فهرست مطالب علی ماهیگیر

The large scale pullout test is used to investigate the pullout behavior of the geogrid in the anchorage zone. When pullout force is applied, tensile force gradually transmitted from the front part to end of geogrid. The active length is the part of the geogrid sample on which the activation of interaction mechanism resists against the applied force. In order to investigate more accurately the interaction between soil and geogrid, the geogrid pullout behavior should be evaluated based on the active length. In this study, a series of large scale pullout tests carried out to investigate the distribution of shear stress, pullout interaction coefficient and longitudinal strain of two types of uniaxial PET geogrid with same geometry and different stiffness embedded in two types of sandy soil and sand containing 20% clay based on active length. The results showed that, the pullout force required to move the last transverse bar of geogrid, increased with increasing of vertical effective stress in both type geogrid. Moreover, the active strain of stiffer geogrid embedded in clean sand and clayey was greater than that measured in the more extensible geogrid in both transfer load and pullout stage. At small frontal displacements, increasing the geogrid stiffness decreases the active shear stress in sandy and clayey sand soil. In all pullout tests, the lowest active pullout interaction coefficient occurred at the beginning of the pullout phase.

Geosynthetics are mainly used to stabilize and reinforce different types of earth structures such as slopes, retaining walls, bridge abutments, and foundations. In these cases, the interaction between soil and geosynthetic plays a significant role. In order to investigate the factors affecting the static, cyclic, and post-cyclic pullout behavior of a type of geogrid produced in Iran under the brand name of GPGRID80/30 embedded in uniform sand, an experimental study was carried out using a large-scale pullout apparatus. In order to study the monotonic and post-cyclic pullout behavior of geogrid in different conditions, a series of monotonic pullout tests and multistage pullout tests were performed. Given the effect of vertical effective stress on the pullout resistance, the maximum apparent friction coefficient of the surface of the geogrid and soil and deformation along the geogrid was investigated using monotonic tests. In the multistage pullout test, the influence of vertical effective stress, cyclic load amplitude, frequency, and number of tensile load cycle on the post-cyclic pullout resistance was studied. The results indicated that with an increase in the vertical effective stress, the pullout resistance of the geogrid and the maximum apparent coefficient of friction would increase and decrease, respectively. A comparison of the results of the multistage pullout tests and constant rate pullout tests with the vertical stress of 60 kPa showed that the cyclic loading had no significant effect on the post-cyclic pullout strength compared to the static pullout strength of the embedded geogrid in the sandy soil; however, with vertical effective stresses of 20 and 40kPa, a reduction in the maximum post-cyclic pullout strength was more evident than the pullout strength. Increasing the effective vertical stress and cyclic load amplitude in the second stage of the multi-stage test would enhance the cumulative displacements along the geogrid sample. A comparison between the loading-unloading tensile stiffness at the end of the second stage and tensile stiffness at the beginning of the second stage suggested that the cyclic loading would increase the tensile stiffness and finally, at the third stage of the experimental multistage test, the tensile stiffness would decrease as the displacement increased until it reached the corresponding value in the constant-rate displacement test.

Naturally formation of soil deposits leads to anisotropy and heterogeneity in their properties. The aim of this study is modeling of soil having anisotropic and heterogeneous strength using finite difference method to estimate the effect of these on slope stability in undrained condition. In this study the soil cohesion is considered as an anisotropic and heterogeneous variable and assumed to have a deterministic trend with depth and different values in any directions per depth. However, the anisotropy of cohesion is considered by generalizing the isotropic Mohr- Coulomb model to anisotropic one. The study performs on a slope in undraned condition and results indicate that the importance of anisotropy and heterogeneity of soil strength parameters on slope stability. Taking account of anisotropy and heterogeneity leads to proper and economic analysis and design of slope stability so that the safety of factor in the case of anisotropy equal to 2(AF=2) was approximately 40% greater than in the case of fully isotropic.

Natural formation of soil deposits causes heterogeneity and anisotropy in their strength and stiffness properties. However, most soils in their natural states exhibit some anisotropy with respect to shear strength and heterogeneity with respect to the depth. In this paper, the standard Mohr- Coulomb constitutive law is generalized to anisotropic version in order to consider the effect of cohesion anisotropy of soil. Random field theory coupled with finite difference method was utilized in Monte Carlo simulations with considering the effect of auto-correlation and cross correlation between strength parameters of soil, in order to calculate the bearing capacity of shallow foundation in a strain controlled scheme. The results showed that the bearing capacity of shallow foundation decreases with increasing in variability of strength parameters and increases with increasing in anisotropy ratio.