DIA of centrifuge model tests on geogrid reinforced soil walls with low-permeable backfills subjected to rainfall

Geogrid reinforced soil walls (GRSWs) constructed using low-permeable backfills often experience failures when subjected to rainfall. The objective of this paper is to employ centrifuge modelling to investigate the effect of geogrid types on the performance of GRSW models constructed with low-permeable backfill, when subjected to rainfall intensity of 10 mm/h. A 4.5 m radius large beam centrifuge facility was used, and rainfall was simulated using a custom-designed rainfall simulator at 40 gravities. Digital Image Analysis (DIA) was employed to understand the deformation behaviour of GRSWs with low stiffness geogrid layers with and without drainage provision subjected to rainfall. Additionally, the effect of varying stiffness of geogrid reinforcement layers across the height of GRSW was also investigated. The interpretation of DIA helped to quantify displacement vector fields, face movements, surface settlement profiles and geogrid strain distribution with depth. Irrespective of drainage provision, GRSWs reinforced with low stiffness geogrid layers experienced a catastrophic failure at the onset of rainfall. However, GRSW reinforced with geogrid layers of varying stiffness was observed to perform well. This study demonstrates the effective use of DIA of GRSWs subjected to rainfall along with centrifuge-based physical model testing.     

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.     

Centrifuge model study on geogrid reinforced soil walls with marginal backfills with and without chimney sand drain

The objective of this paper is to investigate the performance of geogrid reinforced soil walls with panel facing using marginal backfill with and without chimney sand drain subjected to seepage. A series of centrifuge model tests were performed at 40 gravities using a 4.5 m radius large beam centrifuge facility available at IIT Bombay. The results revealed that a geogrid reinforced soil wall with low stiffness geogrid and without any chimney drain experienced a catastrophic failure due to excess pore water pressure that developed in the reinforced and backfill zones at the onset of seepage. In comparison, a soil wall reinforced with stiff geogrid layers was found to perform effectively even at the onset of seepage. Provision of chimney sand drain effectively decreased pore water pressure not only at the wall toe but also at mid-distance from toe of the wall and thereby resulted in enhancing the wall performance under the effect of seepage forces. However, a local piping failure was observed near the toe region of the wall. The observed centrifuge test results were further analysed by performing seepage and stability analyses to evaluate the effect of thickness of sand layer in a chimney drain. An increase in thickness of sand layer in chimney drain was found to improve the discharge values and thereby enhancing the factor of safety against piping near the toe region. Based on the analysis and interpretation of centrifuge test results, it can be concluded that marginal soil can be used as a backfill in reinforced soil walls provided, it has geogrid layers of adequate stiffness and/or proper chimney drain configuration.     

格栅条带式加筋胎面挡土墙变形性能数值计算

采用土工格栅加筋的方法提高废旧轮胎挡墙的承载性能,促进废旧轮胎挡墙的推广应用,通过数值计算方法分析了不同墙顶荷载下有无土工格栅加筋的废旧轮胎挡墙的水平变形与竖向沉降反应特征,得出铺设土工格栅加筋的方法可显著减小墙体的水平变形和竖向沉降,提高废旧轮胎挡墙结构的承载能力,随着外荷载的增加,墙体变形模式依次呈凹凸微小变化型、“弯弓”型、“似弯弓”型和“鼓腮”型和直线型。考虑土工格栅的加筋长度、竖向加筋间距以及格栅加筋刚度3种因素对废旧轮胎+土工格栅加筋土挡墙的水平变形的影响,得出在废旧轮胎加筋土挡墙设计中,建议土工格栅的加筋长度选取范围为0.5H～0.7H,土工格栅竖向间距的选取范围为0.4 m~0.7 m,格栅刚度不宜大于5 000 kN/m。     

Centrifuge modeling of geotextile-reinforced slopes subjected to differential settlements

Today, geosynthetic-reinforced soil structures are widely used to support bridge abutments and approach roads in place of traditional pile supports and techniques. In such situations, foundation conditions have been shown to adversely affect the stability and deformation behaviour of overlying geosynthetic-reinforced slopes and walls. This paper addresses the response of geotextile-reinforced slopes subjected to differential settlements in a geotechnical centrifuge. Centrifuge model tests were carried out on model geotextile-reinforced sand slopes with two different types of reinforcement. A wrap-around technique was used to represent a flexible facing. In order to initiate failure in the reinforcement layers, the ratio of length of reinforcement to height of the slope was maintained as 0.85. One of the objectives of this paper is to present about a special device developed for inducing differential settlements during centrifuge test at 40g for a reinforced soil structure. A digital image analysis technique was employed to arrive at displacement vectors of markers glued to the reinforcement layers. The displacements were used to compute and analyze the strain distribution along the reinforcement layers during different settlement stages. Results of the centrifuge test indicate that even after inducing a differential settlement equivalent to 1.0 m in prototype dimensions, the geotextile-reinforced soil structure with a flexible facing was not found to experience a collapse failure. Analysis of geotextile strain results shows that the location of the maximum peak reinforcement strain occurs along the bottom-most reinforcement layer at the onset of differential settlements, at the point directly below the crest of the slope.     

