Persistence and tensile strength of incipient rock discontinuities

Rock discontinuities are fundamentally important to most rock engineering projects but predicting or measuring their properties such as roughness, aperture, shape and extent (persistence) are fraught with difficulty. So far the solution of how to measure or predict persistence is poorly researched partly because the concept of how to investigate the extent of rock discontinuities within a rock mass seems intractable, by any economical methods. In the majority of engineering applications it is a fairly widespread practice to follow a conventional approach, assuming a 100% persistence value. However that is certainly incorrect even if usually a conservative assumption. This project is a small step towards resolving this issue. A series of laboratory and field research activities were carried out to investigate incipient nature of rock discontinuities and the extent of rock bridges. Uniaxial tensile strength of incipient discontinuities was quantified in the laboratory using cylindrical rock samples. The tested samples included incipient joints, mineral veins and bedding. It has been confirmed that such visible yet incipient features can have high tensile strength, approaching that of the parent rock. Factors contributing to the tensile strength of incipient rock discontinuities have been investigated. It is concluded that the degree of incipiency of rock discontinuities is an important factor that should be differentiated as part of the process of rock mass classification to inform more realistic engineering design and that this might best be done with reference to the tensile strength relative to that of the parent rock. An original methodology has been developed in the laboratory using expansive chemical splitters in drillholes, to quantify the tensile strength of large-scale incipient rock joints. In these tests, smaller tensile strengths were obtained, which probably was the result of localised stress concentration, low pressurization rate and unavoidable variations of expansive tensile force arising from the chemical splitter. A technique ‘Forensic Excavation of Incipient Rock Discontinuities (FEIRD)’ was established and employed to investigate areal extent and incipient nature of discontinuities in the field. Large rock blocks, containing incipient features, were split using similar expansive grout techniques as developed in the laboratory. Test results were interpreted and discussed with respect to fracture mechanics, fractographic features (such as hackle and rib marks), as well as geological conditions affecting the incipiency of the tested discontinuities including degree and extent of weathering and mineralisation. One common observation from the tests conducted is that breakage of non-persistent sections of incipient rock joints (rock bridges) leads to the development of rough surfaces over those freshly broken areas, and this may have implications for rock fracture development more generally. Despite rock bridge failure (say as part of rock slope mass movement), the freshly formed surfaces might be expected to have relatively high strength compared to the pre-existing persistent sections. An important conclusion from this research is that areal extent of open rock discontinuities (persistence) can be investigated realistically using the FEIRD technique. It has been found that estimates of persistence from trace mapping on rock exposures can be wildly inaccurate and it is concluded that field studies using FEIRD techniques (perhaps at a larger scale than used for this research to date) can be used to understand and quantify better the true nature of rock mass fracture network connectivity and extent that are important parameters for many rock engineering endeavours

Impact of pile punching on adjacent piles: Insights 3 from a 3D coupled SPH-FEM analysis

Pile punching (or driving) affects the surrounding area where piles and the adjacent piles can be displaced out of their original positions due to horizontal loads, leading to hazardous outcomes. This paper presents a 3D coupled Smoothed Particle Hydrodynamics and Finite. Element Method (SPH-FEM) model, which was established to investigate pile punching and its impact on adjacent piles subjected to lateral loads. This approach handles the large distortions by avoiding mesh tangling and remeshing, contributing greatly high computational efficiency. The SPH-FEM model was validated against field measurements. Results of this study indicated that the soil type in which piles were embedded affected the interaction between piles during the pile punching. A comprehensive parametric study was carried out to evaluate the impact of soil properties on the displacement of piles due to the punching of an adjacent pile. It was found that the interaction between piles was comparatively weak when the piles were driven in stiff clays; while the pile-soil interactions were much more significant in sandy soils and soft clays

An analytical model for shear behaviour of bolted rock joints

Rock bolts have been widely used to reinforce the jointed rock mass. Modelling the mechanical shear behaviors of the bolted rock joints are difficult due to the complex interactions between bolts and rock joints. The applied pretension forces combined with the axial loads developed in the bolt act as the normal forces which are applied to the rock joints. However, these combined normal forces are not considered in the existing analytical models. An analytical model is proposed in this study to predict the shear behavior of the bolted rock joints, by taking into account the pretension forces, the axial forces developed in the bolt, the interfacial bond stress between the bolt and grout, and dowel shear loads acting transversely to the bolt axis. The proposed analytical model is able to provide complete curves of the dowel shear loads, axial loads, and the global shear loads as a function of the joint shear displacement. The analytically predicted axial load vs shear displacement curves and the global shear load vs shear displacement curves are verified by available experimental tests. The validation shows that the proposed model has the capacity to predict the global shear load evolution as well as the axial load evolution. The factors such as the pretension forces, the bolt inclination angles, the concrete strength and the rock joint friction are successfully accounted for in the analytical model

Numerical investigation into the blasting-induced damage characteristics of rocks considering the role of in-situ stresses and discontinuity persistence

This paper presents a 3D coupled Smoothed Particle Hydrodynamics (SPH) and Finite Element Method (FEM) model, which was developed to investigate the extent of damage zone and fracture patterns in rock due to blasting. The RHT material model was used to simulate the blasting-induced damage in rock. The effects of discontinuity persistence and high in-situ stresses on the evolution of blasting-induced damage were investigated. Results of this study indicate that discontinuity persistence and spatial distribution of rock bridges have a significant influence on the evolution of blasting-induced damage. Furthermore, high in-situ stresses also have a significant influence on the propagation of blasting-induced fractures, as well as the patterns of fracture networks. It is also shown that the blasting-induced cracks are often induced along the direction of the applied high initial stresses. Moreover, additional cracks are normally generated at the edges of the rock bridges probably due to the relatively high stress concentration

