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A review on the effective use of geosynthetic reinforcement to increase soil bearing capacity
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Gypseous soil is one of the problematic soils and greatly affects the stability of the engineering structures, especially in Iraq. The main geotechnical problem of this soil is the significant reduction of its bearing capacity upon loading and/or wetting processes due to the dissolution of gypseous cementing bonds. This study aims to improve the soil's bearing capacity by using geosynthetics material in single, double, and triple distribution patterns. The gypseous soil samples were brought from a site near Sawa Lake by coordinates (31°18′42.83″ N, 45°00′49.36″ E) in Al-Muthanna Governorate with gypsum content of 37.35% forms about 3.0 m high under the ground surface. The Soil-Model apparatus of dimensions (60 × 60 × 50) cm is used, while the proposed square footing dimensions are (10 × 10) cm. The main test program investigates the bearing capacity before and after the soil reinforcement with the geotextile layers. The results showed a considerable increase in bearing capacity and the increase of volume change when using the triple phase pattern with the allowable bearing capacity increase for reinforced gypseous soil, especially with the increasing reinforcement layers at the triple reinforcement pattern. The depth of the geotextile layer with the soil mass has a significant effect on the magnitude of the bearing capacity and decreases the settlement. The improvement proportion of soil bearing capacity using Geotextile Reinforcement at dry state is ranged 20–90% for relative densities 30 and 60% and different reinforcement patterns.
A ring footing is found to be of practical importance in supporting symmetrical constructions for example silos, oil storage container etc. In the present paper, numerical analysis was carried out with explicit code FLAC3D 7.0 to investigate bearing capacity of a ring footing on ge-ogrid reinforced sand. Effects of the ratio n of its inner/ outer diameter (Di / D) of a ring footing, an optimum depth to lay the geogrid layer were examined. It was found that an intersection zone was developed in soil under inner-side (aisle) of ring footing, contributing to its bearing capacity. Substantial increase of bearing capacities could be realized if ratio n of a ring footing was around 0.6. Numerical results also showed that, bearing capacity of a ring footing could increase significantly if a single-layer geogrid was laid at a proper depth under the footing. Similar contribution was found if a double-layer geogrid was implemented. However, such increases appeared to be rather limited if a triple-layer geogrid or a four-layer geogrid was used. A double-layer geogrid was recommended to increase the bearing capacity of a ring footing; the depth to lay this double-layer ge-ogrid was also discussed.
A transformed approach has been made to predict the impression of inclusion geogrid on the reinforced soil’s bearing capacity underneath the strip foundation. The influence of the friction factor of the tensile strength of the reinforcement element performs a significant purpose in estimating the tolerance of the strengthened soil. Analytical study and experimental tests have been performed to validate the proposed empirical approach. This study analyzed certain parameters that influence the efficiency of the strip footing placed on reinforced soil, such as effects of two geogrid layers, the efficacy of geogrid embedment depths, the distance between geogrid layers, the tensile strength of geogrid, the contact surface friction angle, and shear stress distribution along the interactions at the soil–geogrid interface. Consequently, a simple new equation was proposed to modify the reinforced soil’s increased bearing capacity, then a comparative study was executed by the analytical method of literature. Finally, the calculations confirmed a good agreement between laboratory and analytical results such that the error rate was less than 2%. It was found that the value of strain-induced at the midpoint of geogrid decreases with depth, and their magnitude is constant at a 0.5B embedment depth. The impact of geogrid with length ratio (L/B = 5–7) on the strain values has similar behavior, but it is a significant effect of shorter geogrid length layers.
A series of dynamic model tests that were performed on a geogrid-reinforced square footing are presented. The dynamic (sinusoidal) loading was applied using a mechanical testing and simulation (MTS) electro-hydraulic servo loading system. In all the tests, the amplitude of loading was ±160 kPa; the frequency of loading was 2 Hz. To better ascertain the effect of reinforcement, an unreinforced square footing was first tested. This was followed by a series of tests, each with a single layer of reinforcement. The reinforcement was placed at depths of 0.3B, 0.6B and 0.9B, where B is the width of footing. The optimal depth of reinforcement was found to be 0.6B. The effect of adopting this value versus the other two depths was quantified. The single layer of geogrid had an effective reinforcement depth of 1.7B below the footing base. The increase of the depth between the topmost geogrid layer and the bottom of the footing (within the range of 0.9B) did not change the failure mode of the foundation.
A study was undertaken to investigate the bearing capacity of rectangular footings on geogrid-reinforced sand by performing laboratory model rests as well as finite-element analyses. The effects of the depth to the first layer of reinforcement, vertical spacing of reinforcement layers, number of reinforcement layers, and the size of reinforcement sheet on the bearing capacity were investigated. Both the experimental and analytical studies indicated that there was an optimum reinforcement embedment depth at which the bearing capacity was the highest when single-layer reinforcement was used. Also, there appeared to be an optimum reinforcement spacing for multi-layer reinforced sand. The bearing capacity of reinforced sand was also found to increase with reinforcement layer number and reinforcement size when the reinforcement was placed within a certain effective zone. In addition, the analysis indicated that increasing reinforcement stiffness beyond a certain value would not bring about further increase in the bearing capacity.
