Comparative evaluation of the failure mechanism of earth slopes under percolation based on limit and numerical equilibrium methods

Construction

Soil and Foundation Tendency

Abstract

Given that all engineering infrastructures and vital urban and non-urban places and arteries such as: foundations of buildings, bridges, silos, dams, retaining walls, marine structures, industrial facilities, etc. . . All spread on a platform called the earth. The importance of investigating the type of instability in earthen roofs under percolation and earthen roofs under percolation and the solutions to deal with them become more apparent when the sliding of roofs can cause huge and irreparable damages. Two important methods for analyzing the stability of roofs are the limit equilibrium method and the numerical method. The common goal that is followed in all these methods is to check the stability of natural and artificial slopes and determine their stability reliability factor. Based on the definition of Terzaghi and Peck (1967 AD), landslides can happen in any imaginable way, slowly or suddenly, and with any triggering factor. Slope failure is usually caused by any sudden decrease in soil resistance or a change in geometric conditions. In general, slopes can be classified into two categories: natural slopes and designed slopes. This thesis, which is presented in the form of a master's thesis, deals with the evaluation and comparison of reliability coefficients and levels of rupture in clay and sand soils obtained by limit equilibrium methods and numerical methods in unconfined and restrained percolation states.

Key words: safety factor, failure mechanism, limit equilibrium, numerical method, soil trench under percolation, containment.

Introduction

One of the most important topics in geotechnical knowledge is the analysis of the stability of soil slopes under percolation and determining the level with the lowest confidence factor, in other words, the critical level. failure In the construction and operation of slopes and earthen walls, the issue of stability and investigation of possible failure mechanisms is due to economic considerations (geotechnical projects are often built with government funding and the capital of the dear nation of Iran) and in some structures to prevent the occurrence of human disasters (sometimes a slide of a natural hill or an earthen dam causes a village to be buried and thousands of people to become victims) and on the other hand Our holy land is facing natural calamities such as floods, earthquakes, etc., and the importance of the religion of Islam, which shows the high position of man and the treasury, increases the duty of civil engineers and the importance of the issue. There are various methods for checking the stability of slopes and gables, among which we can refer to limit equilibrium methods such as: Flinius, Bishop and Morgenstern method as well as numerical methods. The common goal that is followed in all these methods is to determine the minimum reliability coefficient of stability and the level of rupture corresponding to the one along which the gable will slide and rupture. In this research, by modeling and carefully examining a series of soil ridges with conditions close to field conditions, with both mentioned methods, we try to compare these two methods. For this purpose, we first assume a trench with certain characteristics and use the numerical method (Plexis software) to model and determine the confidence factor, the type, location and manner of possible failure and its critical slip level, and then we model and examine the same trench with the limit equilibrium method (Geostudio software), in this modeling the behavior of the soil in an elastoplastic way with the Mohr-Coulomb rupture criterion. It is assumed with plane and two-dimensional strain.

Overview of the research

1-1- Introduction

Investigating the stability of soil slopes and finding a way to more accurately evaluate the stability of slopes and soil trenches has long been the focus of geotechnical experts and specialists. Estimating the increase in the reliability coefficient of the stability of earthen slopes and slopes in different conditions and projects, including dam construction, canals, embankments, highways, wharves, etc. ., in this regard, trying to increase the stability of slopes is a determining factor in the overall stability of structures. The result of sliding or failure of a slope can cause the functional loss of that project (slope) and even cause the loss of some human lives.?Geotechnical engineers must pay special attention to geology, surface drainage, underground water, and soil shear strength in evaluating the stability of soil slopes.

Evaluation of earthen slopes is one of the main issues of geotechnical engineering, earthen dams, excavations, trenches, etc. Earthen structures are considered to be of high importance even for human civilization, so the analysis, design and behavior of these structures over time are considered by engineers, hence, many efforts are made to analyze earthen roofs. But the uncertainties, theoretical problems of analysis, variety of geotechnical issues, and the daily needs for the design of new roofs, each of which has its own problems, have prompted researchers to deal with this issue as much as possible. In checking the stability of the roofs in both limit and numerical balance methods, it is usual that driving and resisting forces are calculated and their magnitude is measured relative to each other. It is that each of the driving and resisting forces is subject to various factors such as: soil parameters, slope geometry, external loading, etc. on the other hand, the presence of water in construction projects has always caused problems for engineers and caused the rupture of structures, especially earthen roofs, in this regard, reinforced soil has always been used by humans as an efficient engineering material, the concept of reinforced soil in its modern form was first proposed by a French engineer named Henry Vidal[1] in 1966 AD, by examining the properties and conditions of the soil, it is evident that one of the main weaknesses of the soil is low tensile strength. It is, and the presence of water also strengthens this weakness, the idea of ??reinforced soil is actually a solution to fix this weakness; In other words, the basis of the reinforced soil system is based on the use of materials in the soil that have the ability to withstand high tensile stresses in such a way that the mass of reinforced soil avoids rupture, or in other words, they are stable. Nowadays, according to the advances in engineering sciences and the increasing construction of complex buildings and structures, which are sometimes built in mountainous and steep areas, the need for the stability of natural and artificial slopes has been given more attention than before. There are many methods to stabilize the slopes, and the common goal of all these methods is to increase the resistance forces against the driving forces and, as a result, increase the value of the reliability factor. The use of reinforcements increases the resistive force in the soil mass by providing tensile force and also reduces horizontal deformation, which ultimately leads to an increase in overall stability in earthen structures. 1-2 statement of the problem In the construction and operation of slopes and earthen walls, the problem of stability and the investigation of possible failure mechanisms, economic considerations and even human catastrophe in some structures. It is very important, so it is important and valuable to know the type of stability and instability in the gables in order to identify the ways to prevent and deal with it. Two important methods in analyzing the stability of slopes and slopes are limit and numerical equilibrium methods. In this research, by modeling and carefully examining a slope with two different angles under conditions of unrestrained and controlled seepage in clay and sand soils, both methods are compared.

Limit equilibrium method

One of the methods for analyzing the stability of slopes is the limit equilibrium method, in this method, the first analysis Coleman presented at the end of the 18th century, in such methods, the main assumption is that a surface along the sliding surface is selected (curve or level), a specific rupture criterion (for example: Mohr-Coulomb) is governed and the sliding part moves separately and rigidly on the rupture surface, then all the forces affecting the Slipperiness is considered to prevent it, and the safety factor is determined by comparing the shear strength required to maintain stability with the shear strength in the soil. Statics of forces is done on each piece separately, due to the clarity of the concepts and simplicity of use, methods based on limit equilibrium are the most practical methods for checking the stability of slopes and are widely used in consulting engineers' offices.

Numerical method

Various software based on the finite element or finite difference method are available in the market, including: Plexis, Flak, 3D Flak, Geoslope, Yudec, TriDec, CITO, and Ansys. .