Dilatancy of granular materials in a strain space multiple mechanism model

A granular material consists of an assemblage of particles with contacts newly formed or disappeared, changing the micromechanical structures during macroscopic deformation. These structures are idealized through a strain space multiple mechanism model as a twofold structure consisting of a multitude of virtual two‐dimensional mechanisms, each of which consists of a multitude of virtual simple shear mechanisms of one‐dimensional nature. In particular, a second‐order fabric tensor describes direct macroscopic stress–strain relationship, and a fourth‐order fabric tensor describes incremental relationship. In this framework of modeling, the mechanism of interlocking defined as the energy less component of macroscopic strain provides an appropriate bridge between micromechanical and macroscopic dilative component of dilatancy. Another bridge for contractive component of dilatancy is provided through an obvious hypothesis on micromechanical counterparts being associated with virtual simple shear strain. It is also postulated that the dilatancy along the stress path beyond a line slightly above the phase transformation line is only due to the mechanism of interlocking and increment in dilatancy due to this interlocking eventually vanishing for a large shear strain. These classic postulates form the basis for formulating the dilatancy in the strain space multiple mechanism model. The performance of the proposed model is demonstrated through simulation of undrained behavior of sand under monotonic and cyclic loading. Copyright © 2010 John Wiley & Sons, Ltd.     

含二阶起伏体的模拟岩体节理试样剪切特性试验研究

利用规则的小尺寸锯齿构造二阶起伏体,对不同二阶起伏体高度的人工节理进行常法向应力下的直剪试验,研究了节理剪切力学特性。对节理的剪切强度、变形特征进行了分析,并对破坏特征做出解释。试验结果表明：二阶起伏体对节理剪切力学特性有重要影响,含二阶起伏体节理的剪切力学特性与只含有一阶起伏体节理的剪切力学特性不同,只含一阶起伏体的节理剪应力只有一个峰值,含二阶起伏体的节理会出现波浪状的剪胀曲线和多峰值剪应力,且峰值剪应力随剪切位移增大依次出现并逐个减小;随着二阶起伏体高度增大,节理峰值剪切强度增大,节理依次出现磨损破坏、多次性剪断破坏、一次性剪断破坏。     

粗粒土的一种应变硬化模型

堆石料等粗粒土在经受剪切时不仅其强度和刚度随应力水平而变化,其体积变化特征也和应力水平密切相关。在Mohr-Coulomb模型的基础上,提出一个针对这类粗粒土剪切特征的应变硬化模型。模型以Mohr-Coulomb准则作为屈服准则,但考虑土体强度、刚度和体积变化特征均随应力水平而变化等重要特性。对Rowe剪胀理论的推导及其试验基础进行了深入讨论,指出该理论对于粗粒土的局限性,并在此基础上针对粗粒土的剪胀特性进一步发展给出其机动剪胀角的变化规律。与试验结果的计算对比表明,该模型能较为全面地反映粗粒土的主要剪切特性,特别是剪切时的体积变化特性,且相对简便实用。     

Shear-induced pressure changes and seepage phenomena in a deforming porous layer-III

We report the results of an analytical investigation into the deformation behaviour of rate-dependent granular material as a refinement of previous studies on seepage phenomena during shear. The rheology has two components—a compliant part of the constitutive law associated with grain contacts as deformation takes place (dilatancy), and a rate-dependent viscous force transmitted by the melt phase. This formulation allows intermediate, time-dependent behaviour to be assessed for the dilatant porous medium. A key result is that during shear, the magnitude of the excess pore pressure first decreases then increases back to its initial value. Two characteristic timescales are identified that control the rate-dependent dilatancy of the mixture, τ₁, the time constant that rules the increase of the magnitude of the excess pore pressure, and τ₀ that controls its decline. We consider the dilatant effect to be an internal constraint in deforming magmas in the lithosphere and other porous (partially molten) regions in the solid earth. When such regions are exposed to external loading, secular pressure changes should drive fluid flow independent of local buoyancy forces, for the duration of the governing rate-dependent timescales. The accumulated heave of the process is also estimated.     

考虑剪胀角的节理化岩体隧道开挖进尺优化研究

合理进尺的选取是影响隧道开挖施工安全和效率的关键问题之一。以杨家坪隧道现场监测数据为根据,通过ABAQUS软件开展数值试算确定了最接近实际隧道围岩的剪胀角,建立了考虑剪胀角影响的Mohr-Coulomb模型的隧道开挖数值模型,并按1.5、2.0、2.5、3.0、3.5 m开挖进尺开展了计算,对开挖过程中隧道围岩的变形和力学响应规律进行了对比研究。结果显示,考虑剪胀角影响的数值模型计算结果的总体规律与实测结果一致性较好;有锚杆支护的情况下开挖进尺选择为3.0 m能较好地兼顾施工安全与效率,应注意控制距开挖面5.0 m范围内的拱顶变形。