基于无侧限抗压强度试验的土壤离散元参数标定

为标定基于Edinburgh Elasto-Plastic Adhesion (EEPA)模型的土壤离散元参数,该研究通过单轴密闭压缩试验和无侧限抗压强度试验,以无侧限抗压强度σ_u为黏性指标,轴向应变ε_n为塑性指标,轴向压力-轴向应变曲线特征参数(a,b)为弹性指标,基于响应面标定了土壤离散元仿真参数。结合文献与实际情况确定参数试验范围,应用Plackett-Burman试验对7个初始参数进行筛选,发现塑性变行比λp和表面能Δγ对黏塑性指标影响显著。根据试验结果,进行2次Central Composite试验,建立σ_u、ε_n与黏塑性参数的二次回归模型和σ_u、ε_n、a、b与弹性参数二次回归模型,并以实测值为目标优化求解,获得的优化参数组合为:塑性变形比为0.36、表面能为15.6 J/m2、恢复系数为0.37、静摩擦系数为0.6、滚动摩擦系数为0.26、黏性分支指数为4.24、切向刚度因子为0.52。最后将该参数组合下的仿真值与实测值进行对比验证,发现仿真ε_n和b与实测值无显著差异,σ_u和a与实测值存在较大差异。结果表明基于响应面法标定的EEPA模型参数可表征试验土壤的轴向塑性变形和3%～45%轴向应变内的应力-应变行为。

Calibration of discrete element parameters of soils based on unconfined compressive strength test

In order to calibrate the discrete element parameters of viscoplastic soil based on the Edinburgh Elasto-Plastic Adhesion (EEPA) model, two constants to describe the stress-strain behavior of the sample soils were defined in this study. The axial strain of the soil under a load of 300 kPa was used to characterize the plasticity of the soil in the uniaxial closed compression test. First, two EEPA model parameters based on the physical properties of the soil and other scholars'' research were determined, i.e. the constant pull-off force and the load branch index. Then, another two factors, surface energy and plastic deformation ratio, that had significant effects on axial strain and unconfined compressive strength based on the Plackett-Burman test results were described. Next, the central composite test based on the response surface method was designed, and the two factors that matched the actual measured axial strain and unconfined compressive strength based on the test results were determined. According to the test results. the four discrete element parameters corresponding to the two measured constants, unconfined compressive strength and axial strain were solved. Finally, the soil discrete element parameters were calibrated based on the EEPA model, and those were that plastic deformation ratio of 0.36, surface energy of 15.6 J/m2, static friction coefficient of 0.6, rolling friction coefficient of 0.26, recovery coefficient of 0.37, adhesion branch index of 4.24 and tangential stiffness factor of 0.52. Verification test results showed that the EEPA model parameters calibrated based on the response surface method could simulate the plastic deformation of the sample soil under a load of 300 kPa and the stress-strain behavior within 3%-45% of the axial strain. In addition, the results of Quadratic orthogonal rotation combined test were analyzed and it showed that the tangential stiffness factor was one of the key parameters affecting axial strain and unconfined compressive strength, and the random error of the unconfined compressive strength simulation test was also one of the reasons. Moreover, it was found that the limit of the value range of the tangential stiffness factor was the cause of the huge difference between the simulation and the measured values of the unconfined compressive strength.