Study On The Characteristies Of Pile Negative Friction In The Large Thickness Loess Area Considering The Collapsibility

As an important form of deep foundation, pile foundation has the advantages of small subsidence and high bearing capacity. It gets very extensive application in large thickness loess area at present. Pile can be considered as cylindrical component buried in the soil. The interaction between soil and pile was produced under the external force. And the interaction was very complex because of the layered characteristics of soil and the geological features of the differente soil. Engineering practice shows that the side friction value of the pile was great differente under different conditions.In order to study the interaction between pile and soil, and master the law of side friction values of the pile, this paper would study the characteristies of pile negative friction in the large thickness loess area considering the collapsibility through the combining method of field test, scale model test and numerical simulation analysis. The main work and achievements are as follows:(1) With the aid of actual engineering, the relationship between the pile settlement and the pile top load was studied through the field test study on large pile foundation under water immersion and natural conditions. The relationship between the soil settlement and the pile load transfer law was studied by the analysis of soil collapsibility, settlement characteristics, and settlement change features of different soil layer depth in loess area. The pile bearing capacity and the positive and negative friction changing rule was analyzed under pre-water immersion, and the testing pile was compared analysis was carried out between prewatering and non-pre-water immersion. The pile axial force distribution, the change of neutral position, and the friction distribution on the pile was studied.(2) According to the similarity theory, through the analysis and research of the domestic and overseas pile foundation testing scheme of negative friction, the prototype is turned into scale model by similarity relation, and then the testing scheme and method of the pile negative friction laboratory scale model considering the collapsibility was designed. The bearing capacity of large diameter sinking pipe was researched by scale model test, and the negative friction test was also carried out. Then, the neutral point position and the produce and development rule of negative friction were get.(3) The negative friction of single pile and pile group in loess area was analyzed by Nonlinear (ADINA). Main research contents include:the FEM model establishment, the water immersion calculation model set, the P-S relationship analysis of the pile foundation, the axial force analysis of pile top loading condition, the soil sedimentation analysis under water immersion condition, and the axial force and negative friction analysis under water immersion condition.(4) Based on the practical engineering in the northwest loess area, nonlinear FEM (ADINA) is used to established the finite element analysis model. The pile axial force and negative friction was calculated under pile top loading and water immersion conditions. And then parameters which influence the axial force and negative friction were analysed. The influences of loess elastic modulus and cohesive force, pile top loading, pile stiffness and water immersion pressure on the pile axial force and negative friction were mainly studied.