Laboratory and numerical modeling of strip footing on geotextile-reinforced sand with cement-treated interface

This paper presents the results of a laboratory and numerical study on the effects of cement treatment of the interface between geotextile and sand on the bearing capacity of a foundation built on geotextile-reinforced sand. The bearing capacity of a 25 cm × 7.5 cm strip footing on a 90 cm × 25 cm × 30 cm sand box reinforced using a single-layer reinforcement of different lengths including, 20, 30, 45, 60, 75 and 90 cm, was studied in a laboratory. A cement-treated zone was created on the geotextile to improve the friction and adhesion of the interface zone. Tests were also conducted on reinforced soil without a cement-treated zone and the results were compared. A finite element model was calibrated and used for further studies. The results of the laboratory tests indicated that cement treatment of the interface between the geotextile and sand increases the bearing capacity of the foundation by 6%–17%, depending on the length of the reinforcement. The effectiveness of the cement-treated interface on improving of the bearing capacity is more evident with shorter-length reinforcements. For a certain bearing capacity, the required length of the reinforcement was reduced by approximately 40% when the interface zone of the sand and reinforcement was cement-treated. The effect of the cement-treated zone on the bearing capacity was more evident in low settlement levels, and decreased as the length of the reinforcement increased.     

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.     

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.     

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.     

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.     

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.     

Behavior of strip footing on fiber-reinforced cemented sand adjacent to sheet pile wall

In urban areas, shallow foundations are often placed along the ground surface above a sheet pile wall. In this research, the potential benefits of reinforcing the active zone behind a model sheet pile wall by using polypropylene fiber and cement kiln dust have been investigated experimentally and numerically. Tests were conducted by varying parameters including fiber ratio (R_F), cement kiln dust (CKD) ratio, thickness of reinforced layer, footing location relative to the sheet pile wall and curing time of reinforced layer. Finite element computer code PLAXIS 2D foundation was used for numerical modeling. Close agreement between the experimental and numerical results was observed (maximum difference 14%). Experimental and numerical results clearly show that fiber insertion into the cemented soil causes an increase in ultimate bearing capacity of footing and significant reduction in the lateral deflection of the sheet pile wall. At higher fiber ratios (R_F ≥ 0.75%), the bearing capacity ratio (BCR) increased by about 42% and the effect of CKD ratio on BCR is more pronounced. The addition of fibers changed the brittle behavior of cemented sand to a more ductile one. Critical values of reinforcing parameters for maximum reinforcing effects are established.     

Comparison of bearing capacity of a strip footing on sand with geocell and with planar forms of geotextile reinforcement

Comprehensive results from laboratory model tests on strip footings supported on the geocell and planar reinforced sand beds with the same characteristics of geotextile are presented. The various parameters studied in this testing program include the reinforcement width, the number of planar layers of geotextile and height of the geocell below the footing base. Contrary to other researches, the performance of the geocell and planar reinforcement is investigated at the range of low to medium settlement level, similar to those of interest in practice. The results show that the efficiency of reinforcement was decreased by increasing the number of the planar reinforcement layers, the height of the geocell reinforcement and the reinforcement width. For the same mass of geotextile material used in the tests at the settlement level of 4%, the maximum improvement in bearing capacity (IF) and percentage reduction in footing settlement (PRS) were obtained as 2.73 and 63% with the provision of geocell, respectively, while these values compare with 1.88 and 47% for the equivalent planar reinforcement. On the whole, the results indicate that, for the same quantity of geotextile material, the geocell reinforcement system behaves much stiffer and carries greater loading and settles less than does the equivalent planar reinforcement system. Therefore, a specified improvement in bearing pressure and footing settlement can be achieved using a lesser quantity of geocell material compared to planar geotextile.     

Influence of wire mesh characteristics on reinforced soil model wall failure mechanisms-physical and numerical modelling

This paper presents the details of experimental and numerical analysis performed on three 0.8?m-high reinforced earth model walls with strip footing surcharge near the wall facing. The study investigates how wire mesh strength and geometry affect the failure mechanism. All three walls were nominally identical, except for reinforcement strength and geometry. The displacement field of the entire cross section was captured by high-resolution digital camera through transparent sidewall. The resulting images were analyzed using digital image correlation software. The results indicate that both reinforcement strength and aperture size influence the type of failure mechanism. Numerical modelling was also applied to assess the influence of sidewall friction (3D model) and reinforcement stiffness and strength (2D model) on the failure mechanism of the walls. The parameters for the numerical models were derived from independent tests and results, which were compared with the experimental observations. A good level of agreement with measurements was confirmed, even for the 2D model that excluded sidewall friction.     

