| Embedded rear-row piles combined with full-length fore-row ones is a new type of double-row stabilizing structure which can be used for large bedrock-talus landslide.The composite form by embedded the top of the rear-pile at a certain depth below the slope,so it can fully mobilize the resistance performance of the two rows of piles and the landslide thrust borne by the fore and rear piles can be adjusted reasonably,which is of great significance in the treatment of large thrust landslide.However,the theoretical research on this kind of structure is far from perfect,and the relevant theoretical and technical problems need to be solved urgently in engineering practice.In this paper,based on the national natural science fund project “Study on mechanical mechanism and calculation theory of multi-row of embedded and full-length composite piles for reinforcement of large bedrock-talus landslide(51578466)”,in light of limit equilibrium method,variational method,elastic theory method,the 3D numerical simulation(FLAC3D),as well as indoor physical model tests,the author systematically investigates the following key issues:(1)Study on the stabilizing mechanism of the structure;(2)Calculation of the embedded depth of the rear pile;(3)Analysis method of the slope stability and the thrust on rear piles considering strainsoftening effect of the slip shear band;(4)Study on the stress transfer mechanism between pile rows;(5)Calculation method of the thrust on the fore piles.The main reasearch findings are as follows:(1)The reinforcement mechanism of the layout with embedded rear-row and full-length fore-row piles in bedrock-talus landslide is revealed from the perspective of the landslide thrust transfering process from backward to forward.For the double-row stabilizing piles with an embedded rear pile,the deformation of the soil overtop the rear pile is not constrained,so a portion of the landslide thrust incident of the upslope side of the rear-row piles bypasses over the top of the rear piles and is transferred directly to the fore-row piles,as a result,the thrust borne by the embedded-pile is less than that on the full-length pile at the same place.If the embedded depth of the top of the rear-row piles is reasonable,it is most likely that the sum of the landslide thrust on the double-row piles with “embedded rear piles and full-length fore piles” is consequently smaller than that on single-row piles located in the same place as the fore-row piles.(2)Based on the potential overtopping failure model,adopted by the transfer coefficient method and the variational method within the context of limit equilibrium theory,the theoretical solution of the embedded depth of the top of the rear pile is derived.Based on the thrust and resistance balance of the local sliding mass behind the pile,the relationship between embedded depth of the pile top and the stability coefficient of the reinforced landslide under the overtopping failure mode can be determined.Among them,the ultimate resistance of the local anti-slide section overtop the pile can be calculated by the variational method within the framework of the limit equilibrium theory,and the thrust of the local sliding section upslope can be solved by the transfer coefficient method.The proposed method is applicable to both viscous and non-viscous slide mass.As the embedded ratio increases,the start point of the new slip surface gradually closer to the pile along the original slip surface,and its ending point progressively moves downwards on the slope face,also the curvature of the new slip surface increases to some extent.At the same time,the factor of safety of the piled slope decreases nonlinearly as the embedded depth of the pile top increases.(3)Based on the strain softening effect,the shear strength partition method of the sliding band for natural slope and piled-slope is given,and then the method of stability analysis reflecting the softening effect of the shear band is presented,as well as the calculation method of thrust behind full-length pile and embedded ones.For the stability analysis problem,based on the stress field of the landslide obtained using elastic-plastic finite element method and relative relationship between shear stress and shear strength at any point of the shear band,as well as considering the peak strength,transition strength and residual strength of the shear band,a new method of shear strength partition of the shear band is provided to analyze slope stability of landslide(caused landslide and retrogressive landslide).Further,the analysis method can be carried out via transfer coefficient method with taking into account the various values of shear strength parameters on the corresponding sections of the shear band.For the calculation of thrust behind the pile,based on the same partitioning approach as above,a similar method of shear strength partition of the shear band corresponding to the design safety factor is proposed according to the slope stability analysis by the unbalanced thrust method.Then the residual thrust behind the pile and the residual resistance before the pile under the given design safety factor are calculated so as the design thrust value on the stabilizing pile which considering the differences of the shear strength on the shear band can be easily determined.Analysis results of practical examples show that factors of safety obtained by the proposed method are 14.1-21.6% smaller than those by the traditional method with peak strength parameters of the shear band,and 1.1-10.2% larger than those results with residual parameters.The upslope thrust force on the pile by the proposed method is close to that using the existing methods considering residual strength,but the downslope resistance force against the pile obtained by the two kinds of method is obviously different.The design thrust force by the proposed method is relatively higher than that by the existing methods.(4)An analytical method of elasticity is proposed to determine the stress of interrow sliding mass and the thrust on the fore pile based on the elastic theory.Based on the classical elastic wedge theory,we can firstly obtain the basic stress function of slide mass,then corrected by the angular and marginal eigenfunction,so as to obtain the final stress function considering the constraint effect of fore pile and the bottom sliding surface,then the thrust force and its distribution pattern can be determined easily.Considering the layout of the rear row,also the forms of the sliding surface between rows,the analysis results of the examples show that the landslide thrust on the fore-pile is distributed in a parabolic shape and the vertex is below the middle point of the loading section of the pile;the horizontal normal stress of the interrow sliding mass is distributed in a normal trapezoidal shape on the middle vertical section,and the vertical normal stress is distributed in a triangular shape on the section;The horizontal stress transfer law is complex and its variation near the boundary is more drastic.(5)The limit equilibrium inclined slice method was proposed to calculate the thrust borne by the fore pile based on the hypothesis that the sliding surface between pile-rows is in the limit state.The slide mass between the two pile-rows is divided into many oblique slices parallel to the local slip surface,these slices are involved in four basic types at various depths.Calculation formula of the thrust of each inclined slice on the loading segment of the fore pile can be derived based on static equilibrium principle,then distribution pattern of the thrust and its resultant force can be determined.The thrust on the fore pile determined by the proposed method is approximately parabolic distribution and its peak point is close to the slip surface.Below the peak point,the pressure decreases sharply to nearly zero.Analysis results of examples show that the calculated thrust on the fore piles under the embedded rear-row piles are 8.6% to 10.6% smaller than that under the full-length rear-row piles.(6)Influence law of row spacing?soil elastic parameters and pile stiffness on the thrust characteristics of double row piles is revealed.With the increase of row spacing,the maximum stress on the interrow horizontal plane and the thrust value behind the fore piles both show a parabolic variation pattern of increasing and then decreasing,which means that there is a most unfavorable row spacing between double-row piles that makes the thrust acting on the fore pile reach the maximum.The influence of soil poisson's ratio and elastic modulus on the interrow stress and the thrust distribution pattern behind the fore-pile is relatively small,but the thrust value acting on the fore-pile presents nonlinear increase with the increase of Poisson's ratio,while elastic modulus has little influence on it.With the increase of the rear pile's cross section,the thrust value behind the fore pile decreases and the reduction tends to be gentle.Elastic modulus of the rear-pile has little influence on the stress transfer law of intermediate soil,in other words,it does not change its distribution on the inter-row soil and the fore-pile.There is no simple positive correlation between the thrust value behind the fore-pile and its sectional size,and its sectional size and elastic modulus have relatively little influence on the thrust acting on itself. |
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