考虑分级堆载的真空堆载联合预压砂井地基孔压计算方法

A Method for Calculating Pore Water Pressure of Vacuum-surcharge Preloading Sand Drain Foundation Considering Graded Surcharge

真空堆载联合预压法被广泛应用于加固淤泥及淤泥质土软基,但是其理论滞后于实践应用,软基土体孔压计算方法欠完善,导致有效应力和固结度计算依据不充分。将单砂井地基上、下边界假定为符合实际施工和地质条件的半透水边界,根据轴对称单砂井地基固结方程和等应变假定,利用经典解析理论,考虑真空堆载联合预压工法中真空荷载与堆载的加载次序,合理界定初值条件,推导了符合工程实际的真空堆载联合预压单砂井地基孔隙水压力解析解,并确立了计算方式。在此基础上,以某高铁真空堆载联合预压加固软基工程为例,将塑料排水板地基转化为砂井地基,分析了地基土体孔隙水压力时间空间分布规律。结果表明:每一阶段荷载作用下孔隙水压力计算,应计入前一阶段荷载结束时土层底部的孔隙水压力值;在真空堆载联合预压阶段的孔隙水压力计算中,应将一次性瞬时堆载优化为分级堆载,这更符合实际情况,使得计算值更为合理;在真空堆载联合预压阶段,各分级堆载的施加将打破前一级荷载形成的土中水稳定渗流平衡,使得不同深度处孔压重新分布,孔压消散更快,土层加固效果更明显;无论如何堆载,真空堆载联合预压土体的最终孔隙水压力值为负值,并低于单纯真空预压所形成的负压场的孔压值。

Vacuum-surcharge preloading method is widely used to strengthen silt and silty soil soft foundation,but the theory lags behind practical application.The calculation method of pore pressure of soft foundation soil is imperfect,which leads to insufficient basis for calculation of effective stress and degree of consolidation.The upper and lower boundaries of single sand drain foundation are assumed to be semipermeable boundaries which meet the qualifications of actual construction and geological conditions.According to the consolidation equation of axisymmetric single sand drain foundation and the assumption of equal strain,using the classical analytical theory,considering the loading order of vacuum load and surcharge in the vacuum-surcharge preloading method,the initial value conditions are reasonably defined,the analytical solution of pore water pressure of vacuum-surcharge preloading single sand drain foundation which meet the qualifications of the engineering practice is derived,and the calculation method is established.On this basis,taking the vacuum-surcharge preloading soft foundation consolidation project of a high-speed railway for example,the plastic drainage board foundation is converted into sand drain foundation,and the spatiotemporal distribution regularity of pore water pressure of foundation soil is analyzed.The result shows that(1)The pore water pressure under each stage load should be calculated into the pore water pressure at the soil bottom at the end of the previous stage load.(2)In the calculation of pore water pressure in the stage of vacuum-surcharge preloading,the one-time instantaneous preloading should be optimized to graded preloading,which is more in line with the actual situation and makes the calculated value more reasonable.(3)In the stage of vacuum-surcharge preloading,the application of surcharge of each stage will break the stable seepage balance of soil water formed by the previous stage surcharge,which results in the redistribution of pore pressure at different depths,faster dissipation of pore pressure and more obvious reinforcement effect of soil layer.(4)In any case,the final pore water pressure of vacuum-surcharge preloading soil is negative and smaller than that of the negative pressure field formed by single vacuum preloading.