A Study On Numerical Analysis Of Pile-Soil-Slope Interaction And Simplified Design Procedure Of Stabilizing Piles Against Landslide

The stabilizing piles are widely used in the reinforcement engineering of slopes and mitigation and prevention of natural geological disasters induced by landslides. However, because of its rather sophistication, the working mechanism of stabilizing piles and stability of piles in the slopes are not clarified especially when the piles are subjected to the combined actions of both components of horizontal load and vertical load as well as moment imposed by the superstructures. Under such a circumstance, the piles in slope will play two functions, one is as reinforcement to induce the instability and another is to bear the loads transferred by structures. On one hand, with the large-scale railway and highway constructions have been made in the area where the potential geological disaster such as landslides and debris tends to occur in complicated geological condition, the piles are employed to stabilize the slopes which are located the two sides of railway and highway. On the other hand, since the resource of usable lands gets to reduce considerably in modern urban civilization, the high-rise or multi-rise mansions or buildings are directly constructed in the neighborhood of the top or toe of slopes, the piles are used to both reinforce the slope near the construction and carry the superstructure-induced loading. In fact, the pile-soil-slope will constitute an interaction system. Therefore, it will be theoretically important and practically significance to examine the mechanism of piles in both stabilizing the slope and carrying the loads and to discover the interaction effect of piles-soil-slope. In this dissertation, both analysis methods for analyzing the stability of slopes stabilized by piles against potential sliding and bearing capacity behavior of piles in the slopes subjected to combined loads as well as interaction mechanism of pile-soil-slope and simplified design procedure of stabilizing piles are investigated. The main research work done in this thesis consists of the following parts.1. FEM-based numerical analysis of slope stability and the corresponding failure criterion is used to implement the procedure of automatic searching for safety factor.The shear strength reduction technique is incorporated into the large general FEM analysis software, ABAQUS to automatically search for the most critical strength reduction factor as safety factor. Through numerical computations and comparative analyses based on FEM, four criteria for assessing the critical failure state of slope, including the criteria on the basis of iteration non-convergence of nonlinear FEM numerical calculations, plastic zone extension from the toe to the top of slope, equivalent plastic strain exceeding a certain value developed along the potential slip surface and uncontrollable increase in characteristic nodal displacement on slope surface or the slope of curve interrelating strength reduction factor (SSRF) and horizontal displacement of characteristic nodes on slope top reaches a certain value, are respectively examined. Furthermore, the interrelation and applicability of such four failure criteria are demonstrated. It is found that the instability criterion based on iteration non-convergence of numerical solution is easier to be used in the numerical implementation of automatic searching procedure for safety factor, and is rarely dependant on users' experience. Therefore it is suggested to widely apply this criterion in evaluating the failure mechanism and instability state. Then, the non-convergence criterion of numerical solution is successfully used in the stability analysis of layered clay slope, multi-stage slope, embankment slope and slope under surcharge. Moreover, the effects of element type, FEM mesh density, yield criterion of soil, dilatancy and deformation parameters on the safety factor of slope stability, is examined. Finally, the consistency of Mohr-Coulomb yield criterion used in ABAQUS with that in traditional plasticity theory at six extension-compression corners is demonstrated.2. Numerical analysis of stability on pile-soil-slope system with stabilizing piles, load-bearing piles against landslide under combined load modeBy using elasto-plastic finite element method of slope stability based on shear strength reduction, stability of slope with stabilizing piles and performance of load-bearing piles against landslide as well as behavior of stabilizing piles in slope subjected to combined load are numerically analyzed. The iteration non-convergence criterion conventionally used for assessing the instability state of slopes is employed to evaluate the limit-equilibrium state of pile-soil-slope system and its validity is demonstrated through comparative studies for several typical examples. Considering the complexity of pile-soil interaction, the criterion based on the uncontrolled displacement at a certain characteristic nodes on the slope surface is used for evaluate the limit state as an assistant criterion in addition to the iteration non-convergence criterion of solution in order to get more reasonable and reliable solution from numerical results. Furthermore, the effect of stabilization location, pile diameter, embedment depth of pile, constraint condition of pile heady and bending rigidity of pile on the stability of slope stabilized by piles. Both distribution pattern and variation mode of lateral contact force (frictional shear force and contact pressure) around pile shaft are examined in order to develop a simplified model for engineering design of slope reinforced with piles. Finally, the effect of the combinations of vertical load and horizontal load on the stability of pile-soil-slope system is investigated through