Jingjing Lu

Anisotropies in Mechanical Behaviour, Thermal Expansion and P-Wave Velocity of Sandstone with Bedding Planes

Many geo-engineering applications, such as geological repositories for nuclear waste, geological sequestration of CO2, enhanced geothermal systems, deep mining and deep well drilling involve thermal, hydraulic, mechanical and chemical interactions. Describing these coupling processes requires knowledge of each individual process and their coupling effects (Kohl et al. 1995; Hudson et al. 2005; Poulet et al. 2012; Jobmann and Polster 2007). A number of investigations (Maruyama et al. 2014; Zhu et al. 2015; Duchkov et al. 2014; Nagaraju and Roy 2014; Fortin et al. 2011) on various rock materials have indicated that the elastic modulus, strength, thermal conductivity and seismic properties of rock materials are dependent on their porosity, saturation degree, saturating fluids and damage. Furthermore, there are correlations between the physical and mechanical properties, and some theoretical and empirical relations have been proposed for different rocks (Yasar and Erdogan 2004; Gruescu et al. 2007; Hovis et al. 2008; Chen et al. 2012). Anisotropies are usually observed in rock materials and cause the physical and mechanical properties to vary with direction because of the presence of bedding planes, schistosity planes, structural discontinuities and stress-induced defects (Gao et al. 2015; Davis et al. 2007; Bourret et al. 2015; Qin et al. 2015; Wenk et al. 2012). Some previous studies indicated that thermal expansion of rock materials exhibits anisotropies (Mitoff and Pask 1956; Robertson 1988; Somerton 1992). Indeed, the thermal expansion of rock materials is relatively small in magnitude and may have only minor effects (Robertson 1988). However, the thermal expansion coefficients of rock materials usually vary with their mineral compositions, and these differences in thermal expansion behaviour may have significant effects on the structure of rock masses. For example, anisotropy in the thermal expansion caused by bedding planes in sedimentary rocks can also cause structural damage during heating (Somerton 1992). The aim of the present paper is to study the anisotropic mechanical properties, thermal expansion properties and & Dawei Hu dwhu@whrsm.ac.cn

Evaluation Methodology of Brittleness of Rock Based on Post-Peak Stress–Strain Curves

Brittleness is an important characteristic of rocks, for it has a strong influence on the failure process no matter from perspective of facilitating rock breakage or controlling rock failure when rocks are being loaded. Various brittleness criteria have been proposed to describe rock brittleness. In this paper, the existing brittle indices are summarised and then analysed in terms of their applicability to describe rock brittleness. The analysis demonstrates that the widely used strength ratio or product (σc/σt, σc·σt) of rocks cannot describe rock brittleness properly and that most of the indices neglect the impact of the rock’s stress state on its brittleness. A new evaluation method that includes the degree of brittleness (Bd) and brittle failure intensity (Bf) is proposed based on the magnitude and velocity of the post-peak stress drop, which can be easily obtained from the conventional uniaxial and triaxial compression tests. The two indices can accurately account for the influence of the confining pressure on brittleness, and the applicability of the new evaluation method is verified by different experiments. The relationship between Bd and Bf is also discussed.

Analysis of rockburst mechanisms induced by structural planes in deep tunnels

A rockburst is defined as damage to an excavation in a sudden or violent manner, frequently occurring during the excavation of civil engineering tunnels and deep-level mining. Small-scale structural planes in the vicinity of tunnels have been found to play an important role in controlling certain rockbursts in the deeply buried tunnels of the Jinping-II Hydropower Station. In order to study the mechanisms of bursts related to the structural plane, four typical examples were selected to illustrate the temporal and spatial characteristics of the burst and the exposed plane. Three types of rockbursts were classified based on these examples and on a preliminary analysis of the various mechanisms—namely, fault-slip burst, shear rupture burst, and buckling burst. Model experiments using cement mortar revealed three failure mechanisms of the structural plane under shear stress: slip dislocation of the asperities, tensile failure of the footwall and hanging wall initiated from the root of the asperities, and impact fracture of the front end of the hanging wall and back end of the footwall. These mechanisms are used to explain the development processes of fault-slip and shear rupture bursts. Analysis of the source mechanisms of buckling bursts caused by structural planes is also provided, and three proposed source mechanisms are put forth to illustrate the factors that may have triggered the buckling bursts: self-adjustment and accumulation, disturbance from machine or blasting excavation, and energy input from remote seismic sources. Issues that must be addressed in the future are outlined and discussed in the final section. The research results contribute to a mechanistic understanding of and control method for structure-type rockbursts in deep hard-rock tunnels.

Development of a Hollow Cylinder Torsional Apparatus for Rock

The mechanical characteristics of rock subjected to the changing of the principal stress magnitude and orientation caused by excavation are significant for the construction of larger and deeper underground engineering. However, there have been few experimental studies on rock mechanical characteristics under the changing principal stress orientation due to the lack of the test device. Hence, in this paper, a new rock mechanical experimental technique and device was developed to conduct the complex stress path with coupling variations of stress magnitude and orientation. The theoretical principle and apparatus composition were introduced in this work, and two test cases were conducted to verify its feasibility and reliability. This study has important practical significance and scientific value for promoting the technical level of rock mechanical test and enriching the theoretical frame of rock mechanics.