Reinforced soil is a composite material in which elements with tensile strength were utilized for reinforcement. Geotextile is the most common material in group of geosynthetics for soil reinforcement. This paper presents the effect of a non-woven geotextile which have a higher failure strain on bearing capacity of rigid footing constructed on sand. A research has been done to investigate the bearing capacity of granular soil with plates which have standard width according to ASTM D 1194. In this study, a total number of 62 model tests were carried out in a laboratory using two square rigid steel plate with the sides of 270 mm and 350 mm. A broad series of conditions was tested by varying parameters such as the location of upper layer of geotextile, number of geotextile layers, width of reinforcement and vertical spacing between layers. In second step a series of tests were additionally carried out by varying of spaces between layers and width of geotextile layers in proportion to increase of depth. The results demonstrated that in all cases non-woven geotextile increases bearing capacity and the maximum bearing capacity was obtained in 4-layer reinforcement system. It is also shown that the optimum value of vertical spaces between layers after the upper one are respectively, 0.30 B, 0.35 B, 0.45 B. In addition, the results indicate that optimum width of the first two layers of reinforcement are 4 B and for the third and fourth one are 3 B and 2.5 B respectively.
Bio-based materials are widely used recently in order to introduce a more sustainable construction material. Kenaf is a type of bio-based material that can be easily obtained in a tropical country, which could be a potential material to be utilised as a geotextile material because it has good tensile strength. The geotextile could be used to improve the bearing capacity of a loose soil. This paper presents a series of small-scale physical modelling tests to investigate the bearing capacity performance of Kenaf fibre geotextile laid on and inside the sand layer. A rigid footing was used to replicate a strip footing during the loading test, and sand was prepared based on 50% of relative density in a rigid testing chamber for ground model preparation. In order to treat the soil, Kenaf fibre geotextile was laid at four difference locations which are on the soil surface and underneath the ground model surface at 50, 75 and 100 mm deep. It was found that the usage of the Kenaf fibre geotextile has improved the bearing capacity of the sandy soil up to 414.9% as compared to untreated soil. It was also found that the depth of the Kenaf fibre geotextile treated into the soil also affects the soil performance.
Geosynthetics have been successfully used to fulfill a number of functions that contribute significantly to the good performance of roadways. They include the functions of separation, filtration, reinforcement, stiffening, drainage, barrier, and protection. One or more of these multiple functions have been used in at least six important roadway applications. The applications include the migration of reflective cracking in asphalt overlays, separation, stabilization of road bases, stabilization of road soft subgrades, and lateral drainage. This paper illustrates the mechanisms as well as key advances in each one of these multiple applications.
Geosynthetic-Reinforced Soil (GRS) mass, comprising soil and layers of geosynthetic reinforcement, is not a uniform mass. To examine the behavior of a GRS mass by a laboratory test, a sufficiently large-size specimen of soil and reinforcement is needed to produce a representative soil-geosynthetic composite. This paper presents a generic test, referred to as the Soil-Geosynthetic Composite (SGC) test, for investigating stress-deformation behavior of soil-geosynthetic composites in a plane strain condition. The specimen dimensions, 2.0 m high and 1.4 m wide in a plane strain configuration, were determined by the finite element method of analysis. The configuration, specimen dimensions, test conditions, and procedure of the SGC test are described. In addition, the results of a SGC test with nine sheets of reinforcement, as well as those of an unreinforced soil test conducted in otherwise identical conditions, are presented. In the test, the soil mass was subject to a prescribed value of confining pressure, applied by vacuum through latex membrane covering the entire surface area of the mass in an air-tight condition. Vertical loads were applied on the top surface of the soil mass until a failure condition was reached. The behaviors of the soil masses, including vertical displacements, lateral movement, and strains in the geosynthetic reinforcement, were carefully monitored. The measured data allow the behavior of reinforced and unreinforced soils to be compared directly, provide a better understanding of soil-geosynthetic composite behavior, and serve as the basis for verification of numerical models to investigate the performance of GRS structures. J. Ross Publishing, Inc.
This study evaluated the effects of single, double, and triple reinforcing layers on the bearing capacity ratio (BCR) of strip footing on a sand slope system. Seventy-two laboratory-loading tests were conducted on a stripfooting model on a reinforced sand slope. Moreover, this study illustrated the effects of the different parameters of two reinforcing layers on the bearing capacity of a double-reinforced sand slope. The BCR increased from 1.06 to 3.00 for single-reinforced slope soils, 1.09 to 7.73 for double-reinforced slope soils, and up to 8.00 for three-layered reinforced systems. For double-reinforced soil slopes, the most effective spacing between the two reinforcing layers is 0.3 B.
In current design methods for reinforced soil walls, it has been tacitly assumed that reinforcement strength and reinforcement spacing play an equal role. This fundamental design assumption has led to the use of larger reinforcement spacing (0.3-1.0 m) in conjunction with stronger reinforcement to reduce construction time. Recent studies, however, have clearly indicated that the role of reinforcement spacing is much more significant than that of reinforcement strength. With closely spaced (reinforcement spacing <= 0.3 m) reinforcement, the beneficial effects of geosynthetic inclusion is significantly enhanced, and the load-deformation behavior can be characterized as that of a composite material. A reinforced soil mass with closely spaced geosynthetic reinforcement is referred to as geosynthetic-reinforced soil (GRS). In this study, an analytical model is developed for predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass. The model was developed based on a semiempirical equation that reflects the relative roles of reinforcement spacing and reinforcement strength in a GRS mass. Using measured data from field-scale experiments available to date, it is shown that the analytical model is capable of predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass with good accuracy. (C) 2013 American Society of Civil Engineers.
