Rock Mechanics Bulletin最新文献

To explain the effect of joint roughness on joint peak shear strength (JPSS) and investigate the effect of different contact states of joint surface on JPSS, we try to clarify the physical mechanism of the effect of joint cavity percentage (JCP) on JPSS from the perspective of the three-dimensional (3D) distribution characteristics of the actual contact joint surface, and propose a JPSS model considering the JCP. Shear tests for red sandstone joints with three different surface morphologies and three different JCPs were performed under constant normal load condition. Based on test fitting results, the reduction effect of the JCP on JPSS is investigated, and a JPSS model for cavity-containing joints is obtained. However, the above model only considers the influence of JCP by fitting test data, and does not reveal the physical mechanism of JCP affecting the JPSS. Based on the peak dilation angle model for consideration of the actual contact joint morphology, and the influence of JCP on the roughness of the actual contact joint surface, a theoretical model of the JPSS considering the JCP is proposed. The derivation process does not depend on the test fitting, but is entirely based on the joint mechanical law, and its physical significance is clear. It is proposed that the essence of the influence of the JCP on JPSS is that the JCP first affects the normal stress of the actual contact joints, further affects the roughness of actual contact joints, and then affects the shear strength.

Shale reservoirs have been a significant focus of hydrocarbon production over the past few decades, and the mechanical assessment of target shale reservoirs has been critical to successful field operations, especially in hydraulic fracturing and well completions. The Unconfined compressive strength (UCS) and Poisson's ratio ($\nu$) are critical mechanical properties in shale reservoir assessment. The estimation and measurement of shale mechanical properties are often erroneous by not accounting for their heterogeneous and pre-existing features, which yield variability of shale mechanical properties along their lithostratigraphy. Thus, there is a need to investigate the degree of correlation and accuracy in multiscale mechanical evaluations of heterogeneous shales, and the correlation between such micromechanical and macromechanical measurements. This study investigated the impact of inherent heterogeneity on the measurement of continuous micromechanical and macromechanical properties of shale reservoirs using scratch test (ST) and uniaxial compression test (UCT) methods, and the degree of correlation (correlation coefficient, r) of measurements in shale was further assessed for the variability of their measured properties. Shale core samples from three distinct shale formations were utilized and studied, and the core samples were subjected to ST and UCT, respectively. The results from this study showed that despite inherent heterogeneous anomalies and natural fractures in the shale samples analyzed, there is a good degree of correlation (UCS: r ​= ​0.73; $\nu$: r ​= ​0.89) in the micro- and macro-mechanical properties of shales using two independent experimental tests (ST and UCT). This study provides insights for improving the accuracy of mechanical evaluations and numerical modeling in shales with a high degree of heterogeneity and pre-existing natural fractures. The results indicate that when considering the structural complexity and heterogeneity of unconventional reservoirs such as shales, the ST method can provide a better continuous micromechanical assessment of shales. In contrast, the UCT can provide a better bulk macromechanical measurement of shales.

Fractures widely exist in crustal rocks and form complex networks dominating the bulk behaviour of geological media. Thus, understanding how fracture networks affect subsurface processes/phenomena is highly relevant to many rock engineering applications. However, the large-scale behaviour of a fractured rock mass consisting of numerous fractures and rocks cannot be predicted by simple applications of the knowledge of individual fractures and/or rocks, due to upscaling complexities involving the hierarchy of scales, heterogeneities, and physical mechanisms as well as the possible emergence of qualitatively different macroscopic properties. In other words, macroscopic phenomena in fractured rocks arise from the many-body effects (i.e. collective behaviour) of numerous interacting fractures and rocks, such that the emergent properties at the fracture system scale are much richer than those of individual components. Hence, more is different! This paper gives a discussion on the mechanism of emergence in fractured media from a combined statistical physics and rock mechanics perspective, and further presents a multiscale conceptual framework to link microscopic responses of single fractures/rocks to macroscopic behaviour of rock masses consisting of many fractures and rocks. This framework can serve as a useful tool to bridge experimentally-established constitutive relationships of fracture/rock samples at the laboratory scale to phenomenologically-observed macroscopic properties of fractured rock masses at the site scale.