A New Rock Brittleness Evaluation Index Based on the Internal Friction Angle and Class I Stress–Strain Curve

B i Brittleness index p Peak strength r Residual strength E Elasticity modulus M Post-peak modulus p Peak strain r Residual strain k1 Ratio of the elasticity modulus to the post-peak modulus k2 Ratio of the post-peak stress drop to the peak stress Density Poisson ratio t Tensile strength c Uniaxial compression strength 1 Major principal stress 3 Confining pressure m Material constant for a specific rock in the Hoek– Brown criterion s Material constant for a specific rock in the Hoek– Brown criterion Internal friction angle

An elastoplastic coupling mechanical model for hard and brittle marble with consideration of the first stress invariant effect

The mechanical properties of hard and brittle marble have three typical characteristics: (1) elastoplastic coupling, (2) strain hardening and softening and (3) shear dilatancy. To accurately describe the deformation and failure characteristics of marble, this paper developed an elastoplastic coupling mechanical model based on cyclic loading tests. The proposed mechanical model includes the following three characteristics. First, the elastic modulus was quantitatively related to both the first stress invariant and a specially defined internal variable. Second, evolution laws for strength parameters (c and φ) with the internal variable were suggested based on Mohr–Coulomb yield criterion. Third, an evolution law of the dilatancy angle that considered both the first stress invariant and the internal variable was proposed, and its identification method was provided. Then, the proposed mechanical model was embedded in FLAC3D using C++ programming language. A triaxial compression experiment and an engineering case were simulated using the proposed model, respectively, and good agreements between the simulated results and test data were observed. Thus, the proposed model had a capability to accurately simulate the main mechanical characteristics of marble. Effort of this work could provide an important reference to accurately predict the mechanical response of rockmass in deep rock engineering.

Physical model tests of the surrounding rock deformation and fracture mechanism of mixed-face ground under TBM tunneling

Abstract To investigate the surrounding rock deformation and fracture mechanism of mixed-face ground under tunnel boring machine (TBM) tunnelling, tests of a mixed-face ground model with TBM tunnelling were conducted by using a self-developed 3D large-scale simulation test machine and TBM excavation equipment. A fibre Bragg grating sensor and a distributed optical fibre sensor have been installed along the micro-pressure cell and are used to monitor the variation of stress, strain and displacement in the surrounding rocks while ‘uncoordinated deformation is observed at the interface’ during the excavation process. By comparing the experimental observation obtained in the soft and hard rocks located near the interface, a significant difference is observed in the deformation and stress measurement. The fractures observed in the shallow layer of the surrounding rock mass were mainly tensile fractures and tensile shear fractures, and pressure shear fractures were observed at a certain depth of the surrounding rock during TBM tunnelling. The results not only provide reference data on the mechanical behaviour of mixed surrounding rocks during tunnelling by TBM but also can guide construction units to prevent and control the disaster of TBM in tunnelling through mixed-face ground.

Acoustic emission monitoring on damage evolution of surrounding rock during headrace tunnel excavation by TBM

Abstract The acoustic emission (AE) characteristics of the damage evolution of surrounding rock during tunnel-boring machine (TBM) excavation were studied using AE monitoring and ultrasonic testing. The results indicated that the distribution of the AE signals in the surrounding rock were obtained by the reasonable arrangement of the positions of the probes and the multi-parameter filtering method during TBM excavation. For engineering I, rock damage at different degrees along the direction of the TBM advancement was observed within 5 m ahead of the tunnel face during TBM excavation, while the most severe rock damage appeared 1 m ahead of the tunnel face. The difference in the AE events and energy rates helped distinguish the embedding depths of the loose zone, EDZ and disturbance zone, which were 1, 1–3 and 3–8 m from the tunnel wall, respectively. For engineering II, different degrees of rock damage along the axial tunnel direction were observed within 6 m ahead of the tunnel face, with the most severe rock damage occurring 1 m ahead of the tunnel face. The results can provide significant reference values for the safe and efficient application of TBM excavation in engineering processes.

Estimation of the effective thermal properties of cracked rocks

In geo-engineering applications, the effective thermal properties (ETPs) of cracked rock are strongly dependent on the configuration of the cracks and the thermal properties of the saturating fluid, which differs from that of the rock matrix. In this study, methods for predicting the ETPs of cracked rock are investigated. The effects of various factors such as the crack distribution, the type of saturating fluid and the applied stress on the ETPs are analysed. Formulas for the effective thermal conductivity (ETC) and the effective thermal expansion (ETE) are developed in a discrete form based on the homogenisation method, where the interactions of the cracks in different directions are neglected. To analyse the effects of the crack distribution and the type of saturating fluid on the ETC and the ETE, an example of saturated rock with one family of cracks is considered. The results reveal that the crack distribution has a significant anisotropic effect on both the ETC and the ETE. The difference in the bulk moduli of the rock matrix and the fluid in the cracks also has an important effect on the ETE. The behaviour of the ETC and the ETE of granite at various levels of stress is analysed with triaxial compression tests. The growth of microcracks induced by the applied stresses causes significant anisotropy in both the ETC and the ETE. This study provides insight into the factors that affect the ETC and the ETE and methods to estimate the ETC and the ETE of cracked rock in geo-engineering applications.