Intact Rock Properties Research Articles

Improved concept of lithospheric strength and earthquake activity at shallow depths based upon the fan-head dynamic shear rupture mechanism

The typical depth–frequency distribution of earthquake hypocentres (DFDE) demonstrates that, below an upper cutoff, the earthquake frequency increases with depth up to a maximum value and then decreases and ceases at a lower cutoff. Such regular behaviour of earthquakes implies the existence of some fundamental mechanisms responsible for the distribution. Conventional models of lithospheric strength based upon the assumption that the frictional strength along pre-existing faults represents a lower limit on the rock shear strength do not provide any intrinsic logic for the observed DFDE. The paper shows that these models ignore the specific properties of intact hard rocks which can exhibit extremely low transient strength (significantly lower than the frictional strength) during failure under the high confining stresses corresponding to seismogenic depths. The low transient strength is provided by a recently identified fan-head shear rupture mechanism which can be initiated in intact rocks in the proximity of pre-existing faults. The low transient shear strength of intact rock determines the correspondingly low transient strength of the lithosphere, which favours generation of new earthquake faults in the intact rock mass adjoining pre-existing faults in preference to frictional stick-slip instability along these faults. The efficiency of the fan-mechanism within the seismogenic layer is variable, with maximum efficiency at the middle range between the upper and lower cutoffs, thus providing minimum transient strength of the lithosphere and maximum earthquake frequency at that depth. We believe that this intrinsic property of hard rocks is responsible for the observed DFDE. Importantly, the formation of new faults in intact rock generated by the fan-mechanism can be accompanied by very small stress-drops (similar to, or lower than, stress-drops for frictional stick-slip instability) combined with abnormally high energy release. The paper proposes an improved concept of lithospheric strength and earthquake activity at seismogenic depths.

Correlating physico-mechanical properties of intact rocks with P-wave velocity

The physico-mechanical properties of five different intact rock types including sandstone, arkoses and limestone were determined through standardized laboratory tests. Ninety six specimens were tested to obtain the relationships between P-wave velocity (Vp), dry unit weight (DUW), uniaxial compressive strength (UCS), point load index Is(50), Brazilian tensile strength (TS), porosity (Φ), and Schmidt hardness (RN). This study also reviews some recent correlations between P-wave velocity and physico-mechanical properties as well as investigates the reliability of these correlations. Findings show the results of the experimental tests are in good agreement with previous studies. Statistical equations have been determined for estimating the physico-mechanical properties of rocks using nondestructive and indirect test methods. Results of regression analysis showed satisfactory correlations. Based on the results, new strong correlation with correlation coeffecients above (R2 > 0.80) are introduced for predicting the UCS, Is(50), Φ and RN and reasonable good correlations (R2 ≥ 0.78) are introduced to predict TS and DUW from P-wave velocity of different intact rock core samples. There is a discrepancy between P-wave velocity (Vp) in situ values with laboratory results. The large reductions in Vp in situ values are clearly the functions of fractures and natural joints.

Prediction of Rock Mass Deformations in Three Dimensions for a Part of an Open Pit Mine and Comparison with Field Deformation Monitoring Data

