剪切速率对深部黏土直接剪切强度影响的数值模拟研究

NUMERICAL STUDY OF THE EFFECT OF THE SHEAR RATE ON THE DETERMINATION OF DIRECT SHEAR STRENGTH OF DEEP CLAY

日益增多的深部地层开发,要求掌握所遇到的深部黏土工程强度性质。然而由于黏土高压三轴试验存在过于耗时、容易失败等困难,直接剪切试验仍是确定深部黏土工程强度指标的首选方法之一。现行规范推荐的直接剪切试验慢剪剪切速率上限以及固结快剪剪切速率范围,是否适合渗透性极低的深部黏土值得研究。本文采用考虑土体各向异性影响的本构模型,建立了渗流-应力耦合的直接剪切数值模型,并经过了华东重塑深部黏土的试验结果验证。对不同剪切速率下3个固结应力等级下的黏土直接剪切模拟表明,直接剪切破坏剪切应力随剪切速率增大而非线性减小,且减小的幅度随固结压力的增加而明显增大,其原因在于剪切带上随剪切速率和固结压力非线性变化的超孔隙水压;对于渗透系数为10^(-12)m·s^(-1)量级的深部黏土,即使采用0.5倍的推荐慢剪剪切速率上限值,所得内摩擦角与三轴固结不排水内摩擦角接近,而若采用规范推荐的固结快剪剪切速率,会得到明显偏小的内摩擦角。

The strength properties of deep clay are crucial for developing deep strata.However,the direct shear test remains a preferred method for determining the engineering strength of deep clay due to challenges with triaxial tests under high pressure,which can be time-consuming and prone to failure.Whether the upper limit of shear rate in the slow direct shear test and the shear rate range in the consolidation quick direct shear test recommended by national codes are suitable for deep clay with very low permeability needs further study.Using a constitutive model that accounts for anisotropic effects,a seepage-stress coupling numerical model of the direct shear test was developed,and it was validated by the consistency between numerical and experimental results of deep clay in East China.Based on this model,direct shear tests were simulated for clay under three different consolidation pressures and three shear rates.The results show that due to the nonlinear pore pressure variations within the shearing band with shear rate and consolidation pressure,the shear stress at failure decreases nonlinearly with increasing shear rate,and the decrease is more pronounced with higher consolidation pressure.For the simulated remolded deep clay with a permeability coefficient around 10^(-12) m·s^(-1),the internal friction angle from simulations at 0.5 times the upper limit shear rate is close to the one obtained from undrained triaxial shear tests.In contrast,using the recommended shear rate for consolidation quick direct shear tests results significantly underestimated internal friction angle.