Investigation of fracture behaviour during rock mass failure

The main characteristic behaviour of rock mass fractures are studied. Their importance in the application and investigation of rock mass failure is demonstrated through some preliminary investigation of rock mass fracture mechanics by the authors. Firstly, in the study of rock fracture features, the mechanisms and non-linear phenomenon of a fragmented belt of rock fracturing have been investigated. Several new ideas can be used to construct a new system of rockmass fracture mechanics. Secondly, in the investigation of rock mass structure controlled rock mass fracturing, the importance of true mixed mode fracturing is emphasized. A primary criterion to predict the propagation of rock mass cracks has been established. Thirdly, in the investigation of non-singular stress effects on rock mass fracture, a new fracture criterion for the generalized crack opening displacement of rock is proposed. This can be used to analyze the influence of the normal stress and the crack orientation on the fracturing process in a rock mass. Finally, the rock mass fracture mechanics investigation and methodology was used to analyze the stability of the Mabukan high slope in the upper reaches of a large hydro-power station in the south-west of China. It is shown that the results of this study have created a more solid foundation for the application of rock mass fracture mechanics to rock engineering.     

An improved model for hydromechanical coupling during shearing of rock joints

This paper presents some experimental results from hydromechanical shear tests and an improved version of the original model suggested by Barton (Office of Nuclear Waste Isolation, Columbus, Ohio, ONWI-308, 1982. 96pp.) for the hydromechanical coupling of rock joints. The original model was developed for coupling between mechanical and hydraulic aperture change during normal loading and unloading. The method was also suggested for the coupling of shear dilation and hydraulic aperture changes. The improved model has the same appearance as the original and is based on hydromechanical shear experiments on granite rock joints. It includes both the mechanical and hydraulic aperture and the mobilised joint roughness coefficient (JRC_mob).     

复合温度条件下石灰岩多场耦合裂隙渗透侵蚀试验研究

深部岩石工程处在高应力、高水压力、高地热等复杂地质环境中,这些因素相互作用将对岩石的渗透特性产生重要影响,进而影响深部岩石工程的安全和生产效率。通过开展不同温度(25℃~90℃)条件下的同一块石灰岩裂隙多场耦合渗透特性变化试验,得到了温度因素对石灰岩裂隙渗透特性的影响规律。试验结果表明:在恒定有效压力作用下,升温阶段的初始时刻有一个流量峰值过程,温度恒定时,流量缓慢减小并最终趋于稳定状态;温度升高使得岩石裂隙渗透率单调下降,裂隙开度进一步减小;此外,温度越高,初始阶段裂隙开度闭合速度快,趋于稳定开度值历时越短且最终稳定开度值越小。石灰岩的侵蚀溶解速度随温度的升高而加快,裂隙面溶解出的矿物质增多,因此,渗出液中各离子的浓度随温度的升高变大。     

The hydromechanical behaviour of a fracture: an in situ experimental case study

Relationships between flow distributions and mechanical opening in a single natural fracture are investigated in situ through field experiments, at a scale of about 1 m, in a granitic quarry. Experiments have been conducted at various injection flow-rates while the normal stress applied to the fracture was controlled by hydraulic flat jacks. Variations of the collected flow-rate (monitored through contiguous flow collectors distributed along the fracture periphery) with the injection pressure are fully reproducible. They show that the fracture opens only above a threshold pressure which increases with the externally applied stress. This threshold is non-zero with no applied flat jacks pressure which raises questions on the reliability of hydraulic jacking techniques for the measurement of the normal stress on preexisting fractures. It is shown that equivalent hydraulic aperture and mechanical opening are comparable only above a critical mean fracture opening estimated to be around 15–20 μm for the tested granite. For mean fracture openings smaller than this value, the standard time scale used for stress measurements distorts the results. It is also shown that channeling effects may control flow away from the injection hole so that the hydraulic jacking stress measurement technique may overestimate the mean normal stress acting on the fracture plane by as much as 4 MPa. It is concluded that hydraulic testing techniques for normal stress measurements should not include results from the fracture opening phase. Moreover, criteria should be established for validating results from the closing phase in order to demonstrate the absence of channelling effects. Finally, it is shown that, because of the elastic response of the rock, water injection in a fracture system decreases the interstitial pressure ahead of the increasing pressure front associated with the water flow.     

