Journal of Engineering Geology

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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. Therefore, the stone column can receive the load from the structure, and transfer it to the resistant layers. In the soils with low shear resistance, the lateral constraint crated by the surrounding soils is not enough for preventing the sideway buckling of the column under which is subjected to the loads. Thus, special measures should be considered for the use of stone columns in these soils. One of these methods is the use of reinforcement shelves such as geogrid and geotextile. Investigating the previous studies, the lack of evaluation of the design parameters such as the settlement ratio of the soil improved by the reinforced stone column to the geogrid, and provision of design graphs in this regard, has been revealed. Therefore, by extension of the studies conducted by Chub Basti et al. in 2011, the design graphs were provided in this regard.
Material and methods
The PLAXIS V8 Software was used for modelling the soft soil improved by the stone column. For increasing the precision of the results, the 15-knot element was used in the current study. The fine mesh was used in the models made for the analysis of the problem. For simulation of the improved soft soil with the stone column in a single cell, the modelling was implemented in a two-dimensional environment in axial symmetry conditions. In the current study, it was assumed the rigid foundation is on the improved bed. Thus, for analysis of the simulated model, a vertical strain up to 2% of the soft soil height has been applied on the ground. Also, for simulation of the soil behavior, an appropriate model of soil and parameters proportional to the materials should be allocated to the construct geometry. The non-linear stress-strain of the soil in different levels of the problem can be simulated. The number of model parameters increases with the level of problem rupture. For precise simulation, we need the proper parameters of the materials. For modeling of soft soils and stone columns, elastic-plastic model with Mohr-Coulomb rupture criterion was used. In the current study, it was assumed the soft bed is located on a very hard layer of soil. Therefore, the vertical deformation was prevented on this horizontal boundary. Also, the horizontal deformation in two vertical edges was prevented and only deformation in vertical direction was allowed. The soft bed close to saturation was considered without the determined free water level. For models with stone columns, the element of interface between the stone column and soft soil, has been used. The reason behind using this element is that the stone column rupture is of shear form and due to this, a significant shear stress is created on the common surface between the stone column and soft soil. The percentage of the replacement area is defined as the ratio of the total area of the stone columns to the total area of the non-improved area. In the current study, the percentage of the replacement area is utilized between 10 to 30%, which is used in implementation. Also, the diameter of the stone columns is from 0.6 to 1.2, in the analyses.
Results and discussion
The results of the numerical study were compared with the existing theoretical relationships provided by Poorooshasb and Meyerhof (1997), and Pulko et al. (2011). Figure 1 shows the comparison of the replacement percentage (RP) and settlement ratio (SR) in the non-reinforced state in the current study as well as theoretical relationships proposed by the previous researchers. Based on this figure, there is a difference between the results of the current study and those of Poorooshasb and Meyerhof (1997), and Pulko et al (2011). Poorooshasb and Meyerhof (1997) calculated the settlement ratio in their proposed material with the assumption of linear elasticity of the materials without consideration for plastic settlement. Therefore, the settlement of the improved soft soil with stone column, calculated by Poorooshasb and Meyerhof, would not show the real amount. However, Pulko et al. (2011), with consideration for the elastoplastic behavior of the materials, the lateral expansion of the stone column, and the primary stress of the soil around the column, provided more realistic results that correspond closely with the present study. Also, for designing the stone column, the results of its reinforcement have been also provided in the graph presented in Figure 2. Thus, by the use of these graphs, the ratio of settlement reduction can be obtained for each distance between columns and with different percentages of alternatives../files/site1/files/124/2jalili%DA%86%DA%A9%DB%8C%D8%AF%D9%87.pdf

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Introduction
Estimation of Liquefaction is one of the main objectives in geotechnical engineering. For this purpose, several numerical and experimental methods have been proposed. An important stage to predict the liquefaction is the prediction of excess pore water pressure at a given point. In general, there are two important methods for soil dynamics analyses, fully coupled effective stress and uncoupled total stress analysis. The main purpose of this study is to evaluate the model capacity of the finite difference software, FLAC, based on effective stress analysis methods to predict the excess pore water pressure during seismic loading. A level ground centrifuge test conducted during the VELACS project on the Nevada sand with a density of 40%, was utilized to calibrate the numerical model. After the validation of the numerical model, a model was conducted to predict excess pore pressure and consequently the liquefaction for the site of Bandar Abbas Mosque.