Physical and numerical modelling of strip footing on geogrid reinforced transparent sand

This paper presents the results of laboratory scale plate load tests on transparent soils reinforced with biaxial polypropylene geogrids. The influence of reinforcement length and number of reinforcement layers on the load-settlement response of the reinforced soil foundation was assessed by varying the reinforcement length and the number of geogrid layers, each spaced at 25% of footing width. The deformations of the reinforcement layers and soil under strip loading were examined with the aid of laser transmitters (to illuminate the geogrid reinforcement) and digital camera. A two-dimensional finite difference program was used to study the fracture of geogrid under strip loading considering the geometry of the model tests. The bearing capacity and stiffness of the reinforced soil foundation has increased with the increase in the reinforcement length and number of reinforcement layers, but the increase is more prominent by increasing number of reinforcement layers. The results from the physical and numerical modelling on reinforced soil foundation reveal that fracture of geogrid could initiate in the bottom layer of reinforcement and progress to subsequent upper layers. The displacement and stress contours along with the mobilized tensile force distribution obtained from the numerical simulations have complimented the observations made from the experiments.     

Static structural behavior of geogrid reinforced soil retaining walls with a deformation buffer zone

To understand the structural behavior of geogrid reinforced soil retaining walls (GRSW) with a deformation buffer zone (DBZ) under static loads, the model tests and the numerical simulations were conducted to obtain the wall face horizontal displacement, vertical and horizontal soil pressures, and geogrid strains. Results showed that compared with the common GRSW, the horizontal displacement of GRSW with DBZ decreased, and the horizontal soil pressure acting on the face panel of GRSW with DBZ increased. The vertical and horizontal soil pressures showed a nonlinear distribution along the reinforcement length, and the value was smaller near the face panel. The horizontal soil pressure acting on the face panel of GRSW with DBZ was greater than that of the common GRSW in the middle portion. The cumulative strain of the geogrid had a single-peak distribution along its length; the maximum strain of the geogrid was 0.45%, the maximum tension was approximately 29.12% of ultimate tensile strength.     

Hydro-mechanical behavior of geogrid reinforced soil barriers of landfill cover systems

This paper examines the hydro-mechanical behavior of soil barriers with and without the inclusion of geogrid reinforcement within the soil barrier of landfill cover systems. The effect of geogrid type on the deformation behavior of the soil barrier subjected to various ranges of distortion levels was examined through centrifuge tests carried out at 40 g. An overburden pressure equivalent to that of landfill cover systems was applied to all the soil barriers tested in this study. The performance of the soil barrier with and without geogrid layer was assessed by measuring water breakthrough at the onset of differential settlements during centrifuge tests. Un-reinforced soil barriers of 0.6 m and 1.2 m thickness were observed to experience single narrow cracks penetrating up to full -depth of soil barriers at distortion levels of 0.056 and 0.069 respectively. In comparison, soil barriers reinforced with geogrids restrained cracking better than unreinforced soil barriers. However, degree of restraining of cracks in the soil barriers was found to be strongly depending on the geogrid type and the thickness of the soil barrier. Limiting distortion levels for 0.6 m and 1.2 m thick soil barriers reinforced with a low strength geogrid was found to be 0.095 and 0.108 respectively. When the soil barrier of both thicknesses was reinforced with a geogrid having relatively high tensile load-strain characteristics, the integrity of the geogrid reinforced soil barrier was observed to be retained even after inducing a distortion level of 0.125. The results from the present study suggest that the hydro-mechanical behavior of the soil barriers can be improved with a suitable geogrid layer having adequate tensile load-strain characteristics.     

