| Karst landform is one of the unfavorable geological conditions which have a great impact on the construction of deep foundation of bridges.It will not only add additional technical difficulties to the construction of bridge foundation,but also bring risks to the safe operation of infrastructure.Especially under the action of earthquake,the existence of karst cave may lead to foundation failure,and then cause serious earthquake damage to bridge pile foundation.At present,the research on Dynamic Response Law and mechanical properties of rock socketed pile and friction pile in karst area under earthquake action is still insufficient.Therefore,this paper takes the major bridge pile foundation engineering in karst area as the research background,based on shaking table test and numerical simulation,studies the seismic response law of pile body under the condition of complex karst cave factors.The main contents and achievements are as follows:(1)A shaking table test system for seismic response of piles in sandy soil is established.According to the dimensional analysis of the relevant theory of model dynamic test and Buckingham π theorem,the model similarity ratio of pile-soil interaction of sand foundation in shaking table test is derived,the scale model of pile in karst area is designed,and the comparison and optimization design as well as manufacture of layered hanging shear box with the volume of 2.21 m × 1.51 m ×1.67 m and shakeout equipment are completed.By comparing the working conditions of the prototype pile,four kinds of test piles are designed and manufactured,and the design of the test contents,such as data acquisition,layout of measuring points,selection of seismic waves and test loading system,are completed.(2)This paper analyzes the strain distribution characteristics of rock socketed piles in karst cave under earthquake action.The shaking table tests of two rock socketed piles and one friction pile in dry sand soil at different karst cave heights are completed,and the change rule of sand properties during shaking table tests is studied by introducing CPT technology.By utilizing the strain response of pile body under the stimulation of seismic wave,the distribution laws of the peak value of dynamic strain of pile body under different test conditions in seismic wave,peak acceleration and the height of pile-side karst cave,are compared and analyzed.The results show that: in dry sand foundation,the peak strain of pile body presents a larger distribution trend at the both ends than that in the middle;due to the different spectrum characteristics,under the same peak acceleration seismic wave,the peak strains of pile bodies are different.As the input seismic wave peak acceleration increases,the peak strain of pile body is increasing: the higher the height of the karst cave is,the greater the strain of pile body is.(3)The dynamic response characteristics of saturated sand ground under earthquake are studied.Based on the shaking table test,the dynamic response characteristics of saturated sand foundation under the action of earthquake are studied,and the influence of seismic wave form,peak value of maximum acceleration and other factors on the pore water pressure,pore pressure ratio and acceleration response law of soil around the pile are compared and studied.The test results show that the pore water pressure response of the soil around the pile body at different depths increases with the burial depth,and the pore pressure ratio of the saturated sand around the pile body increases faster.Moreover,the rising section basically shows a linear increase trend,and the curve of the dissipation part of the pore pressure shows a non-linear characteristic.The peak acceleration of the saturated sand at different depths decreases with the increase in depth,the bottom-up saturated sand layer can amplify the input wave.At the same time,it is found that the closer to the pile body it is,the stronger the acceleration change of the saturated sand layer is,which indicates that the ability of the soil to transmit earthquake is greatly reduced due to the influence of pile-soil interaction in the saturated sand,the peak strain of friction pile is smaller in the middle and larger at both ends,and the strain response of friction pile in saturated sand layer is larger than that of dry sand foundation.(4)The influence of the height of karst cave and the relationship between the position of karst cave and pile on the seismic performance of rock socketed pile is studied.The rock socketed piles with different karst cave height and relative position are selected as the research object.The distribution of peak strain of rock socketed piles with different seismic wave,peak acceleration,karst cave height and other factors are compared and analyzed,and the test results are compared with the friction pile test results.The results show that the strain distribution of the pile body tends to be large in the middle and small at both ends,and the maximum strain peak of the pile body increases with the increase of the height of the karst cave,which indicates that the increase of the height of the karst cave leads to the earlier failure of the pile foundation;for the karst cave type rock socketed pile under the pile end,the peak strain of the pile body increases first and then increases with the increase of the depth The peak strain of the pile is in the middle of the pile,which is related to the embedment of the rock stratum at the bottom of the pile.The constraint of saturated sand foundation on pile body is weakened,which changes the pile-soil interaction mode in dry sand state and affects the distribution rule of peak strain of pile body;because of the change of foundation state,the failure mode of bridge pile foundation is changed,which affects the overall seismic performance of bridge pile foundation.The research results can provide experimental basis for theoretical analysis and engineering practice of bridge pile foundation in karst area.(5)Based on the numerical simulation analysis method,the dynamic response characteristics of sand soil foundation pile are studied,and the influence of karst cave parameters on the seismic response of pile foundation is analyzed.Based on a practical bridge pile foundation project in karst area,the seismic response law of rock socketed pile considering karst cave factor under ground motion is analyzed by simulating the distribution of actual foundation soil layer,and the actual complex geological conditions are further explored The seismic response characteristics and failure mechanism of rock socketed pile under the condition of earthquake are analyzed.The results show that: the finite element numerical simulation analysis method reasonably reflects the dynamic response characteristics and strain distribution law of rock socketed pile in dry sand soil layer and saturated sand soil layer.In the actual working condition,with the increase of cave height,the increase of cave width and the decrease of cave roof height,the restraint effect of bedrock near the pile body is gradually reduced,while the peak strain of rock socketed pile body will gradually increase;in the case of string bead karst cave,the peak strain of pile body gradually increases with the increase of the number of string bead karst cave;the solution is buried under the bead shaped cave and pile tip In the case of common caves,the presence of the underlying karst cave will reduce the restraint effect of the bottom bedrock and increase the peak strain of the pile to a certain extent.The research results have a certain engineering guiding significance for anti-seismic design of pile foundation in karst area. |
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