Bearing capacity of geogrid reinforced sand over encased stone columns in soft clay

Stone columns develop their load carrying capacity from the circumferential confinement provided by the surrounding soils. In very soft soils, the circumferential confinement offered by the surrounding soft soil may not be sufficient to develop the required load carrying capacity. Hence a vertical confinement would yield a better result. The load carrying capacity is further increased with the addition of a sand bed over the stone columns. In the present study, a series of laboratory model tests on an unreinforced sand bed (USB) and a geogrid-reinforced sand bed (GRSB) placed over a group of vertically encased stone columns (VESC) floating in soft clay and their numerical simulations were conducted. Three-dimensional numerical simulations were performed using a finite element package ABAQUS 6.12. In the finite element analysis, geogrid and geotextile were modeled as an elasto-plastic material. As compared to unreinforced clay bed, an 8.45 fold increase in bearing capacity was observed with the provision of a GRSB over VESC. The optimum thickness of USB and GRSB was found to be 0.2 times and 0.15 times the diameter of the footing. A considerable decrease in bulging of columns was also noticed with the provision of a GRSB over VESC. Both the improvement factor and stress concentration ratio of VESC with GRSB showed an increasing trend with an increase in the settlement. It was observed that the optimum length of stone columns and the optimum depth of encasement of the group of floating VESC with GRSB are 6 times and about 3 times the diameter of the column respectively.     

Effect of aspect ratio of footing on behavior of two closely-spaced footings on geogrid-reinforced sand

This paper presents the results of laboratory model tests carried out on two closely-spaced interfering footings resting on the surface of geogrid-reinforced and unreinforced sand bed. The effect of aspect ratio (or shape) of the footing on interference behavior is studied by adopting three pairs of model footings of different sizes. The length (L) to width (B) ratio (i.e., aspect ratio) of the footings is varied from 1.0 to 2.0. The effects of single layer of geogrid on footing interference and bearing capacity improvement are investigated. The optimum depth of the geogrid layer for both interfering and isolated footings is found to be one-third of the footing width and it is not dependent on the aspect ratio of the footing. The optimum spacing between the interfering footings is found to be 1.5 times the width of the footing. Lower efficiency factor is observed for interfering footings resting on the reinforced sand compared to the unreinforced sand. Higher bearing capacity ratio (BCR) is observed for isolated footing than that of interfering footings when BCR is measured based on ultimate bearing capacity values of reinforced and unreinforced cases and BCR value increases as the aspect ratio of the footing increases.     

Bearing capacity of circular footing on geocell–sand mattress overlying clay bed with void

The potential benefits of providing geocell reinforced sand mattress over clay subgrade with void have been investigated through a series of laboratory scale model tests. The parameters varied in the test programme include, thickness of unreinforced sand layer above clay bed, width and height of geocell mattress, relative density of the sand fill in the geocells, and influence of an additional layer of planar geogrid placed at the base of the geocell mattress. The test results indicate that substantial improvement in performance can be obtained with the provision of geocell mattress, of adequate size, over the clay subgrade with void. In order to have beneficial effect, the geocell mattress must spread beyond the void at least a distance equal to the diameter of the void. The influence of the void over the performance of the footing reduces for height of geocell mattress greater than 1.8 times the diameter of the footing. Better improvement in performance is obtained for geocells filled with dense soil.     

Cyclic response of footing on geogrid-reinforced sand with void

This paper describes laboratory tests on footing constructed on unreinforced and geogrid-reinforced sand with circular a void subjected to a combination of static and repeated loads. The settlement of the footing was measured for up to 5000 cycles of loading and unloading. The variables examined in the testing program include the number of geogrid layers, the location of the void within the soil, the amplitude of cyclic load, and the number of load cycles. The results show that the footing performance due to cyclic loading is better for thicker geogrid reinforced sand with a void than for unreinforced sand with no void. In addition, a critical region was found to exist under the footing, under which a void results in increased footing settlement. Overall, the results indicate that the reinforced soil-footing systems with sufficient geogrid-reinforcement and sufficient void embedment depth behave much more stiffly and are thus capable of handling greater loads with lower settlement than those in unreinforced soil without a void. The undesirable effect of the void on the footing behavior can be eliminated. In addition, the results show that the values of footing settlement increase rapidly during the initial loading cycles; thereafter the rate of settlement is reduced significantly as the number of loading cycles increases.     

