Diansen Yang

Numerical Assessment of the Progressive Rock Fracture Mechanism of Cracked Chevron Notched Brazilian Disc Specimens

The International Society of Rock Mechanics (ISRM) suggested cracked chevron notched Brazilian disc method falls into a major testing category of rock fracture toughness measurement by virtue of chevron notched rock samples. A straight through crack front during the whole fracturing process is assumed in the testing principle but is never assessed. In this study, the progressive rock fracture mechanism of cracked chevron notched Brazilian disc rock specimens is numerically simulated for the first time. Two representative sample types with distinct geometry of notch ligaments are modelled. The assumption of a straight through crack front for chevron notched fracture samples is critically assessed. The results show that not only the notch tip but also the saw-cut chevron notch cracks during the experiments. The straight through crack front assumption is never satisfied in the realistic rock fracture progress of chevron notched disc samples. In addition, the crack features prominent curved front, far from being straight. In contrast to the sample type with narrow notch ligament, the acoustic emission (AE) of the simulation on the sample with wide notch ligament depicts obvious biased fracturing of the prescript fracturing route of the notch. The numerically observed progressive fracture mechanism calls for more attention on how to accurately calibrate the critical dimensionless stress intensity factor for a better measurement of Mode I fracture toughness via chevron notched samples.

Comprehensive evaluation of excavation-damaged zones in the deep underground caverns of the Houziyan hydropower station, Southwest China

The disturbance of a rock mass by blasting or stress redistribution can significantly influence the overall performance of an underground excavation. The characteristics of excavation-damaged zones (EDZs) during the excavation of underground caverns at the Houziyan hydropower station in Sichuan Province, China, were investigated using various in situ tests. This study presents a comprehensive evaluation of the evolution of EDZs in the surrounding rock mass in the underground powerhouse caverns using microseismic (MS) monitoring and conventional testing methods, including multi-point extensometers, acoustic wave testing and borehole TV. First, by analyzing a series of conventional testing and MS monitoring results, the deformation and failure characteristics of the surrounding rock mass were determined. Next, the formation mechanisms of fractures in the surrounding rock mass subjected to excavation-induced unloading in the underground powerhouse caverns were determined. The relationships between the EDZs of the surrounding rock mass, crack evolution and construction status were then analyzed to investigate the crack formation, development and coalescence processes. Finally, the thicknesses of the EDZs were quantitatively determined, the relationships between the fracture evolution and construction progress were established, and the EDZ formation and evolution mechanisms were evaluated. The results not only provide direct data for geological exploration but also contribute to optimizing excavation design and support for analyzing the deformation behaviors of underground powerhouse caverns.

Experimental Investigation on the Mechanical Behavior of Bovine Bone Using Digital Image Correlation Technique

In order to understand the fracture mechanisms of bone subjected to external force well, an experimental study has been performed on the bovine bone by carrying out the three-point bending test with 3D digital image correlation (DIC) method, which provides a noncontact and full field of displacement measurement. The local strain and damage evolution of the bone has been recorded real time. The results show that the deflection measured by DIC agrees well with that obtained by the displacement sensor of the mechanical testing machine. The relationship between the deflection and the force is nearly linear prior to reaching the peak strength which is about 16 kN for the tested bovine tibia. The full-field strain contours of the bone show that the strain distribution depends on not only the force direction, but also the natural bone shape. The natural arched-shape bovine tibia bone could bear a large force, due to the tissue structure with high strength, and the fracture propagation process of the sample initiates at the inner side of the bone first and propagates along the force direction.

