Research On The Safety Of Karst Cave Roof Under Rigid Pile Composite Foundation

Karst geology is widely distributed in China and it is complicated,constantly accompanied by various bad geological phenomena which mean a great threat to the safety of buildings over it.In the karst area,rigid pile composite foundation can effectively make the soil between piles to share the load and reduce the load of the pile,so it is widely applied.In the karst region of northern Guangdong Province where karst is highly developed,the surface layer of limestone broke seriously,and there are always a great number of irregular karst caves inside the limestone.The shape,size and location of caves have great randomness,and caves are often interconnected in a form of a beaded or honeycomb.The thickness of karst cave roof vary a lot,and there is a problem of the safety of the cave roof under the load of the pile,traditional methods are inadequate,if adopting impact-cone concrete pile,there is difficult to pile,and if grouting,the amount of grouting is very large and it is difficult to guarantee the quality,so a new method is proposed to evaluate the safety of karst cave roof,this method applies vertical load to the foundation during the construction phase,so that the additional stress generated in a certain area of the karst cave roof is greater than or equal to the additional stress generated in the same area after the building is completed,and the load multiple of the vertical load required to be applied compared to the load of the building in the normal state will be obtained,which has engineering reference significance and can be used to check the safety of karst cave roof.First of all,an overview of karst geology and its formation process as well as the concept of rigid pile composite foundation is stated in this paper,the mechanism of cave collapse in karst area and methods to analyze it are summarized.According to the mechanical properties of rock,based on the Mohr-Coulomb criterion,a method for evaluating the stability of limestone karst cave by comparing the average additional stress is proposed.Secondly,a simulation of pile test is done,and its results including stress contour,pile-soil stress ratio,soil sharing ratio of the loading,the influence of the cushion on the performance of rigid pile composite foundation are analyzed,then,the validity of the simulation method is verified,which provides a powerful tool for the study.Furthermore,the local models including one pile,4 piles or 9 piles and the overall models including upper structure are established.The calculation result is analyzed and compared.The average additional stress including S33 and Mises in an area of 2m×2m of 1 pile models are compared correspondingly to overall models with respect to piles beneath columns,the load multiples needed to be applied to the local models would be obtained when the average additional stresses are equal in the local model and overall model.Similarly,The average additional stress of the 4 pile models in an area of 2m×2m,9 pile models in an area of 4m×4m are compared correspondingly to overall models with respect to piles beneath columns,comparison of 1 pile models and overall models with respect to all piles is done as well.Then,the influence of the soil thickness,the number of the upper structure layers,the bearing capacity of the overlying soil and the position of the chosen area on the load multiples are analyzed.Finally,the concept of variance analysis and regression analysis is introduced,and the variance analysis is carried out to obtain the results which is used to be discussed whether these factors have significant influence on the load multiples.The regression equations is obtained by regression analysis,moreover,the fitting degrees and the ranking of each factor in all factors that affect the load multiples are analyzed.