Influence of the applied pressure waveform on the dynamic fracture processes in rock

Dynamic fracture process analyses for different waveforms of borehole pressure are conducted using a proposed numerical simulation method in order to verify the dynamic fracture mechanism related to blast-induced borehole breakdown. The fracture processes are affected more by the rise time increases than by the decay time. A higher stress-loading rate increases the number of radial cracks and leads to intense stress release around running cracks. The stress release caused by adjacent cracks interferes with crack extension and results in shorter crack propagation. At lower stress-loading rates, the number of cracks and the crack arrest caused by the stress released at adjacent cracks are reduced, leading to longer crack extension. These analyses reveal that the earlier preferential crack development occurs the greater the extension of the crack. The dynamic fracture process analyses are extended to investigate the influence of the waveform of applied pressure on the dynamic fracture process in a free face model. These fracture processes reveal that crack extension increases with the rise time increase, and that when the rise time is sufficiently long, crack extension depends, predominantly, on the rise time and the peak value of applied pressure. Crack arrest occurs after the peak phase of the stress wave in all cases. The effects of rock inhomogeneity on fracture pattern are discussed.     

Deformation and mechanical properties of rock: effect of hydromechanical coupling under unloading conditions

Bulletin of Engineering Geology and the Environment - Mechanical behavior and deformation characteristics of rock under unloading conditions are of particular significance for analyzing...     

Quantifying progressive pre-peak brittle fracture damage in rock during uniaxial compression

This paper presents the findings of an extensive laboratory investigation into the identification and quantification of stress-induced brittle fracture damage in rock. By integrating the use of strain gauge measurements and acoustic emission monitoring, a rigorous methodology has been developed to aid in the identification and characterization of brittle fracture processes induced through uniaxial compressive loading. Results derived from monocyclic loading tests demonstrate that damage and the subsequent deformation characteristics of the damaged rock can be easily quantified by normalizing the stresses and strains observed in progression from one stage of crack development to another. Results of this analysis show that the crack initiation, σ_ci, and crack damage, σ_cd, thresholds for pink Lac du Bonnet granite occur at 0.39σ_UCS and 0.75σ_UCS, respectively. Acoustic emissions from these tests were found to provide a direct measure of the rapid release of energy associated with damage-related mechanisms. Simplified models describing the loss of cohesion and the subsequent development of microfractures leading up to unstable crack propagation were derived using normalized acoustic emission rates. Damage-controlled cyclic loading tests were subsequently used to examine the effects of accumulating fracture damage and its influence on altering the deformation characteristics of the rock. These tests revealed that two distinct failure processes involving the progressive development of the microfracture network, may occur depending on whether the applied cyclic loads exceed or are restrained by the crack damage stress threshold.     

Indentation hardness testing of rock

The indentation testing was used for characterisation of hardness of rock materials. During the test, an indentor under applied load penetrated into the rock surface forming a crater. The testing procedure was developed to be in line with other ISRM Suggested Methods. The results show that standardised indentation testing allows for characterisation of mechanical properties of rock and that there is a relationship between the value of the indentation hardens index and the uniaxial compressive strength. The value of the calculated index was used to classify the hardness of rock.     

Silica sol for rock grouting: Laboratory testing of strength,fracture behaviour and hydraulic conductivity

A recently introduced non-cementitious grout silica sol is a refined product of colloidal silica, where the particle sizes have been reduced to between 5 and 100 nm. Laboratory tests were performed to determine the behaviour of silica sol as a permeation grout in hard rock. The tests have involved methods such as fall-cone, unconfined compression, triaxial, and oedometer tests. Samples were tested at different time intervals and in different storage environments. Results showed that the initial strength of silica sol, a few kPa, increases over time. Silica sol has a ductile behaviour during the first few days and then becomes elastic–plastic. Its hydraulic conductivity ranges from 10^?10 to 10^?11 m/s. When immersed in water, silica sol hardens and a thin layer of weaker strength is formed at the surface. However, this layer only extends a couple of millimetres into the sample; beyond that the silica sol is not affected, rendering breakdown by erosion a negligible risk. The conclusions are: (1) the strength obtained in silica sol after hardening is sufficient to withstand most grouting conditions; (2) when sufficiently confined, silica sol is able to withstand loading and unloading cycles; (3) a pH environment of around 11 does not appreciably change the strength of the silica sol; (4) silica sol is a material with low risk of failure under blasting vibrations; and (5) due to its low hydraulic conductivity, silica sol can be compared to low permeable clays.     

