Experimental Research On Response Law Of Pile Group Foundation With High And Low Cap Under Earthquake Action

As a difficult problem in geotechnical engineering,the dynamic response law of pile foundation under earthquake involves many complex problems,such as the dynamic response of soil,soil-structure interaction,dynamic nonlinearity,elastoplasticity and so on.Based on the existing research results,this paper mainly aims at the dynamic response law of pile group foundation with high cap,which has obvious seismic damage characteristics and is unfavorable to bear horizontal load.Taking the embedded pile group foundation with low cap as the comparative object,large-scale shaking table test and numerical simulation analysis are carried out to reveal the basic seismic response law of pile foundation in general uniform foundation,and the high cap and the pile group are analyzed in detail.The main conclusions are as follows:1.When considering the interaction between pile and soil,the vibration energy of superstructure will be transmitted outwards through the dissipation of foundation and foundation soil(radiation damping effect),and the buried condition of foundation cap will directly affect the vibration characteristics and dynamic response of superstructure.Due to the exposed surface of pile cap and pile body,the radiation damping effect of high pile cap group is relatively small.2.Under the same conditions,the peak acceleration of the high pile cap structure is 1.35?1.65 times of the low pile cap structure,and the apparent amplification coefficient of the peak acceleration of the exposed pile body is about 2.0?2.4.Compared with the foundation soil around the pile,the acceleration response between the pile and soil on the surface is obviously different,and the relative movement trend between the pile and soil in the depth of the foundation is smaller,which reflects the influence mechanism of the pile soil structure interaction.3.The seismic internal force distribution of the pile body will be significantly changed by the embedment effect of the connection between the pile head and the pile cap and the exposure effect of the free section between the pile cap and body.The main bending dangerous points of high bearing platform pile body are located at the connection between pile top and bearing platform and near the surface(1.5D ? 2.0D below the surface);the internal force amplitude of low bearing platform pile body is the largest at the pile top.The exposure of bearing platform and pile body will also affect the embedment effect of pile top.The bending influence depth of high bearing platform pile body is about 1.1m(18D)below the pile top,i.e.0.8m(13D)below the ground surface,while the bending influence depth of low bearing platform pile body is about 2.0m(33D)below the pile top.4.Under the action of horizontal earthquake,the load distribution and internal force of each pile in pile group foundation are quite different.The bending internal force of angle pile(front row pile)is greater than that of side pile(inner row pile).For the pile group foundation with high bearing platform,the uneven degree of the internal force of the pile is greater.The increase of earthquake intensity will enlarge the coefficient of inhomogeneity.5.The dynamic earth pressure increases and decreases with the relative movement trend between the structure and soil.With the increase of seismic intensity,the total earth pressure(including dynamic earth pressure and static earth pressure)on the structure is more and more affected by dynamic earth pressure.The dynamic earth pressure amplitude between the pile body(cap)and the foundation soil near the ground surface is larger,which reflects the law of soil-structure interaction.In the low cap pile foundation,the lateral dynamic earth pressure of the cap structure is exerted to a certain extent,and the superposition effect of the earth pressure in front of and behind the wall should be considered in the contact relationship.6.The effect of earthquake intensity on response amplitude is the most significant.With the increase of the initial dynamic shear modulus of the soil,the acceleration and displacement response of the soil and pile body will be significantly reduced,and the internal force of the pile body will also be reduced to a certain extent.The increase of pile-soil stiffness ratio will reduce the acceleration response of pile body and bearing platform,and enlarge the internal force of pile body.The increase of pile cap mass mainly enlarges the internal force amplitude at the pile top,and the change of superstructure mass mainly affects the internal force response of pile body at the surface.The increase of exposed length of pile body will significantly enlarge the acceleration response of bearing platform and upper structure,and affect the position of control section of internal force design of pile body.There is a critical depth for the influence of the depth of the pile into the soil on the internal force of the pile.When the depth of the pile into the soil increases to 20 d,the influence on the internal force of the pile is not obvious.7.From the practical design point of view,on the basis of the relevant achievements of pile body internal force caused by foundation movement,considering the influence of pile-soil stiffness ratio,structural inertia action,exposed length of pile body and other factors,a correlation coefficient which can reflect the coupling mode of inertia action and motion action is introduced,and an estimation formula for the maximum bending moment of pile body in general uniform foundation is derived.