Load eccentricity effects on behavior of circular footings reinforced with geogrid sheets

In this paper,an experimental study for an eccentrically loaded circular footing,resting on a geogrid reinforced sand bed,is performed.To achieve this aim,the steel model footing of 120 mm in diameter and sand in relative density of 60%are used.Also,the effects of depth of first and second geogrid layers and number of reinforcement layers(1-4) on the settlement-load response and tilt of footing under various load eccentricities(0 cm,0.75 cm,1.5 cm,2.25 cm and 3 cm) are investigated.Test results indicate that ultimate bearing capacity increases in comparison with unreinforced condition.It is observed that when the reinforcements are placed in the optimum embedment depth(u/D = 0.42 and h/D = 0.42),the bearing capacity ratio(BCR) increases with increasing load eccentricity to the core boundary of footing,and that with further increase of load eccentricity,the BCR decreases.Besides,the tilt of footing increases linearly with increasing settlement.Finally,by reinforcing the sand bed,the tilt of footing decreases at 2layers of reinforcement and then increases by increasing the number of reinforcement layers.     

Compressive load response of geogrid-reinforced fine,medium and coarse sands

This paper presents data obtained from a series of laboratory plate load tests performed on geogrid-reinforced sand beds. Fine, medium and coarse sands were used as test sand beds. Circular geogrids of diameter 120 mm were used as reinforcement layers. Improvement in load–settlement response was studied. The test sand beds were compacted to a relative density of 50%. A surface footing plate of diameter 60 mm was used as the shallow foundation. It was found that the horizontal geogrid reinforcement improved the load–settlement response. The applied load for a deformation of 0.5 mm in the case of geogrid-reinforced fine sand, medium sand and coarse sand was, respectively, 83 N, 44 N and 87 N, whereas it was 63 N, 38 N and 47 N when the sands were unreinforced. Load–settlement response was found to improve further with increase in number of geogrid layers and with decrease in spacing between them. Load improvement ratio (LIR), defined as the ratio of load for a given settlement in geogrid-reinforced case to that for the same settlement in unreinforced case, increased with number of geogrids (n) for all sands, but the improvement was significant in the case of coarse sand.     

Load-settlement response of shallow square footings on geogrid-reinforced sand under cyclic loading

To study the settlement and dynamic response characteristics of shallow square footings on geogrid-reinforced sand under cyclic loading, 7 sets of large scale laboratory tests are performed on a 0.5?m wide square footing resting on unreinforced and geogrid reinforced sand contained in a 3?m?×?1.6?m?×?2?m (length?×?width?×?height) steel tank. Different reinforcing schemes are considered in the tests: one layer of reinforcement at the depth of 0.3B, 0.6B and 0.9B, where B is the width of the footing; two and three layers of reinforcement at the depth and spacing both at 0.3B. In one of the two double layered reinforcing systems, the reinforcements are wrapped around at the ends. The footings are loaded to 160?kPa under static loading before applying cyclic loading. The cyclic loadings are applied at 40?kPa amplitude increments. Each loading stage lasts for 10?min at the frequency of 2?Hz, or until failure, whichever occurs first. The settlement of the footing, strain in the reinforcement and acceleration rate in the soil have been monitored during the tests. The results showed that the ultimate bearing capacity of the footings was affected by the number and layout of the reinforcements, and the increment of bearing capacity does not always increase with the number of reinforcement layers. The layout of the reinforcement layers affected the failure mechanisms of the footings. Including more layers of reinforcement could greatly reduce the dynamic response of the foundations under cyclic loading. In terms of bearing capacity improvement, including one layer of reinforcement at the depth of 0.6B was the optimum based on the test results. It is found that fracture of geogrid could occur under cyclic loading if the reinforcement is too shallow, i.e. for the cases with the first layer of reinforcement at 0.3B depth.     

