Study On The Characteristics Of Shaft Soil Resistance Of Pile

As an important deep foundation form, pile foundation has the virtue of small sedimentation and high load bearing, so it has a broad application. Pile body can be seen as a column buried in soil, when pile bearing load, there will be a interaction between pile and soil. Because the soil is layered and the geologic characteristics of different soil layers are different, the interaction between pile and soil is very complex. It is found in engineering practice that the shaft soil resistance of pile will varied in different situation. To study the interaction between pile and soil profoundly and make the variation rule of shaft soil resistance clear, the following studies were carried out.(1) By mechanism analysis it is concluded that the shaft soil resistance of pile is different from the friction force between pile and soil. The influence factors to shaft soil resistance can be reduced to five aspects: the strength of pile-soil interface (including the adherence force and the friction force), the strength of soil, the load and deformation characteristic of soil (such as shear dilation or shear compression of sandy soil), the rigidity of the combination of pile and soil, the asynchronism of the appearance of ultimate shaft soil resistance or ultimate tip soil resistance of different soil layer. It is found by analysis that the strength of pile-soil interface and the shearing strength of soil will increase with the increase of side soil pressure, but the extend of the increase is different for soil with different character. The increase extend of sandy soil is bigger than that of cohesive soil.(2) Because the shaft soil resistance of pile is influenced by many factors, finite element theory is chosen as the analysis method of it. A common method for the research of shaft soil resistance of pile was summed up. The process includes: calculating the original stress of soil; establishing the calculating model of pile and soil; set the pile-soil interface as"tied"to judge the destroy position. If the destroy position is judged to be in pile-soil interface, the parameters of pile-soil interface can be worked out by fitting the testing result with iterative computation. By comparing the test data and the calculated data obtained from the program, the veracity of the program was verified. The influence of the calculating parameters to the calculating result was analyzed.(3) The friction and adherence characteristics of PHC pipe piles were studied using the result of a still load and internal force test of piles in saturation cohesive soil and in sandy soil. The research showed that for saturation cohesive soil, the interface force is mainly composed of adherence force, while for sandy soil the interface force is mainly composed of friction force. The rule of side soil pressure changed with the depth of soil is analyzed by calculation. The value of interface parameters for PHC pipe pile in silt and in fine sand is estimated by fitting the testing data.(4) A contrastive uplift test of 4 PHC pipe piles before and after excavating were accomplished. This test proves that the side soil pressure have a influence to the shaft soil resistance of pile. Finite element analysis is used to study the rule of side soil pressure changing with the depth of pile and changing with the load on pile.(5) The friction and adherence characteristics of Cast-in-site pile were researched using the result of a still load and internal force test of Cast-in-site piles. Research shows that for middle length pile, shaft soil resistance is mainly decided by interface strength and the interface parameters are related to the shearing resistance of soil. The rule of side soil pressure changing with the depth of soil is studied. The value of interface parameters for Cast-in-site pile in multiple layered soil is estimated by fitting the testing data.(6) A set of uplift and downward load test of short piles were used to research the friction and adherence characteristics of concrete and rock. The ultimate value of shaft soil resistance and its critical displacement under uplift or downward pressure load is obtained from the test. The composing of interface strength is analyzed with finite element calculation. It is found that when the interface of pile is coarse, the general adherence force is the main component part of interface strength.(7) The influence of tip bearing stratum and the load direction of pile to the shaft soil resistance are analyzed. It is found that the reasons why the above two factors can cause a variation of shaft soil resistance are actually same, which can be reduced to three aspects: the variation of side soil pressure, the asynchronism of the appearance of the ultimate shaft soil resistance or tip soil resistance of different soil layers, the different circumjacent restriction to the soil.