关于修正剑桥模型预测负孔隙水应力的评析

Analysis on Predicting Negative Pore Stress Using Modified Cambridge Model

在吸力锚和桩靴等海洋基础结构的上拔问题中,负孔隙水应力是抗拔阻力的重要组成部分,开展相关研究具有重要的科学意义和工程价值.修正剑桥模型不但可以合理地解释超静负孔隙水应力产生的机理,而且可以计算三轴卸荷过程中孔隙水应力的发展变化规律.通过开展高岭土地基桶形基础上拔室内模型试验,研究了总上拔力和超孔压随时间的变化情况;结合数值模拟,用修正剑桥模型计算卸荷对负孔隙水应力的影响,分析了上拔过程中负孔压分布特点;总结高岭土土体中受上拔速率影响的上拔承载力试验,并引入归一化速率对试验结果和数值计算结果进行分析,评析并量化了修正剑桥模型在预测负孔隙水应力时的适用条件.研究表明:负孔隙水压力在上拔初期主要集中在桶体下部和内部土体,对应的最大位移出现在桶裙底部;当上拔位移较大时,最大负孔压出现在桶内土体中;随着上拔位移的增大,土体中超静负孔压区域逐渐变大,同时桶形基础外侧土体发生向桶内的水平向位移.修正剑桥模型计算得到的超静负孔隙水应力小于模型试验结果,造成计算误差的主要原因为:剑桥修正计算模型是通过较低速率的三轴试验得到,其速率远远小于模型试验,从而低估了卸荷速率对负孔隙水应力的影响;修正剑桥模型预测负孔压的适用条件为归一化速率低于70.

The negative pore pressure is an important component of the pullout force in the uplifting project of offshore engineering structures,such as a caisson or a jackup spudcan. It has great scientific significance and engineering value to carry out relevant research. The modified Cambridge model not only can explain the mechanism of negative pore water stress reasonably but also can calculate the development of pore water stress during the triaxial unloading process. Based on the pull model tests of bucket foundation in kaolin soil,variations of the total pullout force and excess pore pressure with time were studied. Combined with the numerical simulation,the modified Cambridge model was used to calculate the influence of unloading on the negative pore water stress. Moreover,the distribution of negative pore pressure during the uplifting process was analyzed. The uplift capacities of kaolin soil affected by the uplift rate were summarized,and the normalization rate was proposed to analyze the test and numerical calculation results. Furthermore,the applicable conditions of the modified Cambridge model was evaluated and quantified. The research showed that the negative pore water pressure mainly appeared at the lower part and the internal soil during the initial stage of uplifting,whereas the corresponding maximum displacement occurred at the bottom of the bucket skirt. When the upward displacement was large,the maximum negative pore pressure was observed in the soil inside the bucket. The area of the negative pore pressure increased gradually with increasing uplift displacement. The soil outside the bucket produced horizontal displacement into the bucket. The calculated value of the negative pore pressure using the modified Cambridge model is less than the test value. The modified Cambridge model was obtained from triaxial test with a low rate,much smaller than that of the model tests. Therefore,the influence of unloading rate on the negative pore water stress was underestimated. The applicable condition for calculating the negative pore pressure of the modified Cambridge model is a normalized rate of less than 70.