International Journal of Rock Mechanics and Mining Science & Geomechanics Abstracts

Published by Elsevier
Online ISSN: 0148-9062
Publications
Article
This paper describes the application of seismic techniques to two problems in rock mechanics. The seismic location of failure foci is discussed and some results of the location, to within 5 m, of the failure foci around longwall stopes between 2500 m and 3000 m below surface, are presented. The seismic measurement of the immediate subsidence of the surface above a deep level stope, following a failure near the stope, is described and discussed.
 
Article
In this paper buckling under plane-strain conditions due to a horizontal compression of an elastic, anisotropic half-space containing coplanar cracks at arbitrary locations is considered. A numerical procedure based on the Displacement Discontinuity Method is developed and is validated against the analytical solution of a half-space with a single crack. It is demonstrated that the critical buckling stress decreases dramatically as the distance between the free surface and the cracks diminishes.
 
Article
The development of HDR reservoirs in the Carnmenellis granite has involved HFSM to depths of up to 2.6 km in several test series. The results of these and other stress measurements, and the operational experience gained, are reviewed in terms of proposed developments at 6 km. Potential borehole tangential stresses are examined in detail. HFSM will not be able to provide complete stress information because of geological and equipment limitations. However, in conjunction with other techniques, notably borehole breakout analysis, sufficiently comprehensive results appear possible to the extent that HDR systems can be designed and modelled.
 
Article
The feasibility of excavating caverns of very large span for underground siting of nuclear power stations in Norway was investigated in the early 1970s. In the end, the 1994 Winter Olympic Games provided the necessary impetus for utilizing a very large engineered rock cavern and proving its general feasibility. The 62m span Olympic Ice Hockey Cavern was constructed in Gjøvik by Veidekke-Selmer JV in 1991. It is located in a jointed gneiss of average RQD = 67%. The Q-values range from 1 to 30, with a weighted mean of about 9, i.e. fair quality rock. The cavern has a rock cover of only 25–50m, thus posing challenging design problems. The investigations prior to construction included two types of rock stress measurements, cross-hole seismic tomography, geotechnical core logging, Q-system classification and numerical modelling with UDEC-BB. Predicted maximum deformations were 4–8 mm; these were surprisingly small due to the high horizontal stresses recorded. Extensometer (MPBX) installations from the surface prior to construction, precision surface levelling and MPBX installed from inside the cavern gave a combined measure of maximum deformations in the range 7–8 mm with the 62m span fully excavated, and three adjacent caverns for the Postal Services also completed. Permanent rock reinforcement based on the Norwegian method of tunnelling (NMT), consisted of 10cm wet process steel fibre reinforced shotcrete, and systematic bolting and cable bolting in alternating 2.5 and 5.0 m c/c patterns. Both the cables and bolting were untensioned and fully grouted.
 
Article
Fragmentation of unconfined cylindrical specimens of rock due to stress wave loading has been investigated using a Split Hopkinson Bar (SHB). Photographs were taken during the fracture process using a high-speed framing camera, and incident, reflected and transmitted stress pulses were recorded using a digital transient recorder. The energy absorbed by the specimen during the fragmentation process was evaluated from the recorded stress pulses. Two different rocks were studied, namely Bohus granite and Solenhofen limestone. The specimen length was 50 mm, and the diameter was 25 mm. The incident pulse durations were 50, 100 and 200 μs, and the amplitude was in the range of 0·5−4 kbar. The high-speed photographs show that fracture occurs as previously observed for other rocks using SHB-devices. Notably, the main crack orientation is axial, and the degree of fragmentation increases with increasing load. The energy absorption of the specimen increases markedly when the load applied approaches a certain value. For the Bohus granite specimens the critical load was found to be 1·8 times the static compressive strength, and for Solenhofen limestone it was 1·3 times the static compressive strength. Two simple specimen models were analyzed theoretically, namely, one linearly-elastic and one rigid-plastic. The first model gives the best agreement with experimental results if the degree of fragmentaion is low, the second model gives the best agreement if the degree of fragmentation is high. The rigid-plastic model explains the experimental observation that the specimen always absorbs less than half the incident pulse energy.
 
Article
A procedure is developed for obtaining the optimal spatial distribution of seismic stations in mines. The optimality criterion is derived from the theory of optimal experiment. An optimal seismic network is one such that the spatial distribution of the stations with respect to the hypocenter position of the seismic event gives the best possible precision for the hypocenter parameters. The approach allows, when needed, to take into consideration: (i) different probability of event occurrence at different mining areas; (ii) planning for the best estimates of selected parameter as for example epicenter coordinates; (iii) building a network that is optimal for location of seismic events within a specified range of seismic energy.Four applications, two hypothetical and two in actual mines in Canada and in South Africa, respectively, show that the proposed procedure for optimal distribution of seismic stations provides a powerful and useful tool.
 
Article
The background to the formulae in the two ISRM Suggested Methods for Determining the Fracture Toughness of Rock Material is presented. The Short Rod and the Chevron Bend are the recommended specimens. Both are easily made from core pieces. Their standard proportions and machining tolerances are justified. Values for stress intensity factors and compliances are given, with linear correction terms for specimens within the tolerance limits, an assessment of transverse tensile failure in the Short Rod arms is made and an expression for crack speed vs controlling displacement rate is derived. The influence of rate dependent material properties is discussed briefly.A basic brittle fracture toughness evaluation is suggested which only requires load registration. A correction for non-brittle rate independent energy dissipation at the crack tip is also derived and the necessary steps to evaluate it are discussed. It requires displacement measurements to be made during the testing.Finally specimen calibration and fracture toughness testing work are discussed in the perspective of testing accuracy.
 
