Model Test Of Uplift Bearing Capacity Of Static Pressure Tubular Pile In Sand

Due to environmental protection,low noise,fast construction and the reliable quality,static pressure piles are widely used.However,the construction makes the punching shear failure of soil,and disturbs the soil around pile,also directly affects the actual bearing capacity of the pile foundation.Therefore,it is necessary to study the mechanism of static pressure pile driving and uplift behavior after pile driving.Based on the laboratory model test,the process of static pressure penetration and the uplift behavior after the penetration in sand were studied.The main work and conclusions are as follows:(1)The model tests of static pressure penetration and uplift behavior of single pile were carried out.The results show that friction fatigue exists in lateral earth pressure and pile friction;the static pressure is proportional to the speed of penetration;the ratio of tip resistance to the static pressure is above 80%and in inverse proportion to the depth of penetration and the depth of penetration.The Q-s curve of static pressure single pile in sand is "steep rise";distribution of axial force is that the top is maximum and the bottom is minimum;the pile friction gradually develops from the top to the bottom with the increase of uplift load.During the uplifting process,the pile friction has a peak,but is less than the maximum during the process of penetration.(2)The model tests of static pressure penetration of pile group were carried out.The results show that the Q-s curve of static pressure single pile in sand is also "steep rise".When the pile spacing of pile group is 3D-7D,the ultimate uplift capacity of the pile group is maximum when the pile spacing is 4D and decreases as the pile spacing increases;the displacement of all pile groups is smaller than that of single pile.When the pile spacing of pile group is 4D and 5D,the resistance of pile friction in the middle of pile is more obvious and the effect coefficient of pile group is greater than 1.When the pile spacing of pile group is 3D,6D and 7D,the effect coefficient of pile group is slightly less than l.(3)Three calculation methods for ultimate bearing capacity of single pile were introduced,and the calculated result of the design specification is the most consistent with the test result;nevertheless,the pile compacts the sand is not considered,so those methods are conservative.In addition,based on existing calculation methods for ultimate uplift capacity of pile group,a method for correcting efficiency coefficient of static pressure uplift pile group in sand is proposed;moreover,the applicability is verified by the test results and relevant calculation examples.