Presented are the results of five large-scale Geosynthetic Reinforcement Soil (GRS) experiments referred to as Mini Piers (MP). The MPs were constructed to evaluate the effect of reinforcement spacing and reinforcement strength on the performance of a GRS mass. The experiments were performed at the Federal Highway Administration (FHWA) Turner-Fairbank Highway Research Center (T-FHRC) in McLean, VA in 1997. The results indicate a more pronounced improvement in the bearing capacity for close-spaced reinforced soil systems and that the performance of a GRS mass is more dependent on the spacing of the reinforcement and not necessarily the strength of the reinforcement. Test results are also correlated to a full-scale experiment to suggest the use of a laboratory test to predict the performance of a GRS mass. Innovators have successfully applied Segmental Retaining Wall (SRW) technology to a variety of applications by modifying the design of GRS to include the benefit of reinforcement spacing. However, a problem with implementing the technology into the main stream is that many experts assert the need for a revised design procedure for more closely spaced reinforcement systems because the current design does not include the benefit of reinforcement spacing created by soil-geosynthetic interaction. The purpose of this investigation was to provide general observations about GRS behavior in terms of reinforcement spacing and the improvement reinforcement spacing has on the performance of a GRS mass. It is hopeful that the results of these experiments will encourage the users of GRS technology to rethink design assumptions in current Mechanically Stabilized Earth (MSE) wall policy related to the frequency of reinforcement spacing. Currently the design of MSE walls does not account for the benefit of soil - geosynthetic interaction. The authors acknowledge that current design may be valid for certain MSE systems, but assert the need for a different design method for closely spaced (less than 16″) GRS systems..
Two full-scale segmental facing Geosynthetic-Reinforced Soil (GRS) abutment walls, referred to as "the NCHRP test abutments", were constructed and load-tested at the Turner-Fairbank Highway Research Center in McLean, Virginia. The two experiments were conducted to (1) examine the behavior of segmental facing GRS abutment walls under increasing vertical loads on a bridge sill, and (2) furnish a set of well-defined data for verification of analytical models. This paper describes the two experiments and the measured and observed behavior due to the load applications. The behavior of the test walls were discussed and compared with existing performance criteria that were developed based on experience with real bridges. The comparisons were made in terms of maximum sill settlement, maximum angular distortion, and maximum lateral wall movement. In addition, the safety factors and failure loads of the test walls were evaluated by the prevailing design method. The predicted failure loads obtained from the prevailing design method, even with all safety factors and strength reduction factors being set equal to one, were found to be much lower than the measured values, suggesting that the prevailing design method is likely to be overly conservative. J. Ross Publishing, Inc.
Dry jet mixing (DJM) and prefabricated vertical drains (PVDs) have been successfully used for soft ground improvement worldwide. They both have advantages and limitations, and have been used independently so far. In this study, DJM and PVDs were used in combination to enhance the performance of soft ground improvement. In the combined method, PVDs are first installed and then DJM columns are installed between the PVDs at larger spacing. This combined method improves the effectiveness of ground improvement and creates a more economical solution. This paper presents the concept, mechanisms, construction procedures, and field validation of this new method. In situ standard penetration tests were performed in dry mixed columns and cone penetration tests and field vane shear tests were performed in soils between the columns. Plate load tests were subsequently conducted on the treated ground. Excess porewater pressures were monitored during and after the installation of DJM columns with and without PVDs. Field validation was conducted through a pilot highway embankment on very soft clay in Jiangsu, China. Long-term settlement monitoring was carried out for the construction of this highway embankment. Field tests and monitoring clearly showed that the DJM-PVD method performed well in enlarged spacing as compared with the conventional DJM method with close spacing.
The paper presents a study of the behavior of model strip footings supported on a loose sandy slope and subjected to both monotonic and cyclic loads. The effects of the partial replacement of a compacted sand layer and the inclusion of geosynthetic reinforcement were investigated. Different combinations of the initial monotonic loads and the amplitude of cyclic loads were chosen to simulate structures in which loads change cyclically such as machine foundations. The affecting factors including the location of footing relative to the slope crest, the frequency of the cyclic load and the number of load cycles were studied. The cumulative cyclic settlement of the model footing supported on a loose sandy slope, un-reinforced and reinforced replaced sand deposits overlying the loose slope were obtained and compared.Test results indicate that the inclusion of soil reinforcement in the replaced sand not only significantly increases the stability of the sandy slope itself but also decreases much both the monotonic and cumulative cyclic settlements leading to an economic design of the footings. However, the efficiency of the sand–geogrid systems depends on the properties of the cyclic load and the location of the footing relative to the slope crest. Based on the test results, the variation of cumulative settlements with different parameters is presented and discussed.
Laboratory model test results for the ultimate bearing capacity of strip and square foundations supported by sand reinforced with geogrid layers have been presented. Based on the model test results, the critical depth of reinforcement and the dimensions of the geogrid layers for mobilizing the maximum bearing-capacity ratio have been determined and compared. Key words : bearing capacity, geogrid, model test, reinforced sand, shallow foundation.Laboratory model test results for the ultimate bearing capacity of strip and square foundations supported by sand reinforced with geogrid layers have been presented. Based on the model test results, the critical depth of reinforcement and the dimensions of the geogrid layers for mobilizing the maximum bearing-capacity ratio have been determined and compared. Key words : bearing capacity, geogrid, model test, reinforced sand, shallow foundation.
Various reinforcing techniques have been used till now for the bearing capacity improvement of Shallow Foundations over soft clay. Of all the techniques, Geosynthetic reinforcement has proved to be a cost-effective solution. Many researchers have focused on behaviour of Geotextile or Geogrid alone as a reinforcing material. But less research is available on Geocomposites. In this study, the combination of Geotextile and Geogrid (i.e. Combigrid) is used as a single reinforcing material in a single layer which is more economical than using different materials in different layers, hence the purpose of my research is to provide a cost-effective ground improvement technique in soft soil by using a single layer of geocomposite at optimum depth and length. Laboratory tests are performed on various configurations of the reinforcement layout to investigate the bearing capacity of reinforced clay. The model test tank of size 280 mm × 280 mm × 240 mm and square footing of width 5 cm is used. First the model tank is tested in a Universal Testing Machine with clay only and the Bearing capacity of unreinforced clay is recorded, then for 0.3B depth of reinforcement, three varied lengths of reinforcement (3B, 4B, 5B) are tested. Similarly, for 0.5B, 0.7B and 0.9B depth, the length of reinforcement varied from 3 to 5B (B = width of footing) and for each case, the bearing capacity is calculated. Results are compared in terms of BCR and it is found that Combigrid helps in bearing capacity improvement of soft clay upto four times more than the Unreinforced Bearing Capacity.