Article
As part of a study of damage development and stress-memory effects, we have carried out a series of laboratory acoustic emission (AE) experiments on dry Darley Dale sandstone under triaxial compressive loads. In particular, we studied the shape of damage surfaces in stress space. Like the yield surface in the theory of plasticity, a damage surface is defined as the locus of points in 3D stress space beyond which additional permanent damage develops. The shape of a damage surface thus provides a means of characterising the state of damage corresponding to the state of stress to which the material has previously been subjected.In our experiments, which were limited to the cross-section of stress space where σ2 = σ3 = Pc (Pc being the confining pressure), the points of the damage surface were indicated by the onset of AE. The damage surface in (Pc, σ1)-space corresponding to an initial stress state (Pci, σ1i) is almost linear in the region where Pc > Pci. In the region where Pc < Pci the gradient of the damage surface becomes clearly larger. We compare the experimental results with predictions based on a 2D model of secondary microcrack (wing-crack) formation from frictional surfaces. According to this model, the damage surface is predicted to consist of two linear parts connected by a vertex (or “knee”) located at the point (Pci, σ1i). The comparison shows that the model, although relatively simple, explains many features of the experimentally observed damage surfaces. The results also have implications in terms of a stress-memory effect. If the initially applied stress state (Pci, σ1i) is not known, then it might in principle be derived from the position of the vertex in an experimentally measured damaged surface.
 
Article
An experimental tunnel was excavated at the 420-m depth at Atomic Energy of Canada Limited's (AECL) Underground Research Laboratory (URL) to investigate rock mass response to excavation-induced stress changes. The results show how the development of excavation-induced damage can be monitored and characterized remotely and non-destructively by examining acoustic emission/microseismicity (AE/MS). Geological mapping, conventional instrumentation and video camera monitoring were used to validate the seismic interpretation. In addition, we describe an AE/MS laboratory test designed to study thermally-induced damage from emplacement of a simulated nuclear waste container within the rock.
 
Article
A series of triaxial compression experiments have been conducted on dry Berea Sandstone at confining pressures of 20, 40, 60, 80, 100, and 138 MPa to examine the stress-induced compressional and shear wave anisotropies. The results indicate that a significant compressional wave anisotropy and a marked lateral shear wave birefringence develop during both dilatant brittle shear failure and during shear-enhanced compaction. These anisotropies decrease in tests at successively higher confining pressures. Compressional wave velocity anisotropy continues to increase with increasing strain during shear-enhanced compaction, an effect not observed after brittle shear failure. Polarized shear wavess parallel to the principal stress axis do not exhibit birefringence until after brittle shear failure.
 
Article
Three large blocks (52 × 52 × 22 cm) of Lac du Bonnet granite from the Cold Springs Quarry were tested under various load conditions to investigate the effect of stress path on the formation of damage around holes. Two blocks had throughgoing central holes, and were subjected to uniaxial and biaxial loads, respectively. Acoustic emission (AE) count rates were monitored during loading of these two blocks. The 3rd block initially had a partially drilled central hole and was subjected to a biaxial load. Following loading the hole was lengthened and then the sample was reloaded, until on the 4th cycle the hole was throughgoing. AE studies were used to determine the location and mechanism of microcracking during the load cycles on the incrementally drilled sample, in addition to the AE count rate. Results indicate that activity was associated both with fracturing close to the hole and with the boundary conditions of the applied load. Most AE events had either shear of complex source mechanism, though many of the events close to the hole were apparently implosive events. Notch formation initiated in this sample when the tangential stress in the hole was approx. 160 MPa, significantly higher than the unconfined compressive strength of the sample (260 MPa). The damage associated with the advancing hole was apparently quite localized and did not appear to have significantly affected notch development under these test conditions. Velocities measured parallel to the hole axis decreased following each load cycle; this decrease correlates primarily with the formation of large crack planes oriented parallel to the unconfined face.
 
Article
Excavation-induced microcrack damage was investigated in the sidewall of a 420-m-deep tunnel at the Underground Research Laboratory. Sixteen channels of 1.0 MHz, whole-waveform acoustic emission and ultrasonic velocity data were collected over a 3.5 week period, during which the tunnel was extended by two half-metre increments. Acoustic emission locations were concentrated near the sidewall free surface. Shear source mechanisms predominated. Compressional velocities were strongly anisotropic, with the maximum velocity parallel and minimum velocity orthogonal to the tunnel. Both compressional and shear velocities rose with depth into the tunnel wall.
 
Article
Recently, a pair of “Suggested Methods for Determining the fracture toughness of rock” have been proposed by the ISRM. The fracture testing methods specify two chevron-notched specimens, the chevron bend (CB) and the short rod (SR). In this study, fracture toughness tests according to the Suggested Testing Methods were conducted on CB and SR specimens of a granite. In addition to these specimens, straight-notched core specimens with three different geometries were tested, using the J-integral approach combined with an acoustic emission technique to determine the fracture toughness. Results from the two testing methods were compared with special reference to specimen geometry compatibility. The fracture toughness determined by the ISRM Suggested Methods showed a specimen geometry effect, and a possible reason for this result was discussed. In contrast, it was shown that the J-iintegral/AE method as applied to the straight-notched specimens provides a suitable evaluation procedure for core-based fracture toughness testing of rock.
 
