吸力式沉箱基础底部土体卸荷蠕变及其长期抗拔承载特性研究

Unloading creep of soft clay and long-term uplift bearing characteristics of suction caisson foundation

饱和软黏土地基中吸力式沉箱基础在上拔过程中其底部土体为轴向卸荷状态,目前对软黏土卸荷蠕变以及吸力式沉箱长期抗拔承载特性研究较少。因此,进行了不同围压下软黏土三轴固结不排水卸荷蠕变试验以及吸力式沉箱基础长期抗拔试验,分析了沉箱底部土体卸荷蠕变特性以及吸力式沉箱基础长期抗拔承载特性。卸荷蠕变试验结果表明:在偏应力较低的情况下,土体卸荷蠕变变形可忽略不计,土样变形主要为瞬时变形。随着轴向卸荷应力增大,软黏土蠕变变形越大且非线性蠕变特性愈加明显。根据卸荷蠕变试验结果,提出与应力水平相关的蠕变模型,揭示了模型中3个参数在相同围压下随应力水平的增大而近似呈线性减小的变化规律。然后将该模型从一维扩展至三维,并开发UMAT子程序,通过有限元验证该模型的合理性。同时将该模型用于吸力式沉箱基础抗拔承载力分析中,并与吸力式沉箱模型长期抗拔试验结果进行了对比验证。

During the uplift process of suction caisson foundation, the soil at the bottom of caisson is in an axial unloading state. But there are few studies on the mechanism of the unloading creep of soft clay. Therefore, the axial unloading triaxial creep test on saturated soft clay was carried out to obtain the variation law of axial strain with time and deformation mechanism of soft clay under different confining pressures. At the same time, the long-term uplift test on suction caisson foundation was carried out to analyze the long-term uplift bearing characteristics of suction caisson foundation. The unloading creep test results show that under the condition of the low deviator stress, the unloading creep deformation of soil can be ignored. The unloading deviator stress is higher, the unloading creep deformation is greater and the nonlinear creep characteristics become more obvious. Therefore, by introducing the creep model and considering the influence of the deviator stress, the stress-dependent creep model is proposed to describe the unloading creep of soft clay. Then, the stress-dependent creep model is extended to a three-dimension constitutive model, and a finite element subroutine is developed to establish a finite element analysis method for analyzing the long-term uplift capacity of suction caisson foundations and validated with the long-term uplift bearing capacity results of caisson model.