饱和多孔白垩岩的孔隙力学性质研究

STUDY ON POROMECHANIC BEHAVIOURS OF SATURATED POROUS CHALK

研究表明,白垩岩具有两种相互独立的塑性力学机制:塑性孔隙坍塌和塑性剪切变形。据此,一个基于Gurson准则和Pietruszczak准则的本构模型被提出用来描述其塑性力学特性。该模型包含2个屈服面方程和相应的塑性流动准则,在排水条件下进行验证并获得满意的结果。然而,白垩岩力学性质除受其矿物组成、结构以及饱和条件等因素的影响外,孔隙压力对其力学特征同样具有重要影响。因此,研究饱和多孔白垩岩的力学特性必须考虑孔隙压力的影响。基于这一认识,将Coussy孔隙塑性理论扩展用于研究饱和白垩岩的孔隙塑性力学特性,根据孔隙弹塑性力学的一般框架,讨论在弹塑性域上有效应力尤其是塑性有效应力概念的有效性。通过将本构模型中两种塑性力学机制的塑性加载函数和塑性潜能表达成塑性有效应力的函数,就能够研究饱和多孔白垩岩在考虑孔隙压力变化条件下的孔隙力学性质,研究在不排水条件下,特别是在不排水三轴压缩条件下白垩岩的塑性力学特性。数值模拟和试验结果之间存在很好的特性上的一致性。研究结果表明,通过采用塑性有效应力概念的方法提出的本构模型能用来描述饱和多孔白垩岩的孔隙力学特性。

The experimental results suggest that the mechanical behaviours of chalk can be modeled in terms of elastoplastic model with two plastic deformation mechanisms：plastic compaction（pore collapse） and plastic shear.From this observation,a constitutive model based on Gurson criterion and Pietruszczak criterion is proposed to study the mechanicals behaviour of chalk.The model contains two yield functions and corresponding plastic flow rules.The performance of the model has been verified for the chalk in drained hydrostatic and triaxial tests;and a good agreement has been obtained.It is well known that minerals,structure and saturation conditions have important influences on the mechanical behaviours of chalk.Moreover,the influence of pore pressure is also very important.Consequently,pore pressure should be taken into account in the modelling of mechanical behaviour of saturated porous chalk.Coussy′s poroplastic theory is used for the extension of this model to describe poromechanical coupling of chalk.According to the general framework of poro-elasto-plastic modelling,the concepts of the effective stress tensors,especially the concept of plastic effective stress tensor,are discussed.In order to study the poromechanical behaviour of chalk in consideration of pore pressure variation,two yield functions and plastic potentials are expressed as the function of the plastic effective stress tensor.Then the plastic behaviour of saturated porous chalk can be studied in undrained conditions,especially in undrained triaxial compression condition.In general,the model′s predictions are in satisfactory agreement with the experimental results.It is shown that the proposed model is able to describe the poromechanical behaviour of saturated porous chalk by using the concept of effective stress.