| Karst development is common in Xuzhou area,and the bearing stratum of rock-socketed pile in pile foundation engineering is often in karst stratum.This thesis takes the site on the north side of Xuhai Road in Xuzhou City as the background.The karst development degree of the site is strong,and the rate of drilling holes is high.Most of the rock-socketed piles need to cross the karst cave,and even some pile foundations will cross the beaded karst cave.In order to study the bearing characteristics of rock-socketed piles crossing karst caves,the influence of embedded depth on the ultimate bearing capacity of rock-socketed piles was studied through field tests,and the influencing factors such as karst cave filling mode and karst cave height were analyzed by numerical simulation.(1)On the basis of collecting and analyzing the characteristics of karst geological development in Xuzhou and the geological survey of the research site,the influence of karst strata on the bearing capacity of rock-socketed piles is analyzed.The existence of karst will reduce the resistance of pile end and pile side friction,increase the settlement of pile top and reduce the compressive capacity of pile body.When the rock-socketed pile passes through the cave,the influencing factors of the cave on the bearing capacity of the rock-socketed pile are summarized,mainly the height of the cave,the span of the cave,the number of the cave,the thickness of the roof of the cave,the distance between the caves and the filling method of the cave.(2)The field static load test was carried out to press the field test pile to the limit state,and the load settlement curve of the pile top of each test pile was obtained.The ultimate bearing capacity of each test pile was predicted by nonlinear fitting.The soil layer and pile end of each pile hole are analyzed respectively,and the overall influence coefficient which can reflect the influence of karst cave and rock weathering degree on the ultimate bearing capacity of rock-socketed pile is introduced.The fitting method is used to analyze the change trend of the overall influence coefficient under the influence of factors such as the embedded depth and the total height of the cave.It is concluded that the embedded depth of the rock-socketed pile through the cave has little influence on the overall influence coefficient when the embedded depth of the rock-socketed pile through the cave is large,while the total height of the cave increases.The value of the overall influence coefficient decreases and the ultimate bearing capacity of the rock-socketed pile decreases.(3)FLAC3D numerical simulation software is used to analyze the influence of karst cave crossing type,pile foundation rock-socketed depth,karst cave roof thickness,karst cave height and spacing between karst caves on the bearing characteristics of rock-socketed piles under different filling conditions of karst caves.The results show that the changes of the above factors have a great influence on the rock-socketed piles of empty karst caves and clay-type karst caves,and the friction resistance of the pile side will decrease,and the pile end resistance and the settlement value of the pile top will increase.If the load on the pile top is large,it may cause damage to the pile end,and even part of the rock-socketed pile crossing the karst cave will increase the bending deformation of the pile body,resulting in buckling instability of the pile foundation.The filling and reinforcement of karst caves can effectively reduce or even eliminate the influence of the above factors,which is conducive to improving the ultimate bearing capacity of single piles.(4)Based on the load transfer principle of ordinary rock-socketed piles,a load transfer calculation model for rock-socketed piles crossing karst caves is established.The model takes into account the filling conditions of different karst caves,such as cavity,clay-type karst cave and filling-reinforced karst cave,and gives the calculation formula of pile shear stress of rock-socketed pile in each loading period,which can be calculated by substituting corresponding parameters according to different geological conditions on site.According to the known pile top load and settlement value of the test pile,the model is used to calculate the axial force distribution of the rock-socketed pile.Through numerical simulation,it is concluded that after the filling and reinforcement of the karst cave in the site,when the embedded depth under the karst cave is 1m and 0.2m respectively,there is no plastic failure in the rock-socketed section,indicating that after reasonable filling and reinforcement,the rock-socketed depth under the karst cave can be appropriately reduced.The thesis has 79 diagrams,21 tables and 98 references. |
PDF Full Text Request