等向固结饱和黏土卸载孔压特性的试验与理论研究

Experiment and theory research on the pore pressure unloading characteristics of saturated clay under isotropic consolidation conditions

负孔压可以为饱和黏土地基的结构基础提供抗拔阻力,准确计算负孔压的大小对研究上拔承载力的意义重大。通过等向固结的三轴伸长试验研究了卸荷状态下饱和黏土中孔压的发展规律,提出了计算超静负孔压的方法;结合修正剑桥模型和Henkel孔压理论,揭示了卸荷作用下超静负孔压产生的机制,合理地解释了在卸荷过程中超静孔压由负转正的过程。研究表明,卸荷试验条件下广义剪应力产生正孔压,而平均主应力对孔压的贡献为负,卸荷初期广义剪应力产生的正孔压不足以抵消平均主应力产生的负孔压,因此,超静孔压呈现负值。研究成果为负孔压预测以及上拔承载力研究提供了理论依据。

When a gravity structure, such as a caisson or a jackup spudcan, is pulled out from the saturated soft clay foundation, a negative pore pressure will be developing, which may greatly increase the pullout force. Therefore, it is very important to estimate the negative pore pressure developed in the soil during the unloading process. The development of excess pore pressure is studied based on the triaxial extension tests under the isotropic consolidation conditions. By combining the modified Cambridge model with the Henkel＇s approach, the mechanism of negative pore pressure generation is revealed and a method is developed to predict the magnitude of the negative pore pressure. Meanwhile, the development rule of pore pressure under the isotropic consolidation conditions is obtained and explained why there will be a change of pore pressure from negative to positive. The research indicates that under the unloading conditions, the generalized shear stress induces positive pore pressure and the averaged principal stress generates negative pore pressure in the soil samples. The superposition of these two effects results in negative pore pressure in the initial unloading stage, and then becomes positive with the increase of strain. The research results can be used to predict the required force to pull out a structure from saturated clay foundation.