Basal Geogrid Research Articles

Geocell Anchor Cage for enhanced load support in soil structures

Geocells are established as effective load support systems in many soil structures including foundations, retaining walls and embankments. This manuscript proposes a structural modification to geocell reinforcement through a newly developed Geocell Anchor Cage (GAC) system for improved load bearing capacity and verifies its effectiveness in geocell reinforced sand beds. The GAC consists of a polymeric basal geogrid with several anchor pins, each of them positioned at the centre of a geocell pocket. Plate load tests were carried out on sand beds with a geocell mattress, and a 3D printed polymeric GAC positioned either above or below the mattress. The pressures within the geocell pockets and strains in geocell walls were monitored continuously. With the inclusion of GAC, the performance of geocell reinforced bed is significantly improved in terms of increased load-carrying capacity and reduced footing settlements. The load carrying capacity of a sand bed reinforced with a geocell mattress of width equal to three times the width of the loading plate and a GAC was found to be equal to the load carrying capacity of the bed with a geocell mattress of width equal to four times the width of the plate without a GAC. With the addition of GAC at the bottom, the settlements of the reinforced sand beds reduced by 38%. The measured pressures and strains provided important clues to the internal mechanisms responsible for the beneficial effects of GAC. Hence the GAC system helps in achieving higher load carrying capacities in soil structures reinforced by geocells with lesser additional cost and reduced space requirements.

Geosynthetic encased column-supported embankment: behavior with and without basal geogrid

Centrifuge model tests were performed to investigate the beneficial effect of the basal geogrid in improving the performance of geosynthetic encased column (GEC)-supported embankments. The digital image correlation (DIC) technique was adopted to understand the deformation behavior of GEC-supported embankment models. The mobilized tensile strain at various locations on the model basal geogrid was quantified using strain gauge-based instrumentation. The results from the centrifuge test indicate that vertical and lateral displacement of the GECs and surrounding soil has been significantly reduced with the inclusion of basal geogrid in the embankment. Moreover, the differential settlement between the GECs and surrounding soil due to embankment loading were significantly reduced with the inclusion of basal geogrid, thereby the distress in the embankment resting on soft foundation can be avoided. The basal geogrid in the embankment has enhanced the stability of the embankment and improved the load transfer between the GECs and surrounding soil. The maximum mobilized tensile strain in the basal geogrid occurred near the shoulder of the embankment, which indicates the resistance offered by the basal geogrid in preventing the differential settlement between the GECs and surrounding soil.

Stress Distribution in Basal Geogrid Reinforced Pile-Supported Embankments Under Seismic Loads

Basal geosynthetic reinforced pile-supported embankments are proven as the more appropriate ground improvement technique for constructing embankments for roads over very soft clay deposits and approach roads or embankments to bridges. Numerous experimental and analytical works are available on the soil arching phenomenon of geosynthetic reinforced piled embankments subjected to static loading conditions. This study attempts to evaluate the stress distribution and soil arching in geosynthetic reinforced pile-supported embankments subjected to seismic excitations. Time-history analysis has been performed on the basal geogrid reinforced pile-supported embankments by varying the height of embankment and tensile modulus of geogrid. Analyses of results show that for α (the ratio of height of embankment to pile centre to centre spacing) less than or equal to 4.5, a geogrid tensile modulus of 3000 kN/m is sufficient to withstand vertical stresses due to earthquakes. And for the considered embankment height and pile diameter when α nearly equal to 4.5, differential settlements are very less irrespective of seismic excitations.

Deformation and Stability Analysis of Embankment over Stone Column-Strengthened Soft Ground

Compacted granular columns are commonly used to support embankments over soft soils. Using a reinforcement layer under the embankment causes the total stress to be further transferred to the column rather than soft soil, thus reducing total deformations of the subsoil. In this paper two dimensional (2D) numerical analysis was used to study the influence of stone columns and basal geosynthetic on deformations and stability of an embankment over soft deposit by means of Plaxis 2D finite element code. Unit cell to plane strain conversion approach was applied to transform columns into equivalent walls thus allowing to simulate a full embankment over a group of columns. Comprehensive parametric analysis was then performed to investigate the role of different critical parameters on embankment behavior. Results showed that the use of stone columns yielded the total deformations of the subsoil to significantly reduce, while its influence was less remarkable as a high stiffness geogrid was placed under the embankment. It was also found that the stone column length was the most influential parameter on the embankment total deformations, so that increasing columns length from 0.25Hs to 0.75Hs reduced the vertical and horizontal deformations by about two and five times, respectively. In addition, the use of a high stiffness basal geogrid caused the stability of the embankment to remarkably improve as the value of safety factor at the end of construction increased from 1.25 to about 1.9.