The intact rock properties and discontinuity properties for both DRC and DP rock formations that exist in the selected open pit mine were determined from tests conducted on rock samples collected from the mine site. Special survey equipment which has a total station, laser scanner and a camera was used to perform remote fracture mapping in the research area selected at the mine site. From remote fracture mapping data, the fracture orientation, spacing and density were calculated in a much refined way in this study compared to what exist in the literature. Discontinuity orientation distributions obtained through remote fracture mapping agreed very well with the results of manual fracture mapping conducted by the mining company. GSI rock quality system and Hoek–Brown failure criteria were used to estimate the rock mass properties combining the fracture mapping results with laboratory test results of intact rock samples. Fault properties and the DRC–DP contact properties were estimated based on the laboratory discontinuity test results. A geological model was built in a 3DEC model including all the major faults, DRC–DP contact, and two stages of rock excavation. The built major discontinuity system of 44 faults in 3DEC with their real orientations, locations and three dimensional extensions were validated successfully using the fault geometry data provided by the mining company using seven cross sections. Numerical modeling was conducted to study the effect of boundary conditions and lateral stress ratio on the stability of the considered rock slope. For the considered section of the rock slope, the displacements obtained through stress boundary conditions were seemed more realistic than that obtained through zero velocity boundary conditions (on all four lateral faces). Stable deformation distributions were obtained for k 0 in the range of 0.4–0.7. Because the studied rock mass is quite stable, it seems that an appropriate range for k 0 for this rock mass is between 0.4 and 0.7. The displacements occurred between July 2011 and July 2012 due to the nearby rock mass excavation that took place during the same period were compared between the field monitoring results available from the mining company and the predicted numerical modeling results; the best agreement was obtained for k 0 = 0.4. Therefore, k 0 = 0.4 can be decided as the most appropriate value for the studied mine site. In overall, the successful simulation of the rock excavation during a certain time period indicated the possibility of using the procedure developed in this study to investigate rock slope stability with respect to expected future rock excavations in mine planning.

Fault strength in thin-skinned tectonic wedges across the smectite-illite transition: Constraints from friction experiments and critical tapers

The strength, shape, and ultimately seismic behavior of many thin-skinned fold and thrust belts, including marine accretionary wedges, are strongly controlled by large-scale faults that develop from weak, clay-rich sedimentary horizons (decollements). The increase of temperature with depth along clay-rich faults promotes the so-called smectite-illite transition, which may influence the fault strength, fluid distribution, and possibly the onset of seismicity. Here we report on the frictional properties of intact fault rocks retrieved from two large decollements, which were exhumed from depths above and below the smectite-illite transition. We find that all tested rocks are characterized by very low friction (μ = 0.17–0.26), velocity-strengthening behavior, and low rates of frictional healing, suggesting long-term fault weakness. Combining our experimental results with the critical taper theory, we computed the effective friction, F , of megathrusts beneath several accretionary wedges around the world; the result was extremely low (0.03 < F < 0.14), and in agreement with other independent estimates. Our analysis indicates a long-term weakness that can explain the shape of several tectonic wedges worldwide without invoking diffuse near-lithostatic fluid overpressures.

Prediction of roadheaders' performance using artificial neural network approaches (MLP and KOSFM)

Application of mechanical excavators is one of the most commonly used excavation methods because it can bring the project more productivity, accuracy and safety. Among the mechanical excavators, roadheaders are mechanical miners which have been extensively used in tunneling, mining and civil industries. Performance prediction is an important issue for successful roadheader application and generally deals with machine selection, production rate and bit consumption. The main aim of this research is to investigate the cutting performance (instantaneous cutting rates (ICRs)) of medium-duty roadheaders by using artificial neural network (ANN) approach. There are different categories for ANNs, but based on training algorithm there are two main kinds: supervised and unsupervised. The multi-layer perceptron (MLP) and Kohonen self-organizing feature map (KSOFM) are the most widely used neural networks for supervised and unsupervised ones, respectively. For gaining this goal, a database was primarily provided from roadheaders' performance and geomechanical characteristics of rock formations in tunnels and drift galleries in Tabas coal mine, the largest and the only fully-mechanized coal mine in Iran. Then the database was analyzed in order to yield the most important factor for ICR by using relatively important factor in which Garson equation was utilized. The MLP network was trained by 3 input parameters including rock mass properties, rock quality designation (RQD), intact rock properties such as uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS), and one output parameter (ICR). In order to have more validation on MLP outputs, KSOFM visualization was applied. The mean square error (MSE) and regression coefficient (R) of MLP were found to be 5.49 and 0.97, respectively. Moreover, KSOFM network has a map size of 8 × 5 and final quantization and topographic errors were 0.383 and 0.032, respectively. The results show that MLP neural networks have a strong capability to predict and evaluate the performance of medium-duty roadheaders in coal measure rocks. Furthermore, it is concluded that KSOFM neural network is an efficient way for understanding system behavior and knowledge extraction. Finally, it is indicated that UCS has more influence on ICR by applying the best trained MLP network weights in Garson equation which is also confirmed by KSOFM.