Behaviour of prestressed geotextile-reinforced sand bed supporting a loaded circular footing

In the past, the beneficial effects of prestressing the geosynthetic in reinforced soil foundations have been studied mathematically. It is timely to experimentally investigate the degree of improvement generated by prestressing the geosynthetic layer for several embedment depths of a footing resting on a reinforced sand bed. Therefore, laboratory physical model tests and finite element analyses were conducted to study the behaviour of prestressed geotextile-reinforced sand bed supporting a loaded circular footing. The addition of prestress to the geotextile reinforcement results in significant improvement to the settlement response and the load-bearing capacity of the foundation. For a surface footing, the load-carrying capacity at 5 mm settlement for the prestressed case (with prestress equal to 2% of the allowable tensile strength of the geotextile) is approximately double that of the geotextile-reinforced sand without prestress. The beneficial effects of the prestressed geotextile configuration were evident for greater footing depths, in comparison with unreinforced and reinforced (without prestress) counterparts. Experimental and numerical results were also used to validate a few empirical relationships, which are commonly used for solving soil-structure interaction problems. The results obtained from finite element analysis using the program, PLAXIS are generally found to be in reasonabaly good agreement with experimental results.     

Comparative Study on the Behavior of Encased Stone Column and Conventional Stone Column

Stone columns, one of the most commonly used soil improvement techniques, have been utilized worldwide to increase bearing capacity and reduce total and differential settlements of structures constructed on soft clay. Stone columns also act as vertical drains, thus speeding up the process of consolidation. However, the settlement of stabilised bed is not reduced in many situations for want of adequate lateral restraint. Encasing the stone column with a geogrid enhances the bearing capacity and reduces the settlement drastically without compromising its effect as a drain, unlike a pile. The behavior of the encased stone column stabilized bed is experimentally investigated and analysed numerically. In the numerical analysis, material behaviour is simulated using Soft Soil, Mohr Coulomb and Geogrid models for clay, stone material and encasement respectively and is validated with experimental results. The parametric study carried out on varying the L/D ratio (L = length of the column; D = diameter of the column) of column, stiffness of geogrid and angle of internal friction of stone material gives a better understanding of the physical performance of the encased stone column stabilized clay bed.     

Bearing capacity of strip footings on sand slopes reinforced with geogrid and grid-anchor

This paper presents the effect of a new type of geogrid inclusion on the bearing capacity of a rigid strip footing constructed on a sand slope. A broad series of conditions, including unreinforced cases, was tested by varying parameters such as geogrid type, number of geogrid layers, vertical spacing and depth to topmost layer of geogrid. The results were then analyzed to find both qualitative and quantitative relationships between the bearing capacity and the geogrid parameters. A series of finite element analyses was additionally carried out on a prototype slope and the results were compared with the findings from the laboratory model tests and to complete the results of the model tests. The results show that the bearing capacity of rigid strip footings on sloping ground can be intensively increased by the inclusion of grid-anchor layers in the ground, and that the magnitude of bearing capacity increase depends greatly on the geogrid distribution. It is also shown that the load-settlement behavior and bearing capacity of the rigid footing can be considerably improved by the inclusion of a reinforcing layer at the appropriate location in the fill slope. The agreement between observed and computed results is found to be reasonably good in terms of load-settlement behavior and optimum parameters.     

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.     

Laboratory investigation of bearing capacity behaviour of strip footing on reinforced flyash slope

The paper presents the results of laboratory model tests on bearing capacity behaviour of a strip footing resting on the top of a geogrid reinforced flyash slope. A series of model footing tests covering a wide range of boundary conditions, including unreinforced cases were conducted by varying parameters such as location and depth of embedment of single geogrid layer, number of geogrid layers, location of footing relative to the slope crest, slope angles and width of footing. The results of the investigation indicate that both the pressure–settlement behaviour and the ultimate bearing capacity of footing resting on the top of a flyash slope can be enhanced by the presence of reinforcing layers. However the efficiency of flyash geogrid system increases with the increasing number of geogrid layers and edge distance of footing from the slope. Based on experimental results critical values of geogrid parameters for maximum reinforcing effects are established. Experimental results obtained from a series of model tests have been presented and discussed in the paper.     

