Experimental Study on Creep Characteristics of Structural Soft Clay under Lateral Unloading Condition
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摘要: 通过对温州瑞安软黏土侧向卸荷条件下的三轴不排水蠕变试验，研究侧向卸荷路径和结构性对软黏土时间-应变曲线、等时曲线等典型蠕变特性的影响；建立侧向卸荷条件下软黏土的蠕变稳定时间模型，分析应变率与土体蠕变破坏标准之间的关系，并验证其对不同地区不同应力路径结构性软土的适用性. 研究结果表明：当土样卸荷量与初始固结应力的比值达到0.2～0.3时，土样达到蠕变破坏；变形发展可分为瞬时变形、衰减蠕变、等速蠕变和加速蠕变4个阶段. 当不同围压下的土样进入加速蠕变时，其应变率对数与蠕变时间存在良好的线性关系，该加速蠕变临界线可作为卸荷蠕变破坏的判据；蠕变稳定时间受结构性的制约和影响，侧向卸荷条件下，土样发生蠕变破坏时的轴向应变随固结压力的增大而增大，长期强度指标黏聚力下降超过50%，内摩擦角则基本不变，导致浅层结构性软黏土长期强度衰减严重，蠕变破坏应变较小，施工和监测时需重点关注.Abstract: Triaxial undrained creep tests under lateral unloading condition were carried out on the soft clay obtained from Ruian, Wenzhou, China to study the influence of lateral unloading path and structural properties on time-strain curves, isochronous curves and other typical creep properties. A creep stabilization time model of soft clay under lateral unloading condition was built to analyze the relationship between strain rate and creep failure criterion of soil, and then applied to structural soft clay soils with different stress paths from different regions to verify its applicability. The results show that creep failure occurred as the ratio of unloading to initial consolidation stress reached 0.2–0.3, and that the deformation of the soft clay can be divided into four stages, i.e., instantaneous deformation, decaying creep, isokinetic creep and accelerated creep. When soil samples under different confining pressures are in the accelerated creep stage, there is a linear relationship between logarithm of strain rate and creep time; the critical line of accelerated creep can be used as a criterion for the unloading creep failure of structural soft clay. The creep stabilization time of structural soft clay is dictated by its structural properties. The axial strain of Wenzhou soft clay increased with an increase in the consolidation confining pressure when the lateral unloading creep failure occurred. Influenced by creep, the cohesion of structural soft clay under unloading condition declined more than 50%, while the internal friction angle remained basically unchanged, resulting in severe attenuation of the long-term strength of shallow structural soft clays and small strains of creep failure, which should be paid more attention to during construction and monitoring.
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Key words:
- lateral unloading /
- soft clay /
- creep characteristics /
- structural properties
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图 4 应变-时间对数加载曲线
Figure 4. Curves of
${\varepsilon _{\rm{a}}}$ with$\ln \;t$ of multi-stage unloading
图 6 分级卸荷条件下土体应力-应变等时曲线
Figure 6. Stress-strain isochronous curves of multi-stage unloading
表 1 软土土样的物理力学性质参数
Table 1. Physical and mechanical properties of soft soil sample
含水量
$w$/%比重 密度
$\rho $/（g•cm−3）孔隙比
${e_0}$塑限
${w_{\rm{p}}}$/%液限
${w_{\rm{L}}}$/%塑限指数
${I_{\rm{P}}}$液限指数
${I_{\rm{L}}}$不排水抗剪强度指标 黏聚力$c$/kPa 内摩擦角φ/（$^\circ $） 74.1 2.72 1.56 2.07 28.4 55.5 27.1 1.68 17.0 18.0 
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表 2 侧向卸荷条件下三轴蠕变试验方案
Table 2. Scheme of triaxial creep tests under the condition of lateral unloading
试验
编号${K_0}$固结 蠕变过程（不排水） ${\sigma _3}$/kPa ${K_0}$系数 控制
方式加载
方式卸荷速率/
（kPa•min−1）RB30 30 0.43 应力
控制${\sigma _1}$不变
${\sigma _3}$减小0.1 RB50 50 0.49 RB70 70 0.52
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表 3 土样实际蠕变稳定时间
Table 3. Table of actual creep stabilization
h 固结围压/kPa 侧向卸荷量/kPa 3 6 9 12 30 107 133 50 112 136 112 70 74 109 96 79
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