Differentiation in Sheared Granular Magma

<p>Recent developments in high definition mineral chemistry at the grain scale are shedding new light on the processes and rates of magma storage, differentiation and eruption. However, the complementary physics and fluid dynamics of magma as a granular material are still based on viscous compaction theory, which may not be relevant in sub-volcanic settings where magma is being deformed by external shear. We present a quantitative model for shear deformation of a crystallised dense magma (>70% solid) with poro-elastic properties where the critical link between the mechanics and associated compositional changes in the melt are governed by dilation (volume increase) of the granular skeleton. Key material parameters governing the dilatancy effect include magma permeability, mush strength, the shear modulus and the contact mechanics and geometry of the granular assemblage. Calculations show that dilation reduces the interstitial fluid (melt) pressure to produce a ‘suction’ effect. At shear strain rates in excess of the tectonic background, deformation-induced melt flow can redistribute chemical components and heat between regions of crystallising magma with contrasting rheological properties, at velocities far in excess of diffusion or buoyancy forces, the latter of course the driving force behind fractional crystallisation and compaction. Unlike static magmas, there is no ‘lock-up’ state above which the interstitial melt cannot percolate. Co-mingling of hotter, indigenous melt has the potential to interrupt (or locally reverse) fractionation trends and produce reverse zoning or resorbtion of crystals, mimicking some of the textural effects attributed to magma mixing. Post-failure instabilities include hydraulic rupture of the mush along shear zones with potential for larger scale extraction and redistribution of evolved melt. A novel feature of congested, sub-volcanic granular magma is that the eruption itself helps drive rapid melt extraction, negating the requirement to first segregate large volumes of evolved melt as a precursor. </p>

Modelling of Rock Joints Interface under Cyclic Loading

The problem of numerical simulation of the material interface response under monotonic and cyclic loading is of fundamental scientific and engineering importance. In fact, such interfaces occur in most engineering and geotechnical structures. The present work is devoted to the deformational response analysis of contact interfaces under monotonic and cyclic loads. The class of materials includes rock and structural joints, soil structure interfaces, masonry and cementitious joints, localized shear bands and so on.The aim of the proposed model is to simulate the cyclic shear test under constant normal load. The associated dilatancy effect is associated with the configurational effects of asperity interaction or dilatancy of wear debris layer. The large primary asperities are assumed as responsible for interfacial dilation and small size asperities as governing frictional sliding and hysteresis response. The elliptic loading yield function is assumed to translate and rotate during progressive or reverse loading events. The model formulation is discussed and confronted with experimental data.

Crack shielding in non-planar and frictional discontinuities under mixed-mode loading

In this paper, a two-dimensional model to describe the non-planar features of crack morphologies is presented. The model accounts for frictional tractions along the crack surfaces by considering an elastic-plastic-like constitutive interface law. Dilatancy effect due to crack roughness is described by the model, leading to a Mode I/II coupling between displacements and tractions along the crack surfaces. The non-linear solution of the rough and frictional crack under general remote scenarios is obtained using the Distributed Dislocation Technique (DDT). By considering a linear piecewise periodic profile of the interface crack, the influence of roughness and friction of interface cracks is examined in relation to both the resulting near-tip stress field and the fracture resistance under monotonic mixed-mode loading. The present model is able to quantify the increase of the fracture resistance due to roughnessand friction-induced crack tip shielding and to correlate it with a dimensionless crack size parameter.

Effect of Sporosarcina Pasteurii on the strength properties of compressed earth specimens

Microbial biodeposition of calcite induction for improving the performance of rammed earth is a research area that must be analysed in a representative environment. This analysis must consider the compaction force, particle size distribution and curing process as production variables. This paper investigates the effects of adding specific bacteria, Sporosarcina Pasteurii, into compressed earth cubes and the effect of production variables. Uniaxial compressive tests and direct shear tests have been conducted for 80 specimens. The results indicate that calcite precipitation interacts with the drying process of clay/silt resulting in reducing the compressive strength, the apparent cohesion and the friction angle. Finally, bacterial activity, which is more likely in samples cured in a high humidity environment, tends to reduce the dilatancy effect.

Solution of Strain-Softening Surrounding Rock in Deep Tunnel Incorporating 3D Hoek-Brown Failure Criterion and Flow Rule

In order to investigate the influence of the intermediate principal stress on the stress and displacement of surrounding rock, a novel approach based on 3D Hoek-Brown (H-B) failure criterion was proposed. Taking the strain-softening characteristic of rock mass into account, the potential plastic zone is subdivided into a finite number of concentric annulus and a numerical procedure for calculating the stress and displacement of each annulus was presented. Strains were obtained based on the nonassociated and associated flow rule and 3D plastic potential function. Stresses were achieved by the stress equilibrium equation and generalized Hoek-Brown failure criterion. Using the proposed approach, we can get the solutions of the stress and displacement of the surrounding rock considering the intermediate principal stress. Moreover, the proposed approach was validated with the published results. Compared with the results based on generalized Hoek-Brown failure criterion, it is shown that the plastic radius calculated by 3D Hoek-Brown failure criterion is smaller than those solved by generalized H-B failure criterion, and the influences of dilatancy effect on the results based on the generalized H-B failure criterion are greater than those based on 3D H-B failure criterion. The displacements considering the nonassociated flow rule are smaller than those considering associated flow rules.

The Elasto-Plastic Waves of Solids with Dilatancy Effect

The elasto-plastic waves of solids with dilatancy effect are studied here based on the theory of standard space under physical presentation for anisotropic solids, in which a new yield function is induced, which consider both isotropic hardening and kinematic hardening. The speed equations of elasto-plastic wave in anisotropic solids are deduced, and several new and important results are obtained.

Analysis on the Interfacial Dilatancy Effect in Anchorage Zone of Grouted Bolts

The interfacial mechanical characteristics in anchorage zone of grouted bolts influence the ultimate capacity of bolts directly; a push-out test was carried out at the lab to study the interfacial mechanical characteristics in the case of bond failure happening at the grout-rock interface, leaned from some scholars to test methods for analysis of concrete microstructure instead of rocks. The commercial finite element software ANSYS method was adapted to analyze the dilatancy effect on the concrete during the grout was pushed out. Using conversion between compressive strength and tensile strength of concrete cube, the tensile strength can be calculated, as the basis for destruction of the concrete matrix.