International Journal of Civil Engineering
Volume:12 Issue: 3, Jul 2014

In this paper, different aspects of the behavior of 2×2 pile groups under liquefaction-induced lateral spreading in a 3-layer soil profile is investigated using large scale 1-g shake table test. Different parameters of the response of soil and piles including time-histories of accelerations, pore water pressures, displacements and bending moments are presented and discussed in the paper. In addition, distribution of lateral forces due to lateral spreading on individual piles of the groups is investigated in detail. The results show that total lateral forces on the piles are influenced by the shadow effect as well as the superstructure mass attached to the pile cap. It was also found that lateral forces exerted on the piles in the lower half of the liquefied layer are significantly larger than those recommended by the design code. Based on the numerical analyses performed, it is shown that the displacement based method is more capable of predicting the pile group behavior in this experiment comparing to the force based method provided that the model parameters are tuned.

When geogrid reinforcement is used as a treatment method for improving soft subgrade as a roadway foundation, a top layer of subgrade is usually excavated and backfilled with geogrid-reinforced aggregates. This treatment method produces an adequate platform for the planned roadway construction site, where heavy traffic loading is constantly moving. This paper presents a quantitative assessment of subgrade improvement by geogrid reinforcement based on numerical modelling and parametric studies. First of all, the preliminary numerical models were verified by comparing the analysis results with previous studies. Secondly, the major numerical models in this study were assumed to be a simplified simulation of a geogrid-reinforced two-layer system with an aggregate layer above a subgrade layer. The numerical models were applied a quasi-static loading and unloading cycle, in order to monitor the permanent deformation at the surface of the models. Afterwards, thickness of aggregate layer, and subgrade CBR values were varied in order to summarize the outcomes of each case. This approach makes it possible to quantify the effects of geogrid reinforcement and aggregate material in terms of an enhanced California Bearing Ratio (CBR) of a single subgrade clay layer. Results have shown that when the aggregate thickness is up to 450mm, the contribution of enhanced CBR is mostly from aggregate material. However, when the aggregate thickness is about 150mm with a relatively weak subgrade material, the inclusion of geogrid material can contribute about 50% of the enhanced value.

In comparison with other geomaterials, constitutive modeling of rockfill materials and its validation is more complicated. This is principally due to the existence of more intricate phenomena such as particle crushing, as well as laboratory test limitations. These issues have necessitated developing more complex constitutive models, with many parameters. Regardless of the type of model, the calibrations of the parameters in such models are considered as one of the most important and challenging steps in the application of the model. Therefore, the need for comprehensive and rapid methods for evaluation of optimum parameters of the models is deemed necessary. In this paper, a Neuro-Fuzzy model in conjunction with Particle Swarm Optimization (PSO) is used for calibration of the twelve parameters of Hierarchical Single Surface (HISS) constitutive model based on the Disturbed State Concept (DSC). The Neuro-fuzzy system is used to provide a high-degree nonlinear regression model between the deviatoric stress and volumetric strain versus axial strain that has been obtained from consolidated drained large scale tri-axial tests on rockfill materials. The model parameters are determined in an iterative optimized loop with PSO and ANFIS such that the equations of DSC/HISS are simultaneously satisfied. Material data used in this study are gathered from the results of large tri-axial tests for two rockfill dams in Iran. It is shown that the proposed method has higher accuracy and more importantly its robustness is exhibited through test predictions. The achieved improvement is substantiated in a comparison with the more widely used "Least-Square" method.

This research shows the results of studies carried out to define and analyze the effect of aging on MSW behavior of Kahrizak Landfill, the biggest landfill in Iran. Studied samples consisted of fresh samples and also aged ones with 5.5, 14 and 21 years of age which were obtained by mechanical excavators in aged burial locations. Analyzing variation in MSW components illustrates that paste fraction of MSW decreases due to aging process while fibers show a rising trend. Also the moisture content and the organic content of MSW reduce below half of the initial values while the degree of decomposition (DOD) increase up to almost 60% after 14 years. These variations over the time are significantly related to the burying methods and environmental condition of burying location. Shear strength behavior of MSW material was analyzed by some CU tests using large scale triaxial apparatus (D=150mm, H=300mm) on remolded MSW specimens. General observations depict that with an increase in strain level, loading rises without any peak point on stress-strain curves. Fresh samples represent the lowest shear strength followed by 21, 14 and 5.5 year-old samples respectively. There is a direct relationship between fiber content and shear strength. Internal friction angle of aged samples decreases in comparison with fresh ones while cohesion has an inverse trend and rises over the time. According to the effect of burying condition on MSW characteristics, it seems that DOD factor is a more appropriate factor than age in order to analyze long-term behavior of MSW.

Municipal Solid Waste (MSW) materials are among the most complicated materials for geotechnical engineering as their composition includes an organic fraction, which suffers loss of mass over time, and a fibrous part, which acts as reinforcement, governing the MSW shear behavior. Because of these characteristics MSW can be described as a viscous material which shows time dependent behavior. Since the decomposition of MSW leads to gas and leachate generation, the changes in the MSW’s mechanical behavior could be linked to gas emission and leachate production from landfills. This paper deals with the characteristics of MSW materials to provide the necessary data for efficient and safe landfill design, construction and operation. The MSW physical characteristics such as composition, water content and organic content at varying ages, field and laboratory measurements of methane generation and leachate production, MSW compressibility behavior and its shear strength are covered. By presenting these data the authors hope to promote a better understanding of the mechanical behavior of MSW and provide useful data for use in landfill management tasks.

In this paper, an advanced formulation of the spectral finite element method (SFEM) is presented and applied in order to carry out site response analysis of 2D topographic structures subjected to vertically propagating incident in-plane waves in time-domain. The accuracy, efficiency and applicability of the formulation are demonstrated by solving some wave scattering examples. A numerical parametric study has been carried out to study the seismic response of rectangular alluvial valleys subjected to vertically propagating incident SV waves. It is shown that the amplification pattern of the valley and its frequency characteristics depend strongly on its shape ratio. The natural frequency of the rectangular alluvial valley decreases as the shape ratio of the valley decreases. The maximum amplification ratio along the ground surface occurs at the center of the valley. A simple formula has been proposed for making initial estimation of the natural period of the valley in site effect microzonation studies.

The purpose of this study is to compare the results of different probabilistic methods such as the perturbation method, Stochastic Finite Element Method (SFEM) and Monte Carlo Method. These methods were used to study the convergence of direct approach for slope stability analysis and are developed for a linear soil behavior. In this study, two dimensional random fields are used and both the First Order Reliability Method (FORM) and Limited Step Length Iteration Method (LSLIM) have been adopted to evaluate the reliability index. The study found that the perturbation method of the second order is easy to apply using the field’s theory because accuracy is reached even with different coefficients of variation of input variables, while the spectral finite element method yields accurate results only for high levels of solution development.