| Karst is widely distributed in China,and projects in karstic areas are inevitably affected by karst.As a deep foundation,pile foundation is widely applied to high-rise buildings,highways,railways,wharfs and other engineering projects for its good applicability.Pile foundation has played a unique advantage and has solved many problems for the construction of engineering projects.Especially in the hidden karst area with complex and varied karst cave,the hidden karst has great influence on the bearing characteristics and stability of pile foundation.Many studies have shown that the calculation and design theory,detection and construction technology of pile foundation in karst area are still not perfect.Design and construction control still lack maneuverability.The bearing characteristics of pile foundation in karst area,the failure mechanism and stability evaluation of karst roof need to be further studied.In this paper,based on the laboratory test,model test,theoretical analysis,numerical simulation and field test,combined with the actual construction of a large project with hidden karst development in Shandong,the influence law of the karst cave on the bearing characteristics of the pile foundation is deeply studied.The failure mechanism and stability evaluation of the cave roof under the base load of single pile and pile group pile are studied in detail,and some research results with theoretical value and engineering significance are obtained.The main contents and achievements are concluded as follow:(1)The regularity of karst development is studied,and the disease characteristics and disease mechanism of karst pile foundation are analyzed.The relative position relation of karst and pile foundation is studied,and the factors that affect the diameter of karst cave,the form of karst cave and the position of pile-hole are analyzed,and the geological model system of the karst pile foundation is established.(2)A series of laboratory tests were carried out on the limestone samples obtained from the engineering site,which provided a basis for the selection of limestone parameters for model test and numerical simulation.According to the design method of the similarity theory and separating similar materials,with the experience of similar materials in limestone,the mixture ratio of similar materials of karst roof was determined by multi group test,and the similar materials of limestone were prepared and applied to the model test in this paper.(3)A large-scale combined multifunctional foundation test system has been developed independently.A new model pile,including single pile model,pile group model and supporting pressure sensor,is developed to test the pile end settlement and pile end resistance.The parallel real-time collection of pile top and pile base resistance and settlement is realized.According to the characteristics of the model test system and the model test requirements that can be carried out,modular development is adopted for the test system.The model test system includes the counterforce frame system,the servo control system using dual hydraulic pressure,the pile foundation model test data acquisition and analysis system,the static strain measurement system,the whole process camera system of the cave failure,the three-dimensional laser scanning modeling system of the cave roof flush surface,the cavity model precast system and the packing system.Through PLC programming,automatic compression,automatic acquisition,mechanized loading and unloading of model test is realized,and the efficiency and accuracy of experiment are improved.(4)By establishing a physical model test scheme for the study of the action mechanism of pile foundation and karst roof in karst area,a model test of single pile and pile group under different conditions of different karst areas has been carried out.The failure rules of karst caves at single pile base under the conditions of different karst roof thickness,different pile diameter,different cavern span and different roof dip angle are studied,and the mechanism of the punching and cutting failure of the cave roof is revealed.At the same time,comparing the failure modes of karst caves at different positions at the base of pile groups,the failure law of karst roof under group piles is revealed,and the influence law of karst cave piles on no karst caves at the end of pile is revealed.Through testing pile shaft force,pile end resistance and settlement and pile top bearing capacity and settlement,the load transfer characteristics of pile foundation under the influence of karst is revealed.(5)On the basis of the model test results,the hyperbolic secant function is introduced on the basis of hyperbolic model,and the load transfer model of pile side is established to reflect the frictional resistance characteristics of pile foundation pile in karst area.Based on the subprogram of ABAQUS,UMAT and FRIC,the two development of material base and contact model are carried out,the corresponding numerical simulation research is carried out,and the numerical experiments of other working conditions are supplemented.The reliability of the results is increased by the mutual validation and supplement of the results.The results reveal the bearing characteristics of karst pile foundation and the failure mechanism of karst cave roof:The bearing capacity of the pile increases with the increase of the thickness of the roof of the cave.The bearing capacity of pile decreases with the increase of the diameter of the cave.The ultimate bearing capacity of pile increases with the increase of pile diameter,but the increase of pile diameter is limited by the strength of pile concrete.Under the condition of certain thickness of the cave roof,the ultimate bearing capacity of the pile decreases with the increase of the inclination of the roof of the cave.The horizontal karst roof is the safest.The pile above the cave controls the ultimate bearing capacity of the pile group foundation,and the vertical displacement,stress and plastic zone distribution of the pile group model are related to the number and position of the pile on the cave.(6)Based on the calculation theory of the ultimate bearing capacity of rock socketed pile base proposed by Serrano,5 failure modes of pile end in karst area are put forward,the extreme resistance calculation polar coordinate system is set up for the bottom pile end of the inclined karst roof,and the calculation equation of the ultimate end resistance of the inclined karst roof in karst area is constructed.According to the correlation relation between the thickness of karst roof and the size of karst cave,the formula of limit end resistance under different failure modes is determined,and the correctness of the theory is verified by the result of the model test.According to the model test results,a transfer model for pile tip in karst area is established.Based on the load transfer method,the load transfer model of the pile foundation in karst area and the calculation method of the ultimate resistance of the bottom pile end of the inclined karst roof,a calculation method for the ultimate bearing capacity of the karst pile foundation based on the failure control of the karst roof is proposed.(7)Based on the theory of extenics evaluation and the calculation method of the ultimate bearing capacity of karst pile foundation,the software for stability evaluation and bearing character analysis of pile foundation in karst area is developed.The related factors affecting the overall stability of pile hole are summarized and analyzed,and the evaluation index system for stability of pile foundation in karst area is established.The software is based on the Microsoft Visual Studio2012 design platform,and the C Sharp programming language is used to compile the dynamic evaluation and analysis system for the stability of karst Pile Foundations(KPSBAS).The programming and interface of stability evaluation and bearing property analysis of karst pile foundation have been realized.Each function is presented in different modules,which is beneficial to the operation of the users,and has the function of image and text processing.The software system is beneficial to the timely evaluation of the construction of the pile foundation in the construction site,and it can realize the dynamic optimization design and construction of the karst pile foundation.It has certain practical value.(8)The static field load test of the cave at the end of the pile is carried out.The monitoring elements are set up at the different parts of the pile bottom and the pile body to obtain the pile end resistance,the axial force of the pile and the pile top load and the settlement of the single pile under different load grades.The pile end resistance,the pile side resistance and the load transfer characteristics of the pile body are theoretically analyzed.The results of the test data are in agreement with the calculation results of the evaluation and analysis software,which proves that the bearing mechanism of the karst pile foundation and the calculation method of bearing capacity and the analysis software are reasonable. |
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