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 (S_f/H), normalized crest settlements (S_c/H), and change in maximum peak reinforcement strain (dε_pmax). 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 (dS_fmax/dt) and rate of maximum peak reinforcement strain (dε_pmax/dt) was observed, with an increase in value of normalized reinforcement stiffness (J_g/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.     

Evaluation of Extent of Damage to Geogrid Reinforced Soil Walls Subjected to Earthquakes

The aim of this study is to establish a simple method for evaluating the extent of damage to geogrid reinforced soil walls (GRSWs) subjected to earthquakes. Centrifuge tilting and shaking table tests were conducted to investigate the seismic behaviour of GRSWs, with special focus on the effects of the tensile stiffness of the geogrids, the pullout characteristics and the backfill materials. As a result, it was found that GRSWs showed large shear deformation in the reinforced area after shaking, that such deformation was influenced by the tensile stiffness of the geogrids, the pullout resistance and the deformation modulus of the backfill material, and that finally slip lines appeared. However, the GRSWs maintained adequate seismic stability owing to the pullout resistance of the geogrids, even after the formation of slip lines. It is considered that such slip lines appeared due to the failure of the backfill material. Since the maximum shear strain occurring in the backfill can be roughly estimated from the inclination of the facing panels, using a simple plastic theory, it is possible to evaluate whether the backfill has reached its peak state or not. The formation of slip lines observed in the centrifuge model tests could be well explained by this method. Finally, the method is proposed to estimate the failure sections in the GRSWs using a Two Wedge analysis.     

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.     

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.     

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.     

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.     

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.     

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.     

Experimental and numerical analysis of large scale pull out tests conducted on clays reinforced with geogrids encapsulated with coarse material

The pullout test is one of the methods commonly used to study pullout behavior of reinforcements. In the current research, large pullout tests (i.e. 100 × 60 × 60 cm) have been conducted to investigate the possibility of pullout resistance enhancement of clays reinforced with HDPE geogrid embedded in thin layers of sand. Pullout tests on clay–geogrid, sand–geogrid and clay–sand–geogrid samples have been conducted at normal pressures of 25, 50 and 100 kPa. Numerical modeling using finite element method has also been used to assess the adequacy of the box and geogrid sizes to minimize boundary and scale effects. Experimental results show that provision of thin sand layers around the reinforcement substantially enhances pullout resistance of clay soil under monotonic loading conditions and the effectiveness increases with increase in normal pressures. The improvement is more pronounced at higher normal pressures and an optimum sand layer thickness of 8 cm has been determined for maximum enhancement. Results of numerical analysis showed the adequacy of the box and geogrid length adopted as well as a relatively good agreement with experimental results.     

Evaluation of permeability characteristics of a geosynthetic-reinforced soil through laboratory tests

The objective of this paper is to examine the permeability characteristics of geosynthetic layers under confinement with soils having relatively low permeability. For this purpose, a large permeameter was custom designed and a series of permeability tests were carried-out by varying soil type and number of geosynthetic layers. Further, effect of provision of sand cushion and the thickness of sand cushion on permeability characteristics was also examined. Normal stress was increased in intervals of 50 kPa up to 200 kPa. With an increase in normal stress, a decrease in the permeability characteristics of a geosynthetic-reinforced soil was observed. The permeability characteristics were found to improve significantly with the provision of sand cushion and an increase in its thickness. Based on the definition of equivalent coefficient of permeability of stratified soils for parallel flow, an equation for estimating coefficient of permeability of soil–geosynthetic system with and without sand cushion is proposed. Considering the application of geosynthetics in reinforced slopes and walls with low-permeable backfill soils, a suitable geosynthetic with a thin layer of sand cushion is recommended. This in turn can also help in enhancing the pore-water pressure dissipation.     

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.     

Strength enhancement of clay by encapsulating geogrids in thin layers of sand

Large size direct shear tests (i.e.300 × 300 × 200 mm) were conducted to investigate the possibility of strength enhancement of clays reinforced with geogrids embedded in thin layers of sand. In this paper test results for the clay, sand, clay–sand, clay–geogrid, sand–geogrid and clay–sand–geogrid samples are presented and discussed. Thin sand layers with thicknesses of 4, 6, 8, 10, 12 and 14 mm were used to quantify their effect on the interaction between the clay and the geogrids. In this regard effects of sand layer thickness, normal pressure (i.e. confinement) and transversal members of geogrids were investigated. All the tests were conducted using saturated clay with no drainage allowed. Test results indicate that provision of thin layers of sand for encapsulating the geogrids is very effective in improving the strength and deformation characteristics of saturated clay. Maximum strength enhancement was derived at an optimum sand layer thickness of 10 mm which proved to be independent of the magnitude of the normal pressure used. For a particular sand layer thickness, increasing the normal pressure resulted in enhanced strength improvement. Results also showed that removal of the geogrid transversal members resulted in reducing the strength of the reinforced samples by 10% compared to geogrids with transversal members. Encapsulating geogrids in thin layers of sand not only will improve the performance of clays if used as backfill it would also provide drainage paths preventing pore water pressure generation on saturation of the backfill.     

