Presenting a new analytical method to determine the behavior of surface foundations based on reinforced soil

Civil Engineering Master's Dissertation

Trend of Soil and Foundation Engineering

Abstract

In this thesis, in order to analyze the behavior of surface foundations based on reinforced soil, a simple physical method based on the resistance of materials called "cone method" is used, which is actually used as an alternative to the exact solution methods that are based on the theory of three-dimensional elastodynamics. The cone method has the ability to combine the complexity of layered soil conditions and investigate how waves propagate in these environments, and has acceptable engineering accuracy.

In order to model geocell-reinforced soil using the cone method, each geocell layer and its filling soil were modeled using a composite experimental model based on ring stress theory. In this composite model, the characteristics of geocell and its filling soil were considered and the geocell layer was modeled as a homogeneous layer. Also, the effect of several parameters on the dynamic stiffness of surface foundation located on reinforced soil was investigated, such as: geometric characteristics of geocell, the number of its layers, density of filling soil and also the depth of geocell layer placement. Subsequently, the dynamic stiffness coefficients for each vertical degree of freedom were obtained. In general, the longer the geocell cushion has, the smaller the holes and the higher the modulus of elasticity, the better the results. . By increasing the number of geocell layers, the amount of spring stiffness and damping coefficient and as a result the dynamic stiffness coefficient increases. On the other hand, the closer the geocell is placed to the soil surface, the higher the stiffness of the spring and the lower the damping coefficient. In general, among these two parameters related to dynamic stiffness, the role of damping coefficient in the investigation of foundations based on reinforced soil has been more prominent and decisive. Because one of the most obvious characteristics of geosynthetic materials in the soil, regardless of the hardness it gives to the soil, is the damping percentage of its constituent materials.

Keywords: physical method, cone model, wave propagation, surface foundation, reinforced soil

Chapter 1- General and Introduction

1-1- Preface

Exact solution methods, despite acceptable accuracy They are not applicable for all models. The use of exact solution methods or numerical methods leads to expensive analyzes and in some cases requires a correct and deep understanding of related issues, which is not possible in most cases due to the existing complexities. If the soil environment is heterogeneous and has different layers with different characteristics, the analysis will be complicated and expensive. Considering non-homogeneous soil as homogeneous soil or using average properties for layered soils may result in unrealistic solution. Shear and expansion waves are created by the propagation of forces in each of the soil layers with different amplitudes. The reflection of waves at common boundaries in layered soils and the reduction in amplitude for the transmission wave towards the far field is a phenomenon that complicates the problem. It will be very difficult to give the effect of these phenomena for the complete behavioral analysis of wave propagation in unlimited environments[1]. Because of these problems, these methods can only be used in important projects with critical conditions. For everyday problems, the physical modeling method can be used to study soil without boundaries. One of the advantages of this method is their simple application and presentation of a comprehensible physical view of the problem. The cone method is one of the physical modeling methods that takes into account the salient features and is based on the experience gained from detailed analysis.

In the past 20 years, modeling based on the material strength approach using conical rods and beams, which are called cones, was only available for surface foundations based on a homogeneous half-space of the soil representative, but today it is possible to model based on the same Assumptions are also provided for more complex applications. For example, changes in soil properties can be modeled with depth, and the construction can have any number of horizontal layers.

In fact, this method provides the possibility of soil analysis with planar and three-dimensional reinforcements due to the efficiency and flexibility it provides to change the properties of soil layers. In this research, the cone method has been introduced and developed as a simple and physical method for the analysis of surface foundations based on geocell-reinforced soil [2].. The reason for using geocell as a reinforcement is its three-dimensional nature and its mattress property, which improves the characteristics of the soil bed more than other reinforcements.

1-2- Statement of the issue

As an alternative to the exact solution method, simple physical models can be introduced to evaluate the interaction of soil and structure and determine the force-displacement relationship for foundations.

For example, a massless rigid foundation with the introduced characteristics is considered. To determine the force-displacement relationship, the exact solution method is considered. Therefore, a part of the soil area and a part of the half-space are modeled by the finite element method. Also, to present the wave propagation towards infinity, the boundary of the model is modeled using adaptive transmission boundaries or the numerical method of boundary components. As expected, the exact solution method requires a strong formulated theory, therefore the computational cost will be high even for performing a single analysis, and as a result, the method can be used for some important and vital problems, and this method cannot be used in everyday engineering work. Most engineers do not want to perform complex and time-consuming calculations and always try to interpret the results obtained from various models, which cannot evaluate many models. In most common projects, it is suggested to use physical models to present unconfined soil, and in this research, these models have been used to advance the goals.

(Images are available in the main file)

The basic approach in cone models is based on the theory of material resistance, which in this model, the soil environment is modeled by an incomplete cone[3] [1]. The only approximation used in this method is to limit the three-dimensional environment of the soil inside an incomplete cone, which is common to use such an approximation in geotechnical problems. The reason for choosing the conical shape is to reduce the stresses caused by applying the load with increasing depth. As a result of the application of load, tensions are created in the soil environment, which are spread over a wider area with increasing depth; But by moving away from the axis of load application, their range is reduced.

Some of the advantages of these models are briefly mentioned below:

Simplicity of concepts, existence of physical understanding and compliance with the laws of wave propagation

The ability to generalize the method to general situations such as the foundation buried in layered soil and appropriate adaptation to the specific conditions of the problem such as bilateral convergence for surface foundation

Accuracy Proper engineering: the results obtained from conical models have an error of less than ±20% compared to the exact results. This error limit is suitable for engineering applications due to the fact that some of the error-causing factors cannot be eliminated. 1-3- Necessity of research The use of exact solution methods, due to their acceptable accuracy, is not applicable for all models and therefore physical simulation is inevitably used. In numerical methods, for example, the numerical method of finite elements, considering the problem of unbounded soil, this method cannot have an acceptable efficiency. Other numerical methods such as boundary elements or the use of infinite elements, although they have solved the problem of unbounded soil, but due to their complexity in solving the problem, they cannot be used for all problems.

Using geosynthetic layers[4] for soil reinforcement is a relatively new method in the field of soil reinforcement. In this research, cellular geosynthetic layers called Geocell have been used to strengthen soils, which improves soil behavior by creating confinement. In this research, the geocell-reinforced soil is physically modeled by the cone method and definition of transfer functions, and the soil impedance function is determined.

The approach of material resistance using cones is a physical and simple method that provides a good physical resolution by simplifying the physics of the problem, and despite being less accurate than complex and time-consuming numerical methods, it obtains a good engineering approximation. Based on the findings, this thesis is the first work in the field of foundation analysis on reinforced soil, and no research has been reported in the literature on the subject.

1-4-     Research Objective

The purpose of this research is to introduce a simple physical method and analytical tool for the analysis of surface foundation based on geocell-reinforced soil. With the cone method, it is possible to examine the effects of layered soil