A discrete fracture network approach for the design of rock foundation anchorage

A major consideration in the design of high-capacity tiedown anchors for dams, bridges and tower foundations includes the tensile resistance of the rock mass to pullout typically as a result of overturning moments or hydrostatic uplift. Rock mass pullout capacity for installed anchors is developed from the tensile strength and fracture propagation properties of intact rock, the orientation and physical properties of the discontinuities and anchor confinement at depth. The typically assumed but conservative design approach is to assume that only the dead weight of a uniformly shaped inverted ‘rock pull out cone’ provides resistance to anchor pullout with an assumed initiation point and breakout angle. It is appreciated that the ‘dead weight’ cone assumption may be valid if the discontinuities are continuous and the geometry allows for a discrete block to form in the area of anchorage. However, if these persistent joints are not ubiquitously developed this failure mechanism may not represent the conditions across a foundation and can lead to a very conservative anchor designs. A less conservative approach is to consider the deadweight and the tensile strength across the surface area of the assumed pullout cones. Strength estimates are often estimated by a designer using correlations based on the Hoek–Brown failure criteria or the Barton Q-system. However, fundamental assumption of these empirical methods is that the discontinuities are sufficiently closely spaced such that the rock mass can be considered homogenous and isotropic – and this often not the case (Hoek, Carter and Diederichs 2013). In reality, at the scale of many rock anchor pullout problems, the strength and deformation properties of the rock mass are anisotropic, controlled by a few persistent discontinuities and small ‘rock bridges’ that exist between naturally occurring joints. To accommodate scale effects and the variability of jointing, this paper introduces a reliability-based design (RBD) approach for anchors that uses discrete fracture networks (DFNs) combined with numerical models. With this approach, limitations of the current practice for anchor design are addressed and opportunities are identified to optimise rock anchor designs provided that sufficient information is available to produce a representative DFN. Typically, this requires geological mapping to characterise the rock mass and verify the conditions at discrete anchor locations and in situ anchor testing can be carried out during construction.

A comparison of support systems obtained from the RMR89 and RMR14 by numerical analyses: Macka Tunnel project, NE Turkey

The aim of this study is to highlight the importance of the numerical approach in realistic support design for tunneling. The Macka Tunnel, which is planned to be constructed between the Trabzon-Gumushane highways, located in the Northeastern Turkey, was selected for a field study. The lithology of the study area consists of Late Cretaceous-aged sedimentary rocks, known as the Catak and Esiroglu formations, and the Late Cretaceous-aged Kackar Rhyodacite. In order to determine the geotechnical properties of rock masses and intact rocks, field and laboratory studies were carried out.The rock masses were classified in terms of the Rock Mass Rating (RMR) using the versions of RMR89 and RMR14 and then the preliminary support systems for the tunnel were determined, accordingly. The support systems obtained from the RMR versions were analyzed using the finite element method (FEM). The maximum total displacement and the maximum thickness of plastic zone occurred around the Macka Tunnel after installation the support systems suggested by the RMR89 and RMR14 versions were compared to determine which the RMR version suggests more realistic support system. The results showed that the support systems obtained from the RMR14 version suggests more realistic support element for fair rock masses and great horizontal stress values.

Rock excavation by continuous surface miner in limestone quarry

This recent study reports an investigation on the effectiveness of the cutting performance models applied to the Surface Miner Wirtgen 2200-SM operation in a limestone quarry, Palimanan, Cirebon, Indonesia. The cutting models applied in this investigation are those proposed by [Gehring, K. H. 1989. Bestimmung von Schneidleistung und Meibelverbauch. Working paper of the Voest Alpine Ges.m.b.H.Zeltweg, Austria and Gehring, K. H. 1992. Evaluation of cutting performance for Voest Alpine Surface Miner (Voest Alpine Internal Report BBV, 1992-08-04)], [Kramadibrata, S. 1996. The influence of rock mass and intact rock properties on the design of surface mines with particular reference to the excavatability of rock (PhD thesis), Curtin University of Technology School of Civil Engineering] and [Dey, K., & Ghose, A. K. (2008). Predicting “cuttability” with surface miners – A rockmass classification approach. Journal of Mines, Metals and Fuels, 56]. The prediction results from Gehring's model are always found higher than that of data observed, with deviation ranges from 0 to 30%. The prediction results from Kramadibrata's model are always found lower than that of data observed, with deviation ranges from − 2% to − 23%. The prediction result from Dey & Ghose for each block in Palimanan varies from lower to higher than that of data observed, with deviation ranges from − 9% to 21%. The recent study indicates that Gehring and Kramadibrata models reveal the same trend for all blocks. The application of Dey & Ghose model requires quite accurate measured parameters.