Uniaxial compressive strength (UCS) of rock is an essential parameter in geotechnical engineering. Point load strength (PLS), P-wave velocity, and Schmidt hammer rebound number (SH) are more easily obtained than UCS and are extensively applied for the indirect estimation of UCS. This study collected 1080 datasets consisting of SH, P-wave velocity, PLS, and UCS. All datasets were integrated into three categories (sedimentary, igneous, and metamorphic rocks) according to lithology. Stacking models combined with tree-based models and linear regression were developed based on the datasets of three rock types. Model evaluation showed that the stacking model combined with random forest and linear regression was the optimal model for three rock types. UCS of metamorphic rocks was less predictable than that of sedimentary and igneous rocks. Nonetheless, the proposed stacking models can improve the predictive performance for UCS of metamorphic rocks. The developed predictive models can be applied to quickly predict UCS at engineering sites, which benefits the rapid and intelligent classification of rock masses. Moreover, the importance of SH, P-wave velocity, and PLS were analyzed for the estimation of UCS. SH was a reliable indicator for UCS evaluation across various rock types. P-wave velocity was a valid parameter for evaluating the UCS of igneous rocks, but it was not reliable for assessing the UCS of metamorphic rocks.

The mechanical behavior of host rock for a deep geological repository of high-level radioactive waste plays a key role in ensuring the isolation function of host rock as a natural barrier under the multi-field coupling environment. For a better understanding of granite in China's Beishan pre-selected area for geological disposal of high-level radioactive waste, a series of investigations were carried out on in-situ stress field of rock mass at depth, strength and deformation characteristics of rocks under different stress and temperature conditions, and rock boreability and adaptability to Tunnel Boring Machine (TBM) technology. The results indicate that Beishan granite shows typical characteristics as a hard and brittle rock with a quite low permeability, and it is favorable to geological disposal. Meanwhile, a new rock mass suitability evaluation system was proposed, and the rock mass mainly composed of Beishan granite was proven to be suitable for geological disposal. Besides, the constructability of Beishan granite at engineering scale was tested and verified through field tests in the Beishan Exploration Tunnel (BET). Here, we summarize the main outcomes of rock mechanics research on Beishan granite in the past years and introduced the current progress of Beishan underground research laboratory (URL) for geological disposal.

In response to the existing consolidation theory for stone column composite foundations which cannot consider the time-dependent loading and the well resistance effect of stone columns under time-dependent boundaries, a consolidation model that can reflect these characteristics is developed in this study, and the corresponding analytical solutions are obtained under permeable top surface with permeable bottom surface (PTPB) and permeable top surface with impermeable bottom surface (PTIB), respectively. In addition, the reliability of the proposed solutions is verified by comparing them with existing analytical solutions. Extensive calculations are then performed by the proposed solutions to analyze the consolidation behaviors of stone column composite foundations under time-dependent boundaries, the results show that the interface parameters have a large effect on the distribution of excess pore water pressure (EPWP) along the depth; for projects with longer construction time, the permeability of the top and bottom surfaces of the composite foundation has a smaller effect on the average consolidation rate. Finally, the proposed solution is applied to the settlement calculation in an actual engineering project, and the theoretical results show a general agreement with the measured data by considering the influence of the interface parameters.

This study analyzed the mechanical and thermal properties of various rock types found in South Korea. The results showed that both igneous and metamorphic rocks possess higher strength compared to sedimentary rocks. The Young's modulus of rocks is dependent on the extent of weathering they have undergone. The average cohesion of granites was found to be relatively higher compared to other rock types, and their friction angle also exhibited a relatively high value with a considerable variance. The results of uniaxial compression strength testing with respect to depth revealed that rock strength generally increased with depth, however, there was a large variance in strength distribution in each depth interval. Based on these findings, it can be concluded that South Korea can also secure HLW disposal sites and facilities in terms of rock mechanics by using crystalline rocks, similar to countries such as Sweden and Finland where disposal facilities are being built.

