| With the development of economy and the increasing investment in infrastructure.More and more projects are built in karst areas,and the engineering geological problems in karst areas are becoming more and more prominent.Pile foundation is the most common type of deep foundation used to overcome the bad karst geology.The influencing factors of pile bearing capacity in karst area are complex,which often lead to the pile design inconsistent with the actual working conditions.It can only meet the engineering requirements by changing the design,resulting in the extension of construction period and the increase of engineering cost.In this paper,combined with the National Natural Science Foundation Project "Study on the effect of karst cave characteristics on foundation failure mode and pile bearing capacity",by means of numerical analysis and indoor large-scale model test,the bearing characteristics of pile in karst area are discussed,mainly in the following aspects:(1)On the basis of the existing research,relying on a project,using ABAQUS finite element simulation software,this paper discusses the influence factors of pile bearing capacity with karst cave at the pile end.The results show that the thickness of karst cave roof are the main factors affecting the bearing capacity of pile foundation;under the same working conditions,the adverse effect of square karst cave on the bearing capacity of pile foundation is always greater than that of oval karst cave;the adverse effect of karst cave shape on foundation pile decreases with the increase of karst cave roof thickness;the bearing capacity of side karst cave on foundation pile decreases with the increase of spacing.(2)According to the requirements of karst pile model test,the Brillouin Optical Time Domain Reflectometer(BOTDR)is selected to monitor the change of strain field caused by pile bearing.The influence of optical fiber layout length and type on test results and the influence of optical fiber implantation density on test block strength are discussed by block test.The results show that it is feasible to use BOTDR to collect the relative strain of the model block,and the accuracy of the collected results is closely related to the type and density of the optical fiber.(3)Based on the similarity principle,a large-scale indoor similar model test is designed to study the characteristics of foundation pile settlement,pile strain and foundation strain field in the process of pile bearing.The results show that the indoor model test can better simulate the pile-soil and geological boundary conditions in the actual project,and calculate the bearing capacity of pile foundation according to the measured load-displacement curve;the traditional resistance strain gauge has good practicability,which can effectively measure the strain value of the layout point,but it is easy to be damaged in the loading process,leading to the lack of data;BOTDR has better survival rate when it is used in large model laboratory,and the sensing optical fiber area can be collected.Compared with traditional point sensors,BOTDR is more practical in strain field monitoring.(4)Under the model test condition,when the thickness of the karst cave roof is three times the pile diameter,the roof is shown as the elastoplastic failure mode,which has obvious plastic stage;In the process of vertical load applied on pile top,the load strain curve of pile body reflects the transfer process of pile top load to some extent.The load on the pile top is transmitted from top to bottom in the form of side friction resistance,and the load borne by side friction resistance decreases first and then gradually becomes constant with the increase of load;Under the load of pile top load,the diffusion range of roof strain in the x-axis direction is about 5-6 times range of the pile diameter,and the strain diffusion characteristics in X and Y directions are different;Under the load at the end of pile,the load characteristics of karst cave roof conform to the simplified mechanical model of the fixed beam and the fixed free plate.The strain is the largest at the axis of the beam,and gradually decreases to both ends in the X direction. |