Evaluation of K-Stiffness Method for Vertical Geosynthetic Reinforced Granular Soil Walls in Japan

In this paper the K-stiffness Method as originally proposed by Allen et al. (2003) is re-examined using a total of six new case studies-five from Japan and one from the USA. A common feature of the walls in this new data set is that the walls were all constructed with a vertical face and a granular backfill. However, the walls varied widely with respect to facing type. This new data set together with data for vertical walls previously published by Allen and Bathurst (2002a,b) and Allen et al. (2002) is now used to isolate the effect of the facing stiffness factor on reinforcement loads and to adjust the original equation that was developed to calculate its value. The paper also shows that predicted reinforcement loads using the current AASHTO Simplified Method in the USA and the current PWRC method in Japan give the same reinforcement load predictions, and both grossly over-estimate the values deduced from measured strains. The new data set is used to slightly refine the estimate of the facing stiffness factor used in the original K-stiffness Method. The original and modified K-stiffness Method are demonstrated to quantitatively improve the estimate of the magnitude and distribution of reinforcement loads for internal stability design of vertical-faced geosynthetic reinforced soils walls with granular backfills when compared to the current American and Japanese methods.     

Centrifuge model tests on the behavior of strip footing on geotextile-reinforced slopes

This paper examines the stability of geotextile-reinforced slopes when subjected to a vertical load applied to a strip footing positioned close to the slope crest. Vertical spacing between geotextile reinforcement was varied while maintaining a constant slope angle, load position, soil density and geotextile type. Small-scale physical tests were conducted using a large beam centrifuge to simulate field prototype conditions. After the model was accelerated to 40g, a load was applied to the strip footing until slope failure occurred. Digital image analysis was performed, using photographs taken in-flight, to obtain slope displacements and strain distribution along the reinforcement layers at different loading pressures during the test and at failure. Stability analysis was also conducted and compared with centrifuge model test results. The vertical spacing between reinforcement layers has a significant impact on the stability of a reinforced slope when subjected to a vertical load. Less vertical distance between reinforcement layers allows the slope to tolerate much greater loads than layers spaced further apart. Distributions of peak strains in reinforcement layers due to the strip footing placed on the surface of the reinforced slope were found to extend up to mid-height of the slope and thereafter they were found to be negligible. Stability analysis of the centrifuge models was found to be consistent with the observed performance of geotextile-reinforced slopes subjected to loading applied to a strip footing near the crest.     

Study of the behavior of mechanically stabilized earth (MSE) walls subjected to differential settlements

The limit equilibrium (LE) analysis has been used to design MSE walls. Presumably, the deflection of MSE walls can be limited to an acceptable range by ensuring sufficient factors of safety (FOSs) for both external and internal stabilities. However, unexpected ground movements, such as movements induced by excavations, volume changes of expansive soils, collapse of sinkholes, and consolidations of underlying soils, can induce excessive differential settlements that may influence both the stability and the serviceability of MSE walls. In this study, a numerical model, which was calibrated by triaxial tests and further by a specially-designed MSE wall tests, investigated the behavior of an MSE wall as well as the influence of various factors on the performance of the MSE wall when the wall facing settled relatively to the reinforced zone. The numerical results showed that the differential settlement would cause substantial vertical and horizontal movements for the MSE wall, as well as an increase in lateral earth pressure and geosynthetic reinforcement strain. The maximum horizontal movement and increase of the lateral earth pressure occurred at about 1.0 m above the toe. The differential settlement resulted in a critical plane that coincided with the plane of 45°+?/2. The maximum increase of the strain for each geogrid layer occurred in that plane, and the bottom layer had the greatest strain increase among all layers of reinforcement. The study further indicated that the surcharge, backfill friction angle, tensile stiffness of geogrid, reinforcement length and MSE wall height had noticeable influences on horizontal and vertical movements, and strain in geosynthetics. According to the results, the MSE wall that had a higher factor of safety would have less movements and geosynthetic strain increase. In contrast, only the friction angle, tensile stiffness and MSE wall height showed some degree of influence on the lateral earth pressure due to differential settlements.     

Full-scale mechanically stabilized earth (MSE) walls under strip footing load

This study analyses two full-scale model tests on mechanically stabilized earth (MSE) walls. One test was conducted with a rigid and one with a flexible wall face. Other parameters were the same in these two tests, like the number and type of geogrid layers, the vertical distance between the layers and the soil type. The loads and strains on the reinforcement are measured as function of the horizontal and vertical earth pressure and compared with analytical models. Specifics regarding the behavior of the geogrids under the compaction load during the construction of the model and under strip footing load are included in the study. Results are compared with AASHTO and the empirical K-stiffness method. In this study, an analytical method is developed for the MSE walls taking into account the facing panel rigidity both after backfill construction and after strip footing load. There is good agreement between the proposed analytical method and the experimental results considering the facing panel rigidity. The results indicate that the tensile force on reinforcement layers for rigid facing is less than the flexible facing. The maximum strains in the reinforcement layers occurred in the upper layers right below the strip footing load. The maximum wall deflection for the flexible facing is more than for the rigid facing. The maximum deflection was at the top of the wall for the rigid facing and occurred at z/H?=?0.81 from top of the wall for the flexible facing.     