Article
The application of acoustic emission to industrial ndt is hindered by a lack of exact knowledge of the significance of parameters such as total count, to characterize the emissions. These parameters, and any novel parameters which can be derived, must be critically re-examined for their relation to metallurgical, mechanical and geometrical characteristics of structures if progress is to be achieved. The author describes experimental and theoretical work to this end on emissions from plastic deformation and fracture of various test pieces. The author concludes from his findings that the problems of noise and transducer sensitivity seem to be surmountable.
 
Article
The determination of the in situ stresses in rock is an important factor in the proper design of any structure founded on or within the rock mass. Of the many techniques available for its determination (all having certain advantages and disadvantages), one based on noise measurement during deformation has not yet been explored. This paper represents a first attempt in this regard.The technique uses acoustic emission (AE) monitoring in conjunction with a borehole rock jack. This modified rock jack pressurizes the borehole walls and monitors both the induced deformation and the AE activity. The pressure, applied at a constant rate, should produce emissions at a uniform rate, unless the rock has been previously stressed. In this case, a discontinuously increased emission rate occurs beyond the previously applied stress. This test method has been performed on five massive blocks of quarry rock. These tests show that: •in a cyclic stress test, previous stress levels can be discerned by a sudden AE rate increase as the stress on a particular cycle exceeds the previous applied stress;•in three out of four constant stress rate tests, anomalous behaviour in the form of discontinuous AE rate increases were observed in the 3.0–4.0 MPa range;•other rate changes were observed (both increases and decreases) but were at significantly higher stress levels, i.e. above 12.5 MPa.The final device, which we call an “acoustic rock jack”, is being further tested in the field in boreholes and the AE concepts are being further evaluated and refined in laboratory studies.
 
SPECIFICATIONS OF TRANSDUCERS
TAPE RECORDER SPECIFICATIONS
SUMMARY OF ROCK TEST DATA
Complete circuit diagram for pulse counting system.  
Article
Acoustic emission is generated in stressed materials as a result of internal deformation and may, therefore, be used to examine the failure mechanisms of materials within the laboratory and in situ. The literature relating to acoustic emission is reviewed, particular attention being given to experimental systems for the detection, recording and analysis of such emissions, the interpretation of the acoustic record and the practical application of acoustic techniques in rock mechanics. An experimental acoustic system, developed for use both in the laboratory and the field, and the ancillary loading equipment are described. The methods of processing and analysing the acoustic record are illustrated by reference to a series of laboratory tests on a variety of rock samples. Various parameters are described, including pulse rate, signal energy content and amplitude distribution; the variation of these parameters during uniaxial loading of rock specimens is described.
 
Contour diagrams and 3-D isometric view displaying: (a) the number of events with respect to time difference and amplitude; and (b) the number of events with respect to amplitude and event duration for sample type B potash rock.
(a) Photograph of planar polished vertical section of potash rock type B displaying tectonic cracks. (b) SEM microphotograph of stage la HARC and LARC under moderate magnification, grain boundary junction consisting of intergranular clay particles. (c) SEM microphotograph of stage la HARC and LARC under higher magnification, the shape of the particles are indicative of agglomerate clay material. (d) SEM microphotograph of stage 3b intracrystalline LARC link A type crack sets, displaying en echelon and en passant crack interactions. These crack types are defined by Ref. [12]. (e) SEM microphotograph of intracrystalline LARC at an advance stage 3 load cycle. The left aspect of the photograph displays two parallel LARCs on a planar surface of a crystal grain. The right aspect shows several parallel LARCs on multiple fracture surfaces.
AE counts over 10 successive periods of increasing load levels calculated as a percer/tage of failure under uniaxial compression. Sample calculations are tabulated for small sample S ! B2
Article
Acoustic emission (AE) techniques under laboratory testing conditions are used to understand the mechanical behavior and deformational processes of uniaxially compressed potash rock. Results obtained from the AE measurements are compared to deformational features identified using optical and electron microscopy. Although the potash rock from Saskatoon can be divided into four types based on the compositional ratio between sylvite, halite, clay, textural homogeneity and structural interlocking, it is shown that the analysis of the AE data collected during a complete load cycle can provide a general picture of the AE behavior. Detailed analysis of AE parameters such as b-value, amplitude and event-duration distribution can be used to monitor progressive changes in crack development.
 
Article
Understanding how a propagating fracture interacts with existing fractures, lithologic boundaries and other material interfaces is essential to the interpretation of fracture network geometries. In this paper, a first order analysis of the stresses near a mode I fracture impinging upon a frictional interface oriented normal to the growing fracture results in a simple criterion that predicts whether a growing fracture will terminate at or cross the interface. The analysis uses the linear elastic fracture mechanics solution for the stresses near a fracture tip to determine the compressive stress required to prevent slip along the interface at the moment when the stress on the opposite side of the interface is sufficient to initiate a fracture. A series of experimental investigations designed to assess the conditions required for crossing are presented and shown to be consistent with the criterion. Using data from previously published experiments, the criterion is shown to accurately predict the occurrence of compressional crossing in nine different brittle materials including three types of synthetic materials and six types of natural rock.
 