A wide variety of factors can influence the bearing capacity (BC) and the settlement of foundations constructed in weak soils; therefore, a number of solutions have been thus far proposed to improve these characteristics. In this study, weak soil was replaced with a granular trench, reinforced with geogrids, as well as a reinforced granular pad, to examine the load-settlement behavior of a strip foundation. A series of laboratory tests and finite element (FE) models were further conducted to propose the optimal parameter value in order to achieve the maximum BC ratio (BCR) of the reinforced soil. Investigations carried out by varying important parameters, including number of geogrid elements, placement depth, and effective trench depth, accordingly showed significant impacts on the BC. These studies were also extended to numerical modeling tests in order to determine the ultimate BCR of a strip foundation supported by a geogrid-reinforced granular trench subjected to eccentric-inclined loads. Based on the physical modeling, the BC of a strip foundation could be significantly improved by replacing sand with granular materials up to three times. Moreover, placing one, two, and three geogrids in the trench indicated a rise in the BCR. Experimental results also demonstrated that the optimal width and depth of the trench to achieve the maximum BCR were 1.5 and 3.5 times the width of the foundation, respectively, and the optimal placement depth for one geogrid layer was observed. Considering the numerical results, a slight decrease in the BCR could follow a drop in it up to 20°in the inclined loads on the foundation laid on the trench with one geogrid layer.
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.
A railroad ballast or subballast layer is composed of unbound granular particles. The ballast/subballast initial compaction phase occurs immediately the construction or maintenance of a track structure is finished. The particles are densified into a more compact state after certain load repetitions. Geogrids are commonly used in railroad construction for reinforcement and stabilization. Currently heavy haul trains are increasing the loads experienced by the substructural layers, which changes behavior of reinforced granular particles. This paper presents a series of ballast box tests to investigate the behavior of geogrid-reinforced unbound granular particles with rectangular (BX) and triangular (TX) shaped geogrids during the compaction phase. Three types of tests were conducted: one without geogrid as a control, one with a sheet of rectangular shaped geogrid, and the other one with a sheet of triangular shaped geogrid. The geogrid was placed at the interface between subballast and subgrade layers. A half section of a railroad track structure consisting of two crossties, a rail, ballast, subballast and subgrade was constructed in a ballast box. Four wireless devices - “SmartRocks”, embedded underneath the rail seat and underneath the shoulder at the interface of ballast-subballast, and subballast-subgrade layers, respectively, to monitor particle movement under cyclic loading. The behavior of the unbound aggregates in the three sections under two different loading configurations were compared. The results indicated that the inclusion of the geogrid significantly decreased accumulated vertical displacement on the ballast surface, ballast particle translation and rotation under a given repeated loading configuration. The results also demonstrated the effectiveness of the SmartRock device and its potential for monitoring behavior of ballast particles in the field.
A mechanism for reinforced earth, constructed by introducing thin flat high modulus and strength material layer by layer into the soil, is proposed. This mechanism is based on the change of stress state in the soil induced by the shearing stress on the contact faces between the soil and reinforcement due to difference in their relative rigidity. Laboratory triaxial tests were performed to investigate the deformation and strength characteristics of the reinforced sand. Both short regular triaxial specimens in which the rigid caps were treated as reinforcement and regular triaxial specimens with several horizontal layers of fiberglass nets equally spaced in the horizontal planes were used.
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In general, soils possess a low tensile strength. The main objective of strengthening the soil mass is to increase bearing capacity improve stability and decreased settlements and lateral deformations. One of the approaches is the use of polymeric materials. Geosynthetic is a well known technique in soil reinforcement. The use of it, can significantly improve the soil performance and reduce costs in comparison with conventional designs. In this paper, a review of experimental and numerical tests carried out by different previous researchers on reinforced soil with synthetic materials specially geogrid under static loading had been made. The studies indicate that the inclusion of planar reinforcement in the sand decreased much both the monotonic and cumulative settlements leading to an economic design of the footings. copy; 2013, EJGE.
Laboratory data from some 65 small scale strip footing bearing capacity tests were used as a basis to develop an analytical procedure of predicting the load-settlement and ultimate bearing capacity of a strip footing on sand which contains horizontal strips of tensile reinforcing. The theory is formulated in terms of the ratio of bearing capacity with and without reinforcing, assuming that existing methods are adequate for predicting bearing capacities on sand with 0 reinforcing. A working hypothesis is suggested involving a particular failure mode suggested from the tests. The dimensions, strength, and frictional properties of the sand and reinforcing are incorporated directly with this assumed failure mode to give a direct solution to the bearing capacity. An example calculation is included along with an approximate comparative cost analysis.
The applicability of a method for predicting 'bearing capacity increase' in reinforced sandy ground was examined using tests performed under various test conditions. It was found that the present method predicted, with reasonable accuracy, the bearing capacity increase in sandy ground, reinforced with stiff reinforcement. This method may not be applicable for sandy ground reinforced with extensible reinforcement due to the unsuccessful formation of a semirigid zone under the footing. An investigation into the settlement of a footing on reinforced sandy ground, at ultimate footing load condition, suggested that the settlement of footing for reaching peak footing load may be correlated to the 'deep-footing' and the 'wide-slab' mechanisms. That is, the ultimate settlement ratio between reinforced and unreinforced model sandy ground, SRf, may be linearly correlated to 'BCRD' and 'BCRS', which represent 'deep-footing' and 'wide-slab' effects, respectively, on the ultimate bearing capacity increase in reinforced sandy ground.