On the initiation, propagation and reorientation of simultaneously-induced multiple hydraulic fractures

This study aims to uncover the growth characteristics of simultaneously-induced multiple hydraulic fractures using the discrete element method. We evaluate the influences of in-situ states and operational parameters on the fracture trajectories. Results reveal that reservoir heterogeneity magnifies the stress-shadowing effect and causes severe interactions among fractures. Higher effective stress anisotropy offsets the stress-shadowing effect and force the fractures to propagate in the direction of maximum stress and results in relative long parallel fractures. Increasing the spacing can mitigate the stress-shadowing effect to some degree. Injection rate and fluid viscosity have a less significant influence on the interactions among fractures

Advanced predication of geological anomalous body ahead of laneway using seismic tomography technique

Advanced predication of geological anomalous body (GAB) of laneway can provide scientific references for mine safety production. It refers to reveal the position, shape and size of GAB in advance. Seismic tomography technology (STT) can realize the prediction of GAB using tunnel surface waves. Tunnel Reflection Tomography (TRT) 6000 system, which is on the basis of STT, has the advantages of convenience and high reliability. In this paper, TRT6000 is introduced to forecast the GAB ahead of laneway in underground metal mine. The operation steps, data processing and notes about TRT6000 are detailed. The research results show that there are two water flowing fractures, separately located at around 50meters and 85meters ahead of laneway. And the prediction results match the actual situation well. Therefore, the seismic tomography technology improves the mine safety management, and the TRT6000 provides a new method for predicting GAB in advance

Advances in multifield and multiscale coupling of rock engineering

No abstract available

Dynamic analysis of geomaterials using microwave sensing

Precise characterization of geomaterials improves subsurface energy extraction and storage. Understanding geomaterial property, and the complexities between petrophysics and geomechanics, plays a key role in maintaining energy security and the transition to a net zero global carbon economy. Multiple sectors demand accurate and rapid characterization of geomaterial conditions, requiring the extraction of core plugs in the field for full-field characterization and analysis in the laboratory. We present a novel technique for the non-invasive characterization of geomaterials by using Frequency Modulated Continuous Wave (FMCW) radar in the K-band, representing a new application of microwave radar. We collect data through the delivery of FMCW wave interactions with geomaterials under static and dynamic conditions and show that FMCW can detect fluid presence, differentiate fluid type, indicate the presence of metallic inclusions and detect imminent failure in loaded sandstones by up to 15 s, allowing for greater control in loading up to a failure event. Such precursors have the potential to significantly enhance our understanding of, and ability to model, geomaterial dynamics. This low-cost sensing method is easily deployable, provides quicker and more accessible data than many state-of-the-art systems, and new insights into geomaterial behavior under dynamic conditions

Combined effects of cyclic load and temperature fluctuation on the mechanical behavior of porous sandstones

Rocks in cold regions tend to experience exacerbated degradation under the combined effects of environmental and anthropogenic factors, which may arise from, for example, temperature fluctuation, mechanical excavation, and blasting. Activities related to rock support or open-pit slope optimization in cold regions require a complete understanding of the failure mechanisms of rock under the complex conditions. This paper quantitatively documents the impact of combined cyclic mechanical load and freeze-thaw cycles (i.e., the effect of stress “history”) on the microstructural evolution and mechanical degradation of three porous sandstones with distinct porosity values (from 3.9 to 14.1%). The three sandstone samples were collected from different geological regions in China. The microstructural evolution of the tested samples was quantitatively analyzed using the low-field Nuclear Magnetic Resonance (NMR) technique. To investigate sample degradation arising from the impact of the stress “history”, the cyclic-loaded and freeze-thaw cycled samples were eventually compressed to failure, during which an acoustic emission system was used to monitor microseismic activities. The results of the study show that the porosity of all tested sandstone samples was increased after cyclic load, with a much more rapid and further increase in porosity observed for samples being subsequently treated under the freeze-thaw cycles. More interestingly, the Chuxiong sandstone with relatively small porosity values were much more sensitive to the impact of cyclic load compared with the Linyi sandstone, exhibiting a somewhat larger increase rate in porosity. However, the Linyi sandstone with larger initial porosity values exhibited a relatively large increase rate in porosity under the multiple freeze-thaw treatments. The multiple freeze-thaw treatments mainly resulted in the development of relatively large pores. The results of the uniaxial compression tests show that the strength reduction of the samples being solely treated by freeze-thaw cycles was within the range of 5–10%, whereas it was within the range of 20–40% for those samples subjected to the combined cyclic load and freeze-thaw cycles

Three-dimensional DEM investigation of the fracture behaviour of thermally degraded rocks with consideration of material anisotropy

A complete understanding of the fracture behaviour of anisotropic rocks under elevated temperatures is fundamentally important for rock and reservoir engineering applications. This paper shows a three-dimensional numerical investigation of the fracture behaviour of anisotropic sandstone, with consideration of the effects of temperature and material anisotropy. In the study, a 3D semi-circular bend (SCB) model was established by using the Discrete Element Method (DEM). The thermal responses of different minerals and the strength anisotropy of incipient bedding planes were considered in the model. The DEM model was calibrated against a series of laboratory experiments on Midgley Grit sandstone (MGS) that exhibits intrinsic anisotropy. The pure mode I, mode II, and mixed-mode (I+II) fracture characteristics of the MGS were investigated under elevated temperatures (up to 600 °C) using the established DEM model. The thermal degradation (i.e., fracturing) of the rock, the fracture load, the evolution of micro-cracks, and the stress-strain relationship around notch tips were analysed, with emphasis on enlightening the micro-mechanisms underlying the fracture behaviour. The results of the study were discussed and then compared with experimental observations and theoretical predictions