Influence of Geogrid Layer on the Integrity of Compacted Clay Liners of Landfills

The objective of this paper is to examine the influence of geogrid layer on the integrity of clay liners of landfills. A series of centrifuge model tests were performed on model clay liners subjected to non-uniform settlements with and without a geogrid layer embedded within the top one-third portion of the clay liner moist-compacted on the wet side of its optimum moisture content at 40 g. The model clay liner material has been selected in such a way that it envelopes the material characteristics of the clay liners, which are used for constructing an impermeable barrier in a lining system. By maintaining type and location of the geogrid within the clay liner as constant, the thickness of clay liner is varied to check the possibility of reducing the thickness of a geogrid reinforced clay liner. Digital image analysis technique was employed to ascertain the initiation of cracking and to compute strains both on the surface and along the cross-section of the clay liner with and without any geogrid layer. It was observed that clay liners compacted at moulding water content towards wet side of their OMC found to experience multiple cracking at the onset of non-uniform settlements. Contrary to this, geogrid reinforced clay liner was observed to sustain large distortions and experience only tiny cracks limited up to a location of a geogrid. With an increase in thickness of the clay liner reinforced with a geogrid, geogrid reinforced compacted clay liner was observed to retain its integrity and restrains cracking completely.     

Behavior of geogrid-reinforced ballast under various levels of fouling

This paper presents a study of how the interface between ballast and geogrid copes with fouling by coal fines. The stress-displacement behavior of fresh and fouled ballast, and geogrid reinforced ballast was investigated through a series of large-scale direct shear tests where the levels of fouling ranged from 0% to 95% Void Contamination Index (VCI), at relatively low normal stresses varying from 15 kPa to 75 kPa. The results indicated that geogrid increases the shear strength and apparent angle of shearing resistance, while only slightly reducing the vertical displacement of the composite geogrid-ballast system. However, when ballast was fouled by coal fines, the benefits of geogrid reinforcement decreased in proportion to the increasing level of fouling. A conceptual normalized shear strength model was proposed to predict this decrease in peak shear stress and peak angle of shearing resistance caused by coal fines at a given normal stress.     

Prediction of the Performance of a Geogrid-Reinforced Slope Founded on Solid Waste

An investigation was undertaken to evaluate the integrity of a geogrid-reinforced steep slope subjected to significant differential settlements and seismic loading. The reinforced soil structure under investigation was constructed in 1987 in order to enhance the stability of steep landfill slopes at the Operating Industries, Inc. (Oil) Superfund site, a hazardous waste site in southern California. The site is in an area of high seismicity. The 4.60 m high, 460 m long ge-ogrid-reinforced structure was founded, along most of its length, on concrete piers located towards the front of the structure. However, as the back of the reinforced slope was founded on waste, the structure experienced more than 600 mm of differential settlements ten years after its construction. A geogrid experimental testing program was implemented to evaluate the performance of the reinforcements when loaded rapidly after a period of constant load. A finite element numerical simulation was performed to assess the integrity of the geogrid reinforcements when subjected to 30 years of additional differential settlements followed by the design earthquake. The maximum geogrid strains predicted for a sequence of expected static and extreme seismic loadings were found to be well below the geogrid allowable strain values, indicating that the integrity of the structure should be maintained even when subjected to large differential settlements and severe earthquake loads. The numerical results show that the critical reinforced zone (i.e., the reinforcement layers that are strained the most) that corresponds to different loading mechanisms (construction, differential settlement, seismic loading) occurs at different elevations within the reinforced soil structure.     

Pullout tests conducted on clay reinforced with geogrid encapsulated in thin layers of sand

The interaction between reinforcement and backfill materials is a significant factor for analysis and design of reinforced earth structures which is simplified as pullout or direct shear resistance. This paper presents the results of pullout tests aimed at studying the interaction of clays reinforced with geogrids embedded in thin layers of sand. Pullout tests were conducted after modification of the large direct shear apparatus. Samples were prepared at optimum moisture content and maximum dry densities obtained from standard Proctor compaction tests. Tests were conducted on clay-geogrid, sand-geogrid and clay-sand-geogrid samples. A unidirectional geogrid with sand layer thicknesses of 6, 10 and 14 mm were used. Results revealed that encapsulating geogrids in thin layers of sand under pullout conditions enhances pullout resistance of reinforced clay. For the clay-sand-geogrid samples an optimum sand layer thickness of 10 mm was determined, resulting in maximum pullout resistance which increased with increasing confining pressure. The optimum sand layer thickness was the same for all the normal pressures investigated. For sandy soils the passive earth pressure offered the most pullout resistance, whereas for clayey soils, it was replaced by frictional resistance. It is anticipated that provision of thin sand layers will provide horizontal drainage preventing pore pressure built up in clay backfills on saturation.     

