One of the efficient ways for reinforcing the earth’s slope is Geogrid Encased Stone Column (GESC). This technique can increase bearing capacity and decrease the settlement rate of the slope. The goal of this study is to perform a three-dimensional finite-difference numerical study on the behavior of GESC in the stabilization of sand slope. According to the results of the three-dimensional finite-difference analysis, the existence of GESC in the middle of the sand slope, as the optimal location for stone column placement, increased stability to an ideal level compared with the ordinary stone column (OSC). Different parameters including stone column diameter, coupling spring cohesion, coupling spring friction, center to center distance of columns (S/D ratio), and the layout of encasements were evaluated and discussed in this paper. The results indicated that the efficient parameter in geogrid is coupling spring cohesion; in which by increasing this parameter, slope bearing capacity increased linearly (i.e. the bearing capacity of slope reinforced using GESC could enhance up to 1.8 times of slope reinforced using OSC). In the case of row stone implementation, the maximum bearing capacity was that of S/D=2. However, a decrease in the S/D ratio did not necessarily increase the bearing capacity of slopes.

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.

Introduction: Stone column installation method is one of the popular methods of ground improvement. One of the common uses of stone columns is to increase slope stability. Several studies have been performed to examine the behavior of stone columns under vertical loads. However, limited research, mostly focused on numerical investigations, has been performed to evaluate the shear strength of soil reinforced with stone column. The study presented herein is an experimental program, aimed to explore the shear strength of loose sand bed reinforced with stone column. Direct shear tests were carried out on specimens of sand bed material, stone column material and sand bed reinforced with stone column, using a direct shear device with in-plane dimensions of 305*305 mm2 and height of 152. 4 mm. Experiments were performed under normal stresses of 35, 55 and 75 kPa. In this study, 4 different area replacement ratios (8. 4, 12, 16. 4 and 25%), and 3 different stone column arrangements (single, square and triangular) were considered for investigation. The obtained results from this study showed that stone column arrangement had an impact on improving the shear strength of stone columns. The most increase in shear strength and stiffness values was observed for square arrangement of stone columns and the least increase was for single stone columns. This study also compares the equivalent shear strength values and equivalent shear strength parameters (internal friction angle and cohesion) measured during experiments with those predicted by analytical relationships. Results show that shear strength values and shear strength parameters measured from experiments are higher than those obtained from analytical relationships. Accordingly, a corrective coefficient was calculated for each column arrangement to represent the correlation between experimental and analytical results. Material Properties of Loose Bed and Stone Column: Fine-grained sand with particle size ranging from 0. 425 to 1. 18 mm was used to prepare loose sand bed, and crushed gravel with particle size ranging from 2 to 8 mm was used as stone column material. The sand material used as bed material had a unit weight of 16 kN/m3 and a relative density of 32. 5%, and the stone material used in stone columns had a unit weight of 16. 5 kN/m3 and a relative density of 80%. The required standard tests were performed to obtain the mechanical parameters of bed material and stone column material. As the diameters of model scale stone columns were smaller than the diameters of stone columns installed in the field, the particle dimensions of stone column material were reduced by an appropriate scale factor to allow an accurate simulation of stone columns behavior. Testing Procedure: In this study, large direct shear device with in-plane dimensions of 305*305 mm2 and height of 152. 4 mm was used to evaluate the shear strength and equivalent shear strength parameters of loose sand bed reinforced with stone column. Experiments were performed under normal stresses of 35, 55 and 75 kPa. Two class C load cells with capacity of 2 ton were used to measure and record vertical forces and the developed shear forces during the experiments, and a Linear Variable Differential Transformer (LVDT) was used to measure horizontal displacement. All achieved data from the experiments including data on vertical forces, shear forces and horizontal displacements were collected and recorded using a data logger, and an especial software was used to transfer data between the computer and the direct shear device. All specimens were sheared under a horizontal displacement rate of 1 mm/min. Testing Program: Experiments were performed on single stone columns and group stone columns arranged in square and triangular patterns. The selected area replacement ratios were 8. 4, 12, 16. 4, and 25% for single stone columns, and 8. 4, 12 and 16. 4% for square and triangular stone column arrangements. To eliminate boundary effects, the distance between stone columns and the inner walls of the shear box was kept as high as 42. 5 mm. In total, 12 direct shear tests were carried out, including 2 tests on loose sand bed material and stone column material, and 10 tests on stone columns with different arrangements. From the tests performed on group stone columns, 4 tests were performed on single stone columns, 3 tests on stone columns with square arrangement and 3 tests on stone columns with triangular arrangement. Hollow pipes with wall thickness of 2 mm and inner diameters equal to stone column diameters were used to construct stone columns. To prepare the specimens, first, the hollow pipes were installed in the shear box according to the desired arrangement. Then, bed material with unit weight of 16. 5 kN/m3 was placed and compacted in the box in 5 layers, each 3 cm thick. Stone material was uniformly compacted to construct stone columns with uniform unit weight. The compaction energy was 67 kJ/m3 in all tests. Results and discussion: In this paper, the behavior of stone columns under shear loading was experimentally investigated in large direct shear device by performing tests with different area replacement ratios (8. 4, 12, 16. 4, and 25%), different stone column installation arrangements (single, square and triangular), and different normal stresses (55, 75 and 100 kPa). The key findings of this study are as follows: 1. Shear strength increases with increase of area replacement ratio due to the higher strength of combined soil-stone column system, and due to the increase of stone column area effective in shear plane. The amount of shear strength increase with area replacement ratio is low for ratios lower than 15%. However, this amount is higher for area replacement ratios higher than 15%. 2. For stone columns with equal area replacement ratios, higher shear strength was mobilized in stone columns with square and triangular installation arrangements compared to single stone columns. Among the installation patterns investigated in this study, stone columns with square arrangement experienced the highest increase in shear strength value, while single stone columns experienced the lowest. One of the reasons of shear strength increase in square and triangular patterns is the increase of confining pressure applied by stone columns to the soil between them. Another reason is the increase the total lateral surface by changing the column arrangement from single column to square and triangular patterns. This increased lateral surface increases the lateral force imposed on the stone columns, resulting in higher shear strength mobilization of stone material. 3. The slope increase of shear strength-horizontal displacement curves shows that soil-stone column system has higher stiffness than loose sand bed, and this stiffness varies with area replacement ratio and installation pattern. The maximum stiffness values refer to stone columns installed in square pattern and the minimum values refer to single stone columns. In general, stone column installation pattern has an effective role in increasing stiffness. 4. Results show that shear strength parameters increase in soil reinforced with stone column. The maximum increase in internal friction angle refers to stone columns with square pattern and the minimum increase refers to single stone columns. 5. The equivalent shear strength values measured from experiments are higher than those obtained from analytical relationships. Accordingly, it is conservative to use analytical relationships to calculate shear strength parameters. It is worthy to mention that these relationships assume that the value of stress concentration ratio is equal to 1. Results from this study indicate that the value of stress concentration ratio should be accurately calculated and used in the relationships. 6. As discrepancy was observed between values measured from experiments and those obtained from analytical relationships, corrective coefficients were calculated to modify analytical relationships. These coefficients were computed and presented based on stone column installation pattern, area replacement ratio and the applied normal stress values.

