In the present study utilizing large scale direct shear test, the effect of ballast encasing with geogrid has been investigated. In this mater two ballast grading of 1 and 4 have been considered in conjunction with three types of geogrids GP35/35, GP40/20 and GP60/20. All direct shear test have been carried out under the vertical surcharges of50, 100 and 150 kPa with shear deformation rate of 1mm/min. The outcomes of the results reveals that in absence of geogrid encasing, the shear strength of ballast depends on the maximum particle size, uniformity coefficient and the vertical surcharge value. In the case of geogrid encasing of the ballast has promoted the shear strength behavior whereas for the grade 1 gradation of the ballast, the maximum friction angle corresponding to 50 and 150 kPa surcharges have been 74.5 and 68.96 degrees while the maximum reduction percentage in dilation angle has been 32.22. On the other hand for ballast grading 4, the mentioned variations have been correspondingly obtained as 30.12, 14.67 and 27.86 percentages.

One of the appropriate methods to improve the roadway that is built on the weak subgrade is use of geosynthetic reinforcement that was already highly regarded. Nowadays, using new materials such as gesynthetic Polymer in the pavement layers, is highly regarded due to technical specifications and better distribution of traffic loads and prevents local settlement. Existence of the reinforcement layer at the base-subgrade interface results in lower vertical pressure on the underlying layer (subgrade) due to wider distribution of load from truck traffic. Many field and laboratory test conducted to investigate the behavior of reinforced subgrade and achieve to the suitable design methodology has been implemented. However, due to limitations such as high cost of laboratory or field test, evaluate the performance of reinforced roads under traffic loads by numerical simulation methods have been developed. Numerical analysis are capable that with simulation of paved and unpaved road, develop the parametric studies for complex structures such as reinforced subgrade. In this study, the finite element method (FEM) is used to investigate the behavior of the geosynthetic reinforced roads. Finite Element Method is able to analyze stability, time-dependent problems and those problems with non-linear properties for the material. The simulation procedure is done in several steps. First the exact geometry and dimensions and other data of field test were extracted to produce the numerical model, then the Elastoplastic behavior is used to introduce materials behavior of pavement structure. Third, quasi static loading is also performing to acquire real simulation of experimental model. Therefore, the three-dimensional model in ABAQUS that has several advantages compared to the two-dimensional model conducted and the results of the 3d numerical study with field tests results in Montana university-Bazmn, is compared, then the effect of reinforcing mechanical properties, elastic module of base layer and reinforced subgrade on settlement was evaluated. The results showed that the simulation by finite element method, according to the assumptions used in three-dimensional modeling includes the quasi-static loading and frictional interaction parameters in terms of the contact surfaces, have a good agreement with the results of field test. FE analysis indicated that geogrid improves pavement life in terms of rutting. The numerical study results also show that the settlement of reinforced road strongly influenced by the mechanical properties of reinforcement. So with the existence of reinforcements, the section settlement will reduce up to 84 percent. FE analysis showed that benefits of reinforcement are more noticeable when stiffer geogrid are used. With reduce the elastic module of geogrid (about 50%) the surface settlement about 61% increased. If the base layer has a low stiffness, Existence of the reinforcement has more effect on reducing the settlement and the strain. Whereas increasing in the base layer stiffness, indicates the larger amounts of reduction on surface settlement with the existence of geogrid at the interface of base layers and subgrade and also effect the stiffness of subgrade and base layer in reinforced test section on the surface settlement compared to unreinforced test section about 50% reduced.