Article
The principles of rock cutting are reviewed and differentiation made between linear and impact cutting. Theoretical aspects of cutting are discussed with particular reference to the tensile splitting theory. The role played by the rock fabric in relation to its cutting characteristics is examined. Consideration is given to how a rock's impact cutting property can be established and to this end a description is given of an experimental rig designed to test different rock types. Results from a study with the impact cutting rig are presented and discussed. It is concluded that this form of test is of great value in comparing the impact cutting properties of different rocks. The practical implications of the results are discussed, in particular their bearing on the design aspects of current rock cutting machines.
 
Article
A new model is introduced for rock bolt support of a single block. The model assumes that the bolt supports the block in much the same way that another block would. It therefore includes approximations to the geometric as well as the force constraints imposed by the bolt. Depending on its mode of support, the bolt either is represented as in the traditional model or else acts like a keystone to make the block geometrically stable.The latter analogy provides a useful computational test of the bolt's orientation, entailing only the addition of the appropriate fictitious fixed face into a stability analysis of the block. An associated procedure enables either the tension in the bolt or the factor of safety to be determined if the other has been specified. In some circumstances the lower values of these two quantities violate the block's equilibrium and are therefore physically unattainable, although they would be valid in the traditional model. But all other tensions and factors of safety are identical in both models.The paper also reviews a number of the block types in the existing Warburton and Goodman-Shi methods of analysis and points out the correspondence between them. This review leads to a reassessment of the roles of some types in supporting other blocks in an assemblage. In particular, the key blocks identified in Goodman and Shi's sample calculations could also be identified by Warburton's method, but support of all such blocks in an assemblage could not in practice be relied on to stabilize the excavation. Although blocks that could fill the role intended for key blocks undoubtedly do exist, there would generally not be enough information to identify them.Goodman and Shi's method can identify removable blocks as an intermediate type, but Warburton's method is more direct when the resultant driving forces (or even their directions) are known. However, other features of the two methods make Goodman and Shi's best suited to design and Warburton's to construction.
 
Article
This paper provides a review of the rock mechanics activities in Japan. After a short history of the development of rock mechanics geotechnical features of the geology and industrial activities relating to rock mechanics are indicated. In terms of the geotechnical features in mines and construction sites of important projects, recent research activities in several research institutes are briefly reviewed.The present state of rock mechanics education in departments of mining and civil engineering at the universities is given, and it is suggested that the interaction between various related special fields should be promoted more actively.The features and results of rock mechanics research activities in the field of mining and civil engineering during the last 5 years are classified into eight areas and described briefly. Many research activities in mining have been conducted for prediction and prevention of rock bursts and/or gas outbursts, while the most important developments and research activities in civil engineering have been accomplished in the field of tunnelling, that is, the effect of rock bolts and sprayed concrete as tunnel support have been evaluated by various field measurement techniques, and some design methods for tunnel support have been proposed. New fields in the application of rock mechanics are being developed and some new research projects are being actively conducted.
 
Article
Floor heave occurring in a roadway behind a longwall advancing face has been analysed and analytical predictions have been compared with observed field results. The heave analysis consists of first obtaining the ribside and goaf loading on the floor, for which a method to estimate the zone of ribside crushing and abutment pressure distribution has been given. The method of face element stress analysis has been used for obtaining the stresses resulting from the loading on the floor. The zone of failure in the floor can be then determined from safety factor contours. The correspondence principle of linear viscoelasticity was applied to the broken rock to give the time-dependent variation of floor lift with face advance. The final floor heave picture was obtained after superposing an apparent increase in the floor rock volume due to breaking. The influence of roadside pack width, load and face length on the extent of floor fracturing has also been studied.
 
Article
A number of cast iron tunnel lining segments have cracked in two sections of the London Underground between Old Street and Moorgate Stations that are located in the Palaeocene Woolwich and Reading Beds facies. There are also several slow seepages of highly corrosive acidic groundwater. Investigation of the ground has shown that water derived from the consolidation of the overlying London Clay is collecting in a lens of sand which is crossed by the tunnels. Such water, together with air from the tunnels, is oxidising pyrite in the sand lens to yield a sulphate-rich acidic water which is damaging the tunnel, both by corrosion of the cast iron lining and by producing expansive pressures from the precipitation of gypsum crystals, causing the tunnel to be squeezed horizontally and move up at the crown.
 
Article
This paper describes results of experimental research in which the fracture toughness of a granite is determined by means of the J-integral approach and an acoustic-emission (AE) technique. Compact tension specimens and three-point bend specimens of various sizes were tested, and the effects of specimen size and loading condition on the fracture toughness were examined. Concurrently with the fracture toughness tests. AE monitoring was conducted, and the relation between the AE characteristics and fracture behavior was examined. Energy measurements of AE signals and frequency spectral analyses were made to characterize the AE signals. The AE measurements were used to determine a fracture toughness evaluation point and to obtain the critical J-integral value, denoted by JiAE. It is demonstrated that the JiAE value is reasinably constant for a wide range of specimen sizes and specimen types. Based on the result, the fracture toughness of the granite KIc was then computed from the JiAE value using the conversion equation under the assumption of linear elastic response. A comparison of linear elastic fracture toughness values with KIc showed that the J-integral approach combined with the AE technique can be applied to determine a valid fracture toughness of the granite. Finally, a reason for the specimen size dependency observed in the linear elastic toughness value is discussed on the basis of the experimental results.
 
