Dynamic Response Of Composite Pile Foundation System In Liquefied Soil

The damage of pile foundation is one of important disasters in liquefiable soil layer during earthquakes. Study on bearing capacity of pile foundation in liquefiable soil layer has generally become important subject in seismic design of soil and foundation engineering in the word. There are obvious deficiencies in seismic design method of pile foundation in liquefiable site used at present. However, the existing theory and experience for seismic design method of pile foundation depends on static condition. Except of that, the shortage or misunderstanding to mechanism of liquefied soil-pile-cap interaction are the other troubles.Based on the literature reviews of domestic and abroad scholars, the vertical bearing properties of composite pile foundation system are analyzed deeply and summarized systematically by shaking table tests of single and group of pile and numerical simulation. Following conclusions are drawn out:(1) Based on the theory about dimensionless similarity of dynamic model tests, a kind of simulated material is made to replace the concrete pile after several trial testing by measuring the density and elastic modulus of different proportions materials.the proportions is cement:sand:soil:water= 1:5.8:1.45:1.9, similar ratio is1.86. The test results prove that this material of model pile is feasible, and can be used in similar experimental study.(2) The model shaking table tests show that:the pore water pressure and the pore pressure ratio are increasing with vibrating time, the growth rate for the surface soil is faster than that at the bottom soil, so the liquefaction takes place from surface to bottom of the soil. After the vibration is stopped, the pore water pressure in the bottom soil layer is dissipated faster than the upper layer because the pore water pressure in the deeper soil layer is higher than that in the upper layer which makes the bottom pore water flow upward.(3) The measuring data of the strains installed in pile body show that pile side friction is reducing with pore water pressure increasing during vibration process, but there is a delay after pore pressure increase. Furthermore, the pile side friction is increasing again with pore water pressure dissipating after vibration stop.(4) The results of shaking table test with3×3pile groups system in which the pile cap is contacted with the soil closely show that the composite pile groups system posses an inhibiting effect for the soil between piles. Such inhibiting effect is more obvious with pile spacing decreasing, because the smaller pile spacing is, the denser the soil between piles, the more significant clamping effect, which inhibits the soil liquefaction. This phenomena is proved by the shear wave velocity of the soil measured during testing by using the method invented by authors. (5) Based on the settlement process analysis of pile groups shaking table tests in the different working conditions, a parameter, settlement dynamic amplification factor (SDAF) is proposed in this study. SDAF is defined as the ratio of the total dynamic settlement at giving time and total static settlement under the same loading action. The variation relationship of SDAF with vibration process is analyzed; a linear statistical expression is set up, which provides the theory foundation for the static calculation of composite pile foundation under the dynamic loading.(6) The results of finite element numerical simulation further prove the inhibiting effect, and the inhibiting effect almost disappears when the pile spacing is6D (D is pile diameter). The analysis results of finite element numerical simulation also show that soil liquefaction will be delayed with stiffness of the pile caps enhancing.(7) Based on the model shaking table testing and finite element analysis, a formula of vertical bearing capacity considering dynamic loading effect is finally established, ie. Q=β1ηsQsk/γs+β2ηpQpk/γp+β3ηcQck/γc. Hereβ1、β2β3are reduction coefficients of pile side friction, bottom resistance, and soil resistance under the cap, respectively. They are related with the characteristics of dynamic loading and soil conditions. In this study,β1、β2may take as0.7when the soil surrounding the pile is partially liquefied,β1、β2may take as (0～0.55) when the soil surrounding the pile is complete liquefied.β3may take as1when the soil under the cap is not liquefied,β3may take as0when the soil under the cap is complete liquefied.