Extension of numerical manifold method for coupled fluid flow and fracturing problems

The present study extends the numerical manifold method to include the hydro‐mechanical model to investigate the effect of water flow in fractures on the stability of rock structures, particularly slopes. The proposed flow model is verified by a simple 2‐D flow problem in a homogeneous aquifer. Combining the water flow model with the earlier developed fracture evolution technique, the entire failure process of the rock slope due to a heavy rain is simulated. The results illustrate that the developed numerical manifold method can not only determine the trigger factor of the crack initiation but also model the failure processes related to crack initiation, propagation, block formation, detachment and sliding due to the water effect successfully. Copyright © 2014 John Wiley & Sons, Ltd.     

Simulating effect of anthropogenic activities and climate variation on Liulin Springs discharge depletion by using the ARIMAX model

Based on the groundwater development process, and regional economic and social developing history, we divided the spring hydrological process of the Liulin Springs Basin into two periods: pre‐1973 and post‐1974. In the first period (i.e. 1957–1973), the spring discharge was affected by climate variation alone, and in the second period (i.e. 1974–2009), the spring discharge charge was influenced by both climate variation and human activities. A piecewise analysis strategy was used to differentiate the contribution of anthropogenic activities from climate variation on karst spring discharge depletion in the second period. Then, the ARIMAX model was applied to spring flow time series of the first period to develop a model for the effects of climate variation only. Using this model, we estimated the spring discharge in the second period solely under the influence of climate variation. Based on the water budget, we subtracted observed spring discharge from the estimated spring discharge and acquired the contribution of human activities on spring discharge depletion for the second period. The results of the analysis indicated that the contribution of climate variation to the spring discharge depletion is?0.20 m³/s from 1970s to 2000s. The contribution of anthropogenic activities to the spring flow depletion was ?2.56 m³/s in 2000s, which was about 13 times more than that of climate variation. Our analysis further indicates that groundwater exploitation only accounts for 29% of the spring flow depletion due to the effects of human activities. The remaining 71% of the depletion is likely to be caused by other human activities, including dam building, dewatering during coal mining, and deforestation. Copyright © 2012 John Wiley & Sons, Ltd.     

Fracturing and Failure Behavior of Carrara Marble in Quasistatic and Dynamic Brazilian Disc Tests

The tensile strength and fracturing behavior of Carrara marble subjected to the dynamic Brazilian disc test using the split Hopkinson pressure bar technique are determined and compared with those obtained by the conventional quasistatic Brazilian disc test. Detailed observation of the cracking processes is aided by high-speed video footage captured at a frame rate of 100,000 frames per second. The dynamic increase factor is computed, revealing a strong strain rate dependence of the Carrara marble when subjected to strain rates above 1 s^?1. Similar to the quasistatic loading tests, conspicuous white zones/patches commonly appear prior to the initiation of visible cracks in the dynamic loading tests. Identification of the white patch initiation and evolution is aided by image comparison software. Comparing the cracking and failure processes under quasistatic and dynamic loading, some distinct differences in the white patch geometry and initiation load are observed. In addition, the extent of the compressive failure zones around the contact points between the loading platens and specimens is found to increase with the strain rate.     

Critical phenomena in thermal conductivity: Implications for lower mantle dynamics

Microscopic mechanisms for heat transport in dense minerals (phonon scattering and photon attenuation) exhibit aspects of threshold behavior, discussed qualitatively here. For all minerals examined so-far using laser-flash analysis, the lattice component of the thermal conductivity of the mantle asymptotes to a constant above a critical temperature of 1500 K. Radiative transfer calculated from absorption spectra has thresholds in both grain-size and Fe content, and a rather complex dependence on temperature. These critical phenomena impact convection of the lower mantle, because the lattice contribution tends to destabilize the cold boundary layers, whereas radiative transfer mostly promotes stability in the lower mantle, unless the grains are large and Fe-rich, which makes convection chaotic and time-dependent. The specific behavior suggests that flow in the lower mantle is sluggish, whereas flow in the upper mantle-transition zone is time-dependent. The decrease in k_rad as Fe/(Fe + Mg) increases beyond 0.1 may be connected with formation of lower mantle, thermo-chemical plumes through positive feedback.     

Regional Stress Fields under Tibet from 3D Global Flow Simulation

Tibetan area is the most active continental collision zone on earth.Several major earth-quakes occurred around the boundaries of Tibetan plateau and caused massive damages and casualties.The dynamics of this area is not well understood due to the complex structure of Tibet and its sur-rounding area.In this study,a 3D global flow simulation with only viscous rheology is applied to studying the stress distribution in this area,and the interaction between Tibet and its surrounding areas is investigated.Finally...     

