Bearing Mechanism And Design Method On Rock-socketed Piles In Karst Areas

As a significant ground improvement technique,pile foundation has been widely used in karst areas.How to weigh the influence of the existence of caves on rock-socketed pile has become a tricky problem,due to the complex geological condition of karst areas and the intricate pile-soil or pile-rock interaction mechanism.Moreover,current research still stays on preliminary stage.The corresponding design and computation approaches cannot be found in current codes and standards.Based on reasons mentioned above,this dissertation proposes a new analytical framework for rock-socketed pile in karst areas.The work on bearing mechanism,stress and strain analysis for single pile-single void system under vertical or inclined loads and single pile-multiple voids system under vertical loads of rock-socketed pile in karst areas is carried out on aspects of theoretical analysis,laboratory model tests and numerical simulation.This dissertation is funded through the National Natural Science Foundation of China “Research on bearing mechanism and computational approach on pile in karst areas”(Contract No.51278187).Firstly,based on the analysis of the working mechanism and the failure mode of void under pile tip,an approach on stability evaluation of void under pile tip is proposed by adopting the state of the void’s boundary as evaluation index.Based on the complex variable function theory and Mindlin’s solution,the formation stresses in semi-finite space containing square cave under gravity and pile tip load are obtained respectively.Then,the crustal stress of cave under pile tip is introduced by superimposing the formation stress in semi-finite space containing square cavity under gravity and the formation stress due to pile tip load.With the introduction of the latest Hoek-Brown failure criterion,the stability analysis for square cave under pile tip load is evaluated,considering the influence of the crustal stress field,pile-tip load,the size and deflection of the cave,etc.The rationality of the proposed approach is verified by numerical analysis and an engineering example.Secondly,based on the stress and deformation conditions of cave roof in karst areas,the laboratory model tests are designed and completed,according to the similarity theory.With the analysis of test results,the load transfer law of pile and the failure modes of roof of the cave under varied roof spans and roof thickness of caves are discussed,the comparison of the traditional method(bearing capacity determined by the roof thickness of the cave)with test results is conduct ed as well.Results indicate that with the decrease of the roof span or the increase of the roof thickness,the ultimate bearing capacity of pile increases in a certain range.The failure pattern changes from the punching failure to bending tensile failure with the increase of roof thickness.In practical engineering,the ultimate bearing capacity of pile can be evaluated effectively by proper increase of roof thickness.Thirdly,in view of deformation characteristics and bearing mechanism of rock-socketed pile in karst areas,based on Hoek-Brown failure criterion,a new calculative method of the ultimate bearing capacity of rock-socketed single pile in karst areas is proposed by obtaining the average constraint stress at pile tip.With the introduction of the complex variable function theory,the average constraint stress is solved.Then an approach is proposed considering gravity,cave size,cave shape,the horizontal and vertical distance between the pile tip and the center of the cave and the depth of the cave,etc.Furthermore,the influence factors for the ultimate bearing capacity of single rock-socketed pile in karst areas are analyzed,such as cave size,the horizontal and vertical distance between the pile tip and the center of the cave and the depth of the cave,etc.The results show that the increase of cavity radius and cavity depth as well as decrease of distance from the pile tip to the center of the cavity make bigger impact of cave on ultimate bearing capacity for rock-socketed pile in karst areas.Fourthly,based on the C-method assumption(the resistance coefficient increases with depth by a parabolic pattern),a power series solution of the internal forces and displacements of a flexible pile in a single homogeneous soil has been obtained under inclined loads,taking the weight of the pile and its shaft resistance into account.The results indicate the internal forces and displacements for single pile calculated by the power series solution are well consistent with those in practical engineering.The P-(35)effect is so apparent that cannot be ignored in design when pile is partially embedded in soil and the axial force is large.The shaft resistance and the weight of pile have little influence on the calculation of the internal forces and displacements for single pile.The finite element analysis software ABAQUS is adopted to build the computational model for single pile in karst areas considering the influence of void beside the pile under inclined loads.The influences of horizontal force,void size,the horizontal distance of void,the depth of void on the bearing mechanism of the pile are analyzed at length.Finally,the ultimate bearing capacity of rock-socketed pile above multiple voids is analyzed using the upper bound and lower bound finite element limit analysis.Considering the coupling mechanism of pile and voids,the hypotheses are proposed based on practical engineering;the computational model is therefore built.Using the finite element limit analysis programs developed by our research team,several influence factors on the bearing capacity of pile foundation above voids such as rock-socketed length,the surcharge of soils,horizontal distance,vertical distance and void size are discussed in detail.Considering the varied relative positions between the pile and the voids,six cases are divided to discuss the bearing capacity of pile.By analyzing the velocity fields and the energy dissipation fields,the typical failure patterns of rock-socketed pile above multiple voids are obtained.