Experimental and numerical study of the geometrical and hydraulic characteristics of a single rock fracture during shear

Coupled shear-flow tests were conducted on two artificial rock fractures with natural rock fracture characteristics under constant normal loading boundary conditions. Numerical simulations using the 3-D Navier–Stokes equations taking account of the inertial effects of fluid were conducted using the void space geometry models obtained from the coupled shear-flow tests. The test and numerical simulation results show that the evolutions of geometrical and hydraulic characteristics of rock fracture exhibit a three-stage behavior. Transmissivity of a certain void space geometry within a fracture is related to the Reynolds number of fluid flow due to the inertial effects of fluid, which can be represented by the Navier–Stokes equations, but cannot be represented by some simplified equations, such as the cubic law, the Reynolds equation or the Stokes equations. The mechanical aperture is usually larger than the hydraulic aperture back-calculated from measured flow rate, and the difference between them is found strongly related to the geometrical characteristics of the fractures. A mathematical equation is proposed to describe the relation between hydraulic aperture and mechanical aperture by means of the ratio of the standard deviation of local mechanical aperture to its mean value, the standard deviation of local slope of fracture surface and Reynolds number.     

Multifractal characterization of rock fracture surfaces

The multifractal behavior of rock fracture surfaces is studied taking into account fractal heterogeneity and anisotropy of surface structures. A projective covering method (PCM) is proposed and used to estimate directly the fractal dimension D_s[2, 3) of a fracture surface. The study indicates that the multifractal spectrum of the fracture surfaces provide much additional information on the fracture mechanism and the distribution of asperity concentration on the surface. The fracture surfaces induced in rocks under different failure mechanisms display distinct multifractal behavior.     

岩石中断裂力学的研究

在断裂力学的基础上,研究了岩石破坏类型和受压裂缝的扩展,并对微裂缝演化进行了探讨,提出利用断裂力学中的裂纹尖端应力和应变场的分布情况,可以预测和制止岩体的失稳。基于能量平衡建立岩石裂纹的扩展条件,进而导出断裂稳定性准则。指出岩石断裂力学中存在的一些问题,并对研究要点进行了总结,为岩石断裂问题研究提供了理论依据。     

Numerical simulation of thermal-hydrologic-mechanical-chemical processes in deformable,fractured porous media

A method is introduced to couple the thermal (T), hydrologic (H), and chemical precipitation/dissolution (C) capabilities of TOUGHREACT with the mechanical (M) framework of FLAC^3D to examine THMC processes in deformable, fractured porous media. The combined influence of stress-driven asperity dissolution, thermal-hydro-mechanical asperity compaction/dilation, and mineral precipitation/dissolution alter the permeability of fractures during thermal, hydraulic, and chemical stimulation. Fracture and matrix are mechanically linked through linear, dual-porosity poroelasticity. Stress-dissolution effects are driven by augmented effective stresses incrementally defined at steady state with feedbacks to the transport system as a mass source, and to the mechanical system as an equivalent chemical strain. Porosity, permeability, stiffness, and chemical composition may be spatially heterogeneous and evolve with local temperature, effective stress and chemical potential. Changes in total stress generate undrained fluid pressure increments which are passed from the mechanical analysis to the transport logic with a correction to enforce conservation of fluid mass. Analytical comparisons confirm the ability of the model to represent the rapid, undrained response of the fluid-mechanical system to mechanical loading. We then focus on a full thermal loading/unloading cycle of a constrained fractured mass and follow irreversible alteration in in-situ stress and permeability resulting from both mechanical and chemical effects. A subsequent paper [Taron J, Elsworth D. Thermal-hydrologic-mechanical-chemical processes in the evolution of engineered geothermal reservoirs. Int J Rock Mech Min Sci 2009; this issue, doi:10.1016/j.ijrmms.2009.01.007] follows the evolution of mechanical and transport properties in an EGS reservoir, and outlines in greater detail the strength of coupling between THMC mechanisms.     

A testing system to understand rock fracturing processes induced by different dynamic disturbances under true triaxial compression

In this context, a testing system to understand rock fracturing processes induced by different dynamic disturbances under true triaxial compression was developed. The system is mainly composed of a static loading subsystem, a dynamic loading subsystem, a specimen box subsystem, and a data measurement subsystem. The static loading subsystem uses low stiffness loss frame structure technology, which greatly improves the frame stiffness in the three principal stress directions (up to 20 GN/m) and ensures the demand of the disturbance experiment in both the prepeak and postpeak stages. The disturbance loads with frequency of 0–20 Hz and stress level of 0–30 MPa were applied using large flow parallel oil source technology characterized with high heat dissipation efficiency. For the disturbance loads with frequency of 100–500 Hz and stress level of 0–30 MPa, they were realized by using high-frequency and centimeter-per-second-scale low-speed disturbance rod technology. Three rigid self-stabilizing specimen boxes were utilized to provide support for the specimen and deformation sensors, ensuring the stability and accuracy of the data obtained. To verify the performance of the true triaxial test system, disturbance experiments were conducted on granite specimens. The results show that the experimental device satisfies the requirements of original design, with an excellent repeatability and reliable testing results.     