Experimental Studies on Permeability of Intact and Singly Jointed Meta-Sedimentary Rocks Under Confining Pressure

Three different types of permeability tests were conducted on 23 intact and singly jointed rock specimens, which were cored from rock blocks collected from a rock cavern under construction in Singapore. The studied rock types belong to inter-bedded meta-sandstone and meta-siltstone with very low porosity and high uniaxial compressive strength. The transient pulse water flow method was employed to measure the permeability of intact meta-sandstone under a confining pressure up to 30 MPa. It showed that the magnitude order of meta-sandstone’s intrinsic permeability is about 10^?18 m². The steady-state gas flow method was used to measure the permeability of both intact meta-siltstone and meta-sandstone in a triaxial cell under different confining pressures spanning from 2.5 to 10 MPa. The measured permeability of both rock types ranged from 10^?21 to 10^?20 m². The influence of a single natural joint on the permeability of both rock types was studied by using the steady-state water flow method under different confining pressures spanning from 1.25 to 5.0 MPa, including loading and unloading phases. The measured permeability of both jointed rocks ranged from 10^?13 to 10^?11 m², where the permeability of jointed meta-siltstone was usually slightly lower than that of jointed meta-sandstone. The permeability of jointed rocks decreases with increasing confining pressure, which can be well fitted by an empirical power law relationship between the permeability and confining pressure or effective pressure. The permeability of partly open cracked specimens is lower than that of open cracked specimens, but it is higher than that of the specimen with a dominant vein for the meta-sandstone under the same confining pressure. The permeability of open cracked rock specimens will partially recover during the unloading confining pressure process. The equivalent crack (joint) aperture is as narrow as a magnitude order of 10^?6 m (1 μm) in the rock specimens under confining pressures spanning from 1.25 to 5.0 MPa, which represent the typical ground stress conditions in the cavern. The in situ hydraulic conductivity measurements conducted in six boreholes by the injection test showed that the in situ permeability of rock mass varies between 10^?18 and 10^?11 m². The lower bound of the in situ permeability is larger than that of the present laboratory-tested intact rock specimens, while the upper bound of the in situ permeability is less than that of the present laboratory-tested jointed rock specimens. The in situ permeability test results were thus compatible with our present laboratory permeability results of both intact and jointed rock specimens.     

The Brazilian Disc Test for Rock Mechanics Applications: Review and New Insights

The development of the Brazilian disc test for determining indirect tensile strength and its applications in rock mechanics are reviewed herein. Based on the history of research on the Brazilian test by analytical, experimental, and numerical approaches, three research stages can be identified. Most of the early studies focused on the tensile stress distribution in Brazilian disc specimens, while ignoring the tensile strain distribution. The observation of different crack initiation positions in the Brazilian disc has drawn a lot of research interest from the rock mechanics community. A simple extension strain criterion was put forward by Stacey (Int J Rock Mech Min Sci Geomech Abstr 18(6):469–474, 1981) to account for extension crack initiation and propagation in rocks, although this is not widely used. In the present study, a linear elastic numerical model is constructed to study crack initiation in a 50-mm-diameter Brazilian disc using FLAC^3D. The maximum tensile stress and the maximum tensile strain are both found to occur about 5 mm away from the two loading points along the compressed diameter of the disc, instead of at the center of the disc surface. Therefore, the crack initiation point of the Brazilian test for rocks may be located near the loading point when the tensile strain meets the maximum extension strain criterion, but at the surface center when the tensile stress meets the maximum tensile strength criterion.     

A Revised Evaluation of Tsunami Hazards along the Chinese Coast in View of the Tohoku-Oki Earthquake

Japan’s 2011 Tohoku-Oki earthquake and the accompanying tsunami have reminded us of the potential tsunami hazards from the Manila and Ryukyu trenches to the South China and East China Seas. Statistics of historical seismic records from nearly the last 4 decades have shown that major earthquakes do not necessarily agree with the local Gutenberg-Richter relationship. The probability of a mega-earthquake may be higher than we have previously estimated. Furthermore, we noted that the percentages of tsunami-associated earthquakes are much higher in major events, and the earthquakes with magnitudes equal to or greater than 8.8 have all triggered tsunamis in the past approximately 100 years. We will emphasize the importance of a thorough study of possible tsunami scenarios for hazard mitigation. We focus on several hypothetical earthquake-induced tsunamis caused by M _w 8.8 events along the Manila and Ryukyu trenches. We carried out numerical simulations based on shallow-water equations (SWE) to predict the tsunami dynamics in the South China and East China Seas. By analyzing the computed results we found that the height of the potential surge in China’s coastal area caused by earthquake-induced tsunamis may reach a couple of meters high. Our preliminary results show that tsunamis generated in the Manila and Ryukyu trenches could pose a significant threat to Chinese coastal cities such as Shanghai, Hong Kong and Macao. However, we did not find the highest tsunami wave at Taiwan, partially because it lies right on the extension of an assumed fault line. Furthermore, we put forward a multi-scale model with higher resolution, which enabled us to investigate the edge waves diffracted around Taiwan Island with a closer view.