Finite Element Analysis On A New Method For Pile Static Loading Test Using Vacuum Negative Pressure

Firstly, this paper analyzes three types of static loading test of pile-Piling-up weights method, Anchor-pile loading test method and Self-Balanced loading pile test method. Piling-up weights method takes much time and great effort;Anchor-pile loading test method needs extra expenses of anchor piles and couldn't used in the condition that pile shaft friction is small;Self-Balanced loading pile test method is difficult to realize the random inspection and not suitable to detect an end bearing pile. Additionally, the working behavior in this method differs from the real conditions, etc. The paper also points out the necessity of developing a new method for pile static loading test by using vacuum negative pressure.Secondly, the paper analyzes the key problem which may occur in this new method, and does pilot calculation on natural ground with FEM. Then, the ground stability is analyzed. It is shown that natural saturated soil with general permeability could not bear the pore pressure of -80kPa.Thirdly, the paper analyzes the new method under the condition that the ground under vacuum area is improved by sand drain. Through varying the permeability coefficient, the buried depth of sand drain, void ratio, modulus of deformation(poisson's ratio) and cohesion(angle of internal friction), many numerical calculations are made and following conclusions are drawn:i) The key of the new method is keeping hydraulic gradient in the ground smaller than the criteria hydraulic gradient there;ii) The maximum hydraulic gradient increases as the coefficient of permeability decreases. In terms of the hydraulic gradient, this new method is more suitable to the soil which permeability is greater than le-8m/s;iii) With the soil's permeability being greater, the hydraulic gradient is much easier to be smaller than the criteria hydraulic gradient there. The paper suggests drilling through the layer of soil using sand drain if the permeability of superficial soil is smaller than le-8m/s;iv) Void ratio have little effect on the pile test, while modulus of deformation(poisson's ratio) influences the settlement of the pile and ground greatly.Finally, the paper compares the piling-up weights method with the new method, and reaches the following conclusions:i) In loading-settlement curve: Due to ground being improved by sand drain, the settlement calculated by the new method is smaller than the traditional method in the same load step;ii) The settlement of ground surface due to the piling-up weights method could be ignored, but the new method's is much greater and couldn't be ignored.