Effect of Column Configuration on the Performance of Encased Stone Columns with Basal Geogrid Installed in Lithomargic Clay

Lithomargic clay is extensively found along the Konkan belt in peninsular India and serves as a foundation for most of the structures. The reduction in strength under saturated conditions makes this soil problematic causing a lot of engineering problems such as uneven settlements, erosion, slope failures, and foundation problems. This paper presents the effect of column configuration (i.e. equivalent number of columns with reduced diameter for the same surface area) on the performance of lithomargic clay reinforced with geogrid encased stone columns and basal geogrid layer. The investigations were performed both experimentally through small-scale models and through finite element analyses. The results were compared with the performance of lithomargic clay reinforced with ordinary and encased stone columns. A single geogrid encased stone column with a basal geogrid layer improved the load-carrying capacity of lithomargic clay by 180% while the percentage of increment in the case of a group of three geogrid encased stone column with basal geogrid layer having the same surface area was 210%. It was also observed that the geogrid encasement of stone columns reduced the maximum column bulging by 38%, whereas geogrid encased stone columns along with basal geogrid layer reduced the bulging by 82% compared to ordinary stone columns.

A comparative study for the performance of encased granular columns

This paper describes a three-dimensional (3D) numerical analysis of a test embankment on geotextile-encased columns (GECs), in comparison with two-dimensional (2D) axisymmetric and plane strain analyses. The 3D numerical analysis was performed considering a rectangular strip under the embankment centerline. The 2D analysis was also carried out using axisymmetric unit cell and plane strain approaches. Numerical results indicated that the adopted 3D strip model represented well the measured deformations and pore pressure evolution during embankment construction and post-contraction periods. Unlike the unit cell model, both plane strain and 3D analyses could properly determine the settlement profile along the embankment base as well as the profile of the horizontal soil deformation beneath the embankment toes. The plane strain analysis, however, was not able to compute the geotextile ring force which might be simply calculated using the axisymmetric unit cell approach. The paper also showed that, due to horizontal boundary fixities applied to the embankment borders, the unit cell model clearly underestimated the tension developed in the basal geogrid.

Ground improvement of soft soil by geotextile-encased columns

This paper compares the field response of two test embankments constructed on similar soft soil deposits. The un-strengthened foundation had vertical drains underneath the embankment and the strengthened foundation had geotextile-encased granular columns, both provided with basal geogrid reinforcement. The effectiveness of the encased granular columns was assessed by comparing the settlement below the embankments, the horizontal displacement of the soft foundation and the excess pore pressures in the middle of the soft clay layer. The results showed that the granular encased columns significantly reduced the maximum settlement occurring below the embankment as well as the maximum horizontal displacement in the clayey foundation. At the same settlement level, the load-carrying capacity of the strengthened foundation was found to increase by as much as around 2·5 times that of the un-strengthened one. It was also observed that the geotextile-encased granular columns caused a noticeable reduction in the maximum excess pore pressure following load application. Furthermore, the degree of consolidation achieved at the end of embankment construction was quite high when encased granular columns were used.

Behaviour of geocell reinforced soft clay bed subjected to incremental cyclic loading

The paper deals with the results of the laboratory cyclic plate load tests performed on the reinforced soft clay beds. The performances of the clay bed reinforced with geocells and geocells with additional basal geogrid cases are compared with the performance of the unreinforced clay beds. From the cyclic plate load test results, the coefficient of elastic uniform compression (C-u) was calculated for the different cases. The Cu value was found to increase in the presence of geocell reinforcement. The maximum increase in the Cu value was observed in the case of the clay bed reinforced with the combination of geocell and geogrid. In addition, 3 times increase in the strain modulus, 10 times increase in the bearing capacity, 8 times increase in the stiffness and 90% reduction in the settlement was observed in the presence of the geocell and geogrid. Based on the laboratory test results, a hypothetical case of a prototype foundation subjected to cyclic load was analyzed. The results revealed that the natural frequency of the foundation-soil system increases by 4 times and the amplitude of the vibration reduces by 92% in the presence of the geocells and the geogrids.

Three-dimensional numerical analysis of geocell-reinforced soft clay beds by considering the actual geometry of geocell pockets

Due to its complex honeycomb structure, the numerical modeling of the geocell has always been a big challenge. Generally, the equivalent composite approach is used to model the geocells. In the equivalent composite approach, the geocell–soil composite is treated as the soil layer with improved strength and stiffness values. Though this approach is very simple, it is unrealistic to model the geocells as the soil layer. This paper presents a more realistic approach of modeling the geocells in three-dimensional (3D) framework by considering the actual curvature of the geocell pocket. A square footing resting on geocell reinforced soft clay bed was modeled using the “fast Lagrangian analysis of continua in 3D” (FLAC3D) finite difference package. Three different material models, namely modified Cam-clay, Mohr–Coulomb, and linear elastic were used to simulate the behaviour of foundation soil, infill soil and the geocell, respectively. It was found that the geocells distribute the load laterally to the wider area below the footing as compared to the unreinforced case. More than 50% reduction in the stress was observed in the clay bed in the presence of geocells. In addition to geocells, two other cases, namely, only geogrid and geocell with additional basal geogrid cases were also simulated. The numerical model was systematically validated with the results of the physical model tests. Using the validated numerical model, parametric studies were conducted to evaluate the influence of various geocell properties on the performance of reinforced clay beds.

