Journal of Engineering Geology

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Introduction
Many studies have shown that the lime stabilization method can increase the strength and hardness of cohesive soils. Increasing these parameters is dependent on several factors such as curing time, lime content, clay minerals, soil particle size and moisture content.
When lime is added to moisture clay soils, a number of reactions occur to improve soil properties: 1- short-term and 2- long-term reactions. The short-term reactions include cation exchange, flocculate and carbonation; whereas, the long-term reactions include pozzolanic reactions. Since adding lime changes clay particles structure, it can change shear strength parameters.
Using geogrids as reinforcement in soil mass creates a composite system in which the soil tolerates compressive stresses. The elements of the reinforcement are also responsible for tensile stresses and interaction the reinforcement elements and soil increases the strength and ductility. The mechanism of stress transfer is based on interaction between soil and reinforcement. Accordingly, one of the most important issues in the analysis and design of reinforced soil structures is determination of frictional resistance parameters in soil-geogrid interface (adhesion and friction angle) which is discussed in this paper.
Stability and performances of reinforced earth structures significantly depend on the shear behavior of interface soil-geogrid in different weather conditions. Factors such as rainfall, seepage of groundwater and seasonal changes influence on soil moisture content. Changes in moisture content or soil dry density change interface soil-geogrid resistance. Increasing moisture content reduces the shear strength of reinforced soil and sometimes leads to large deformation or failure of system.
In this study, clayey soil with low plasticity (CL), hydrated lime for soil stabilization and two types of geogrid with different aperture size for reinforcing were used. In order to improve the brittle behavior of lime stabilized soils and to increase ductility of the samples, in the present study, lime stabilization and geogrid reinforcement was investigated, simultaneously. The interface shear strength parameters of treated soil with different lime content-geogrid and reinforcement coefficient were determined by direct shear tests. In addition, to study the effect of moisture content on interface shear strength soil-geogrid, all samples were subjected to shear in optimum and higher moisture content because the long-term performance of reinforced cohesive soils exposed to seasonal variations is evaluated.
Material and methods
The selected soil for the study is clayey soil from south region of Tehran, Iran. According to Unified Soil Classification System (USCS), the soil was classified as CL (clay of low plasticity).
In this study, three series of specimens were prepared and tested as follows:

• Stabilized samples with 0, 2, 4 and 6% lime for 7 days curing time
• Reinforced samples by geogrid (with and without transverse ribs of geogrid)
• Reinforced stabilized samples with different lime contents (0, 2, 4, 6 and 8%) by geogrid (with and without transverse ribs of geogrid) for 7 days curing times

To investigate the effects of bearing resistance provided by the transverse members of the geogrid and their contribution to the overall strength for reinforced soil sample, numerous tests were conducted with the geogrid without transverse members (all the samples had the same number of longitudinal members of the geogrid).
Direct shear tests were carried out on specimens based on ASTM D5321 at constant horizontal displacement rate of 1 mm/min.
Results and discussion
The results reveal that the shear strength of the stabilized soil increased and there are maximum values in an optimum lime content which is about 4%. Increasing lime content to an optimum lime content of clay caused the maximum changes in clay minerals because of cementitious and pozzolanic reactions and increases the strength of the clayey soil. Reduction of strength by adding lime to the soil more than the optimum content may be caused by the following reasons:
1. Stopping pozzolanic reactions because of finishing reactance during reaction
2. Making difficult the release of limewater (Ca OH ₂) in the cementitious context of soil.
Until SiO₂ and AL₂O₃ are not finished, pozzolanic reactions continue and produce cementitious product, thus the shear strength increases and improves the long-term performance of the stabilized soils.
Reinforced soil samples have higher shear strength relative to samples without reinforcement subjected to the same normal stress. This increase in shear strength is mainly attributed to the interlocking of soil particles that penetrate through geogrid apertures. In addition, geogrids restrain particles´ movement and thus increase the mobilized frictional resistance at particle contact points.
Increasing in lime content to 4% (optimum lime content in this study) has significant effect on the development of adhesion and then decreases gradually with increasing of lime content from 4 to 6%, while friction angles remain constant approximately.
Adhesion and friction angles decrease with increasing moisture content.
The results show that the reinforced stabilized specimen with 4% lime has the maximum value of reinforcement efficiency. The increase in moisture content can significantly reduce the reinforcement efficiency.
It is clearly observed that the reinforcement coefficient of reinforced stabilized sample by geogrid that has smaller aperture opening size (4Í4 mm) is higher than reinforced stabilized sample by another geogrid (10Í10 mm) in optimum and higher than optimum moisture content.
Conclusion
One hundred and twenty samples in 3 specimen categories including lime treated, reinforced and reinforced treated samples were prepared for the current study for 7 days curing time in optimum content and higher than optimum content. The main results can be concluded as:
The test results indicate that the shear strength of stabilized clayey samples increases after 7 days curing time due to pozzolanic reactions.
The results show that reinforced samples have higher shear strength relative to unreinforced samples.
Adhesion and friction angles and reinforcement efficiency decrease with increasing moisture content.
The reinforcement coefficient of reinforced stabilized sample by geogrid 1 that has smaller aperture opening size is higher than by geogrid 2. In general, interaction between particles and geogrid with smaller mesh size is stronger because of matching the size of soil particles and meshes../files/site1/files/123/8Extended_Abstract.pdf
 

