| The mountainous area in southwest China is well-know for its Karst landform;nowadays, with the development of highway-construction, workers need to build pilefoundations on top of the karst area. Thus, the safety and stability of the piling requireclose attention and care. This thesis is based on sichuan traffic technology project“karst area of subgrade disease prevention and built bridge and tunnel relying ontechnology”as the basis, then use NaQian expressdway in which there are hidden karstcaves under the subgrade as engineering backround. Bearing characteristics of the caveroof under pile foundation load is systematically analyzed.The main research results in this paper are as follows:①The main research contents of this paper are decided through site survey andinspection of domestic and foreign literature. Based on strength theory as well as thesize, integrity, boundary conditions of cave roof and the position relationship betweenpile and roof, the impacting system of cave roof and pile foundation is reasonablysimplified, then mechanical analysis is conducted on the ten simplified models, finallythe checking formulas for safe thickness of the models are deduced.②The factors affecting cave roof stability are analyzed and summarized, they canbe generalized as internal and external factors, and current stability evaluation methodsare introduced in details.③According to the similarity theory, sand, cement and gypsum are selected as themain analog materials, through simulating material strength test, the mix proportion isdetermined as482(weight ratio of cement, sand and gypsum is4:0.8:0.2),555,564,582,673,682. Intact massive limestone cave roof is taken as a rectangular plate, andthe simulation of indoor cave roof is carried, its bearing characteristics is studied.④The three-dimensional model of the piles and caves is set up using the finitedifference software FLAC-3D. Efforts are made to find out the rules affecting thestability; in this part, various factors such as the multi-scenario, the cave span pile load,rock-socketed depth, high cross-cave diameter and the location of the cave roof arecombined in calculation to draw the conclusion. Thus, the safety thickness of the pilingfoundation is obtained.⑤Based on the finite difference software FLAC-3D, the stability of caves beneathpier6#and8#of Hua She Dong BridgeⅡis analyzed. These two pile foundation are conducted by sleeve reinforcement method to ensure the security of project design andimplementation. |
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