Theoretical bases
A fully coupled u–P formulation, where pore pressures and displacements are computed simultaneously and interactively at each time step, is used in FLAC software. This feature is used to simulate the excess pore water pressure time histories during cyclic loading.
The finite difference based software, FLAC, used the Finn model that incorporates two equations correlating the volumetric strain induced by the cyclic shear strain and excess pore water pressure produced during cyclic loading. As mentioned above, the pore water pressure generation can be computed from two sets of equations: Martin et al. (1975) and the Byrne (1991) formulations in which the volumetric strain that was produced in any cycle of loading is depended on the shear strain that was formed during that cycle as well as the previously accumulated volumetric strain.
Modeling and Results
The VELACS model # 1 centrifuge test representing a level ground site constituted of the Nevada sand at 40% relative density has been numerically simulated in the current study to validate the numerical model. The centrifuge model contains a laminar box with slipping “rings” that allows differential horizontal displacements. This was simulated in the FLAC model by free-field boundary conditions which prevent reflection of the waves in the side walls. Figure 1 shows comparison of EPWP time histories ratio of numerical modeling and centrifuge test. Static analysis was carried out before dynamic analysis in order to find initial stress and strain state. At the next stage, the dynamic loads were applied at the base of the model and dynamic analysis was performed. The Bandar Abbas mosque project is located approximately 500 meters from the coast. In the project, due to the groundwater level and the existence of loose layers of silt, investigating the potential of liquefaction is necessary.
For numerical modeling the results of the general soil mechanics test on soil samples and standard penetration test performed on the site were used to calibrate the parameters and select the model constants.
Conclusion
The results of numerical modeling have been matched to experimental results of the centrifuge test using both Martin and Byrne formulations, except for the case of 5 m the numerical model has predicted lower excess pore water pressure values than the experimental values. This may be originated from the fundamental assumption of the Martin et al. (1975) EPWP theory, in which excess pore water pressure is directly related to the relevant volume changes. On the other hand, the Martin et al. (1975) model was adopted for one-dimensional measures of shear strain, while, in a 2D analysis under both horizontal and vertical shakings, there are three strain rate measures. FLAC uses some assumptions to solve this problem and it can affect the results.
The results of the numerical model showed liquefaction to a depth of about 5 meters that is almost compatible with the results from the lab, which has declared that the depth 2 to 5 m is liquefiable.
With careful selection of numerical model parameters one can generally use the simulation results to have a general sense on the pore water pressure generation and liquefaction prediction.
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Introduction
Dispersive soils are problematic and they cause a great many of local damages and destructions in hydraulic structures such as dikes and irrigation channels. The correct identification and recognition of divergence are fundamental measures taken in line with preventing the early destruction of the hydraulic structures. The soil improvement using lime, especially in clayey soils (CL), brings about an increase in the optimum moisture percentage, reduction of the maximum dry unit weight, reduction of swelling potential, increase in the strength and elasticity module. The effect of lime on soil can be classified into two groups, namely short and long-term stabilization. Raise of the soil’s workability is counted amongst the short-term modification measures and it is the most important factor in the early improvement stages. The increase in the strength and stability can be considered as the lime utilization on long-term results occurring during curing and afterwards. Also, according to the reports, swelling and damages occur in the lime-stabilized soil containing sulfate. The effective role of the iron furnace slag has been well recognized in increasing the strength against sulfates and corrosive environment conditions of the mortar containing lime and sulfates.