numerical analyses.3. Upper-bound solution of limit analysis of complicated slope stability based on shear strength reduction techniqueNowadays, limit equilibrium method is often adopted to analyze the stability of slope in practical engineering. However the result from this procedure is neither the upper-bound nor the lower-bound of the true solution based on the principle of limit analysis. Combined the kinematic method of limit analysis with shear strength reduction technique, the equation for expressing the limit-equilibrium state is formulated and is employed to define the factor of safety and its corresponding critical failure mechanism for a given slope. Through numerical analyses for typical examples, the solutions computed by the proposed approach are compared with the results available given by limit-equilibrium methods and finite element methods to verify the reasonability of the method. Furthermore, the complicated conditions such as non-homogeneity and anisotropy of soil strength, surcharge on slope top, pore water pressure in slope, earthquake-induced inertial load and other external loads are respectively considered. And the effects of these factors on slope stability are individually examined.4. Upper-bound solution of limit analysis of the lateral effective earth pressure acting on stabilizing piles and determination of the most optimal location of pile.On the basis of the distribution mode of lateral contact force from elasto-plastic finite element method based on the technique of shears strength reduction factor, considering the engineering experience and current investigations available, lateral force of surrounding soils on stabilizing piles are analyzed by using upper-bound theorem of limit plasticity and the concept of shear strength reduction and the upper-bound solution for defining the lateral effective earth pressure acting on stabilizing piles against landslide is established in which the mobilized strength parameters are given by the actual strength parameters with a reduction by the desirable overall safety factor. The lateral effective earth pressure acting on the piles can be chosen as the objective function with respect to the related parameters used to describe potential failure mechanism and then mathematical programming is formulated and the optimization technique are utilized to define the critical state. Then the lateral effective earth pressure acting on the stabilizing pile is determined. In the analyses, the effects ofnon-homogeneity and anisotropy of soil strength, surcharge on slope top, pore water pressure in slopes and earthquake-induced inertial load are taken into account. Furthermore, numerical computations are made to examine the optimum location of pile placement on the basis of the dimensionless lateral limit effective earth pressure obtained. The position of the critical slip surface and some factors affecting the reinforced force are also analyzed respectively.5. Lower-bound solution of the lateral allowable bearing capacity of soils around stabilizing piles and determination of anchorage depth of stabilizing piles.Using the lower-bound technique of limit analysis of plasticity, the lower-bound procedure is established to define the lateral allowable bearing capacity of soil around stabilizing piles, when soil cohesion and inclination of slope, surcharge and seismic acceleration coefficient are considered. Through numerical analyses, comprehensive active and passive earth pressure coefficient are given in a tabular form for different combination of the relevant parameters such as internal friction angle of soil 0, inclination of slope a, dimensionless cohesion cjyz., dimensionless density of surcharge q/}2 or horizontal seismic acceleration coefficient kh. The effect of various parameters on comprehensive coefficients of active and passive earth pressure are discussed, and the computed earth pressure coefficients can be directly adopted to assess lateral allowable bearing capacity of soil around stabilizing piles in practical engineering. Furthermore, the anchorage depth of stabilizing piles can be determined for rigid or elastic piles to offer an instructive guideline for the design of stabilizing piles. The effects of slope inclination, soil parameters and computational depth on the lateral allowable bearing capacity of soil around stabilizing piles are discussed. When two different soil layers respectively upper and lower the potential slip surface are considered, the modification is made for the anchorage depth of stabilizing piles in layered slope.6. Simplified design of pile-stabilized slope and uncoupled procedure for analyzing the performance of pile-soil-slope interaction system.Based on the dimensionless lateral limit effective earth pressure acting on stabilizing piles determined by the above proposed method, the numerical algorithm based on finite-difference method is developed to numerically solving the partial differential equation for governing deflection of stabilizing piles. Then a simplified design procedure of stabilizing piles is given. Therefore, for the pile-soil-slope interaction system, stability analysis of slopes reinforced with piles can be firstly performed and the proposed checking method can be used for evaluating anchorage depth of stabilizing piles, and finally the numerical procedure can be employed to calculate the deflection of stabilizing piles and simplified design can be accomplished. In spite of its simplification and uncompleteness, such an uncoupled procedure is practically effective for evaluating the overall performance of the pile-soil-slope interaction system. Finally, as a preliminary application, such a procedure is used to check the design of stabilizing piles in slope for a slip-prevention control engineering in Dayaowan port of Dalian Port Authority.