The increasing demand for engineered cut and fill slopes on construction goals has increased the need of understanding the analytical methods, investigation tools and the most important stabilization methods to solve slope stability problems. The first step to maintain the stability of soil slope is performing excavation in the slope crest or/and filling the slope toe. This is the cheapest method for stabilization of soil slopes. If the method cannot provide the required factor of safety, it is necessary to use other stabilization methods. Numerical and laboratory approaches are useful for modeling soil slopes stabilization. Modeling the stability of earth slopes using numerical methods is a common practice in geotechnical engineering. Moreover, stabilization of soil slopes using piles has been practiced by many researchers in numerical and analytical approaches. Although numerical and analytical methods have special capabilities, laboratory modeling is more reliable. Stability slope analysis has attracted lots of researchers around the world and it shows the significance of this matter. When suspicious about stability of soil slopes, immediate actions and preventative steps should be used for suppression of instability occurrence. Many projects intersect with valleys and rides, which can be prone to slope stability problems. Natural slopes that have been stable for many years may suddenly fail because of many reasons; therefore, finding useful techniques for stabilizing them is a great concern for geotechnical engineers. In all soil slopes, the primary way for stabilization is the excavation in slope crest and/or filling slope toes. If this would not increase safety factor, other procedures should be applied. Three common styles of stabilization methods are; vertical reinforcement (such as stone columns and piles), horizontal reinforcement (like Geo-grids), oblique reinforcement (such as nailing). One of the common methods that is used to increase the safety factor of slopes is stone columns. All of the experimental tests were modeled and compared using the limit equilibrium (LE) and finite element (FE) methods, which are compliant with each other. Understanding soil properties is crucial for analysis of soil slopes. In this study, the effect of cohesion in embankment is investigated. This is carried out by performing laboratory tests and using finite element method software (PLAXIS2D) and finite difference method software (FLAC3D). A sand slope is reinforced with a stone column at the middle of slope. It is then saturated by precipitation and loaded up to the failure. Experimental studies in this article have the potential to give valuable information about the effects of embankment cohesion and penetration depth of stone column into the stiffer layer, in stability of stone column reinforced soil slopes.