Recycled asphalt pavement (RAP) has been increasingly used as an energy efficient and environmentally friendly paving material and is currently the most reused and recycled material in the United States. RAP has been used in new hot mix asphalt (HMA) mixtures and in base courses for pavement construction. When RAP is used as a base course material, the presence of asphalt in RAP may cause excessive deformation under traffic loading. Geocell, three-dimensional (3D) polymeric geosynthetic cells, was proposed in this study to minimize the deformation by confining the RAP material. Full-scale accelerated pavement tests were conducted to evaluate the effect of geocell reinforcement on RAP base courses over weak subgrade. Two types of RAP were used and a total of seven geocell-reinforced and unreinforced RAP sections were tested under full-scale traffic loads. The road sections were excavated and examined after each moving wheel test. The benefits of geocell reinforcement were evaluated in rut depths for a specific number of passes of the wheel load and the angle of stress distribution from the surface to the base course-subgrade interface. The test results demonstrated that the novel polymeric alloy geocell reinforcement improved the performance of unpaved RAP sections by widening the stress distribution angle and reducing the rut depth if the base courses were equally compacted in unreinforced and reinforced sections. DOI: 10.1061/(ASCE)MT.1943-5533.0000286. (C) 2011 American Society of Civil Engineers.
A new simple analytical procedure (AASHTO modified) that includes the effect of the induced stress due to backfill compaction for use with conventional design methods of geosynthetic reinforced soil (GRS) walls is proposed. The proposed analytical procedure may be used with any conventional design methods that do not take into consideration the effect of the compaction-induced stress in their calculations. This approach is based on an equation suggested by Wu and Pham (2010) to calculate the increase in lateral stress in a reinforced soil mass due to compaction. Additionally, two numerical procedures for modeling of compaction are described. Analyses using these procedures were performed to evaluate the capability of the proposed analytical procedure. The results were compared with values predicted using the Ehrlich and Mitchell (1994) method, the modified version of the K-stiffness method (Bathurst et al., 2008) and the AASHTO simplified method. The results show that the AASHTO modified method and the numerical analyses, in which the compaction-induced stress was modeled using two distributed loads at the top and bottom of each soil layer, resulted in values of the maximum reinforcement tension, Tmax, that agree with those from the full-scale test and those calculated by Ehrlich and Mitchell (1994). On the other hand, the K-stiffness method under-predicts the measured Tmax values. Moreover, numerical modeling of compaction using a distribution load only at the top of each soil layer overestimated the measurements.
Reinforced soil foundations (RSFs) have been employed in engineering practice to increase the soil bearing capacity and to reduce the potential footing settlement. The aim of this study is to develop analytical solutions for estimating the ultimate bearing capacity of strip footings on RSFs. A general failure mode for RSFs was first proposed based on previous studies conducted by the authors and test results from literature study. A limit equilibrium stability analysis of RSFs was performed based on the proposed failure mechanism. New bearing capacity formulas, which consider both the confinement and the membrane effects of reinforcements on the increase in ultimate bearing capacity, were then developed for strip footings on RSFs. Several special cases of RSFs were presented and discussed. The proposed model was verified by the experimental data reported in the published literature. The predicted ultimate bearing capacity was in good agreement with the results of model tests reported in the literature. The study showed that the depth of the punching shear failure zone (DP) depends on the relative strength of the reinforced soil layer and the underlying unreinforced soil layer, and is directly related to the reinforced ratio (Rr).
The geosynthetic reinforced soil (GRS) performance test (PT), also called a mini-pier experiment, was developed by the Federal Highway Administration (FHWA) to evaluate the material strength properties of GRS composites built with a unique combination of reinforcement, compacted fill, and facing elements. The PT consists of constructing a 1.4-m square column of alternating layers of compacted granular fill and geosynthetic reinforcement with a facing element that is frictionally connected up to a height of 2 m, then axially loading the GRS mass while measuring deformation to monitor performance. The results can be directly used in the design of GRS abutments and integrated bridge systems. Considering that the geometry of the PT is square in plan, the equivalency of the results to a bridge application, which more resembles a plane strain condition, is evaluated and presented in this paper. The analysis indicates that the PT closely approximates the bearing resistance, or capacity, of a typical GRS abutment, and is a conservative estimate when predicting stiffness. These results indicate that the PT can be used as a design tool for GRS abutments at both the strength and service limit states.
Reinforcing soils with biaxial geogrids have been shown to be an effective method for improving the ultimate bearing capacity of granular soils. The pull-out resistance of reinforcing elements is one of the most significant factors in increasing bearing capacity. In this research a new reinforcing element that includes attaching elements (anchors) to ordinary geogrid for increasing the pull-out resistance of reinforcements is introduced. Reinforcement is therefore consists of geogrid and anchors with cubic elements that attached to geogird, named (by the authors) Grid-Anchor. Three-dimensional numerical study was performed to investigate the bearing capacity of square footing on sand reinforced with this system. The effect of depth of the first reinforcement layer, the vertical spacing, the number and width of reinforcement layers, the angle of anchors, the stiffness of reinforcement and anchors and the distance that anchors are effective were investigated. Three-dimensional finite element analysis by "PLAXIS 3D Tunnel" software indicated that when a single layer of reinforcement is used there is an optimum reinforcement embedment depth for which the bearing capacity is greatest. There also appeared to be an optimum vertical spacing of reinforcing layers for multi-layer reinforced sand. The bearing capacity was also found to increase with increasing number of reinforcement layer, if the reinforcement were placed within a range of effective depth. In addition analysis indicated that increasing reinforcement and anchor stiffness beyond a threshold value does not result in further increase in the bearing capacity. Finally the results were compared with the bearing capacity of footings on non-reinforced sand and sand reinforced with ordinary geogrid and the advantages of the Grid-Anchor are highlighted.