Investigating the Permeability of Marble under Moderate Pressure and Temperature

The permeability of intact marble samples collected from the depth of 1.6 km in southwestern China is investigated under moderate confining pressures and temperatures. No microcracks initiate or propagate during the tests, and the variation of permeability is due to the change of aperture of microcracks. Test results show a considerable decrease of permeability along with confining pressure increase from 10 to 30 MPa and temperature increase from 15 to 40°C. The thermal effect on the permeability is notable in comparison with the influence of the stress. A simple permeability evolution law is developed to correlate the permeability and the porosity in the compressive regime based on the microphysical geometric linkage model. Using this law, the permeability in the compressive regime for crystalline rock can be predicted from the volumetric strain curve of mechanical tests.

Estimation of Elastic Moduli of Non-persistent Fractured Rock Masses

Abbreviations E0 Young’s modulus of intact rock G0 Shear modulus of intact rock m0 Poisson’s ratio of intact rock Kn Fracture normal stiffness Ks Fracture shear stiffness E Effective elastic modulus of fractured rock mass E\i (Eki) Effective elastic modulus of fractured rock mass in the normal (shear) direction of the ith fracture sets G Effective shear modulus of fractured rock mass m Effective Poisson’s ratio of fractured rock mass a Half-length of fracture r The distance from fracture center along its length q Fracture density, defined as the number of fracture central points per square meter h Angle between loading direction and normal direction of fracture plane u Normal displacement jump of fracture faces u Average normal displacement jump of fracture faces V Shear displacement jump of fracture faces rn Resolved normal stress on fracture from far-field stress ss Resolved shear stress on fracture from far-field stress rk Tension stress of fractures rp Normal stress acting on fracture faces rp Averaged normal stress acting on fracture faces S Far-field shear stress DR The total excess elastic strain energy due to the existence of fractures DRfn The elastic strain energy stored in fracture due to normal stress DRrn The excess elastic strain energy stored in rock due to normal stress DRrs The elastic strain energy stored in rock due to shear stress DRfs The excess elastic strain energy stored in fracture due to shear stress A * D Parameters of excess strain energy terms for non-uniform deformation mode A D Parameters of excess strain energy terms for uniform deformation mode rn1rk1 The subscript 1 denotes the ultimate quantities in integrals

Application of the orthogonal design method in geotechnical parameter back analysis for underground structures

The back analysis method has been widely used as an indirect method of determining geotechnical parameters based on field measurements. The number of parameters and their initial values greatly influence the reliability and efficiency of back analysis. Therefore, sensitivity analysis is often employed to select high sensitivity parameters that have more greater impact on measured back analysis values. The orthogonal design method was first utilized to select geotechnical parameters for back analysis. The optimized parameter values obtained from an orthogonal design table can be used as the initial back analysis values, so as to avoid optimisation algorithm searching in local parameter spaces. By introducing a penalty function to the objective function, back analysis of the geotechnical parameters is changed into an unconstrained optimisation problem, whereby the Nelder–Mead method can then be employed. To verify the feasibility of the proposed back analysis method, a case study was conducted to determine the rock mass parameters for the Houziyan underground powerhouse complex.

Nonlinear seepage–erosion coupled water inrush model for completely weathered granite

ABSTRACT Water inrush from completely weathered granite has presented a significant challenge for tunnel construction, suggesting the urgency and importance of revealing the water inrush mechanism. In this paper, a seepage erosion model is proposed to describe the water inrush. Assuming that completely weathered granite consists of solid grain phase, fluid phase, and fluidized grain phase, the three-phase interaction was constrained by mass balance equations, and the erosion of fluidized grains was described by a modified porosity evolution equation. The fluid flow is governed by a coupled Darcy–Brinkman/Navier–Stokes equation, which responds to the variation of the flow pattern in the evolution process. Then, the validity of the model has been proved, and the superiority has been studied by comparing with the previous models. The comparison results showed that the flow pattern has a significant impact on pore pressure, water velocity, and mutation time, and the proposed model can more accurately predict the development of velocity. Furthermore, this model was used to simulate the development of water inrush and achieved good results in predicting the direction, channel size, and whole evolution process of water inrush. The research findings from the paper can benefit tunnel engineering in the case of water inrush disasters.