Study On Negative Skin Friction Characteristics Of Rock Socketed Pile In Deep Fill Site

In the western mountainous and hilly areas with complex topography,in order to reasonably plan land resources and improve urban space utilization,the method of "digging high and filling low" is often used to form a large number of deep filling sites directly backfilled with earth rock mixture from excavation.Rock socketed pile is widely used in this kind of site engineering because of its excellent performance of controlling foundation settlement.The self consolidation settlement of filler will produce considerable negative friction on pile foundation,which may cause excessive deformation of pile foundation,damage of pile body and yield of pile end foundation,The interaction mechanism of pile-soil mixture system is also different from that of pilesoil system such as plain fill foundation and soft soil foundation.Therefore,it is of great engineering value and theoretical significance to study the negative friction characteristics of rock socketed piles in deep earth rock mixture fill foundation.In this paper,the representative sand and mudstone in Southwest China is selected as the filling material for the indoor model test of pile foundation.The comparative analysis is carried out by means of model test and numerical simulation,and the sectional calculation model of pile side friction is constructed.Based on the law of functional reciprocity,the control equations of axial force and displacement of pile body are derived,In this paper,the mechanism and distribution of negative skin friction of rock socketed piles in deep fill are discussed systematically.The results are as follows(1)The bearing capacity characteristics and load transfer mechanism of rock socketed pile are expounded,and the assumptions,theoretical basis,service conditions and characteristics of various calculation methods of negative friction of single pile and estimation methods of neutral point are systematically analyzed.(2)The model test scheme of rock socketed pile in deep fill site is designed.The test results show that the different filling around the pile has a greater impact on the soil settlement and a smaller impact on the pile settlement under the load.After the completion of surcharge loading,the surface settlement of soil in the soil rock mixture filling foundation is reduced by 15.22% compared with that under the plain filling condition,The settlement of the pile in the earth rock mixture filled foundation is 1.47% less than that in the plain filled foundation;Under the same load condition,the axial force of the pile increases first and then decreases along the depth direction of the pile,regardless of the soil rock mixture filling or plain filling conditions,but the negative friction of the pile in the soil rock mixture filling foundation is smaller;The position of neutral point moves up linearly with the increase of pile top load.For the same type of soil rock mixture fill foundation,the depth of neutral point can be obtained by the same type of plain fill through the correction coefficient of 0.92;The existence of rock socketed section can effectively improve the vertical bearing capacity of rock socketed pile.(3)The finite element model is established and compared with the indoor model test,which verifies that the experimental model and the numerical model have good consistency in the basic characteristics of pile axial force,pile side friction and neutral point.Further analysis shows that with the increase of rock socketed depth,the load shared by pile end resistance decreases,which is not conducive to play its bearing role.The appropriate rock socketed depth can be determined according to the strength of bedrock;The strength of bedrock is an important factor affecting the strengthening effect of pile side friction.The smaller the strength of bedrock is,the more obvious the increase of pile side friction near the pile end is.(4)The hyperbolic model is used to calculate the pile side friction before the pilesoil shear displacement reaches the limit state.Combined with the effective stress method,the sectional calculation model of the pile side friction is established.Based on the functional reciprocity law,the theoretical derivation is carried out,and the calculation expressions of the axial force and displacement of the pile body are obtained considering the relative displacement of the pile-soil and the change of the potential energy of the pile-soil system,Then,the improved design and calculation method of rock socketed pile considering negative skin friction in deep fill site is given.Combined with engineering examples,it is verified that this method can accurately analyze the mechanical properties of pile foundation in deep fill foundation.