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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../files/site1/files/133/2Extended_Abstracts.pdf 

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Sandy soils usually contain different amounts of fines like silt and clay, causing some changes to their shear strength and dilation characteristics. Bolton [1] conducted  some experiments on the different sands and suggested a relation between the parameters of the soil shear strength. In this paper, some experiments were performed on fine contained sand and the extended Bolton's relation was has been proposed. In this paper, shear strength and dilation behavior of a pure sand mixed with different amounts of silt or clay fines were studied using direct shear test device (100*100*30 mm), and a total of 96 tests were carried out. The samples were prepared separately using clay and silt contents of 0, 10, 20 and 30% in different relative densities of 70, 80, 90 and 100%. They were tested under three surcharge pressures of 90, 120 and 150 kPa, under particle crushing threshold. Variations in shear strength, maximum friction angle, critical state friction angle and cohesion, as well as dilation angle were investigated by increasing in the mentioned amounts. The results demonstrate that shear strength, dilation angle, maximum friction angle decreased by clay content increase, however, they increase with increase in silt content. In addition, a new form of the Bolton's relation for fine contained sandy soils was presented.

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Introduction
Geofoames are used as a light weight fill material in those places which soil borrows is not cost effective for engineering or economic purposes. In general, geofoames are highly capable of improving some of geotechnical properties of soils such as inflation creation, reduction of density, and etc., due to their light weight, no change of volume against water, low permeability, and relatively proper strength. Using mixture of geofoam beads and soil has been recently taken into consideration by researchers. The mixture causes tangible reduction of soil density and severe drop of active pressure of retaining walls. Also, using the mixture in seismic zones is of special importance. In the paper, effect of mixing geofoam (4 different percent) and three types of poorly graded sandy soils have been dealt with. The research innovation has been compared to previous ones is using poorly graded sandy soil, separating geofoam beads based on their diameter, and reviewing the effect of adding various percentage of geofoam on improvement of poorly graded sandy soil’s properties.
Materials and Test Method
Tests have been performed in direct shear box (10 ^cm x10 ^cm) under three stress levels of 50, 100, and 150kPa. First type of soil has been Firoozkooh sand (#161) with specific gravity of 2.65, as uniformly graded sand (SP). Second type of soil has been mixture of uniformly graded sand and 10% silt (SM-SP); and, third type of soil has been mixture of Firoozkooh sand and 20% silt (SM). The three above types of soils have been named as soil 1, soil 2, and soil 3, respectively.
Geofoam beads have been all fine grained, passing through sieve No. 10; and, their added weighted values have been 0, 0.2, 0.4, and 0.6% of weighted percentage of soil. All of tests have been performed with optimum moisture content of geofoam and soil mixture. Due to diversity of soil types and ratio of geofoam-soil mixtures, soil compaction test has been performed on each direct shear test’s sample to specify optimum moisture content of various types of mixtures; because there have been various types of soils used, and also various ratios of soil and geofoam mixtures.
Results
According to the results, using geofoam beads leads to considerable reduction of soil density. Decrease made in density will be more tangible when higher percentages of geofoam are added to the soil. Also, as far as geofoam absorbs water, optimum level of moisture will be increased through increase of geofoam percentage in soil-geofoam mixture.
Since geofoam beads are less rigid compared to grains of sand, sand and geofoam interlocking and friction level is lower than sand interlocked to sand; and shear strength has been decreased through increase of geofoam percentage in soil. The point to be remembered is that, reduction level of shear strength in soils containing various percentages of geofoam is not so tangible compared to the soil itself. In its worst case, the reduction would be about 12%.
Adding geofoam beads to all of the three types of soil has led to their increase of apparent cohesion. Moreover, through increase of mixture percentages, more increase has been made in apparent cohesion of mixture. The results are indicative of significant effect of mixing geofoam and soil 1 in increase of soil cohesion up to 9 times. The cohesion increase has been about 4 and 2 times for soils type 2 and 3 respectively. So, it could be concluded that the lower the soil cohesion, the higher would be effect on cohesion increase of soil, through increase of geofoam percentage.
In figure 1, chart of internal friction angle is shown based on mixture percentage of geofoam for those types of soils being tested. Considering decrease of internal friction angle through increase of geofoam percentage, the important point is slope drop observed when geofoam percentage added has been 0.4%. Therefore, reduction speed of internal friction angle has become slower, after this level. Considering the figure, internal friction angles of soils type 1, 2, and 3 have shown respectively 15, 16, and 18% of reduction, through highest percentage of geofoam added (0.6%).
Figure 1- Internal friction angle based on geofoam percentage mixed with different soils
Comparing the results from present and previous researches, it could be concluded that adding higher percentages of geofoam results in cohesion increase of sandy soils; however, the increase level is different for various types of soils. The lower the initial cohesion of sandy soils and the more uniform their gradation, the more the effect of adding geofoam on increase of cohesion coefficient of soil. Also, downward trend of internal friction angle for well graded and poorly grades sandy soils is almost similar.
Using the results from present research and considering acceptable level of reduction made in internal friction angle of the soil mixed with geofoam against cohesion increase and reduction of soil density; mixture of geofoam beads and soil could be used in construction of embankments, retaining walls and other earth structures, appropriately.
 