Seismic performance of a whole Geosynthetic Reinforced Soil – Integrated Bridge System (GRS-IBS) in shaking table test

A scaled plane-strain shaking table test was conducted in this study to investigate the seismic performance of a Geosynthetic Reinforced Soil-Integrated Bridge System (GRS-IBS) with a full-length bridge beam resting on two GRS abutments at opposite ends subjected to earthquake motions in the longitudinal direction. This study examined the effects of different combinations of reinforcement stiffness J and spacing S_v on the seismic performance of the GRS-IBS. Test results show that reducing the reinforcement spacing was more beneficial to minimize the seismic effect on the GRS abutment as compared to increasing the reinforcement stiffness. The seismic inertial forces acted on the top of two side GRS abutments interacted with each other through the bridge beam, which led to close peak acceleration amplitudes at the locations near the bridge beam. Overall, the GRS-IBS did not experience obvious structure failure and significant displacements during and after shaking. Shaking in the longitudinal direction of the bridge beam increased the vertical stress in the reinforced soil zone. The maximum tensile forces in the upper and lower geogrid layers due to shaking happened under the center of the beam seat and at the abutment facing respectively.     

Behavior of geogrid–reinforced sand and effect of reinforcement anchorage in large-scale plane strain compression

This paper presents experimental investigations on the behavior of geogrid–reinforced sand featuring reinforcement anchorage which simulates the reinforcement connected to the wall facings in numerous in-situ situations. A series of large plane strain compression tests (the specimen 56 cm high × 56 cm wide × 45 cm long) was conducted. Standard Ottawa sand and 4 types of PET geogrids exhibiting 5% stiffness in the range of 750–1700 kN/m were used in this study. The specimens were tested by varying the relative density of sand, confining pressures, geogrid types, and reinforcement-anchorage conditions. Experimental results indicate that relative to unreinforced specimens, both anchored and non-anchored geogrid reinforcements can enhance the peak shear strength and suppress the volumetric dilation of reinforced soil. The studies on anchorage revealed that anchoring the reinforcement can restrain the lateral expansion of reinforced specimens, resulting in a substantial increase in shear strength and a reduction in volumetric dilation. The strength ratios of non-anchored specimens appeared to be insensitive to the reinforcement stiffness, whereas the strength ratios of the anchored specimens increased markedly with increases in soil density, reinforcement stiffness, and system deformation (i.e., axial stain). Geogrid anchorage contributed a large percentage of the total shear-strength improvement, nearly 3-times more than the contribution of the soil–geogrid interaction in non-anchored specimens. Lastly, an analytical model was developed based on the concept that additional confinement is induced by reinforcement anchorage, and the predicted shear strength of the anchored soil was verified based on the experimental data.     

Centrifuge model study on low permeable slope reinforced by hybrid geosynthetics

The objective of this paper is to study the performance of hybrid geosynthetic reinforced slopes, with permeable geosynthetic as one of its components, for low permeable backfill slopes subjected to seepage. Four centrifuge tests have been performed to study the behavior of hybrid geosynthetic reinforced slopes subjected to seepage, keeping the model slope height and vertical spacing of geosynthetic reinforcement layers constant. Centrifuge model tests were performed on 2V:1H slopes at 30 gravities. One unreinforced, one model geogrid reinforced and two hybrid geosynthetic reinforced slope models with varying number of hybrid geosynthetic layers were tested. The effect of raising ground water table was simulated by using a seepage flow simulator during the flight. Surface movements and pore water pressure profiles for the slope models were monitored using displacement transducers and pore pressure transducers during centrifuge tests. Markers glued on to geosynthetic layers were digitized to arrive at displacement vectors at the onset of raising ground water table. Further, strain distribution along the geosynthetic reinforcement layers and reinforcement peak strain distribution have been determined using digital image analysis technique. The discharge for the performed model tests is determined by performing seepage analysis. It was confirmed by the centrifuge tests that the hybrid geosynthetics increases the stability of low permeable slope subjected to water table rise. The hybrid geosynthetic layers in the bottom half of the slope height play a major role in the dissipation of pore water pressure.     

Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments with different facing conditions

This paper presents an experimental study on reduced-scale model tests of geosynthetic reinforced soil (GRS) bridge abutments with modular block facing, full-height panel facing, and geosynthetic wrapped facing to investigate the influence of facing conditions on the load bearing behavior. The GRS abutment models were constructed using sand backfill and geogrid reinforcement. Test results indicate that footing settlements and facing displacements under the same applied vertical stress generally increase from full-height panel facing abutment, to modular block facing abutment, to geosynthetic wrapped facing abutment. Measured incremental vertical and lateral soil stresses for the two GRS abutments with flexible facing are generally similar, while the GRS abutment with rigid facing has larger stresses. For the GRS abutments with flexible facing, maximum reinforcement tensile strain in each layer typically occurs under the footing for the upper reinforcement layers and near the facing connections for the lower layers. For the full-height panel facing abutment, maximum reinforcement tensile strains generally occur near the facing connections.     

Post-construction performance of a two-tiered geogrid reinforced soil wall backfilled with soil-rock mixture

There have been very few studies on the application of soil-rock mixtures as the backfills of geogrid reinforced soil retaining walls with due concern for their long-term performance and safety. In this study, a 17-m high two-tiered reinforced soil wall backfilled with soil-rock mixture was instrumented for its performance under gravity load after construction. The instrumentation continued for 15 months. It is found that soil-rock mixtures with small rock content (<30%) have the potential to be used as the backfill materials of geogrid-reinforced retaining walls, but special attentions should be given to compaction quality, backfill–geogrid interaction, and installation damage to geogrids. Reinforcement slippage is possible because of the large particles, but it was small in this case and ceased to develop nine months after the end of construction. Compressibility difference between reinforced and unreinforced backfill might led to rotation of the upper tier. Using the estimated soil strength, the predictions of reinforcement loads by the FHWA methods were 100% higher than the estimated ones from measured strains.     

Seismic behavior of geosynthetic-reinforced retaining walls backfilled with cohesive soil

This study investigates the seismic performance of geosynthetic-reinforced modular block retaining walls backfilled with cohesive, fine grained clay-sand soil mixture. Shaking table tests were performed for three ½ scaled (wall height 190 cm) and ¼ scaled model walls to investigate the effects of backfill type, the influence of reinforcement length and reinforcement stiffness effects. The El Centro and Kobe earthquake records of varying amplitudes were used as base acceleration. Displacement of the front wall, accelerations at different locations, strains on the reinforcements, and the visual observations of the facing and the backfill surface were used to evaluate the seismic performance of model walls. The model walls were subjected to rigorous shaking and the walls did not exhibit any stability problems or signs of impending failure. The maximum deformations observed on the models with cohesive backfill was less than half of the deformation of the sand model. The load transfers between the geogrid and cohesive soil was comparable to that of sand and hence the needed reinforcement length was similar as well. As a result; the model walls with cohesive backfills performed within acceptable limits under seismic loading conditions when compared with granular backfilled counterparts.     

An experimental evaluation of the behavior of footings on geosynthetic-reinforced sand

This research was performed to investigate the behavior of geosynthetic-reinforced sandy soil foundations and to study the effect of different parameters contributing to their performance using laboratory model tests. The parameters investigated in this study included top layer spacing, number of reinforcement layers, vertical spacing between layers, tensile modulus and type of geosynthetic reinforcement, embedment depth, and shape of footing. The effect of geosynthetic reinforcement on the vertical stress distribution in the sand and the strain distribution along the reinforcement were also investigated. The test results demonstrated the potential benefit of using geosynthetic-reinforced sand foundations. The test results also showed that the reinforcement configuration/layout has a very significant effect on the behavior of reinforced sand foundation. With two or more layers of reinforcement, the settlement can be reduced by 20% at all footing pressure levels. Sand reinforced by the composite of geogrid and geotextile performed better than those reinforced by geogrid or geotextile alone. The inclusion of reinforcement can redistribute the applied footing load to a more uniform pattern, hence reducing the stress concentration, which will result reduced settlement. Finally, the results of model tests were compared with the analytical solution developed by the authors in previous studies; and the analytical solution gave a good predication of the experimental results of footing on geosynthetic reinforced sand.     

Load transfer mechanism and deformation of reinforced piled embankments

A series of twenty-eight centrifuge tests was performed on piled embankments with basal geosynthetic reinforcement to assess the influence of pile spacing, embankment height, pile cap size and geosynthetic stiffness on the load transfer mechanism and surface settlements. The measurements of the forces on the piles made it possible to assess the load transfer mechanisms, and 100% efficiency was achieved for all tests performed. The results showed that for the thicker mattress and/or closer piles, the surface settlements were smaller or negligible. Geosynthetic maximum deflections were also examined experimentally and analytically, the latter based on BS8006 (2010) and its further corrigendum in 2012. Close agreement in the predictions of the maximum reinforcement deflection was reached with BS8006 (2012) by adopting a slight modification in the ratio of diagonal and orthogonal maximum deflection (y_d/y = √2).