Soft and loose soils have always been the focus of attention due to their high settlement and insufficient bearing capacity. The foundations on soft soils and soil slopes containing this type of soil should be modified for high probability of failure. There are various ways to improve these soils, which vary greatly depending on the environmental conditions, site priority, the degree of softness, cost of materials used. Stone column is one of soft soil modification and soil slopes safety factor improvement methods which is economical and easy to implement. The aim of this study is to conduct a numerical study to investigate the effect of stone columns on the bearing capacity of the clay slope and its stability using the FLAC3D finite difference software. the slope is modeled in three dimensions with appropriate dimensions and boundary conditions. The bearing capacity was obtained by stress control procedure and slope safety factor was calculated by shear strength reduction method. In general, two types of failures observed in stone columns. The columns were Located under the footing Were exposed to phenomenon of bulging and the columns were Located outside the footing Were exposed to phenomenon of lateral displacement. The results showed that in general, the stone column will improve the stability of clay slope and its bearing capacity. The use of stone column below the foundation will affect the most on increasing bearing capacity and the effect will decrease by increasing center to the center distance of foundation and columns, so that at 4 times the diameter of stone column it will not have much effect on the bearing capacity. In the case of group columns, the arrangements are along the length of footing is preferred over the arrangements are along slope due to the higher replacement area.

Introduction: The design engineers usually follow a specific decision-making process for optimal selection of the type of required foundation and its design. In this state, in case the surface foundation is not appropriate for the project conditions, before making any decisions about the use of deep foundations, the proper methods for optimization of the liquefied soil should be evaluated in order to compare the advantages and disadvantages of each of them with those of deep foundation, in terms of efficiency, implementation problems, costs, and finally to select the best choice. One of the best methods of soil improvement is the use of stone columns. The rationale behind the use of stone columns is the high shear strength of materials and the provision of lateral grip by surrounding soil...

In recent decades, the growth of infrastructure in urban and metropolitan areas has led to a significant increase in the value of land and the lack of suitable places for development. These factors cause the construction industry to look for cheap land for construction; As a result, the building is run on the loose ground against the previous tendency. One of the methods that have been used in recent years in loose deposits and fine soils is stone columns, which are widely used to reduce subsidence and increase the soil bearing capacity of structures such as fluid storage tanks. Embankments, broad foundations, and light structures are used. The use of rock columns as a soil improvement method has been the subject of research by many researchers for the past 40 years. Numerous analytical methods, numerical analyze, and laboratory tests have been performed to investigate the effect of different soil and rock column parameters on the bearing capacity and subsidence of rock columns. One of the most significant research achievements in this field is the idea of reinforcing stone columns with geosynthetic cover, which was proposed more than 25 years ago. During this period, relations have been provided for designing these reinforced stone columns and considering the effect of geosynthetic cover. This paper presents the results of researchers' research on the effect of various parameters on the bearing capacity and subsidence of stone columns.In recent decades, the growth of infrastructure in urban and metropolitan areas has led to a significant increase in the value of land and the lack of suitable places for development. These factors cause the construction industry to look for cheap land for construction; As a result, the building is run on the loose ground against the previous tendency. One of the methods that have been used in recent years in loose deposits and fine soils is stone columns, which are widely used to reduce subsidence and increase the soil bearing capacity of structures such as fluid storage tanks. Embankments, broad foundations, and light structures are used. The use of rock columns as a soil improvement method has been the subject of research by many researchers for the past 40 years. Numerous analytical methods, numerical analyze, and laboratory tests have been performed to investigate the effect of different soil and rock column parameters on the bearing capacity and subsidence of rock columns. One of the most significant research achievements in this field is the idea of reinforcing stone columns with geosynthetic cover, which was proposed more than 25 years ago. During this period, relations have been provided for designing these reinforced stone columns and considering the effect of geosynthetic cover. This paper presents the results of researchers' research on the effect of various parameters on the bearing capacity and subsidence of stone columns..

The aim of this research is to experimentally investigation of ordinary stone column and reinforced stone column using horizontally laminated geotextile disks in stabilizing sandy slopes. The sand slope model saturated through precipitation and reinforced by installing ordinary stone column and horizontal layers of geotextile within the stone column, and then slope crest undergo loading. Accuracy of experimental results confirmed using 3-D finite difference method (FLAC3D). Both experimental and numerical results show that geotextile reinforced stone column in the middle of sandy slope have an impressive impact on increasing stability of reinforced slope. In such way that reinforced stone column, increase shear strength of sandy slope up to 1. 5 times more than ordinary stone column.