水下双面真空预压离心模型试验及数值模拟分析

Centrifuge modelling and numerical analysis on underwater two-way vacuum preloading

水下双面真空预压既有低位真空比顶部真空更大降低土体内孔隙水压力的特点,又有水下真空预压可以更好利用上覆水荷载的优点。然而双面真空预压目前仅在处理疏浚土时应用,在固结机制和固结行为方面研究尚不深入。为探讨水下双面真空预压的加固特征和加固效果,采用离心模型试验模拟软土在顶部与底部同时受真空联合砂井预压,并利用有限元分析离心模型试验结果,对孔隙水压力和变形进行了比较分析,探讨了水下双面真空预压总水头分布的变化、应力路径的发展,并评价了固结速率。通过试验和分析发现,水下双面真空预压比陆上真空预压有更大的有效预压荷载,砂井能加速土体固结并减少土体最终沉降,加固区中部应力路径基本按K0发展;同时在重力与真空双重作用下,底部孔隙水压力下降值远大于顶部孔隙水压力下降值,加固后期底部为低水头区。上述发现增强了对水下双面真空预压的认识和开发利用。

Underwater two-way vacuum preloading not only reduces more excess pore water pressure comparing to conventional vacuum preloading with vacuum acting only on the top face,but also has the advantage of utilizing the effective load from the overlying water.However,two-way vacuum preloading is currently only applied in the treatment of dredged soil,thus the mechanism and performance of consolidation of two-way vacuum preloading are hardly studied.To explore the consolidation characteristics and efficiency of underwater two-way vacuum preloading,centrifuge modelling has been performed to simulate soft soil subjected to underwater two-way vacuum preloading in collaboration with group sand drains and single sand drain.The finite element method is also used to analyze and compare the results with those from centrifuge modelling test.Variation of pore water pressure and development of deformation are compared.Meanwhile,changes in total head of pore water and stress path of soil element,as well as degree of consolidation are discussed and evaluated.It is found that more effective load can be obtained from underwater two-way vacuum preloading comparing to conventional vacuum preloading,and the rate of consolidation is larger and the ultimate settlement can be reduced in group sand drains zone comparing with single sand drain zone.The stresses for soil in the center of treated zone follow a path close to K0 line.Under the combination effects of vacuum and gravity,the reduction of pore water pressure at the bottom layer is larger than that at the top layer,and it is found that a lower water head exists at the bottom at final consolidation stage in this experiment.These findings may enhance the understanding and practical application for two-way vacuum preloading.