Material and methods
Adding the slag products of the melting furnaces and lime is a method used to stabilize dispersive soils. The present study makes use of a mixture of clay featuring low plasticity with 1% and 2% lime and slag, for 0.5%, 1%, 3% and 5% of the weight, to improve dispersivity, shear strength and plasticity. The samples were kept in constant temperature and humidity for a day and then were subjected to direct shear, uniaxial strength and pinhole tests.
Results and discussion
It was observed based on pinhole experiment of the initial dispersive soil sample, denoted as D1, that the sample, shown by ND2, containing lime, for 2% of the weight, and slag, for 5% of the weight, turned out to have become non-divergent. The results of the direct shear test showed that the adhesion coefficient of the slag-free samples stabilized using 1% lime has been increased from 0.238 kg/cm² to, respectively, 0.251 kg/cm², 0.373 kg/cm², 0.41 kg/cm² and 0.48 kg/cm²  per every 0.5%, 1%, 3% and 5% slag added. The adhesion of the samples stabilized using 2% lime as determined in the direct shear experiment were 0.615 kg/cm², 0.671 kg/cm², 0.724kg/cm² and 0.757kg/cm² per every 0.5%, 1%, 3% and 5% slag added. Also, the internal friction angle of the samples stabilized using 1% lime was found an increase from 14.3° for slag-free samples to 18.11°, 21.3°, 21.86° and 21.92° per every 0.5%, 1%, 3% and 5% added slag. As for the samples stabilized using 2% lime, the internal friction angles were found in direct shear test equal to 23.15°, 23.53°, 23.76° and 24.12° per every 0.5%, 1%, 3% and 5% slag added. The uniaxial strength of the slag-free samples stabilized using 1% lime was found an increase  from 1.0014 kg/cm² to, respectively, 1.0616 kg/cm², 1.0782 kg/cm², 1.2127 kg/cm² and 1.2246 kg/cm² per every 0.5%, 1%, 3% and 5% slag added. The uniaxial strength rates has been determined in the direct shear test of the samples stabilized using 2% lime were 1.1367 kg/cm², 1.1885 kg/cm², 1.2322 kg/cm² and 1.2872 kg/cm² per every 0.5%, 1%, 3% and 5% slag added. The amount of axial strain of the slag free samples stabilized using 1% lime was found decreased from 9.6842% to, respectively, 9.3333%, 9.2683%, 9.6364% and 8.4444% per every 0.5%, 1%, 3% and 5% slag added. Moreover, the axial strain amounts obtained for the samples stabilized using 2% lime were 7.7333 kg/cm², 7.6316 kg/cm², 7.1517 kg/cm² and 4.7619 kg/cm² per every 0.5%, 1%, 3% and 5% slag added.
The study results indicate that slag and lime have the capacity of improving the studied soil’s dispersivity. Furthermore, it was figured out that adding slag to the soil causes an increase in the soil strength and improves the shear strength parameters. It can be stated according to the observed results that the use of slag, a byproduct of iron smelting industry, as a substitute for a given percentage of lime is effective on the reduction of the clay soil’s divergence potential. The results of the experiments carried out to determine Atterberg limits are suggestive of the idea that the increase in the slag and lime fractions brings about a decrease in the liquid limit and plasticity and improves the plasticity properties of the soil. The reason why the soil plasticity has been reduced after being mixed with lime and slag is the cationic exchange and coarsening of the soil texture. Addition of lime to the soil causes an increase in the plasticity limit and a reduction in the liquid limit. Therefore, the plasticity index is decreased and the plasticity characteristics of the soil are improved. Adding 1% lime to the dispersive soil leads to small reduction of the liquid limit from 32.43% to 31.73%, a small increase in the plasticity limit from 13.42% to 14.66% and a insignificant decrease in the plasticity index from 19.01% to 17.07%.