Experimental data for porous media exhibit nonlinear pressure-volumetric strain relations and a strong dependence on the Terzaghi pressure defined as confining pressure minus pore pressure. However, a clear explanation of why this pressure plays such a dominant role appears to be missing. Several authors have suggested that shear must be a significant factor in predicting change in porosity even for purely hydrostatic loading. Here this idea is explored in detail by analyzing a representative volume element consisting of a hollow sphere within a unit cube subjected only to hydrostatic compression. The results are presented independent of this particular geometry with the use of volume fractions. The analysis shows that the stress field within a relatively small region around a pore contains a measure of shear stress, called the Terzaghi shear, which is similar, but not equal, to the Terzaghi pressure. Shear strain in the hollow sphere does not affect the volumetric strain of the hollow sphere itself but is a major contribution to the volumetric strain of the pore, and hence to the bulk volumetric strain. The constitutive equation between Terzaghi shear and shear strain within a conditioned state is nonlinearly elastic, whereas transitioning from one conditioned state to another is governed by strain-hardening plasticity. The plastic strain that develops introduces a residual Terzaghi shear, which provides an explanation as to why the elastic response in a second conditioned state is different from that in the first. The formulation also explains the interesting phenomenon of shear-enhanced compaction under hydrostatic loading. Explicit expressions are given for the compressibilities relating increments of confining and pore pressures to increments of bulk and pore volumetric strains. Relations between the compressibilities are similar to classical equations with the exception that the new formulation extends beyond a single conditioned state.

多孔介质的实验数据表现出非线性压力-体积应变关系，并且强烈依赖于定义为围压减去孔隙压力的 Terzaghi压力。然而，似乎没有明确解释为什么这种压力起着如此重要的作用。几位作者提出，即使对于纯静水载荷，剪切力也必须是预测孔隙率变化的重要因素。这里通过分析一个有代表性的体积元素来详细探讨这个想法由一个单位立方体内的空心球体组成，仅受静水压。结果的呈现与使用体积分数的特定几何形状无关。分析表明，孔隙周围相对较小区域内的应力场包含剪切应力的度量，称为 Terzaghi 剪切，它类似于但不等于 Terzaghi 压力。空心球中的剪切应变不影响空心球体本身的体积应变，但它是对孔隙体积应变的主要贡献，因此对体积体积应变有主要贡献。Terzaghi 剪切和条件状态下的剪切应变之间的本构方程是非线性弹性的，而从一种条件状态到另一种状态的转变受应变硬化塑性控制。产生的塑性应变引入了残余 Terzaghi 剪切，这解释了为什么第二个条件状态下的弹性响应与第一个不同。该公式还解释了静水载荷下剪切增强压实的有趣现象。给出了显式表达式将围压和孔隙压力的增量与体积和孔隙体积应变的增量相关联的压缩性。压缩率之间的关系类似于经典方程，不同之处在于新公式超出了单个条件状态。