A series of centrifuge model tests were carried out using a 4.5 m radius beam centrifuge facility available at IIT Bombay. A seepage condition was imposed to all models to simulate rising ground water condition. Displacement and pore water pressure transducers were used to monitor the performance of all centrifuge models. A geogrid reinforced soil wall without any geocomposite layer experienced catastrophic failure soon after applying seepage due to the development of excess pore water pressure within the reinforced soil zone of the wall. In comparison, reinforced soil wall with two geocomposite layers at the bottom portion of the wall was found to have a good performance at the onset of seepage and by embedding four geocomposite layers up to the mid-height of the wall from bottom as a result of lowering phreatic surface much more effectively. For analysing further the observed behaviour of centrifuge model tests, stability and seepage analysis were conducted using SLOPE/W and SEEP/W software packages. A good agreement was found between the results of numerical analysis and observation made in centrifuge tests. The effect of number of geocomposite layers as well as its transmissivity was further analysed using parametric study. The results of parametric study revealed that the number of geocomposite layers plays a main role on the good performance of the geogrid reinforced soil walls with marginal backfill.

Natural frequencies of full-height panel reinforced soil walls of variable cross-section

Natural frequency has a significant influence on the dynamic response of structures. In this study, a new transfer matrix method is proposed to calculate the natural frequencies of reinforced soil retaining walls by modeling the facing and reinforcement layers as a beam-and-spring assembly using beam-on-elastic foundation theory. Among the advantages of the proposed method are that the method can be applied to both the uniform and tapering facing cross-sections, and it can be used to calculate higher-order natural frequencies of reinforced soil retaining walls. The accuracy of the proposed method is demonstrated by comparing its predicted frequency results with those using analytical and numerical methods reported in related literature. A parametric analysis is carried out, which indicates that the size and geometry of the facing panel have more significant influences on the natural frequencies of the full-height panel reinforced soil walls than their reinforcement properties.

Behaviour of Two-Tiered Geosynthetic-Reinforced Soil Walls

The literature studies and current design methodologies of reinforced soil wall show increases in tensile stresses in reinforcement with increase in height of wall. This results in decrease of spacing between reinforcement and ultimately increases in cost of construction. Alternatively, the reinforced soil wall can be constructed in tiered fashion to improve aesthetic and reduce cost of construction. This paper focuses on the performance of two-tiered-reinforced soil-retaining walls with different offset lengths. A numerical model of geosynthetic-reinforced soil-retaining wall with a concrete-block facing reported in literature is simulated using commercial finite element software ANSYS. The backfill soil is modelled using the Drucker–Prager plasticity model. The concrete facing is simulated as elastic material and reinforcement with non-linear material properties. The results obtained from numerical model are validated with those from physical model studies reported in literature. Using validated model parameters, two-tiered-reinforced soil walls with different offset lengths are simulated. Four models of 0 offset, 1.2-m offset, 2.0-m offset and 3.0-m offset are developed to study the effects of tiered wall. The offset lengths are determined as intermediate offset distance and large setback distance as per FHWA (2010). The walls are studied for horizontal displacement of facing, strain developed in backfill soil and in reinforcement layers. The maximum horizontal displacements of reinforced soil wall decrease with increase in offset length of tiered wall. By considering the strain variation in soil, two deformation zones—shear deformation zone near the facing and compaction zone below the upper tier facing—are identified. The strains in compaction zone of lower tier increase with increase in offset length. The reinforcement strains in top reinforcement layer of lower tier decrease with increase in offset length.

Characteristics of Geosynthetic Reinforced Soil (GRS) Walls: an Overview of Field-Scale Experiments and Analytical Studies

This article starts with an overview of reinforced soil retaining walls, in that the fundamental differences in design concepts between externally and internally stabilized reinforced soil walls were explained. Mechanically stabilized earth (MSE) walls belong to the former, whereas geosynthetic reinforced soil (GRS) walls belong to the latter. MSE walls with geosynthetics as reinforcement have sometimes been referred to as GMSE walls. GMSE walls have seen to suffer from an unusually high failure rate of about 5 to 8%. GRS walls, on the other hand, have seen a very low failure rate. This article addresses 11 major features of GRS which help to explain the very low failure rate. The features, learned from field-scale experiments and analytical studies, include the following: (1) behavior of internally stabilized reinforced soil mass, (2) effect of tight reinforcement spacing, (3) the lateral earth pressure (“bin pressure”) against facing, (4) facing connection strength of GRS, (5) relative roles of reinforcement spacing vs. reinforcement strength in GRS, (6) the Wu-Pham GRS load-carrying capacity equation, (7) approaching a K0 stress condition in GRS, (8) rational evaluation of pullout failure of GRS, (9) seismic resistance of GRS, (10) creep and stress relaxation of GRS, and (11) GRS with truncated base. It is the author’s hope that the article will add to reader’s understanding of GRS and make better use of this novel technology in wall construction.

