考虑次压缩时分级超载预压时间的确定方法

采用能够同时考虑主固结和次固结沉降的一维弹黏塑性等效时间线模型,近似采用地基处理规范中推荐的固结度计算公式,建立等速分级加载时预压持续时间与固结度和超载量之间的相互关系式,确定当超载量一定时超载预压的卸载时间.理论分析表明,堆载预压的持续时间不但与固结度、超载比和加载计划有关,而且与初始有效应力和黏塑性指数也有关.算例表明,针对特定的加载方案,受软土地基强度增长规律的影响,超载预压存在最短持续时间.对次固结变形显著的软土地基,按纯主固结理论计算的持续时间过小,考虑次固结沉降后无法满足工后沉降的要求;而按有效应力面积比法计算持续时间时有时会出现固结度大于1.0的错误,应全面考虑主固结和次固结的沉降特性才能合理地确定超载预压的持续时间.

Method of determining duration of stage constructed surcharge preloading with a view to second settlement

A one-dimensional elastic visco-plastic equivalent time model was used to describe both the primary consolidation and the secondary consolidation. The formula of consolidation degree recommended by the ground improvement specification was utilized. The relationships between the duration of stage constructed surcharge preloading and the consolidation degree and the surcharge preloading were established. Then the unloading time for a specific surcharge loading can be determined. The theoretical results show that the surcharge preloading duration not only relates to the consolidation degree, the surcharge pressure ratio and the constructed plan, but also relates to the original effective stress and the visco plastic index. Examples show that the shortest duration of stage constructed surcharge preloading exists for a specific constructed plan because the surcharge loading is restricted by the increase of soft clay’s strength. Since the soft clay has obvious creep, the preloading duration determined by the pure primary consolidation is too short to satisfy the post construction settlement and the consolidation degree determined by the effective stress area ratio is sometimes bigger than 1.0, which is obvious wrong. A suitable surcharge preloading duration must be determined by both the primary consolidation deformation and the secondary consolidation deformation.