It is very difficult to predict weathering characteristics of rock because of limitation caused by time and space. An experimental study of accelerated weathering was performed on Cretaceous granitic rock samples from Jinhae and Gimhae area in southeastern Korea to investigate physico-mechanical property changes including revelation characteristics of microfractures due to laboratory accelerated weathering process. Total of 140 cycles of freeze-thaw were completed on deteriorated rock specimens with measuring the index properties as well as geometries of microfractures. Each complete cycle of freeze and thaw implemented 24 hours, comprising 2 hours of saturating in vacuum chamber, 8 hours of freezing at 16±1°C and 14 hours of thawing at room temperature. The seismic velocity was found to decrease with increasing freeze-thaw cycle. Effective porosity and absorption tend to increase with freeze-thaw cycle. The amount of deterioration of rock samples depend on pre-test degree of weathering. Effective porosity, absorption and seismic velocity can be used as the measure of physical weathering for granitic rocks of the study area. The size and plane density of the microfracture on rock specimens were obviously changed with increasing freeze-thaw weathering. The conducted research in this study has shown that accelerated weathering test has strong capability to capture the weathering characteristics of deteriorated rocks.
The determinations of the peak strength of rocks have been comprehensively investigated and applied to the simulation of tunneling. However, only few achievements have been made to estimate the post-peak strength of rocks, which is becoming more and more important due to the rapidly increase of tunneling depth. Understanding both the peak and the post-peak strengths of brittle rock are necessary for the design of deep tunneling. After reviews of various estimation methods, the post-peak form of the Hoek-Brown failure criterion by introducing a strength loss parameter (ß) was adopted in the paper. The relationship between the strength loss parameter (ß), the post-peak strength and the confining stress were established on the basis of the triaxial compressive tests of marble. A numerical simulation of the tri-axial compressive test was conducted by the axisymmetric modeling in FLAC to verify the accuracy of the method suggested. The post-peak strength estimation method, so-called strength loss experiment method, may estimate the post-peak strength reasonably well.
Zoorabadi, Mahdi (The University of New South Wales) | Saydam, Serkan (The University of New South Wales) | Timms, Wendy (The University of New South Wales) | Hebblewhite, Bruce (The University of New South Wales)
In order to investigate the influence of water vapor pressure of surrounding environment on fracture toughness of rock, a series of Semi-Circular Bend (SCB) tests under various water vapor pressures were conducted. The water vapor is the most effective agent which promotes stress corrosion of rock. The rocks used in this study were granodiorite and sandstone, which are known as anisotropic rocks. Measurement of elastic wave velocity and observation of thin section of these rocks were performed to make clear orientation of inherent micro cracks and grains. Then two types of specimens were prepared based on the results of the observed anisotropy. The experimental results show that the fracture toughness of the granodiorite is dependent on the water vapor pressure of the surrounding environment and that that of the sandstone is independent on the water vapor pressure. Furthermore the fracture toughness of both rocks has anisotropic properties. Then crack initiation was analyzed using images of fracture within fractured specimen taken by X-ray CT scanner. Together with the results of the experiments and the analyses of CT images, it was made clear that the anisotropy of fracture toughness is dependent on the micro structure of the rocks.
By excavating an underground space, the state of stress and displacement in the surrounding medium are changed in comparison to the inital state. As time passes, the variation of the displacement mainly depends on the creep behaviour of the hosting rock mass. In this paper, an elasto-viscoplastic creep model is proposed. In the proposed model, the main purpose is to consider plastic deformations increasing with time. The viscoplastic behaviour of rocks plays a key role in the tunnelling works, especially for deep tunnels subjected to large in situ stresses. Using non-linear Hoek-Brown yield criterion in a creep model is the other important aim of this paper, which eliminates estimating specific equivalent Mohr-Coulomb strength parameters from the Hoek-Brown parameters. The equations related to the proposed model are derived and then, to reach a numerical solution of the equations, the finite difference software (FLAC2D-FISH Editor) is used. The application of the proposed model is illustrated through an example analyzed numerically using the finite difference software FLAC2D.
Active production in oil and gas fields tends to cause reservoir compaction and consequent surface subsidence. The compaction and the associated subsidence are often time-dependent in various reasons, and are reported, in some cases, to accelerate even after depletion of reservoir. In order to explain the accelerated post-depletion subsidence, we propose a possibility of compaction in the reservoir-bounding shale, which is induced by slow drainage of pore fluid from shale to reservoir sands after depletion. To examine the significance of the compaction in the reservoir-bounding shale, we build a simple one-dimensional compaction model that consists of sand reservoir embedded within the surrounding shale. The model assumes a poroelastic compaction in sand reservoir during production, and then shale compaction due to pore pressure diffusion in the bounding shale after depletion. The shale compaction is described by a superposition of two rheological constitutive laws: time-dependent poroelasticity and time-dependent viscoplasticity. The analytical result shows that despite extremely low permeability of shale, the total amount of shale compaction associated with a slow drainage of pore fluid can be comparable to that of the sand compaction within a decade, and that the resulting subsidence rate can accelerate even after depletion.
Koyama, Tomofumi (Kyoto University) | Katayama, Tatsuo (KANSOTechnos, Co. Ltd.) | Tanaka, Tatsuya (Obayashi Corporation) | Kuzuha, Yuji (Japan Atomic Energy Agency (JAEA)) | Ohnishi, Yuzo (Kyoto University)