Experimental Study On PHC Piles With Liquefied Soft Soil Under Shaking-Table In Coastal Area

Recently,the prestressed high-strength concrete pipe pile(PHC pile)has been widely used in soft soil foundation for high-rise buildings,especially in the coastal areas of China(Fuzhou,Quanzhou and Putian,Fujian Province).In these areas,most of the pile foundations are adopted to PHC piles.However,because of the thin wall of PHC pile,it has poor ductility,horizontal resistance as well as bear capacity,and the damage is often occurred in the liquefaction soft soil foundation under earthquake.In order to give insight into the dynamic behavior,response as well as pile-soil interaction of PHC piles in the liquefaction soft soil foundation,tests under the shaking table were conducted.Main achievements are described as follows:(1)The PHC pile-soil resistance,peak ground acceleration and other response characteristics were analyzed based on the shaking table tests.The results show that the upper area of PHC piles responds more strongly than bottom and the dynamic interaction between PHC pile and soil is more intense when the frequency of input wave is close to its natural frequency.After the sand are liquefied,the soil pressure at the place of liquefaction is less,that is to say the horizontal constraint of soil to PHC pile decrease,but there still are some constraint.The lateral displacement of upper PHC pile is increased significantly,and the peak acceleration for the pile would appear in the liquefied area.(2)By the conversion of strain,it can get the distribution of bending moment in PHC pile,where the characteristics and distribution of bending moment in different conditions were analyzed.The results show that the bending moment increases firstly,and then decreases along the length of PHC piles in soil with or without the overlying clay layer.When there is an overlying clay layer that covers the soil,the peak bending moment will be occurred at the interface.However,if the sand is liquefied,the peak bending moment and relevantly peak acceleration would appear in the liquefaction area.Furthermore,the magnitude of peak acceleration would be increasing with the process of liquefaction.(3)On the basis of test results,the dynamic p-y curve of PHC pile was constructed and the reaction characteristics of dynamic p-y curve under the liquefaction were focused on.The results show that the pore pressure ratio is increasing with the increase of peak ground motion.When the pore pressure ratio is close to 0.4,the soil has a certain degree of softening and its lateral stiffness decreases.With the increase of pore pressure ratio,the soil lateral stiffness is smaller and smaller.When the pore pressure ratio of saturated sand is around 0.6,the saturated sand is becoming liquefied and there is an obvious decrease for the horizontal constraint,but not completely disappears.With the process of sand liquefaction,the main energy consumption is no longer concentrating on the upper part of PHC pile,which has a certain degree of moving down.