Stability Assessment of the Gökgöl Karstic Cave (Zonguldak, Turkey) by Analytical and Numerical Methods

Turkey has many cave formations and some caves are important tourist attractions. Theses caves have natural and historical characteristics that need to be investigated and protected. An important rock engineering topic is the identification of possible rock falls and collapses in caves that are open to the public and planning preventive measures. In this study, the locations and types of instability problems were identified along a section of 875 m of the Gokgol cave in Zonguldak, Turkey. The main instabilities were identified as block falls controlled by discontinuities, rock block sliding, and flexural toppling. To obtain the intact rock and rock mass properties of the stratified limestone which is the host rock of the cave, field and laboratory studies were conducted. Stability assessments for four different sections inside the cave were carried out using numerical and analytical methods, and the results were compared. The results of the analyses revealed that some countermeasures and precautions must be implemented to increase the stability in some areas of the Gokgol cave, especially those which have potential for regional failure.

Utilizing partial least square and support vector machine for TBM penetration rate prediction in hard rock conditions

Rate of penetration (ROP) of a tunnel boring machine (TBM) in a rock environment is generally a key parameter for the successful accomplishment of a tunneling project. The objectives of this work are to compare the accuracy of prediction models employing partial least squares (PLS) regression and support vector machine (SVM) regression technique for modeling the penetration rate of TBM. To develop the proposed models, the database that is composed of intact rock properties including uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), and peak slope index (PSI), and also rock mass properties including distance between planes of weakness (DPW) and the alpha angle (α) are input as dependent variables and the measured ROP is chosen as an independent variable. Two hundred sets of data are collected from Queens Water Tunnel and Karaj-Tehran water transfer tunnel TBM project. The accuracy of the prediction models is measured by the coefficient of determination (R 2) and root mean squares error (RMSE) between predicted and observed yield employing 10-fold cross-validation schemes. The R 2 and RMSE of prediction are 0.8183 and 0.1807 for SVMR method, and 0.9999 and 0.0011 for PLS method, respectively. Comparison between the values of statistical parameters reveals the superiority of the PLSR model over SVMR one.

Drilling rate prediction of an open pit mine using the rock mass drillability index

Besides intact rock properties, structural parameters of rock mass have strong effect on drilling rate. In this research, 11 different zones of an open pit iron mine were studied precisely to classify the area based on rock drillability point of view. Laboratory tests were conducted on the rock samples to determine strength parameters. Geological mapping of the rock facies was carried out and rock mass structural parameters as joint inclination, spacing, aperture and filling were recorded along with net drilling times of drill holes. Using these data, an empirical relation was developed to predict drilling rate (DR) using the rock mass drillability index (RDi) and also a relation that can predict uniaxial compressive strength (UCS) of rocks in terms of Schmidt hammer rebound values at this mine. In conclusion, all 66 zones of the mine area were classified according to the RDi. It was observed that RDi can reasonably predict drilling rate of rock masses. A new penetration rate model is defined based on the measured data and then compared with previous model of penetration rate from literature. Since the new model which involves not only intact rock mechanical properties but also structural properties of rock masses could attain better predictions in relation to the previous model.

