Study On Progressive Failure Of The Excavated Slope And The Pre-reinforced Measurement With Pile And Anchor

Due to nonuniform distribution of stress field and different mechanical property of material, local failure will occur in excavated slope and bearing capacity of failured parts will descend, accompanied with stress adjustment interior slope and resulting in local failure expanding, ceasing and change of stability of total slope. Starting with characteristics of stress distribution and slope material, such contents are systematically studied and analyzed as determination of local failure scope, calculation of rainfall infiltration depth, analysis of slope stability, pre-reinforcement of slope with progressive failure, etc, and main research results and findings are achieved as follows:1. Numerical calculation is applied to analyze stress field of excavated slope and summarize characteristics of stress field and variation laws with change of slope toe. It is shown that the steeper slope toe is, the more concentration stress distribution is, the more nonuniform the stress distribution is and the more easily the progressive failure occurs. The characteristics of progressive failure in excavated slope are revealed and it is describled through concept of progressive failure rate that the greater progressive failure rate of material is, the more easily the progressive failure of slope occur. Thus, conventional limit equilibrium method used for single-time failure analysis can not be applied in stability analysis and should be improved. At the same time, mechanism of progressive failure is put forward and progressive failure procedure is diagrammed.2. Combined with stress distribution laws of excavated slope, stress distribution assumption is brought forward and through this assumption, initial local failure scope is determined, and then based on limit equilibrium method, slope stability after excavated are analyzed. It shows that, when stability analyzed, residual shear strength index should be used for slope with local failure and peak shear strength index should be used for slope without local failure. Destabilization mechanisms and corresponding stability calculation methods are analyzed for pure traction, traction and thrust type slope. Contrast method by zoning for determinating quasi-sliding face is advanced and concepts of longterm steady slope angle and critical selt-stabiliztion height, by which slope is zoned into anti-slide zone and thrust zone, are brought forward by variation of support power of single slice. By comparison between anti-slide zone and thrust zone, base face on base of which, quasi-sliding face at some point is competed, is determinated for hunting quasi-sliding face. It shows by variation of displacement of slices that it is important for controlling progressive failure to control displacement variation during excavation.3. It is the rainfall infiltration that results in difference of physical mechanic property of slope material and presence of the progressive failure. It is generally agreed that all slope material are in saturation state when stability of excavated slope is analyzed under rainfall working condition. Most of present analysis for rainfall infiltration is based on nonsaturated soil mechanics, analysis procedure of which is overloaded with details and difficult of application. In this paper, depth of rainfall infiltration is determined and it is supposed that soil in within range of rainfall infiltration depth is in saturated state and soil under rainfall infiltration depth is in natural state. When stability analysis carried out, different properties are selected for different slope material according rainfall infiltration depth. Therefore, decreasing the depth of rainfall infiltration is a good method for improving stability of slope under rainfall working conditions, which is explained by tests.4. Controlling deformation of slope during construction is key for voiding progressive failure of the slope, and for nearhorizontal redbed slope to prevent from crazing and expanding of controlling crack. Based on fundamental principle of fracture mechanics, intensity factor of vertical tension crack is determined and stress field at at the tip of crack is calculated. Based on energy criterion of Griffith, crazing expanding stress is derived. Based on conservation of energy principle, drastic moving distance after block breaking is calculated. Pseudo-static method is applied to simulate the effect of blasting load on a point interior the slope, and inverse computation is used to determine explosive quantity and blasting distance through which crazing and expanding of controlling crack can be prevented from.5. Analytic method of progressive failure is applied to analyze the downslide thrust of excavated slope with ealier pre-reinforcement and later excavation, and ealier excavation and later reinforcement. It is shown that downslide thrust of the slope with ealier pre-reinforcement and later excavation is greater than that of the slope with ealier excavation and later reinforcement, and the downslide thrust determined by single-time failure analysis based on limit equilibrium is corresponding to that that of the slope with ealier pre-reinforcement and later excavation. For the conventional embedded piles, monitoring example shows that maximum downslide thrust occurs after excavation of slope and the measured maximum downslide thrust is corresponding to calculated value, and downslide thrust correspondingly decreases after a period of time of excavation finish. Space between embedded piles can be calculated by big and small earth arch theory and magnitudes of space are different. Space of piles determined through big earth arch is smaller than that determined by small earth arch. Thus, in order to increase the stability of soil body between piles, big earth arch between piles can be applied to determine the spaces between piles.6. During the slope excavation with possible progressive failure, it is key to carry out reinforment while excavation so as to ensure stability of the slope, In order to decrease downslide thrust endured by anchorage body, adjusting excavated height of each level slope according to practical situation is favourable to control the downslide thrust endured by anchorage body. When pre-reinforcement is applied to the slope of each level excavation, considered downslide thrust should compromise the downslide thrust occurring in slope after next level excavation. Pressure (dispersion) type anchored cable is a conventional measures used for slope reinforcement due to big anchoring force, reasonable distribution and flexible application in all kinds of construction yard and construction stage. Stress characteristics of pressure type anchored cable are summarized in this paper and shows the characteristics of sidewall shear stress of anchored section of pressure type anchored cable. Both lastic model and ideal elastic-plastic model are applied to determine the length of anchored section and discuss the difference between computing method of anchored section length given in code and elastic method. When the length of anchored section determined according to code is smaller than that determined according to elastic theory, safety effect required by anchoring is not achieved, and in reverse, invalid anchored section will occur.7. Displacement characters of excavated slope are monitored by geologic model. Through comparisons between displacement without support and displacement with support when excavating, and variation with step-by-step excavation, it shows that progressive failure occur in excavated slope and it is necessary for pre-reinforcement so as to prevent progressive failure occurring when excavating.