It has been of great interest among the researchers to investigate the behavior of soil_geogrid interface. Due to the wide use of geogrids between different layers of soil; these investigations are very important. This paper shows the results of experimental tests on soil_geogrid interface. This study conducts a series of large scale direct shear tests to investigate the interface shear strength of granular soil with various degrees of compaction and various sizes of geogrid apertures. The shear stress versus shear displacement curves and peak shear strength are important for evaluating the results. The interactions between soil and geogrid may include the following mechanisms: 1) shear resistance between soil and the surface of the geogrids; 2) internal shear resistance of the soil in the opening area; and 3) passive resistance of the transverse ribs. The value of α was defined to evaluate the effect of geogrid on the shear strength of the soil which is the ratio of geogrid_soil shear strength to internal shear strength of soil. The results showed that a larger degree of compaction reduces the resistance in soil_geogrid interface and shear strength of soil_geogrid interface will be reduced further by reducing the distances of transverse ribs of geogrid.

When soil beneath a foundation is weak and cannot bear applied loads, an appropriate method of soil improvement is essential. In cases where weak soil is improved by a replacing method, the improvement depth, due to the stress distribution pattern, is of great importance.However, in some cases, it is very difficult or too expensive to provide enough improvement depth. Hence, additional bearing capacity or other improvement methods are required. Using a geogrid to reinforce soil is an effective alternative way to improve the bearing capacity more effectively and economically.In this paper, the effect of replacing a loose layer by a dense one on the bearing capacity and settlement of a strip footing has initially been investigated. Then, the influence of a geogrid layer placed on the interface of loose and dense layers is analyzed and investigated using finite difference FLAC software. The numerical model is verified and calibrated using experimental data from a physical model developed in the Soil Laboratory of Amirkabir University.Analyses results show that increasing the depth of compacted soil layer by more than times the width of the foundation, does not have a considerable effect on the ultimate bearing capacity of strip footings, and only settlements will decrease. Using a geogrid layer between loose and compacted soil will improve the bearing capacity greatly, for a modified depth less than times the width of the foundation' The most efficient improvement in this method is when the geogrid layer is placed on the boundary of loose and dense layers at a depth of the foundation width. The efficiency of reinforcement is reduced by increasing the improvement depth. Using a reinforced layer with a width of less than times the foundation width is not recommended.

Piping is an erosive process that occurs in hydraulic structures under the influence of upward seepage. Lack of sufficient notice of this phenomenon may seriously affect the stability of hydraulic structures. In this research work the effect of reinforcement soil on the variations of critical hydraulic gradient and seepage force were investigated through experimental tests. Reinforcement of samples was performed by two geo grids with mesh diameters of 6mm (No. 1) and 2mm (No. 2). One dimensional piping test were carried out on non-reinforced vs. reinforced sandy soil samples, compacted and fabricated through static methods, and in a specially designed apparatus. The results indicated that the critical hydraulic gradient and resistance against seepage force increased by reinforcement of the samples and that the resistance is a function of the number of the geogrid sheets as well as their location. In addition, the results indicated that the effect of the two geo grids is nearly the same for treatment of the soil against piping.

The purpose of this research is to investigate the effect of geogrid layers on the seismic behavior of stone columns. In this article, it has been addressed by both manual and numerical methods. In addition, the seismic behavior of stone columns covered with geogrid was investigated. By using numerical modeling and finite element method and by using two-dimensional Plexis software, the seismic behavior of stone columns with geogrid covering in the near and far field compared to normal stone columns was evaluated. The results show that the bearing capacity of a stone column increases with the use of a geogrid cover, and the smaller the diameter of a normal stone column or one with a geogrid cover, the lower the load capacity in a static state, and the use of a geogrid cover increases the bearing capacity of a stone column. In the case of plate loading, the smaller the diameter of the normal or geogrid-covered stone column is, the less load-bearing capacity it has in static mode, and the use of geogrid cover increases the load-bearing capacity of the stone column. Based on the results of numerical analysis, geogrid reduces the lateral deformation and displacement of stone columns during an earthquake. By examining different types of geogrid with different hardness, it was found that the type of geogrid had no effect on the seismic behavior of the stone column. Also, covering the upper half of the stone column with geogrid was enough to improve the seismic behavior of the stone column, and of course, this work is also economical.