A series of axi-symmetry models using finite element analyses were performed to investigate the behavior of circular footings over reinforced sand under static and dynamic loading. Geogrid was modeled as an elastic element and the soil was modeled using hardening soil model which use an elasto-plastic hyperbolic stress–strain relation. Several parameters including number of geogrid layers, depth to the first geogrid layer, spacing between layers and load amplitude of dynamic loading are selected in this paper to investigate the influence of these parameters on the performance of reinforced systems under both static and dynamic loads. The numerical studies demonstrated that the presence of geogrid in sand makes the relationship between contact pressure and settlement of reinforced system nearly linear until reaching the failure stage. The rate of footing settlement decreases as the number of loading cycles increases and the optimum values of the depth of first geogrid layer and spacing between layers is found 20% of the footing diameter. Some significant observations on the performance of footing-geogrid systems with change of the values of parametric study are presented in this paper.
Soil reinforcement is defined as a technique to improve the engineering characteristics of soil. In this way, using natural fibers to reinforce soil is an old and ancient idea. Consequently, randomly distributed fiber-reinforced soils have recently attracted increasing attention in geotechnical engineering for the second time. The main aim of this paper, therefore, is to review the history, benefits, applications; and possible executive problems of using different types of natural and/or synthetic fibers in soil reinforcement through reference to published scientific data. As well, predictive models used for short fiber soil composite will be discussed. On other words, this paper is going to investigate why, how, when; and which fibers have been used in soil reinforcement projects.
This paper presents a physical model study of the influence of compaction on the behavior of geogrid-reinforced soil walls. Experiments were accomplished in a facility at the Geotechnical Laboratory of COPPE/UFRJ. For soil compaction, two different types of hand-operated compactors were used: a vibrating plate and a vibratory tamper. Equivalent vertical stresses for the vibrating plate (referred to as the “light compactor”) were much lower than the corresponding value of the vibratory tamper (referred to as the “heavy compactor”). Mobilized tension along the reinforcements and external and internal displacements of the wall were monitored. The results showed that the effect of soil compaction is not limited to a reduction of the soil void ratio. Compaction has led to a significant increase in the horizontal stress inside the reinforced soil mass and generates a kind of pre-consolidated material. Analyses of results showed that compaction has played a decisive factor in terms of the reinforcement tensions and post-construction displacements. The connection load was much less in a wall with heavy compaction than that in a wall with light compaction. Results also showed that the position of maximum tensile force mobilized in the reinforcements was nearer to the face in the wall with heavy compaction. On the other hand, the mobilized tension measured along the reinforcement layers at the end of construction in the wall where heavy soil compaction was used was much higher than the values of tension measured in the wall where light soil compaction was applied. Nevertheless, it was observed that the difference in the mobilized tensions in the reinforcements of these two walls decreased with increase in the value of the external surcharge load.
This paper presents experimental investigations on an innovative construction method for reinforced soil structures by geosynthetics called prestressed reinforced soil. The concept of prestressed reinforced soil, (PRSi) developed to increase the bearing capacity of a reinforced soil structure and to improve its displacement behaviour is introduced. The concept of PRSi is validated by experimental studies. Large scale experimental tests conducted at the Institute of Soil Mechanics and Foundation Engineering at Graz University of Technology, Austria and their results are presented. Over 60 path-controlled static load displacement tests have been performed to investigate the load displacement behaviour of 10 different reinforced soil structures. The reinforced soil structures have been constructed under homogeneous laboratory conditions with respect to construction sequence, compaction, temperature and measurement equipment to assure high quality reproducible test results. The overall results show a considerable improvement of the macroscopic load displacement behaviour of the soil structure by utilizing the concept of prestressed reinforced soil. In addition 80 cyclic load displacement tests have been conducted in Weimar, Germany to validate the concept of PRSi under cyclic loading conditions. A soil element, theoretically taken out of a reinforced soil structure, is used to investigate its behaviour under vertical cyclic load and horizontal support conditions. The macroscopic research shows that displacements occurring under cyclic loading can be reduced tremendously by installing a geogrid with the concept of PRSi. Besides investigating the macroscopic load displacement behaviour of the reinforced soil structure a detailed mesoscopic analysis using the Particle Image Velocimetry (PIV) method has been performed. From the PIV analysis it was demonstrated that the vertical and horizontal displacements under cyclic loading and below the geogrid layer decreased rapidly.
Since the late 1990s, riverbank revetments constructed of sand-filled geotextile bags (geotextile bags) have been developed in Bangladesh in response to the lack of traditional erosion-protection materials, particularly rock. After independence in 1971 and the related loss of access to quarries, rock was replaced by concrete cubes, but those are expensive and slow to manufacture. Geotextile bags on the other hand, first used as emergency measures during the second half of the 1990s, can be filled with local sand and therefore provide the opportunity to respond quickly to dynamic river changes.Geotextile bags also provide the potential for substantial cost reduction, due to the use of locally available resources. The use of the abundant local sand reduces transport distance and cost, while local labor is used for filling, transporting, and dumping of the 75–250kg bags. Driven by the need for longer protection, the idea of using geotextile bags for permanent riverbank protection emerged in 2001. Eight years of experience have enabled systematic placement of geotextile bag protection along about 12km of major riverbanks at a unit cost of around USD 2M per km. By comparison, concrete-block revetments cost around USD 5M per km. In addition, there are strong indications that geotextile bags perform better than concrete blocks as underwater protection, largely due to their inherent filter properties and better launching behavior when the toe of the protected underwater slope is under-scoured.This article reports the outcome of the last eight years of development work under the ADB-supported Jamuna-Meghna River Erosion Mitigation Project (ADB, 2002), implemented by the Bangladesh Water Development Board. Besides substituting geotextile bags for concrete blocks as protective elements, the project involved development of a comprehensive planning system to improve the overall reliability and sustainability of riverbank protection works.