Bearing capacity of square footings on geosynthetic reinforced sand

The results from laboratory model tests and numerical simulations on square footings resting on sand are presented. Bearing capacity of footings on geosynthetic reinforced sand is evaluated and the effect of various reinforcement parameters like the type and tensile strength of geosynthetic material, amount of reinforcement, layout and configuration of geosynthetic layers below the footing on the bearing capacity improvement of the footings is studied through systematic model studies. A steel tank of size 900 × 900 × 600 mm is used for conducting model tests. Four types of grids, namely strong biaxial geogrid, weak biaxial geogrid, uniaxial geogrid and a geonet, each with different tensile strength, are used in the tests. Geosynthetic reinforcement is provided in the form of planar layers, varying the depth of reinforced zone below the footing, number of geosynthetic layers within the reinforced zone and the width of geosynthetic layers in different tests. Influence of all these parameters on the bearing capacity improvement of square footing and its settlement is studied by comparing with the test on unreinforced sand. Results show that the effective depth of reinforcement is twice the width of the footing and optimum spacing of geosynthetic layers is half the width of the footing. It is observed that the layout and configuration of reinforcement play a vital role in bearing capacity improvement rather than the tensile strength of the geosynthetic material. Experimental observations are supported by the findings from numerical analyses.     

Centrifuge tests to investigate global performance of geosynthetic-reinforced pile-supported embankments with side slopes

Three centrifuge model tests were conducted to investigate the influence of the number of geosynthetic layers and the pile clear spacing on the global performance of Geosynthetic-Reinforced Pile-Supported (GRPS) embankments with side slopes constructed on soft soil foundations. This study found that the change of the geogrid number from one to two did not significantly affect the foundation settlement, the geogrid deflection, and the vertical stress at the embankment base. For the GRPS embankment with a single geogrid layer, the geogrid strain distribution at the embankment base showed an “M” shape along the transverse direction with the maximum strain near the embankment shoulder. When two geogrid layers with sand in between were used, the upper and lower layers showed different strain distributions with the maximum strains happening near the embankment shoulder and at the center of the embankment for the upper and lower layers respectively. The strains of the upper geogrid were smaller than those of the lower geogrid. Smaller pile clear spacing reduced the geogrid deflection and the foundation settlement. Despite the change of the pile clear spacing, the progressive development of soil arching with the normalized displacement at the embankment base followed a similar trend without an obvious stress recovery stage.     

Load sharing characteristics of rigid facing walls with geogrid reinforced railway subgrade during and after construction

Reinforced subgrade for railways (RSR) is a construction method in which reinforced subgrade is constructed first and a rigid facing wall later to minimize the residual settlement after the service of a roadbed. The RSR was designed and constructed at Osong railway test line in Korea. In this study, load sharing capacities from the reinforced subgrade to the rigid facing wall of it were evaluated through long-term measurement, extending 22 months from the start of roadbed construction to the completion of track construction. Under the condition of 0.4 m geogrid vertical spacing installation, the load sharing proportion of horizontal earth pressure of the rigid facing wall was 9%–22% in the lower part, and lesser in the upper part. The strain of geogrid during construction was 0.607%, which was relatively lower than the designed geogrid tensile strain of 5%. The change in geogrid strain after construction was closely correlated with temperature change in the soil.     

Numerical study of the impact of climate conditions on stability of geocomposite and geogrid reinforced soil walls

Geosynthetic-reinforced soil (GRS) walls using marginal soils can operate under unsaturated conditions depending on climate conditions and drainage inside the reinforced zone. Geocomposite reinforcements have been suggested to act as internal drainage layers, but their hydraulic behavior can also be strongly affected by climate conditions. Numerical analyses were conducted to observe the impact of four distinct tropical climate conditions (arid, semi-arid, humid subtropical and humid tropical) on suction profiles and stability of reinforced soil walls constructed using geogrid and geocomposite reinforcements. The climate simulation involved the incorporation of a soil-atmosphere interaction on water balance and on the unsaturated transient infiltration. Results indicate the GRS walls can operate under relatively high suction levels under arid climates in which cumulative evaporation overcomes infiltration. Any climate that has rainy seasons with consecutive rainfalls with intensities close to the infiltration capacity of soil and/or monthly cumulative precipitation higher than 200 mm/day led to critical conditions in terms of soil water saturation and stability. Under unsaturated conditions of soil, the drainage effectiveness of geocomposites is significantly reduced and adverse capillary break effects become critical.