Article
To date, it has not been possible to take accurate in-situ measurements of the stress rise in the goaf as a result of longwall mining, due to the difficulty in maintaining contact between in-situ equipment and data loggers. For this reason, research into stress distributions within the goaf, loaded by overlying strata, has been addressed through numerical modelling, with a strain stiffening constitutive law for the goaf. The modelling results have been compared with existing methods and significant differences noted.
 
Article
Drilling, mining and blasting of rock-like materials, in situ fracturing, rock fragmentation size and permeability extension are strongly affected by strain rates of loading. For low strain rates, fragment sizes are large, and cracks propagate further. For high strain rates, fragment sizes are small and cracks are localized. The purpose of the present investigation is to determine the effects of strain rate on the initial (linear) value of Young's modulus and on Poisson's ratio for different grades of New Albany oil shale, a reference oil shale designated by the U.S. Department of Energy. Material anisotropy is considered. Cylindrical specimens cored in a particular orientation of the oil shale formation are tested in compression, and the rate effects are investigated. An efficient data acquisition/reduction system is developed, interfacing with an Instron closed-loop hydraulic testing machine. Strain gauge instrumentation is used to measure lateral and longitudinal strains. Strain rates of about 10−5–100/sec are studied.
 
Article
Theory as well as small- and large-scale experiments indicate that tunnel excavations in rock result in an increased axial permeability close to the tunnels. This is due to stress relief as well as to blasting, of which the former is generally assumed to be most important, at least when smooth blasting is applied. The Buffer Mass Test in Stripa offered a possibility of quantifying the increase in axial permeability and it was concluded from this experiment that the increase may have been as high as 1000–10,000 times in a 0.5–1.0 m wide zone adjacent to the excavation, assuming that the value 10−10 m/sec derived from the preceding, large-scale “Macropermeability Test” is representative of the hydraulic conductivity of the undisturbed rock. If this figure is correct, disturbance by blasting would be the major cause of the increased perviousness.
 
Article
Joints in a given rock mass are subjected to a wide variety of boundary conditions and also to various cycles of loading and unloading in both normal and shear directions. This paper presents an extension of the Saeb and Amadei model to take into account joint loading and unloading in both normal and shear directions. In the normal direction, the cyclic behavior is hyperbolic and the irrecoverable normal closure depends on the joint loading history. Concerning the shear direction change, two assumptions are supposed: the pre-peak behavior is elastic; and, during the residual behavior, the joint is smooth (all the asperities are crushed) and the shear band developed by these fragments is not taken into account. Finally the applicability of the proposed model to predict the behavior of a joint under loading-unloading paths is verified using existing experimental results.
 
Article
A compilation of natural and residual stress data from published and unpublished sources is presented. The data are primarily from the United States and Canada, and were obtained in the last twenty years. Measurement techniques in the compilation include overcoring using the U.S.B.M. borehole deformation gage, undercoring employing the South African doorstopper, photoelastic techniques, electrical strain gage methods and hydrofracture.The description of each measurement includes its location, environment, modulus of elasticity, depth and stress values as reported by the author. The original reference source is also listed.This compilation is the first step in a three-part analysis to better understand the current natural in-situ stress field in North America for engineering purposes.
 
Article
The amplitudes of shear waves transmitted across interfaces between granite samples during forced frictional sliding show a distinct difference between stable sliding and stick-slip movement. When the sliding is stable, the shear-wave amplitude decreases gradually and continuously. During stick-slip movement along the interfaces, a rapid decrease in shear wave amplitude occurs before or at the sudden slip of the interfaces. The changes in shear-wave amplitude during sliding and stick-slip events reflect the changes in the specific stiffness of the fracture.
 
Article
An analysis of ground-support interaction in a tunnel during excavation is presented. A time-dependent model taking into account the face advance effect is developed. A closed form solution for the wall displacements and the ground pressure acting on the lining is given for the case of a circular tunnel driven in an homogeneous and isotropic medium with time-dependent behaviour. A generalization of the “convergence confinement method”, including the time effects, is proposed. In the last chapter, this model is applied to the case of a real tunnel and it is shown how the time-dependent behaviour of the lining can be predicted.
 
Article
A solution is obtained for predicting tunnel water inflows for the commonly observed case of an exponential decrease of hydraulic conductivity with depth. The analytical solution is applicable to the prediction of ubiquitous inflows from a jointed rock mass, and the risk of major isolated inflows from singular conductors such as faults must be separately evaluated.The predictions are verified by finite-element modelling and compared with published inflows for tunnels in a variety of rock conditions. The new solution predicts the commonly observed trend that inflows at first increase then decrease as a tunnel goes deeper, rather than continue to increase as predicted by Goodman's solution where the hydraulic conductivity is assumed constant. Goodman's solution can underestimated inflow by 30% or overestimate it by an order of magnitude, depending on the depth at which hydraulic conductivity is measured.Upper and lower bound predictions corresponding to extreme values of stress-independent factors such as an increase of joint spacing with depth are examined for a range of typical tunnel characteristics. The differences are found to be insignificant when the hydraulic conductivity gradient A is small, but increase to 30% A = 0.01. An]estimate mid-way between upper an lower bound values gives a maximum prediction error of about ±15% which is likely to be adequate in most practical applications.The analytical and finite element models of both predict that inflows are insensitive to tunnel diameter (maximum 2% increase in flowrate per metre increase in diameter). This agrees with the observation that inflows during the driving of a full-bore tunnel can be similar to those from a pilot tunnel or exploratory drillhole ahead of the face.
 
