EVALUATION OF BEARING CAPACITY OF SOIL LAYERS. A CASE STUDY OF OWERRI FEDERAL CONSTITUENCY

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Abstract

This study was carried out on the evaluation of bearing capacity of soil layers: A case study of Owerri Federal Constituency. The bearing capacity of the foundation is a primary concern in the field of foundation engineering. Predicting ultimate bearing capacity of a footing on layered soil is very important as it is a requirement for any design. The failure mechanism of a soil under footing and the bearing capacity value mainly depend on soil properties of each layer and the layer thickness. It is well established that safe bearing capacity of soil is affected by various factors such as the depth of ground water table (GWT), soil properties, layering of soils, size and shape of the foundation, depth of foundation etc. among many other factors. Hence most of the studies focused on homogenous soils but in practice foundations often consist of layered soils, it is very important to consider layered soils.  In this study a numerical model is developed using PLAXIS. Finite element analysis is carried out using Mohr-coulomb failure criteria to represent three dimensional soil models. For Comparison purpose the layered soil bearing capacity has been analyzed by Meyerhof and Hana method which is classical approach method. After that by comparing results from FEM and classical approach the analysis has been presented.  

CHAPTER ONE

INTRODUCTION

1.1. Background of the study

The bearing capacity of the foundation is a primary concern in the field of foundation engineering. The self-weight of the structure and the applied loading such as: dead load, live load, wind load etc. should be transferred to the soil safely and economically. The load at which the shear failure of the soil beneath the foundation occurs is called the ultimate bearing capacity of the foundation. The magnitude of the ultimate bearing capacity depends on the mechanical characteristics of the soil and the physical characteristics of the footing.

Foundation design consists of two distinct parts: the ultimate bearing capacity of the soil under the foundation and the tolerable  settlement that the footing  can undergo without affecting the superstructure. The ultimate bearing capacity aims at determining the load that the soil under the foundation can handle before shear failure; while, the calculation of the settlement caused by the superstructure should not exceed the limits of the allowed deformation for stability, function and aspects of construction.

From previous reviews on the subject, the majority of the bearing capacity theories involve homogenious soils under the foundation soil properties were assumed to remain constant for the bearing capacity analysis, and therefore analytical solutions,like Terzaghi‟s bearing capacity theory,matched with the experimental results. However, in cases where the soil properties vary with depth,  most of these theories cannot be implemented , and the analytical solutions that take into consideration the non-homogeniety of the soils are approximations, and hence the results are inaacurate.

Layered soil profiles are often encountered whether naturally deposited or artificilally made. With in each layer , the soil may be considered as homogeneous.The ultimate load failure surface in the soil depends on the shear strength parameters of the soil layers such as; the thickness of the upper layer to the width of the footing. Therefore, it is important to determine the soil profile and to calculate the bearing capacity accordingly.

Geotechnical engineers often should solve problems in layered soil while the majority of existing studies have mostly focused on homogeneous continuum. Predicting ultimate bearing capacity of footings on layered soil is very important as it is a requirement for any design and the failure mechanism of soil under footing and the bearing capacity value mainly depend on soil properties of each layer and the layer thickness.

In recent years, Finite Element Method (FEM) has been widely used in geotechnical studies to investigate soil behavior. In practice, for bearing capacity analysis engineers are seeking less complicated solutions to simplify computations as experimental analysis is time consuming and commonly used solutions such as limit equilibrium are no longer applicable. Therefore, computer programs developed based on the finite element method have been receiving much attention over recent decades as the powerful tool for solving complex cases.

1.2. Problem Statement

Safe bearing capacity of foundation is one of the important stability problems in geotechnical engineering, which depends upon the bearing capacity and the allowable settlement of foundation.

Finite Element Method allows modeling complicated nonlinear soil behavior through constitutive model, various geometrics with different boundary conditions & interfaces. It can predict the stresses, deformations and pore pressures of a specified soil profile, therefore predicting ultimate bearing capacity of footings on layered soil is very important as it is a requirement for any design and the failure mechanism of soil under footing and the bearing capacity value mainly depend on soil properties of each layer and the layer thickness.