Storage tank foundations with frequent discharges and filling or road embankments under repeated traffic loads are examples of foundations subjected to the cyclic loading with the amplitude well below their allowable bearing capacity. The concern exists for the amount of uniform and non-uniform settlement of such structures. The soil under such foundations may be reinforced with geosynthetics to improve their engineering properties.This paper deals with the effects of using the new generation of reinforcement, grid-anchor, for the purpose of reducing the permanent settlement of these foundations under the influence of proportion of the ultimate load. Unloading-reloading field tests were performed to investigate the behavior of a square footing on the sand reinforced with this system under such loads. The effects of footing size and reinforcement types on the cyclic behavior of the reinforced sand were studied experimentally and numerically by the aid of computer code. The large-scale results show that by using the grid-anchors, the amount of permanent settlement decreases to 30%, as compared with the unreinforced condition. Furthermore, the number of loading cycles reaching the constant dimensionless settlement value decreases to 31%, compared with the unreinforced condition. Another goal of this paper is to present the equations for reinforced soil under cyclic loading to prevent such complicated calculation involved in deformation analysis. According to these equations, calculation of the permanent settlement and the number of load cycles to reach this amount for each foundation with a given size on the geomesh and grid-anchor reinforced sand, without further need to carry out the large-scale test, is supposed to perform easily.
A nonlinear finite element method (FEM) analysis technique is developed to simulate the viscous behavior of geogrid-reinforced sand during loading. In the FEM simulations, the viscous properties of sand and polymer geogrid are described in the framework of a unified nonlinear three-component elasto-viscoplastic model. The results from the plane-stain-compression (PSC) tests on the geogrid-reinforced sand specimen with the dimension of 96 × 62 × 120 mm are simulated using the developed elasto-viscoplastic FEM technique. In the PSC tests, the strain rate was changed step-by-step as well as the creep and stress relaxation tests were performed during monotonic loading (ML) at a constant strain rate. Both creep and stress relaxation tests lasted for 3 h. The FEM simulated average stress ratio-vertical strain-time relationships of geogrid-reinforced sand are compared with the measured ones from the PSC tests. The strain during creep loading stage is also simulated by the FEM. It is shown that the developed FEM analysis technique can simulate the stress-strain behavior of geogrid-reinforced sand well, especially for rate-dependent behavior, creep deformation and stress relaxation. The constraining effects due to the tensile reinforcing of geogrid layers can be observed clearly in the FEM simulation results.
At present an enormous amount of pond ash is being produced by thermal power plants throughout the world. Storage of pond ash requires vast land area and disposal of ash becomes problematic and also it creates environmental hazards. To mitigate these problems, pond ash has been used in the low-lying areas as structural fills for developing residential and industrial sites. To enhance the bearing capacity of pond ash, it may be reinforced with jute-geotextile, a textile made from jute (natural fibre) for the purpose. In the present study an attempt has been made to study the bearing capacity of square footing on pond ash reinforced with jute-geotextile. The effects of different parameters like number of layers (N) of reinforcement, the depth of the upper most layer of reinforcement from the base of the footing (u), friction ratio (f), i.e. the ratio of the pond ash jute-geotextile interface friction angle (ψ) to the direct shear friction angle of pond ash (φd) and jute-geotextile sheet length (Ls) on bearing capacity of square footing (qrs) at any settlement resting on pond ash reinforced with jute-geotextile are discussed. A non-linear power model has been developed to estimate qrs based on 1399 experimental data.
The creep of geosynthetics leads to the increase of Geosynthetic-Reinforced Soil (GRS) wall's deformation. More importantly, the influences of creep of geosynthetics are also affected by the creep properties of soils. In this paper, a Finite Element procedure was validated against a model test on the creep response of a clay–geotextile composite. An extensive parametric study was then carried out to investigate the long-term response of 8-meter-high model GRS walls with marginal backfill soils. The influences of backfill creep, reinforcement creep, reinforcement stiffness, reinforcement length and reinforcement spacing were analyzed. A long-term analysis was conducted for 5years and the results at the end of construction (EOC) and 5years afterwards were compared. It is found from the analysis that the relative creep rate between geosynthetic reinforcement and backfill soil influenced not only wall deformation but also reinforcement loads and stress states in the soils. The load distribution in backfill soil and reinforcement is the result of battling between their time-dependent properties. Large reinforcement creep can lead to large post-construction deformation and increase in soil stress; on the other hand, large soil creep can induce a significant increase in reinforcement load. It is hence necessary to take into account the relative creep rate of reinforcement and backfill soil in the design of GRS walls. It may not be adequate to consider only the long-term strength of reinforcement, which is the state-of-the-practice at present.
1. General Principle. The subject of this paper is,—the mathematical theory of that kind of stability, which, in a mass composed of separate grains, arises wholly from the mutual friction of those grains, and not from any adhesion amongst them. Previous researches on this subject are based (so far as I am acquainted with them) on some mathematical artifice or assumption, such as Coulomb’s “wedge of least resistance.” Researches so based, although leading to true solutions of many special problems, are both limited in the application of their results, and unsatisfactory in a scientific point of view. I propose, therefore, to investigate the mathematical theory of the frictional stability of a granular mass, without the aid of any artifice or assumption, and from the following sole.