Experimental and numerical studies on protection of buried pipelines and underground utilities using geocells

This paper presents the results of the laboratory model tests and the numerical studies conducted on small diameter PVC pipes, buried in geocell reinforced sand beds. The aim of the study was to evaluate the suitability of the geocell reinforcement in protecting the underground utilities and buried pipelines. In addition to geocells, the efficacy of only geogrid and geocell with additional basal geogrid cases were also studied. A PVC (Poly Vinyl Chloride) pipe with external diameter 75 mm and thickness 1.4 mm was used in the experiments. The vehicle tire contact pressure was simulated by applying the pressure on the top of the bed with the help of a steel plate. Results suggest that the use of geocells with additional basal geogrid considerably reduces the deformation of the pipe as compared to other types of reinforcements. Further, the depth of placement of pipe was also varied between 1B to 2B (B is the width of loading plate) below the plate in the presence of geocell with additional basal geogrid. More than 50% reduction in the pressure and more than 40% reduction in the strain values were observed in the presence of reinforcements at different depths as compared to the unreinforced beds. Conversely, the performance of the subgrade soil was also found to be marginally influenced by the position of the pipe, even in the presence of the relatively stiff reinforcement system. Further, experimental results were validated with 3-dimensional numerical studies using FLAC3D (Fast Lagrangian Analysis of Continua in 3D). A good agreement in the measured pipe stain values were observed between the experimental and numerical studies. Numerical studies revealed that the geocells distribute the stresses in the lateral direction and thus reduce the pressure on the pipe. In addition, the results of the 1-g model tests were scaled up to the prototype case of the shallow buried pipeline below the pavement using the appropriate scaling laws.

3-Dimensional numerical modelling of geocell reinforced sand beds

Numerical modelling of the geocell has been always a big challenge due to its complex honeycomb structure. Generally, the equivalent composite approach is adopted to model the geocells. In equivalent composite approach, the geocell-soil composite is treated as the soil layer with improved strength and stiffness values. Though this approach is very simple, it is unrealistic to model geocells as the soil layer. This paper presents a more realistic modelling approach to model geocells in 3-dimensional (3D) framework. Numerical simulations have been carried by forming the actual 3D honeycomb shape of the geocells using the finite difference package FLAC3D (Fast Lagrangian Analysis of Continua in 3D). Geocells are modelled using the geogrid structural element available in the FLAC3D with the inclusion of the interface element. In addition to the modelling of geocells, other two cases, namely, only geogrid and geocell with additional basal geogrid cases were also modelled. It was found that the geocells distribute the load laterally and to a relatively shallow depth as compared to unreinforced case and the geogrid reinforced case. The numerical model was also validated with the experimental studies and the results are found to be in good agreement with each other. The validated numerical model was used to study the influence of various properties of the geocells on the performance of the reinforced foundation beds. The performance of the foundation bed was directly influenced by the modulus and the height of the geocells. Similarly, the pocket size of the geocell inversely affected the performance of the reinforced beds. The geocell with textured surface yielded better performance than the geocell with smooth surface.

Model studies on circular footing supported on geocell reinforced sand underlain by soft clay

The effectiveness of geocell reinforcement placed in the granular fill overlying soft clay beds has been studied by small-scale model tests in the laboratory. The test beds were subjected to monotonic loading by a rigid circular footing. Footing load, footing settlement and deformations on the fill surface were measured during the tests. The influence of width and height of geocell mattress as well as that of a planar geogrid layer at the base of the geocell mattress on the overall performance of the system has been systematically studied through a series of tests. The test results indicate that with the provision of geocell reinforcement in the overlying sand layer, a substantial performance improvement can be obtained in terms of increase in the load carrying capacity and reduction in surface heaving of the foundation bed. An additional layer of geogrid placed at the base of the geocell mattress further enhances the load carrying capacity and stiffness of the foundation bed. Its beneficial effect decreases with the increase in the height of the geocell mattress. A seven-fold increase in the bearing capacity of the circular footing can be obtained by providing geocell reinforcement along with a basal geogrid layer in the sand bed underlying soft clay.