mohammad hazeghian

Shear banding is one of the most significant behavioral characteristics of granular soils, which is actually the result of localization of deformations under loads applied to the soil sample. Many studies have been conducted on the phenomenon of shear band formation. However, the comparison of the behavior of soil inside and outside the shear band has received less attention. The purpose of this study is to compare the behavior of soil inside and outside the shear band in the direct shear test at macro and micro scales using the two-dimensional discrete element method. To achieve this, the micro-material parameters were first calibrated through the comparison of simulation and experimental results. Then a parametric study was conducted by performing 9 direct shear tests with different vertical stresses on three soil samples with different relative densities. During the tests, quantities such as shear stress, porosity, particle rotation, coordination number, and interparticle plastic energy were measured both inside and outside the shear band. The results of the present study showed that the particle rotations at the end of the test inside the shear band were 5 to 17 times higher than those outside the shear band. Furthermore, the results showed that the dissipated energies at the end of the test inside the shear band were 12 to 96 times larger than those outside the shear band. Moreover, it was found that the coordination numbers at the end of the test inside the shear band were on average 3-19% lesser than those outside it.

In today’s world, roads are considered the main arteries of the global community. One of the most typical methods to build these routes is stabilizing the excavations on two sides of the roads. Using multiple helical anchors in excavation stabilization is one of the means recently used by many road and geotechnical engineers. The increasing use of multiple helical anchors has led to several laboratories, numerical, and field studies. Therefore, this study employed the FLAC 3D software also a three-dimensional numerical investigation of bearing capacity under different inclination angles in sandy soil. In the present study, the effect of soil installation, which is caused by installing helical anchors in the soil, has been studied on the load-bearing capacity of these helical anchors. For this purpose, first, a verification test is done to validate the modeling approach. Then, by performing a parametric analysis, the effect of different inclination angles, as well as the installation effect on the bearing capacity of the multiple helical anchors, are studied. The present study results show that the maximum bearing capacity of the helical anchors is achieved by the angle of 5 and 10 degrees. In addition, by investigating the installation effect, the bearing capacity of the helical anchors has been significantly reduced with respect to the number of helical plates and the density of soil.

Today there are numeral methods for supporting an excavation. Using of helical multiple anchors as excavation support has speared recently. Because of wide range of applications of helical anchors, a massive number of experimental, numerical, and field studies have been carried out. This study works on the pullout capacity of helical multiple anchors by using FLAC 3D software. First, a verification test is carried out in order to confirm the method of simulation. Then, with the help of an extensive parametric study, the effect of different parameters on pullout capacity is observed. This study indicates that sharp growth in loading capacity is observed due to an increase in density of soil and diameter of helical plates. In addition, the results show that changes in the cohesion of soil have a few effects on the loading capacity of helical anchors. Moreover, the share of loading on the plates keeps remain if the space between plates is five times of helical plate diameter and more. In this situation, the failure mechanism is individual plate. Nevertheless, if the space between plates is less than five times of helical plate diameter, the share of loading on the first helical plate (near to excavation) goes up, but the share of loading on the rest helical plates drops dramatically. In this situation, cylindrical shear is the main failure mechanism. Therefore, the current study results demonstrate that the critical space between helical plates is five times of helical plates diameter.

In the conventional design method for soil-nailed walls that is based on the FHWA guidance, the nails properties (the length, spacing and diameter of nails) through the excavation depth is usually assumed constant. However, in the design method introduced in the present study (the proposed design method), the nails properties are varied through the excavation depth by step-by-step control of the factor of safety for each row of nails. In the present study, first numerical assumptions are explained and then the numerical modelling methodology is verified through the Clouterre project. Afterwards, the details of the conventional and proposed design methods are described. Next, a parametric study is performed to compare the conventional and proposed design methods. In the parametric study, the effects of the excavation depth, soil density, soil cohesion, surcharge and required factor of safety are investigated. The results of the present study show that the proposed design method compared with the conventional one would decrease on average 30% the nails volumetric density (i.e. the volume of nails divided by the wall area), which is considerable from the economical point of view. Moreover, the results show that the total displacement at the wall crest for the proposed design method is comparatively 10% higher than that of the conventional design method.

Nowadays, helical anchors are one of the fastest methods of supporting excavations. The use of helical anchors is increasing, and recently they received more attention in researches. One of the most important factors for the design of helical anchors is their tensile capacity, to which less attention was paid in the literature compared with the helical piles. The present study uses a three-dimensional numerical modeling approach to investigate the tensile capacity of helical multi-plate anchors. For this purpose, first, the adopted numerical modeling methodology is verified. Then, a comprehensive parametric study is performed to investigate the effects of various parameters involving the soil type, soil cohesion, plate diameter, plate spacing, surcharge, and anchor inclination. The present study results show that the tensile capacities of the helical multi-plate anchors increase by increasing the plate’s diameter, surcharge, and soil relative density. However, the soil cohesion and anchor inclination have negligible effects. Moreover, the results indicate that the load-bearing shares of the shaft increase by increasing the surcharge and decreasing the plate diameter. In addition, the results show that the load-bearing shares of the plates stay about constant for S/D≥4 (S and D represent the plate's Spacing and Diameter, respectively). So that the failure mechanism of multi-plate anchors could be considered as the individual plate for S/D≥4 and cylindrical shear for S/D<4. In other words, the critical S/D ratio is 4. The union of failure zones formed around the plates in displacement contours for S/D<4 confirms this result.

In most excavation projects, the excavation plan is irregular in shape, including concave and convex corners. In practice, the 2D (i.e., plane strain) analysis is often employed to evaluate the factor of safety and displacements induced by excavation for concave and convex corners. However, contrary to concave corners, using the plane strain analysis is not on the conservative side for convex corners. The present paper uses a numerical modeling methodology to study the effects of the convex corner on the displacements induced by excavation for soil-nailed walls. In this regard, a series of parametric studies are carried out, involving 2D and 3D deformation analyses of nine soil-nailed excavation models with three wall heights and three types of soil. The results of the paper show that the lengths of the affected zone (i.e., the zone adjacent to the convex corner along which the 3D settlements at the wall crest are higher than the 2D one) increase by decreasing the soil strength. Moreover, the results indicate that the maximum ratios of 3D settlement to 2D one along the affected zone are independent of the wall height and soil type. In addition, the results suggest that giving azimuth to soil nails along the affected zone causes the wall displacements along this zone to increase significantly.

Soil nailing is one of the common methods used to stabilize excavation walls. This technique has been in attention in recent years. A variety of studies have been performed to optimize the pattern, length and inclination of nails in the design of soil nailing. The nail inclination has been one of significant optimization variables considered in previous studies. As a constraint of the optimization process, it has been assumed in previous studies that the inclinations of all nails through the depth of a soil-nailed wall must be equal. On the contrary, the present study presumes that the inclination of each row of nails could be different; in other words, the inclination of each row of nails is an independent optimization variable. The present study takes into account two approaches to optimize the design of soil nailing. In the first approach, the ratio of the maximum length of nails to the wall height is unconstrained but in the second approach, it is limited to the ratio proposed by the FHWA manual. The results of the present study show that two optimization approaches lead to different optimized inclinations for nails. For the first approach, the optimized inclination of all rows of nails equal to be 5 degree with respect to the horizon but in the second approach, these inclinations are different for rows of nails in the way that they reduce from the top row to the bottom one.