Experimental study on settlement and scour characteristics of artificial reef with different reinforcement type and soil type

An artificial reef has been actively installed to restore marine ecosystems and increase fish catching. The artificial reef which installed on soft ground loses its function due to the settlement and scour of seabed. This study performed a series of laboratory tests to investigate settlement and scour characteristics of seabed according to different reinforcement type, reinforced area and soil type. Two reinforcement types with different reinforced area were applied to reduce settlement and scour of ground: geogrid and geogrid-bamboo mat. Soil types of ground are clay, silt and sand deposits. A series of laboratory tests includes California bearing ratio test, large size settlement test, and two-dimensional wave channel test. The test results indicated that the reinforced artificial reef had less settlement and scour depth than the unreinforced artificial reef. Especially, the artificial reef reinforced with geogrid-bamboo mat had more improved stability than that with geogrid due to high bending stiffness of bamboo mat.     

Variable-diameter deep mixing column for multi-layered soft ground improvement: Laboratory modeling and field application

Deep mixing columns are commonly employed for soft ground improvement. However, the diameter of a single conventional column is a constant and the area replacement ratio does not vary with depth. Hence, the conventional column is not the ideal solution for multi-layered soft grounds, where different layers have remarkably different soil properties. Accordingly, this study proposes a better solution, which is the variable-diameter deep mixing column with a large diameter in the soil layer having high compressibility and a small diameter in the soil layer having relatively low compressibility. In this study, small-scale laboratory model tests were firstly employed to compare the performance of two-layered soft grounds improved by a variable-diameter column and a conventional column. The additional vertical stress in the soil and the column, the excess pore water pressure, and the ground settlement were analyzed. Then, a field application of variable-diameter columns for multi-layered soft ground improvement was presented; the design considerations, column installation, and monitored settlement were introduced and analyzed. The results indicated that the additional stress in the soil and the column in the highly compressible soil layer were much lower in the variable-diameter column-improved ground than in the conventional column-improved ground. Consequently, the variable-diameter column-improved ground yielded less total settlement and less post-construction settlement compared to the conventional column-improved ground.     

Influence of relative density of soil on performance of fiber-reinforced soil foundations

An experimental study has been carried out for studying the influence of combinations of relative densities of two layered soil system. The model tests have been performed for the case of circular and ring footings resting on randomly distributed fiber reinforced sand (RDFS) layer overlying unreinforced sand bed. The influence of relative density on, different type of footings i.e. circular and ring (r_i/r_o = 0.3, 0.4, 0.5, 0.6) footings; percentages of fiber in RDFS layer i.e. 0.5%, 0.75%, 1.00%, and 1.25%; and thickness of RDFS layer i.e. 0.5B, 0.75B, and 1.00B have been studied. Results have indicated that relative density, of both the RDFS layer as well as the bottom unreinforced sand layer, significantly influences the ultimate bearing capacity as well as the settlement. Improvement in terms of bearing capacity ratio (BCR) is more when top RDFS layer is compacted at 70% relative density with bottom unreinforced sand having 30% relative density. Moreover, in terms of settlement reduction, maximum improvement is observed when both the layers were compacted at 70% relative density.     

Influence of geosynthetic stiffness on bearing capacity of strip footings seated on thin reinforced granular layers over undrained soft clay

Thin granular fill layers are routinely used to aid the construction of shallow footings seated over undrained soft clay foundations and to increase their load capacity. The influence of time- and strain-dependent reduction in reinforcement stiffness on the bearing capacity and load-settlement response of a footing seated on a thin reinforced granular fill layer over undrained soft clay foundations is examined in this paper using finite-difference method (FDM) numerical models. The time- and strain-dependent stiffness of the reinforcement described by a two-component hyperbolic isochronous tensile load-strain model is shown to influence the bearing capacity and load-settlement response of the reinforced granular base scenario. The additional benefit of a reinforced granular layer diminishes as the time-dependent stiffness of the geosynthetic reinforcement increases. An analytical solution for the ultimate bearing capacity of strip footings seated on thin unreinforced and reinforced granular layers over undrained clay is proposed in this study. The main practical outcome from this study are tables of bearing capacity factors to be used with the analytical solution. The bearing capacity factors were back-calculated from the numerical analyses and account for the influence of rate-dependent properties of geogrid reinforcement materials and clay foundations with soft to very soft undrained shear strength.     