Article
Over the last few years, a number of research papers have been published which propose the use of mathematical models to describe the rock blasting process. These models consider a large number of physical processes to be important; however, this paper concentrates on the treatment of the stress field within the rock. Previously this has been modelled both by extrapolation of simple situations and by numerical solutions for stresses throughout the rock mass. In this paper, an improved model of the stress field around a cylindrical borehole with radial cracks is introduced. This model takes the form of an analytical expression for the crack height as a functional of the gas pressure and the in situ stress. The model is valid for the physically interesting cases where the rock acts as a linear elastic material, there are a large number of radial fractures and the rock is held together by in situ stresses and fracture toughness. It accounts for cracks opening due to stresses created both by the borehole gas and by the gas in the fractures, as well as determining the change in the borehole size as the stress distribution changes. To illustrate the behaviour of this model some very simple idealized situations are considered which allow analytical expressions to be derived for the crack propagation and the gas motion. These expressions show that the improved model predicts important effects such as the cracks tips starting ahead of the gas front, the gas front catching up with the crack tips, the borehole radius reducing as the stress field relaxes and the rapid reduction in borehole pressure.
 
Article
The present paper examines the linear elastostatic problem related to a cylindrical elastic anchor which is embedded in partial bonded contact with an elastic medium. The base of the elastic anchor contains a flat circular crack, the boundary of which extends beyond the elastic anchor. The analysis focusses on the utilization of the boundary element technique for the evaluation of the axial stiffness of the anchor and the stress intensity factors at the boundary of the crack.
 
Article
The problem of axially symmetric loading of a spheroidal rigid inclusion embedded in bonded contact with an isotropic linear viscoelastic infinite medium is investigated. This particular problem is of interest in connection with the analysis of the time-dependent anchorage efficiency of deep ground anchors and rock anchors embedded in soil and rock media susceptible to creep. The solution to the spheroidal rigid anchor problem pertaining to a linear viscoelastic medium is facilitated by the application of the Laplace transform-based correspondence principle. The transformed viscoelasticity problem is then solved by the use of Boussinesq's three-function approach developed for the solution of three-dimensional problems in classical elasticity. For the purposes of illustration, the particular viscoelastic material behavior is restricted to one which exhibits a dilatational response which is elastic and a deviatoric response which is that of a standard linear solid. Explicit analytical results are presented for anchors of both prolate and oblate spheroidal shapes which are subjected to either loads or displacements which vary as step functions of time. Numerical results are presented for cases where the rigid anchor embedded in typical geological materials is subjected to a step function of displacement. These results illustrate the manner in which the tension in the anchor rod can be influenced by the creep of the geological material in the vicinity of the anchor region.
 
Article
The paper reports the findings of a laboratory study on six different types of cement-based grouts and two types of steel rock anchors. The physical and mechanical characteristics of cement grouts employing silica fume, aluminium powder, superplasticizer or sand are compared with those of conventional cement grouts. Pullout tests of grouted, 7-strand steel cable and solid steel thereadbar were conducted under similar conditions for different grouts and embedment lengths. From the results obtained, an empirical equation is derived for the estimation of anchor pullout resistance for a given embedment length. A simple trilinear constitutive model for shear bond stress-slip relation at the anchor-grout interface is proposed and discussed.
 
Article
A literature survey has revealed that current rock anchor design methods have been empirically derived and are conservative in nature. The design stability of a rock anchor depends on an assumed yield surface which in practice is difficult to predict. Additionally, rock parameters such as shear stress at the yield surface are commonly ignored. In the fixed anchor design, although both theoretical and experimental evidence indicates bond distribution to be nonlinear at the tendon/grout/rock interfaces, current design still assumes the bond to be uniformly distributed.To provide a more fundamental approach to rock anchor design a simple isoparametric finite element (FE) program has been developed. The stress-strain pattern in the fixed anchor zone has been obtained for a rock/grout modular ratio of 1:2 and the results obtained are compared with those derived experimentally. The results show the most severe position of shearing to be at the rock/grout interface and that partial debonding is not a serious problem.Having shown that the FE method can be used to successfully predict rock anchor behaviour, the authors recommend further refinement of the numerical solution. The need for further experimental research work to investigate the load transfer mechanism in a wide range of rock types to obtain full validation of the approach is stressed, as is the dependence of the solution on accurate determination of the properties of the host rock.
 
Article
Hydraulic fracturing at the Hot Dry Rock Geothermal Project in Cornwall stimulated extensive cracking in the Carnenellis granite. The acoustic emissions were monitored by a surface network of three-component seismometers which recorded acoustic events with double-couple mechanisms indicating the release of significant shear stress. Three-component shear wavetrains display splitting diagnostic of crack-induced anisotropy. Equal-area projections of the shear-wave polarizations display patterns characteristic of propagation through parallel vertical liquid-filled microcracks, where the orientations of the leading shear-wave arrivals are parallel to the strike of the vertical cracks. This strike is confirmed by the elongated distribution of the acoustic events, and indicates the stress direction at the depth of the fractures. This direction is approximately parallel to one of the sets of joints at the surface and si 20° away from the axis of compression measured about 10km northwest of the HDR site.
 