Regarding the thermal properties of rocks, they are influenced by the distribution of the parent rock. The thermal conductivity is highly concentrated in the southwest and central regions of South Korea, while the geothermal gradient is high in the northeast, west, and some parts of the southeast regions. The southeast region of Korea has a high geothermal heat flow, and some central northern regions also exhibit relatively high geothermal heat flow. In light of these distributional characteristics, it is crucial to continue conducting precise studies on the mechanical and thermal properties of rocks in the future disposal depths of spent nuclear fuel.

In Japan, high-level radioactive waste and specific low-level radioactive waste which includes long-lived radionuclides are planned to be disposed of in the geological formations at depths greater than 300 ​m. The disposal site will be selected through a stepwise site investigation process that consists of a Literature Survey, Preliminary Investigation, and Detailed Investigation phases. In October 2020 a Literature Survey was launched in Japan at two municipalities in Hokkaido for the first time since NUMO initiated a nationwide call for volunteer municipalities in 2002, and the outcomes are currently being compiled. To enhance the public’s understanding of how to implement safe geological disposal in Japan based on the latest scientific knowledge and technology, NUMO, as the implementing organisation, developed and published a safety case for geological disposal at the pre-siting stage. This safety case provides multiple lines of arguments and evidence to demonstrate the feasibility of the geological disposal and a basic structure for a safety case that will be applicable to any potential sites in Japan. The safety case also presented some R&D challenges to enhance the technical confidence of the project, including the R&D topics related to rock mechanics. This report presents the current status of the geological disposal programme in Japan, together with the status of the Literature Survey phase and an overview of the NUMO safety case.

Safe disposal of high-level radioactive nuclear waste (HLW) is crucial for human health and the environment, as well as for sustainable development. Deep geological disposal in sparsely fractured crystalline rock is considered one of the most favorable methods for final disposal of HLW. Extensive research has been conducted worldwide and many countries have initiated their own national development programs for deep geological disposal. Significant advancements of national programs for deep geological disposal of HLW in crystalline rock have been achieved in Sweden and Finland, which are currently under site development stage, focusing on detailed site characterization, repository construction, and post-closure safety analysis. Continued research and development remain important in the site development stage to ensure long-term safety of the HLW disposal repository. This work presents an overview and discussion of the progress as well as remaining open scientific issues and possibilities related to site development for safe disposal of HLW in crystalline rock. We emphasize that developing a comprehensive and convergent understanding of the coupled thermal, hydraulic, mechanical, chemical and biological (THMCB) processes in fractured crystalline rock remains the most important yet challenging topic for future studies towards safe disposal of HLW in crystalline rock. Advancements in laboratory facilities/techniques and computational models, as well as available comprehensive field data from site developments, provide new opportunities to enhance our understanding of the coupled processes and thereby repository design for safe geological disposal of HLW in crystalline rock.

The aim of the present research was to establish a case study for the prediction of the unknown EDZ (Excavation Damaged Zone) distribution using a numerical analysis calibrated by replicating the trends in the EDZ observed from one of the representative underground research fields in Japan (Horonobe URL). In this study, a 2D numerical analysis using a damage model, which can determine rock deformation and fracturing simultaneously, is presented. It was calibrated to reproduce the excavation of the gallery at the Horonobe URL at a depth of 350 ​m. Simulated results show an excellent agreement with the extent of the measured EDZ and capture the failure modes of EDZ fractures suggested by the in-situ observations. Finally, the calibrated numerical analysis was used to realistically estimate the EDZ formation for the geological disposal of high-level radioactive waste (HLW) under the same environment as that of the above-mentioned galley at the Horonobe URL. Consequently, it was shown that the tensile/shear hybrid fractures dominantly constituted the EDZ and propagated to a maximum extent of about 0.3 ​m from the cavity wall during the cavity excavation for the HLW disposal. Overall, the calibrated numerical analysis and resulting estimations, targeted for the environment at the depth of 350 ​m at the Horonobe URL, where mudstone is located, should be useful for predicting the trends in the EDZ distribution expected in the implementation of HLW disposal projects under deep geological conditions, such as those that exist in Japan, which are dominated by sedimentary rocks, including mudstone.