Numerical simulation and sensitivity analyses of full-scale test embankment with reinforced lightweight geomaterials on soft Bangkok clay

A full-scale test embankment was constructed on soft Bangkok clay using rubber tire chip–sand mixture as a lightweight geomaterial reinforced with geogrid under working stress conditions. The facing of the embankment was made of segmental concrete blocks with rock filled gabion boxes as the facing to the sloping sides. This paper attempts to simulate the behavior of the full-scale test embankment using PLAXIS finite element 2D program by means of undrained analysis in the construction stage and thereafter consolidation analysis was performed during the service stage. The settlement predictions of the soft clay foundation mostly depended on the assumed thickness of the weathered crust and the OCR values of the soft clay layer. The predicted excess pore water pressures were sensitive to the OCR values of the soft clay layer. The lateral wall movements were overpredicted by the analysis due to the partially drained consolidation process at the early stage of the construction. The FEM computed geogrid movements were smaller than the observed field data due to the use of lightweight tire chips-sand backfill. The maximum tension line agreed reasonably well with the coherent gravity bilinear failure plane. The sensitivity analyses of settlements, excess pore water pressures, lateral wall movements, geogrid movements and tensions in geogrid were performed by varying the weathered crust thickness, the OCR values of soft clay, the permeability values of the soft clay and the interface coefficient of the geogrid. The settlements and the excess pore water pressures changed significantly when the OCR and the permeability values of soft clay were varied. The interface coefficient of the geogrid reinforcements affected the lateral wall movements, geogrid movements and tensions in the geogrids. The higher interface coefficient yielded less wall/geogrid movement and resulted in higher tensions in geogrids as expected. The results of analyses show that the FEM analysis using 2D plane strain conditions provided satisfactory predictions for the field performance.     

Visualisation and quantification of geogrid reinforcing effects under strip footing loads using discrete element method

Geogrids have been commonly used in reinforced soil structures to improve their performance. To investigate the geogrid reinforcement mechanisms, discrete element modelling of unreinforced and geogrid reinforced soil foundations and slopes was conducted under surface strip footing loads in this study. For unreinforced and reinforced soil foundations, the numerically obtained footing pressure-settlement relationships were validated by experimental results from the literature. In the numerical modelling of unreinforced and reinforced soil slopes, identical models and micro input parameters to those used in the numerical modelling of unreinforced and reinforced soil foundations were used. The geogrid reinforcing effects under strip footing loads were visualised by the qualitative contact force distributions in the soil structures, as well as the qualitative and quantitative tensile force distributions along the geogrids. In addition, the qualitative displacement distributions of soil particles in the soil structures and the quantitative vertical displacement distributions along soil layers/geogrids also indicated the geogrid reinforcing effects in such practical reinforced soil structures. The discrete element modelling results visualise and quantify the load transfer and spreading behavior in geogrid reinforced soil structures, and it provides researchers with an improved understanding of geogrid reinforcing effects at microscopic scale under strip footing loads.     

Effect of footing shape and load eccentricity on behavior of geosynthetic reinforced sand bed

This paper presents the results from a laboratory modeling tests and numerical studies carried out on circular and square footings assuming the same plan area that rests on geosynthetic reinforced sand bed. The effects of the depth of the first and second layers of reinforcement, number of reinforcement layers on bearing capacity of the footings in central and eccentral loadings are investigated. The results indicated that in unreinforced condition, the ultimate bearing capacity is almost equal for both of the footings; but with reinforcing and increasing the number of reinforcement layers the ultimate bearing capacity of circular footing increased in a higher rate compared to square footing in both central and eccentrial loadings. The beneficial effect of a geosynthetic inclusion is largely dependent on the shape of footings. Also, by increasing the number of reinforcement layers, the tilt of circular footing decreased more than square footing. The SR (settlement reduction) of the reinforced condition shows that settlement at ultimate bearing capacity is heavily dependent on load eccentricity and is not significantly different from that for the unreinforced one. Also, close match between the experimental and numerical load-settlement curves and trend lines shown that the modeling approach utilized in this study can be reasonably adapted for reinforced soil applications.     

Strip footing on geocell reinforced sand beds with additional planar reinforcement

The results from laboratory model tests on strip footings supported by geocell reinforced sand beds with additional planar reinforcement are presented. The test results show that a layer of planar geogrid placed at the base of the geocell mattress further enhances the performance of the footing in terms of the load-carrying capacity and the stability against rotation. The beneficial effect of this planar reinforcement layer becomes negligible at large heights of geocell mattress.