Experimental and Analytical studies of reinforced crushed limestone

This research study aims at investigating the behavior of spread footings on reinforced crushed limestone using laboratory model tests. The model tests were conducted inside a steel box with dimensions of 1.5 m (length) × 0.91 m (width) × 0.91 m (height) using a steel plate with dimensions of 152 mm × 152 mm (6 in. × 6 in.). The parameters investigated include the number of reinforcement layers and the tensile modulus and type of reinforcement. An analytical solution is also developed to estimate the ultimate bearing capacity of reinforced crushed limestone. The test results show that the inclusion of reinforcement can appreciably improve the bearing capacity of crushed limestone up to a factor of 2.85 at a settlement ratio of 10% and reduce the footing settlement down to 75% at a surface pressure of 5500 kPa. The results also demonstrate that reinforcements with higher tensile modulus performed better than reinforcements with lower tensile modulus and that steel reinforcement performed better than geogrid reinforcement. The proposed analytical solution gave a good predication of experimental results of geogrid reinforced sections, while Huang and Menq's solution (1997) gave a good predication for steel reinforced crushed limestone. The analytical results also showed that the reinforced ratio (R_r) should be high enough to form the “deep footing” effect.     

Effect of reinforcement form on the bearing capacity of square footings on sand

This paper presents the results of laboratory model loading tests and numerical studies carried out on square footings supported on geosynthetic reinforced sand beds. The relative performance of different forms of geosynthetic reinforcement (i.e. geocell, planar layers and randomly distributed mesh elements) in foundation beds is compared; using same quantity of reinforcement in each test. A biaxial geogrid and a geonet are used for reinforcing the sand beds. Geonet is used in two forms of reinforcement, viz. planar layers and geocell, while the biaxial geogrid was used in three forms of reinforcement, viz. planar layers, geocell and randomly distributed mesh elements. Laboratory load tests on unreinforced and reinforced footings are simulated in a numerical model and the results are analyzed to understand the distribution of displacements and stresses below the footing better. Both the experimental and numerical studies demonstrated that the geocell is the most advantageous form of soil reinforcement technique of those investigated, provided there is no rupture of the material during loading. Geogrid used in the form of randomly distributed mesh elements is found to be inferior to the other two forms. Some significant observations on the difference in reinforcement mechanism for different forms of reinforcement are presented in this paper.     

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.     

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.     

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.     

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

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

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.     

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.     

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

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

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.     

Behavior of closely spaced square and circular footings on reinforced sand

This paper describes an experimental investigation conducted to evaluate the ultimate bearing capacity, the settlement and the tilt of two types closely spaced footings, one having square shapes and the other having circular shapes, on unreinforced and reinforced soil. To decrease the objectionable influence of interference on the performance of the closely spaced footings, the foundation soil is reinforced by geogrid layers. The results of this reinforcement show both positive and negative effects, namely, a positive effect because there is a considerable increase in the ultimate bearing capacity, and a negative effect because there is an increase in settlement and tilt. Regarding the experimental results, the negative effect of interference can be decreased considerably through the use of soil reinforcements. The ultimate bearing capacity of the interfering footings increased by about 25–40%, whereas the settlement of the interfering footings at the ultimate load increased in the range of 60–100%. However, the closely spaced footings tilted by approximately 45% and 75% for reinforced sand with one and two layers of geogrid, respectively.     

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.     

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.     

Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments on yielding foundation

This paper presents an experimental study of the load bearing behavior of geosynthetic reinforced soil (GRS) bridge abutments constructed on yielding clay foundation. The effects of two different ground improvement methods for the yielding clay foundation, including reinforced soil foundation and stone column foundation, were evaluated. The clay foundation was prepared using kaolin and consolidated to reach desired shear strength. The 1/5-scale GRS abutment models with a height of 0.8 m were constructed using sand backfill, geogrid reinforcement, and modular block facing. For the GRS abutments on three different yielding foundations, the reinforced soil zone had relatively uniform settlement and behaved like a composite due to the higher stiffness than the foundation layers. The wall facing moved outward with significant movements near the bottom of facing, and the foundation soil in front of facing showed obvious uplifting movements. The vertical stresses transferred from the footing load within the GRS abutment and on the foundation soil are higher for stiffer foundation. The improvement of foundation soil using geosynthetic reinforced soil and stone columns could reduce the deformations of GRS abutments on yielding foundation. Results from this study provide insights on the practical applications of GRS abutments on yielding foundation.     

土工合成材料加筋砂土三轴试验研究

本文以 5种国产土工合成材料为加筋材料 ,它们分别是针刺无纺土工织物、涤纶纤维经编土工格栅、玻璃纤维土工格栅、聚丙烯双向土工格栅和聚乙烯土工网 ,用三轴试验比较各种土工合成材料对砂土的加筋效果 ,得到一些有益的结论 ,可指导土工合成材料的优选和研究加筋机理 ,同时指出部分国产土工合成材料产品的不足。     

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.     

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.     

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.     

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.     

Mechanism of Bearing Capacity of Spread Footings Reinforced with Micropiles

The Hyogoken-Nambu Earthquake in 1995 caused extensive damages to the foundations of bridges. Ever since, methods to improve the bearing capacity of existing foundations have become an important aspect of foundation engineering in Japan. Micropiles are considered to provide promising solutions. The mechanism which enhances the bearing capacity of surface footings reinforced with micropiles is the subject of investigation in this study. As an initial phase, model tests were conducted to understand the load-displacement behavior of surface footings with and without micropiles on loose, medium dense, and dense layers of sands. Salient factors which influence the behavior of the footings were selected and their influence on bearing capacity was examined through a comprehensive series of model tests. Notable improvements in the bearing capacity of surface footings reinforced with vertical micropile groups were observed in the case of dense sand which is dilative during shear. To assess quantitatively the degree of improvement in the bearing capacity of surface footings reinforced with micropiles, an index R called “Network Effect Index” was introduced in this study. The index R of unity means that the bearing capacity of footings reinforced with micropiles is simply equal to the summation of the individual value of the surface footing and that of the micropile group. An index R of more than two is achieved in this study where surface footings reinforced with a group of vertical micropiles bear on a dense layer of dilative sand. By contrast, with loose and medium dense sand, which are contractive in nature, the index R is found to be less than unity.