Laboratory testing of a new type of energy absorbing rock bolt

Energy absorbing rock bolts are used as part of rock support systems in underground constructions that are exposed to e.g., rock bursts and detonating explosives. A rock bolt capable of absorbing kinetic energy from these loads must be able to yield with the ground movements and also deform plastically over large distances, at high displacement rates. A new type of energy absorbing rock bolt has been developed and tested in laboratory. The bolt is without a casing and consists of a steel bar that has an inner ribbed-like anchorage section and an outer nut that transfers the load from the rock via a circular disc. When subjected to a dynamic load, the lengthening of the steel bar leads to a decrease in diameter whereby the adhesive bond between bar and grout is lost and the outer end of the bolt is free to yield. The rock bolt is given a very good protection from corrosion when fully grouted in cement. In a laboratory, rock bolts in concrete cylinders were subjected to free fall tests to achieve a loading velocity of 10 m/s. The tests demonstrated that the distribution of plastic strain along the length of a grouted rock bolt is not constant when dynamically loaded. The sections where plastic yielding was allowed were not fully utilized in any of the cases, opposite to that in previous static tests which show almost constant elongation of the bolts. The tests also verified that the load-carrying components of the bolt, the nut and the anchorage, are reliable when dynamically loaded. Elastic and plastic waves will start to propagate through the rock bolt as it is suddenly loaded, resulting in permanent deformation along a section of the bolt. This yield process is demonstrated through a combined graphical and numerical method.     

Effects of loading rate on rock fracture

By means of a wedge loading applied to a short-rod rock fracture specimen tested with the MTS 810 or SHPB (split Hopkinson pressure bar), the fracture toughness of Fangshan gabbro and Fangshan marble was measured over a wide range of loading rates, =10⁻²–10⁶ MPa m^1/2 s⁻¹. In order to determine the dynamic fracture toughness of the rock as exactly as possible, the dynamic Moiré method and strain–gauge method were used in determining the critical time of dynamic fracture. The testing results indicated that the critical time was generally shorter than the transmitted wave peak time, and the differences between the two times had a weak increasing tendency with loading rates. The experimental results for rock fracture showed that the static fracture toughness K_Ic of the rock was nearly a constant, but the dynamic fracture toughness K_Id of the rock ( ≥10⁴ MPa m^1/2 s⁻¹) increased with the loading rate, i.e. log(K_Id)=a log +b. Macroobservations for fractured rock specimens indicated that, in the section (which was perpendicular to the fracture surface) of a specimen loaded by a dynamic load, there was clear crack branching or bifurcation, and the higher the loading rate was, the more branching cracks occurred. Furthermore, at very high loading rates ( ≥10⁶ MPa m^1/2 s⁻¹) the rock specimen was broken into several fragments rather than only two halves. However, for a statically fractured specimen there was hardly any crack branching. Finally, some applications of this investigation in engineering practice are discussed.     

岩质边坡中预应力锚索抗拔试验研究

在某高危边坡治理工程开始时，通过对16m长600kN的预应力锚索的抗拔试验，对预应力锚索抗拔试验过程进行了详细的分析，并验证和调整了相关设计参数，调整后的设计采用了18m长的预应力锚索。     

Evaluation of cutting efficiency during TBM disc cutter excavation within a Korean granitic rock using linear-cutting-machine testing and photogrammetric measurement

In TBM excavation, estimation of cutting performance is of great importance in design stage as well as during construction. The performance is highly dependent on the geological conditions, i.e. characteristics of rock and discontinuities, and operational conditions, i.e. selection of cutter, cutting forces, cutter spacing, etc. For performance estimation, full scale test is most reliable and accurate since it takes full advantage of using real cutter and real size specimen. Linear cutting machine (LCM) is usually used for a full size test to evaluate the cutting performance. This paper presents the results of LCM tests carried out under various cutting conditions to assess the cutting performance of a TBM disc cutter for granitic rock in Korea. In LCM test, the excavated rock volume was determined by ShapeMetrix3D photogrammetric measurement system. This system was employed to ensure the accurate determination of cutting volume and subsequently calculated specific energy (SE). The optimum cutting condition for the Korean granitic rock was obtained at the minimum value of SE. In addition, three-dimensional numerical analysis was performed to simulate the rock cutting behavior in the LCM test. The results of the numerical simulation were closely comparable with the results of the LCM test. This study presents the cutting performance of a disc cutter by LCM test for a Korean granitic rock and demonstrates the applicability of numerical analysis as an alternative for the prediction of the cutting performance.