Study On Progressive Failure Mechanism And Determination Of The Toe Segment Slippage Length For Large Consequent Slope

In current calculation method of thrusting force for large consequent slope, the whole gliding mass is considered. In this situation, the thrust force is too large to make the general supporting structure meet the demand of design. Investigation shows that many large consequent slopes fail not as a whole but partly in one time. So it is suggested that if the first part of the consequent slope is reinforced, the slope will be safe. But this design concept is lack of theoretical supporting. The reinforcing design of large consequent slope has great effect on stability of slope; it also affects the cost of engineering. To study the mechanism of failure of large consequent slope and the length of first failing part is very important to determine a rational reinforcing design. In this paper, the failing process and mechanism of large consequent slope are analyzed systemically. A preliminary calculation formula of first failing part of large consequent slope is obtained. The reinforcing design principle based on the failing mechanism of large consequent slope is put forward, and the present design theory is supplemented and perfected. Based on field investigation, numerical analysis, model test, field survey and field monitoring, the contents of this paper are as follows:(1) Considered the progressive failing mechanism and reinforcing difficulty of consequent slope, the slope whose length is longer than 100 m is defined as consequent slope in this paper based on the point of view of engineering.(2)The characteristic of the elastic-plasticity of weak intercalated layer, peak-residual strength, friction mechanism of structural plane and shear property is systemically analyzed and summarized. The mechanics parameter of sliding surface of large consequent slope is obtained. The stress character of sliding surface is analyzed in detailed. The influence factors that affect the length of stress concentration on sliding surface are analyzed. Those factors include slope angle, slope length, thickness of slide body and so on. A length calculation formula of stress concentration segment of sliding surface in large consequent slope is put forward as following:L_max=0．3938α+0．0643 14L+1．505569H-2．18222α≤35⁰ H≤40m 100m≤L≤1000m(3)A model to stimulate the slide mechanism of large consequent slope is designed and established. The failure mode and failing process are stimulated through model test. The relation between the length of the first part of failing slope and length of slope and thickness of slope is obtained. The result is compared with the result from the length calculation formula of stress concentration segment of sliding surface, which shows that it is rational to determine the length of the first part of failing slope by the length calculation formula of stress concentration segment of sliding surface.(4) By field geologic investigation, the lengths of the first part offailing slope of five large consequent slopes are obtained. The geometric parameters of those slopes are measured in field and the lengths of stress concentration segment are calculated. The result shows the length of stress concentration can be the basis for reinforcing design for large consequent slope.(5) The design principle of large consequent slopes is systemically studied. And a preliminary design principle for large consequent slopes is established. The design principle is divided into four steps: the step of excavation stability valuation of slope, the step of rock mass mechanics parameter value collection, the step of slope classification and the step of design.(6) A comprehensive integrated method for determining mechanical parameter of sliding surface of large consequent slope is put forward. That is to determine mechanical parameter by large-sized shear test, back analysis based on field investigation, engineering analogy, code requirement and engineering experience of experts. Applied this method in large consequent slopes in Yichang-Wanzhou railway, the value range of mechanical parameter of sliding surface is obtained.(7) Based on the relationship between comprehensive friction angle and dip angle of rock stratum, the large consequent slope can be divided into three types: the overall stable, the progressive failure and the overall failure. The calculation formula for side pressure of rock mass against supporting structure for the large consequent slope is obtained, and in the formula, the comprehensive friction angle of sliding surface equals to residual strength parameter.(8) Taking Badong slope in Yichang-Wanzhou railway as an example, the reinforcing measure of this large consequent slope is designed based on the supporting principle in this paper. By field monitoring, the deformation and stress character of the slope rock mass is obtained, and the length of deformed section by field monitoring is similar to the length of stress concentration segment. By back analysis of the first part of sliding body based on rock pressure and pile deformation, the degree of safety of the BaDong large consequent slope, which is 2.16, is obtained, which shows that the reinforcing design principle in this paper is rational.The reinforcing design principle in this paper is supported by clear theory. It can be applied to engineering practices.