Article
This paper presents closed-form solutions for the stress field induced by gravity in anisotropic rock masses. These rocks are assumed to be laterally restrained and are modelled as a homogeneous, orthotropic or transversely isotropic, linearly elastic material. The analysis, constrained by the thermodynamic requirement that strain energy be positive definite, gives the following important result: inclusion of anisotropy broadens the range of permissible values of gravity-induced horizontal stresses. In fact, for some ranges of anisotropic rock properties, it is thermodynamically admissible for gravity-induced horizontal stresses to exceed the vertical stress component; this is not possible for the classical isotropic solution. Specific examples are presented to explore the nature of the gravity-induced stress field in anisotropic rocks and its dependence on the type, degree and orientation of anisotropy with respect to the horizontal ground surface.
 
Article
The geological structural features of a rock mass not only affect its strength and deformability but also its cuttability and drillability. Results of wedge penetration tests and disc cutting experiments on rocks with distinctive strength anisotropy clearly show that anisotropy and discontinuities have to be fully taken into account to attain a realistic prediction of the net advance rate of a full-face tunnelling machine. Based on a simplified theoretical analysis and the experimentally proven assumption that tensile rather than shear failure is the dominant chip forming mechanism, equations for the prediction of disc cutter performance in bedded and schistose rocks are derived.
 
Article
The anisotropy and stress-dependency of the strength and deformability of rock joints were investigated experimentally through shear tests of concrete replicas of natural rock joints. 50 concrete replicas of two natural granite joints were sheared on a servo-controlled hydraulic test frame under different magnitudes of normal stress and in different shear directions. Significant anisotropy in both the friction angle and shear stiffness of concrete replicas of joints were observed and both of them change with the variation of normal stress. The rates of dilatancy and contraction of joint samples appear to be different during cyclic shear tests, depending very much on the initial conditions of sample surfaces. Empirical relations were then postulated to describe these newly found properties of joint samples and used in developing new constitutive models for rock joints.
 
Article
Foliated metamorphic rocks and laminated, stratified or bedded sedimentary rocks have properties (physical, dynamic, thermal, mechanical, hydraulic) that vary with direction and are said to be anisotropic. Rock mass anisotropy can be found in volcanic formations and sedimentary formations consisting of alternating layers or beds of different rock types. Rock masses cut by one or several regularly spaced joint sets are anisotropic in addition to being discontinuous. This paper deals with the subjects of rock anisotropy and rock stress. Both topics are important in rock engineering and are interrelated. Rock fabric controls the build-up in in situ stresses in the Earth's crust, their magnitude and orientation. On the other hand, stresses and in particular compressive stresses tend to close microcracks or discontinuities thus making rock behavior non-linear and rock anisotropy pressure dependent. This paper is divided into three parts. In the first part, existing models to describe the deformability properties of anisotropic rocks as well as the laboratory and field methods to determine those properties are reviewd. Then, it is shown how to account for both rock anisotropy (intact and joint induced) and the curvature of the Earth when estimating in situ stresses in rock masses. Finally, the effect of anisotropy on the analysis of overcoring measurements with cells similar to the CSIR Triaxial Strain Cell is discussed.
 
Article
Ultrasonic P- and S-wave velocities have been measured for propagation in three orthogonal directions through a sample of Berea sandstone as a function of maximum compressive stress applied perpendicular to the bedding plane. The sample was a 50mm cube and was stressed to peak in a new triaxial loading frame, with the principal stress components parallel to the bedding plane held constant at 4.1 MPa. The measured velocities are compared with velocities calculated for a medium containing an anisotropic orientation distribution of microcracks. With a recently developed technique, the P- and S-wave measurements can be inverted separately for the microcrack density and orientation distribution, thus enabling a consistency check to be made on current theories of elastic wave scattering by microcracks and fractures. Inversion of the measured velocities to obtain the microcrack orientation distribution function suggests the growth of cracks parallel to the direction of the maximum compressive stress. This supports the current view that failure is preceded by the growth of microcracks parallel to the direction of the maximum principal compressive stress. The large stress-dependent changes in ultrasonic velocities observed suggest the usefulness of ultrasonics for studying damage processes in sedimentary rocks.
 
Article
A novel approach using digital processing is described for the study of shear-wave velocities and attenuation anisotropy in some sedimentary rocks and a slate. Broadband ultrasonic pulses in the frequency range 0.3–0.6 MHz for polarized shear waves are transmitted through the specimen to be characterized for comparison with a reference (aluminium) having low attenuation. The attenuation is calculated in terms of the quality factor Q from the Fourier spectral ratios, using a digital signal processing technique. Experimental results are presented and discussed for measurements made on two anisotropic rocks (a slate and a sandstone) and isotropic samples (a limestone and a granite). The observed attenuation anisotropy is generally accompanied by anisotropy in velocity. Compared with the magnitude of the velocity, the magnitude of the shear wave attenuation is more sensitive to changes in anisotropy and grain size. These laboratory observations are expected to be helpful in the interpretation of seismic and acoustic well logging data.
 
