Karst Roof And Pile Mechanism Of Action Research, Analysis And Simulations

Karst area is widely in our country, all kinds of Karst condition always be come across as our country's fundamental construction is greatly developed, which bring a series of problems in the building construction. Cast in-situ pile foundation is more popular in Karst area. How to choose suitable pile pattern and loading lays of pile-tip, how to determine the best socketed depth and roof safety thickness, would produce important theorem meaning and economic value.Based on a lot of international and our country's papers, research level of socketed-rock piles in Karst area has been proposed and discussed. The socketed depth and roof safety thickness determine the cost of engineering projects .design method and construction of socketed-rock piles. Studying the mechanism of pile and cavity roof and doing simulated test become the most important thing which we should to do. In this paper, an extensive study is carried out and the results have been obtained as follows:Firstly, the paper synthetically analyses the limestone properties in Karst area, make a statistics for the roof width and space distribution laws, draw the distribution scheme of cavities, also discuss the socketed-depth and roof safety thickness in many engineering projects.Many reasonable hypothesizes have been proposed, the cavity roof has been simplified, the system of piles and cavity roof can be simplified for ten mechanics models according to the boundary conditions and mechanical station. The ten models have been discussed and analyzed respectively, the stress and strain formula has been deduced. At the same time, the influence of Poisson's ratio also is discussed.With the vertical loading, the formula of loading capacity of socketed-rock piles' has been talked over synthetically. The model of socketed-rock piles can be simplified to an only-tip loading pile. Taking the roof as a whole plate, the rock's anti-bend strength is the key factor which determine the stableness of pile foundation.According to the analyses of the mechanism of pile and cavity roof, we design and make the test model of pile foundation by using the similar theory, also work out the auto-measure system. Through a lot of tests, the model and system can reach our purpose.The choice of similar material is the key problem in the simulated model test. With the uniform design methods, we choose the mixture of cement with gypsum and sand to simulate the limestone properties according to the similar theory. This kind of mixture is verified to be good.According to the test result and theoretical analysis, the paper has proposed the approach to examine the safety roof thickness, half-theory and half-experience formula associated with vertical loading is proposed as follow:In karst area, the socketed depth seems difficult to determine, considering synthetically the correlation between the pile skin friction resistance and the bedrock internal cohesion and the influences on loading capacity of pile, the mine socketed length is suggested 0.2 pile diameters or at least 0.2m when the rock surface is level and the upper soil is thick relatively.