Performance of two-tiered reinforced-soil retaining walls under strip footing load

In the current study, an attempt was made to investigate the performance of two-tiered mechanically stabilized earth walls (T-TMSEWs) under static footing loading using reduced-scale model tests. For this purpose, twenty-four T-TMSEW models were constructed with three different types of reinforcement (metal strips, geogrid and geostraps) and were loaded using the rotatable and non-rotatable strip footings in different distances to the wall crest. Findings indicated that, although decreasing the reinforcement stiffness and the soil-reinforcement interaction reduces the ultimate bearing capacity of footings, the use of extensible reinforcements with low pull-out capacity and allowing the footing to tilt can be two effective solutions in T-TMSEWs to minimize deformations of backfill surface and connection loads as well as lateral pressures. It was observed that the use of a two-tiered configuration in MSE walls and also reducing tensile stiffness and soil-reinforcement interaction simultaneously, not only lead to change in the slip surface geometry but also prevent the development of deep slip surfaces in the lower tier. On the other hand, increasing the footing distance to the wall crest in the range of reinforced zone was found to be another influential solution to improve the bearing capacity, reduce wall deformations and also minimize lateral pressures.     

Load-settlement characteristics of large-scale square footing on sand reinforced with opening geocell reinforcement

This paper describes load-carrying characteristics of a series of large-scale steel square footing tests performed on sand reinforced with two types of reinforcement methods. These are full geocell reinforcement (FGR) and geocell with an opening reinforcement (GOR). A thick steel square plate with 500?mm by 500?mm dimensions and 30?mm thickness was used as foundation. The parameters varying in the tests include the depth of geocell mattress (u), width of opening in geocell in the GOR type (w), relative density of sand (D_r) and number of geocell layers (N). The results revealed that the use of GOR and FGR methods enhances significantly the footing load carrying capacity, decreases the footing settlement and decreases the surface heave. It has been found that the use of GOR with an opening width of w/B?<?0.92, has the same improvement effect on the footing load-carrying response as the FGR has (B?=?footing width). Furthermore, with increasing the number of geocell layers from 1 to 2 in both GOR and FGR methods, the footing bearing pressure increases and footing settlement, surface heave and difference of performance between FGR and GOR mattress decrease.     

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.     

Behaviour of a square model footing on loose sand reinforced with braided coir rope

The effectiveness of horizontally placed braided coir rope reinforcement on the strength improvement and settlement reduction of loose sand is investigated for modeling footings using plate load tests in the laboratory. The influence of parameters such as depth of reinforcement embedment, length, number of layers and number of plies of braided coir rope was examined. The model test results indicate that up to about a six-fold improvement in strength and about ninety percent reduction in settlement (vertical displacement) can be achieved through the use of the proposed reinforcing method. The optimum value of embedment depth of a single layer of braided coir rope reinforcement was identified as 0.4 times the footing width. It was also found that optimal benefit was realized for a length ratio equal to about 3 and by reinforcing the zone of soil directly beneath the model footing upto a depth equal to about 0.6 times the width of footing. Increase in the number of layers within the significant depth leads to a proportionate increase in strength improvement ratio, while the optimal settlement reduction is realized with three layers of braided coir rope reinforcement. Regression analysis carried out with limited experimental data suggests the possibility of developing a predictive model to quantify the strength improvement.     

Experimental study on the behavior of eccentrically loaded circular footing model resting on reinforced sand

In this study, an experimental investigation has been conducted on a circular footing model subjected to eccentric load resting on the geonet-reinforced sand. To this end, five series of tests were carried out in order to evaluate the effect of reinforcement dimension and eccentricity on the bearing capacity, settlement, and rotation of the footing. Results show that the bearing capacity ratio (BCR) is in direct relationship with eccentricity and the impact of soil reinforcement at low settlements is much more significant in the case of eccentric loading. Additionally, the bearing capacity interaction diagram and variation in the position of rotation line at different load levels for reinforced and unreinforced conditions are presented.     

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.     

Model testing and numerical investigation of interference effect of closely spaced ring and circular footings on reinforced sand

Due to heavy loads and the non-availability of suitable construction sites, engineers are often required to place footings at close spacing. These footings influence each other, including effects on load-settlement and bearing capacity behavior. In this research the bearing capacity of closely located ring and circular footings on reinforced sand has been investigated numerically and experimentally. The goal of this study is to evaluate the interference effect on the bearing capacity of adjacent circular and ring footings. Footings on reinforced and unreinforced sand have been investigated. In this research, interference effect of footings, shape effects, effect of spacing between footings and also the effect of reinforcement layer on the bearing capacity are studied. To achieve these objectives laboratory circular and ring footing models and also numerical models were used. Finite element computer code PLAXIS 3D Foundation was used for numerical modeling. Experimental and numerical analysis results show that the ultimate bearing capacity of two closely spaced circular and ring footings is greatest when they stand exactly beside each other and decreases with increase in the spacing to footing diameter ratio (Δ/D). It is found that for Δ/D > 4, the bearing capacity of each adjacent footing is almost the same as that for single footing. This means that for a center-to-center spacing greater than 4D, no significant interference effect was observed and each footing acted more or less independently, similar to a single footing.     

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.