Article
The aim of the experimental work presented in the paper has been to carry out flow experiments and aperture measurements on the same specimen of a single natural fracture, in order to compare measured flow with predicted flow based on geometrical description of the fracture void space. A technique to measure the aperture was developed utilizing injection of fluorescent epoxy in the fracture specimen. An image analysis system was used to take measurements along sections across the fracture surface. The method was successfully applied to a fracture in granite having a mean aperture of 360 μm at 0.45 MPa normal stress. The spatial correlation of the aperture was about 1 cm. The predicted and measured flow through the fracture specimen are in good agreement.
 
Article
Pressure-test data have been used to estimate the average spacing and the average aperture of water-conducting fractures in undisturbed rock masses. Discharges from packer injection tests show skewed distributions when adjusted to uniform test conditions. Each distribution is a compound of (i), the distribution of numbers of fractures intersected by drill-holes in the intervals between packers, and (ii) the distribution of fracture apertures. The numbers are approximately Poisson-distributed in structurally-uniform rock, thus λ, the mean and variance of fracture frequency, can be estimated from the frequency of intersecting no water-conducting fractures, which is the observed frequency of zero-discharges. Once λ is known, the mean aperture and standard deviation of apertures of equivalent parallel-plate openings can be computed from the parameters of the discharge distribution.Apertures, measured on outcrops by photographing fractures treated with a fluorescent dye penetrant, are lognormally distributed. If the lognormal shape prevails also in undisturbed rocks, then the mean and standard deviation computed from water tests may also disclose fracture sizes at any specified depth in a rock body.
 
Article
Synthetic records of fractional Brownian motion (fBm) with known dimensions were generated using random midpoint displacement, spectral synthesis and deterministic fractal geometry methods. The validity of the methods used to measure the fractal dimensions of self-affine records was evaluated by examining the calculated dimensions of the synthetic fBm records with known fractal dimensions. Two methods of fractal analysis (i.e. variogram and periodogram) were employed to determine the fractal dimensions of the profiles. Analysis of periodograms of fBm records revealed a higher correlation between the expected and calculated dimensions of pre-whitened data. Variograms of those fBm records indicated a better accuracy while smaller lag (i.e. approx. 10% of the profile length) of the synthetic profile length was used for determination.Laser profilometry was utilized as well to provide the measurement of joint roughness. Several natural rock joints were cored parallel to the joint faces. The cores were then analyzed with a laser profilometer for asperity measurement. The fractal dimensions of the natural joint profiles determined by using variograms and periodograms ranged from 1.3 to 1.4. The periodogram method, however, produced slightly higher dimensions than did the variogram method. Synthesized joint profiles of known dimensions were generated using fractal interpolation functions to provide a qualitative comparison with the natural joints. The computer generated profiles were found to mimic many of the visual characteristics of natural rock joints.
 
Article
The study of coupled thermal, mechanical and hydraulic phenomena in fractured rock mass like granite is of major importance for underground wastes repositories. There exist two possible approaches to model a highly fractured rock mass: either represent each fracture individually or use an equivalent continuum. These two approaches have been compared in the frame of a bench mark test (BMT1) on an hypothetic regularly fractured medium hosting nuclear wastes. The methodologies, as well as the algorithms used by the various research teams who participated in the exercise, are presented in detail. The thermal and mechanical results compare rather well, while hydraulic results show pronounced differences which can be partly attributed to the non-linear dependance of the fracture hydraulic conductivity in terms of the aperture.
 
Article
The triaxial test technique represents a convenient form of laboratory testing for the evaluation of both soil and rock parameters for engineering design purposes. Although soil mechanics literature examines in considerable detail the many factors which influence the results obtained, the same depth of understanding cannot be claimed for rock materials.A detailed investigation was undertaken to evaluate the influence of a number of important factors on the results obtained during triaxial testing of a saturated soft rock, namely the Silurian mudstone of Melbourne. Consideration is given to the appropriate triaxial equipment and associated measuring systems for the determination of stresses, strains and pore-water pressures. The preparation of test specimens and their geometry are also examined. Finally, the rates of strain application are investigated in some detail to show that this variable can have a significant influence on the soft rock response and therefore the parameters measured. By considering the influence of these factors in conjunction with the basic soft rock characteristics, it is demonstrated that engineering properties independent of procedures may be derived from the triaxial test.
 
Article
A computational is derived to analyse the stability of a single three-dimensional rock block. It is assumed that the block is rigid and that it is located in an otherwise fixed body of rock and bounded by a combination of flat discontinuities and excavation surfaces. In common with most other solutions for three-dimensional blocks, possible block movements are assumed to be limited to translation only, and rotation is excluded.The block geometries that can be handled by the new procedure are more complex and realistic than those in existing solutions. To start with, the block can be an arbitrary polyhedron with various re-entrant surface features, such as notches and cavities. In addition, any number of its faces may be free and exposed at excavation surfaces.The computational procedure is based on a vector analysis of the block's stability. Part of the computation tests the geometrical configuration of the fixed faces initially in contact with the block to find whether they permit it to move. If so, the procedure can determine the nature and direction of the attempted movement and can usually calculate a factor of safety indicating the likelihood that such movement will be prevented by friction.A method is also outlined for calculating areas of block faces, together with the block's volume and centre of mass. These quantities are relevant to a few of the parameters in the stability analysis.
 
Top-cited authors
Nick Ryland Barton
  • Nick Barton & Associates
Hans Wackernagel
  • MINES ParisTech
Stavros Bandis
  • Aristotle University of Thessaloniki
A.C. Lumsden
  • University of Leeds
Neil Chandler