Current reinforced earth structure designs arbitrarily distinguish between reinforced walls and slopes, that is, the batter of walls is 20 degrees or less while in slopes it is larger than 20 degrees. This has led to disjointed design methodologies where walls employ a lateral earth pressure approach and slopes utilize limit equilibrium analyses. The earth pressure approach used is either simplified (e.g., ignoring facing effects), approximated (e.g., considering facing effects only partially), or purely empirical. It results in selection of a geosynthetic with a long-term strength that is potentially overly conservative or, by virtue of ignoring statics, potentially unconservative. The limit equilibrium approach used in slopes deals explicitly with global equilibrium only; it is ambiguous about the load in individual layers. Presented is a simple limit equilibrium methodology to determine the unfactored global geosynthetic strength required to ensure sufficient internal stability in reinforced earth structures. This approach allows for seamless integration of the design methodologies for reinforced earth walls and slopes. The methodology that is developed accounts for the sliding resistance of the facing. The results are displayed in the form of dimensionless stability charts. Given the slope angle, the design frictional strength of the soil, and the toe resistance, the required global unfactored strength of the reinforcement can be determined using these charts. The global strength is then distributed among individual layers using three different assumed distribution functions. It is observed that, generally, the assumed distribution functions have secondary effects on the trace of the critical slip surface. The impact of the distribution function on the required global strength of reinforcement is minor and exists only when there is no toe resistance, when the slope tends to be vertical, or when the soil has low strength. Conversely, the impact of the distribution function on the maximum unfactored load in individual layers, a value which is typically used to select the geosynthetics, can result in doubling its required long-term strength.
The potential benefits of geosynthetic reinforced soil foundations are investigated using large-scale model footing load tests. A total of 34 load tests were performed to evaluate the effects of single and multiple layers of geosynthetic reinforcement placed below shallow spread footings. Two different geosynthetics are evaluated: a stiff biaxial geogrid and a geocell. Parameters of the testing program include the number of reinforcement layers, spacing between reinforcement layers, the depth to the first reinforcement layer, plan area of the reinforcement, the type of reinforcement, and soil density. Test results indicate that the use of geosynthetic reinforced soil foundations may increase the ultimate bearing capacity of shallow spread footings by a factor of 2.5.
Quantitative evaluations are performed on two failure mechanisms, namely, deep-footing and wide-slab mechanisms, that dominate the bearing-capacity characteristics of sandy ground reinforced with horizontal reinforcing layers. The improvement contributed by reinforcement, creating a quasi-rigid, wide earth slab immediately under the footing to the bearing capacity is analyzed. The maximization of the bearing-capacity increase of a footing is a matter of optimizing the depth and width of the quasi-rigid earth slab. The results of a total of 105 model tests, including reduced-scale model tests and some centrifuge tests under 15 g, are analyzed using calibrated internal friction angles of sand and an experimentally verified failure mechanism in reinforced sandy ground, namely, the deep-footing mechanism. Back-calculations are performed on the load-spreading angle a, which represents the total effect of the deep footing and wide slab for all the tests. Both linear and nonlinear multiple-variable data regressions are performed to find the relationships between the effect of reinforcing and the factors that control the scheme of reinforcement. The analytical procedure and equation derived in the present study substantiate the prediction of the ultimate bearing capacity of sandy ground reinforced with horizontal reinforcement.
Bridge abutments made of geotextile-reinforced soil have been shown to support the bridge load without the use of piles. However, current design procedures are considered to be conservative. To determine the strength, and to understand better the behavior of reinforced soil, large unconfined cylindrical soil samples reinforced with geosynthetics were axisymmetrically loaded. Samples were 2.5 ft (0.76 m) in diameter and 5 ft (1.52 m) in height. Peak strengths of 4.8 kips/ft 2 (230 kPa) to 9.6 kips/ft 2 (460 kPa) at 3% to 8.5% vertical strain were obtained from cylinders reinforced with geotextiles at 6-in. (152-mm) vertical spacing. A strength reduction occurred after the peak strength, but most of the loads were sustained up to at least 10% strain before yielding. Tension in the reinforcement appears to be mobilized first in the middle layers, as determined from the location of tears in the geotextile. An equation to calculate the tensile force in the reinforcement, T max, in a reinforced bridge abutment is proposed. The normalized strains led to the development of the strain distribution factor incorporated in the proposed equation. The proposed equation is slightly more conservative or almost equal, depending on the type of facing, when compared with the K o-stiffness method, but gives values approximately one-half of those obtained using the National Concrete Masonry Association and FHWA Demonstration Project 82 methods.
The effects of compaction at various levels of fill behind a retaining wall are considered analytically. The formulation is adapted for numerical solution and applied to cantilever and foundation walls with different fill geometries and properties. The resulting wall deflexions and earth pressure magnitudes and distributions considerably modify the ‘at-rest’ pressure and are practical significance.
On examine d'une façon analytique l'effect du compactage à divers niveaux de remblai posé derrière un mur de soutènement. On adapte cette formula afin de l'utiliser pour une solution numérique et on l'applique aux murs de fondations et en porte à faux à compactage variable en géométrie et en étéments. Les déflexions du mur et les diverses magnitudes de pression et de distribution qui en résultant amènent une modification considérable aux pressions ‘au repos’. On peut en tirer de conclusions pratiques.
The first part of the paper summarizes the results of recent research on the bearing capacity of spread foundations of various shapes under a central vertical load and outlines the effects of foundation depth, eccentricity and inclination of the load. Simple formulae have been derived for use in practice and their application to the design of rigid and flexible foundations is briefly indicated.The second part of the paper discusses the bearing capacity of single piles under vertical and inclined loads. The bearing capacity of piled foundations and free-standing pile groups is outlined, and the results of model tests on pile groups under central and eccentric loads are briefly analysed in relation to some problems in practice.