Seismic responses of vertical-faced wrap-around reinforced soil walls

A series of shaking table tests on geosynthetic-reinforced walls with a height of H = 0.6 m is performed to investigate the seismic performance of prototype walls with H = 6 m. Ground-wall resonance occurs at small values of horizontal peak ground acceleration (HPGA), namely 0.1–0.5g, with an amplification factor of Am = 1.2–2.2 at the crest of the wall. This factor decreases with increasing HPGA. For walls that attain states of maximum horizontal displacement (Dmax)/H > 5%, the failure mechanism consists of a vertical failure surface at a distance from the facing of 0.3H for the upper half of the wall and a Rankine triangular wedge over the lower half of the wall. Mononobe-Okabe theory gives dynamic earth pressure coefficients that agree well with experimental results based on kh = η·HPGA/g, where khand η are the horizontal seismic coefficient and an empirical constant, respectively, with values of η ranging from 1/4 to 1/3. Furthermore, the maximum tensile forces induced by shaking increase as the depth of reinforcement increases, generating a trapezoidal shape rather than the inverted trapezoidal shape proposed in the literature.

Sliding stability and lateral displacement analysis of reinforced soil retaining walls

Field observations have demonstrated that reinforced soil retaining walls generally have superior seismic performance when compared to traditional gravity retaining walls. However, current design guidelines for reinforced soil retaining walls are typically based on pseudo-static methods of analysis, which involve simplifying assumptions. For instance, the reinforced zone is usually assumed as a rigid body in external stability calculations. As a result, the influences of reinforcement arrangement and properties on the sliding stability and displacement of the wall cannot be accounted for in their design. Additionally, the soil shear strength is assumed to be constant in conventional displacement calculations using the Newmark sliding block method. In this paper, an analysis method is proposed to determine the yield acceleration and lateral displacement of reinforced soil walls that includes soil shear strength mobilization and a two-part wedge planar failure mechanism. The proposed method is validated against the results of laboratory model tests, and influences of factors such as ground acceleration coefficients, and reinforcement and backfill properties on the stability of the wall are examined.

Pseudo-Dynamic Approach to Analyze the Seismic Stability of Reinforced Soil Wall for c–ϕ Soil Backfill

In the design of reinforced soil wall, soil is considered as cohesionless. In earthquake-prone regions, analysis is based on either pseudo-static or pseudo-dynamic approach. Very few studies have considered behavior of reinforced soil walls assuming c–ϕ soil backfill in the pseudo-dynamic approach. Present study is aimed to investigate the seismic stability of reinforced soil wall incorporating the pseudo-dynamic approach. The propagation of primary and shear waves is considered under both horizontal and vertical seismic accelerations incorporating effect of soil amplification. A numerical algorithm is proposed to obtain the required strength of reinforcements, critical angle of failure wedge, and factor of safety of the reinforced soil wall for specific number of reinforcement layers and length of reinforcement. The effects of shear strength parameters, coefficients of ground accelerations, and soil amplification factor on the stability of the reinforced soil wall are critically examined. Variation in the mobilized pullout resistance of reinforcement layers and associated factor of safety with time is considered in the assessment of stability. For c–ϕ soil backfill, numerical predictions are in good agreement when compared with available studies in the literature for validation purpose.

Centrifuge model tests for investigation of fiber reinforced soil walls

In this study, the centrifuge model tests were used to evaluate the geotechnical properties of fiber reinforced soil walls. The reduced-scale centrifuge models were built and the clay barrier was prepared using kaoline amended silty soil. The unreinforced soil barrier was found to lose their water-tightness and integrity at lower distortion levels compared to fiber reinforced soil barriers. The silty soil used in the centrifuge models, frequently considered as having negligible creep, was ultimately found not to prevent the development of time-dependent deformations. The obtained results revealed the significant time-dependent deformations could be occurred in geotechnical structure of fiber reinforced soil walls wall systems.