Large size direct shear tests (i.e.300 x 300mm) were conducted to investigate the interaction between clay reinforced with geogrids embedded in thin layers of sand. Test results for the clay, sand, clay-sand, clay-geogrid, sandgeogrid and clay-sand-geogrid are discussed. Thin layers of sand including 4, 6, 8, 10, 12 and 14mm were used to increase the interaction between the clay and the geogrids. Effects of sand layer thickness, normal pressure and transverse geogrid members were studied. All tests were conducted on saturated clay under unconsolidated-undrained (UU) conditions. Test results indicate that provision of thin layers of high strength sand on both sides of the geogrid is very effective in improving the strength and deformation behaviour of reinforced clay under UU loading conditions. Using geogrids embedded in thin layers of sand not only can improve performance of clay backfills but also it can provide drainage paths preventing pore water pressure generations. For the soil, geogrid and the normal pressures used, an optimum sand layer thickness of 10mm was determined which proved to be independent of the magnitude of the normal pressure used. Effect of sand layers combined with the geogrid reinforcement increased with increase in normal pressures. The improvement was more pronounced at higher normal pressures. Total shear resistance provided by the geogrids with transverse members removed was approximately 10% lower than shear resistance of geogrids with transverse members.

The goal of this research is to measure the interface properties between a geogrid and a soil (i.e. the friction angle and dilatancy angle) as well as the strength parameters of materials used in the tests in order to be able to use the parameters in numerical analysis. A comprehensive set of large shear box tests were conducted on the geogrid. Apart from the main findings, some interesting results concerning the mechanisms behind the behaviour of reinforcement in direct shear tests were obtained.

The use of stone columns is one of the effective ways to increase the bearing capacity of soils. An alternative system that can provide sufficient lateral confinement to support stone columns and increase bearing capacity is geosynthetic encased stone columns. These methods have been well utilized in Europe and South America. If the soil bed requires excessive confinement, the use of geotextile and geogrid encase around the stone columns is one way to improve the performance of these load-bearing members. This study aims to compare the behavior of geotextile and geogrid layers in reinforcing stone columns in standard Ottawa sand. In this study, a series of triaxial experiments in the undrained state was used. In the lowest confining pressure case, the load-bearing capacity for the geotextile reinforced column will be 1. 18 times higher. Whereas for the geogrid-reinforced stone column, the load-bearing capacity is 1. 31 times higher. In this study, standard Ottawa sand, gravel with a unit weight of 17 kN/m3 and a friction angle of 47. 8° , geotextile and geogrid layers, and triaxial test apparatus are used. Triaxial specimens were 10 cm in diameter and 20 cm in height. Stone column dimensions of 2 cm in diameter and 20 cm in height are selected, respectively. Due to the limitations in the laboratory and the simulation of natural conditions, the unit weight of sand samples and stone columns made in triaxial test molds were selected as 15 and 17 kN/m3, respectively. Precipitation is used to fabricate cylindrical sand samples for triaxial testing. In this method, firstly attach the membrane to the underside of the triaxial apparatus and fasten the detachable bifurcation mold to the membrane and attach the membrane to the detachable mold walls by suction pumping about 2 bars. The aim is to create a homogeneous sample with uniform rainfall velocity to obtain a sample with evenly possible porosity. The method of precipitation depends on two parameters, one is the intensity of rainfall (amount of sand poured in a given volume at a specified time), and the other is the height of the sand fall, which is the distance between the sand outlet from the precipitation tank to the sand bed. The important point is that to achieve the same porosity, and this distance must be kept constant throughout the precipitation process. After construction, the test is performed according to ASTM D7181-11. Triaxial CU experiments on Ottawa sand were carried out in three cases: unreinforced, reinforced using geotextile encased stone column and reinforced using geogrid encased stone column. In triaxial experiments, three confining pressures of 200, 300, and 400 kPa were used.