饱和砂土的三剪弹塑性边界面模型研究(三)——PFC3D数值试验验证

Approaches to the triple-shear elasto-plasticity bounding surface model for saturated sands(3)——PFC3D numerical test verifications

为进一步验证所提饱和砂土三剪弹塑性边界面本构模型的适用性,以南昌地区饱和砂土为研究对象,利用PFC3D颗粒流建立了单调及循环荷载作用下不同含泥量饱和砂土外裹柔性膜的常规三轴和真三轴数值试验模型。常规三轴条件下的室内试验、PFC3D模拟及理论模型间的结果对比验证了3者之间在固结排水时土的偏应力与轴向应变、轴向应变与体应变,以及固结不排水时土的偏应力与轴向应变、轴向应变与孔隙水压力之间的关系均具有较好的一致性,达到了可以进行数值试验的程度,表明所建PFC3D模型可以代替各种复杂加载条件下的饱和砂土室内试验。另外,将PFC3D真三轴固结不排水数值试验和理论模型间的结果作了对比,研究了不同含泥量饱和砂土的中间主应力影响问题。其中,在静真三轴固结不排水条件下,单调荷载下的PFC3D模拟与中间主应力影响系数b=0.5时的理论曲线整体吻合均较好,其曲线误差在轴向变形前期基本保持5%以内;在后续的动真三轴边界面理论模型验证时,循环荷载下的理论模型计算均采用了b=0.5。在动真三轴固结不排水条件下,PFC3D数值模拟和理论计算结果在应力-应变、循环次数-孔隙水压力关系曲线也比较吻合,含泥量越多应变发展速度越快、动剪切模量更小、滞回圈面积更大,孔隙水压的振动幅值和绝对值都随之变大。

In order to verify the applicability of the triple-shear elastoplastic bounding surface constitutive model for saturated sands proposed previously,the conventional triaxial and true triaxial numerical test models using PFC3 D were established for Nanchang saturated sands with different clayey contents wrapped in flexible membranes and bearing the monotonic and cyclic loading.Comparisons among the results from the laboratory conventional triaxial CD and CU tests,the PFC3 D simulations,and the theoretical models showed good consistency in relationships existing in the deviatoric stress vs the axial strain and the axial strain vs the bulk strain under the CD conditions,and the same results existed in the deviatoric stress vs the axial strain and the axial strain vs the pore water pressure under the CU conditions.The conclusions mentioned above indicate that the numerical simulations with built PFC3 D models can be used to verify the theoretical models replacing the laboratory tests for saturated sands under various complex loading conditions.In spite of that,results between the PFC3 D true triaxial numerical tests and the theoretical models are compared to study the effects of the intermediate principal stresses for the saturated sands with different clayey contents.Because the curves deduced from the PFC3 D simulation acting on the monotonic loads coincide with those from the theoretical models in the static true triaxial CU tests when b=0.5,giving the error within 5%,b=0.5 is adopted for all to cyclic load computations for theoretical models in the later true dynamic triaxial verifications to the theoretical models of bounding surface.Result comparisons between the PFC3 D simulations and the theoretical models show good curve consistency in the stress vs the strain and the cycle times vs the pore water pressure under the dynamic true triaxial CU conditions.The greater clayey contents lead to the faster strain developing speed;the smaller dynamic shear moduli,the larger hysteresis loop areas,the greater vibration amplitudes and the absolute pore water pressures.