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Introduction
Stone column installation method is one of the popular methods of ground improvement. Several studies have been performed to investigate 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 stress concentration ratio (n) is one of the important parameters that uses in soil improvement by stone column method. Stress concentration ratio is the ratio of the stress carried by stone column to that carried by the surrounding soil. In this paper, the results of a laboratory study were used to examine the changes in the stress concentration ratio when normal and shear stress applied. 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 mm and height of 152.4 mm. Experiments were performed under normal stresses of 55, 77 and 100 kPa. In this study, three different area replacement ratios (8.4%, 12%, 16.4%), and three different stone column arrangements (single, square and triangular) were considered for investigation. Loose sand and crushed gravel were used to make the bed and stone columns, respectively. In this study, the equivalent shear strength and equivalent shear parameters measured from experiments were also compared with those predicted by analytical relationships at stress concentration value of 1 and stress concentration value obtained from experiments.
Material Properties
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 crushed 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 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 55, 75 and 100 kPa. Two class C load cells with capacity of 2 tons 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. The main objectives of this study was to calculate the stress concentration ratio of stone columns in different arrangement. Stress concentration ratio is the ratio of the stress carried by stone column to that carried by the surrounding soil, and can be calculated using Equation 1. For this purpose, the direct shear device was modified. Two miniature load cells with capacity of 5 kN were employed. The load cells were mounted on the rigid loading plate with dimensions of 305*305 mm² and thickness of 30 mm, as shown in Figure 1, 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.
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 and 16.4% for single, 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, 11 direct shear tests were carried out, including two tests on loose sand bed material and stone column material, and 9 tests on stone columns with different arrangements. From the tests performed on group stone columns, 3 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.
Results and discussion

1. The SCR value increases for settlement up to 3 mm and then decreases with increasing the horizontal displacement and then approaches almost a constant value. Results also show that stress concentration ratio decreases with increase of stone column diameter. Results show that the value of stress concentration ratio in square pattern is higher than that in single and triangular pattern. Moreover, results show that stress concentration ratio decreases with increase of normal stress.
2. The value of the internal friction angle in (peak) state, for loose bed increases from 33 to 40 degrees in square arrangement and in the corresponding state of displacement of 10 % from 30 degrees in a loose bed increase to 32 degrees, for loose sand reinforced with stone column. Shear strength increases with the increase of modified area ratio in all stone column installation patterns in both the peak and the corresponding state of the horizontal displacement of 10%.
3. For stone columns with the same modified area ratio, the installation pattern has an effective role in defining the shear strength. Group stone columns mobilize higher shear strength 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.
4. 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 worth explaining that these relationships assume that the value of stress concentration ratio is equal to 1. Results from this study show that the value of stress concentration ratio should be accurately calculated and used in the relationships.
5. Comparison between shear strength parameters obtained from experiments and those predicted by analytical relationships shows that in single stone columns, the value of stress concentration ratio should be 3 to 4.5, and in square and triangular patterns, this value should be 6 to 7 and in triangular patterns 4.5 to 5, respectively, to achieve good agreement between experimental and analytical results in peak condition. In horizontal displacement 10% the value of stress concentration ratio should be 2.5 to 3, in single, square and triangular patterns, to achieve good agreement between experimental and analytical results../files/site1/files/151/2.pdf