Evaluation of the ratio between uniaxial compressive strength and Schmidt hammer rebound number and its effectiveness in predicting rock strength

Schmidt rebound hammer (SRH) test has been used worldwide as an index test for estimating the compressive strength and deformation characteristics of intact rocks. Although there is a high correlation between the surface hardness and the uniaxial compressive strength (UCS) of intact rocks, the SRH provides only a crude estimate for the UCS of rocks. SRH numbers reflect the outer surface of rocks and a depth of 30–50 mm. It is not sensitive to the intrinsic properties of the rocks such as texture, saturation, porosity and micro-fractures controlling the mechanical behaviour of rocks. In order for an empirical equation relating the surface hardness to the UCS to be widely used, the index parameter should characterise the mechanical properties of intact rocks. The ratio of UCS/SRH defined as a function of the UCS is a much better indicator for assessing the mechanical characteristics of rocks because the UCS of rocks defines the strength of the material and the ratio UCS/SRH is strongly affected by the level of the UCS. The ratio of UCS/SRH increases with increasing compressive strength at an increasing rate. A large-scale regression analysis was carried out using experimental data to evaluate the ratio of UCS/SRH for the rocks. The accuracy and reliability of the relationship was assessed by means of the root mean square error. The standard error associated with the empirical relationship is very small and the reliability and accuracy of the relationship to assess the compressive strength indirectly seem to be higher than those of traditional relationships between the UCS and the SRH. The ratio of UCS/SRH was also verified by a large database collected from previous studies. This strong linear relationship is proposed for engineering projects requiring the estimation of the compressive strength for intact rocks.

An approach for characterising the weathering behaviour of Flysch slopes applied to the carbonatic Flysch of Alicante (Spain)

Various studies indicate that most of the slope instabilities affecting Flysch heterogeneous rock masses are related to differential weathering of the lithologies that make up the slope. Therefore, the weathering characteristics of the intact rock are of great importance for the study of these types of slopes and their associated instability processes. The main aim of this study is to characterise the weathering properties of the different lithologies outcropping in the carbonatic Flysch of Alicante (Spain), in order to understand the effects of environmental weathering on them, following slope excavation. To this end, 151 strata samples obtained from 11 different slopes, 5–40 years old, were studied. The lithologies were identified and their mechanical characteristics obtained using field and laboratory tests. Additionally, the slaking properties of intact rocks were determined, and a classification system proposed based on the first and fifth slake cycles (Id1 and Id5 respectively) and an Index of Weathering (IW5), defined in the study. Information obtained from the laboratory and the field was used to characterise the weathering behaviour of the rocks. Furthermore, the slaking properties determined from laboratory tests were related to the in-situ weathering properties of rocks (i.e., the weathering profile, patterns and length, and weathering rate). The proposed relationship between laboratory test results, field data, and in-situ observations provides a useful tool for predicting the response of slopes to weathering after excavation during the preliminary stages of design.

Characterization of bi-planar and ploughing failure mechanisms in footwall slopes using numerical modelling

Footwall slopes refer to unbenched rock slopes in which the slope face is parallel to a set of persistent discontinuities (e.g. bedding planes, foliation, faults). These are commonly encountered in weak, thinly bedded, orthogonally jointed, sedimentary rock sequences. Common failure mechanisms include bi-planar failures where shallow dipping crosscutting structures daylight near the slope toe, enabling sliding to occur along steep dipping bedding planes. In the absence of crosscutting structures, failure occurs through deformation and rock mass yielding involving the formation of inter block shear and toe breakout surfaces. Because of the complexity of the toe breakout mechanism, evaluation methods are not well understood. An improved understanding of the failure mechanism, the role of adverse discontinuities, and characterization of the discontinuity, intact rock and rock mass strength properties are key for a successful footwall stability analysis. This paper investigates the development of the inter block shear and toe breakout surfaces with three approaches: i) continuum-based frictional plasticity theory; ii) discontinuum-based distinct-element modelling with Voronoi tessellation using the commercial software UDEC; and iii) hybrid continuum/discontinuum finite-/discrete-element brittle fracture modelling using the commercial software ELFEN. Numerical simulations using ELFEN and UDEC demonstrated a good agreement with frictional plasticity theory. Ploughing failure of footwall slopes is also evaluated, specifically the influence of cross-cutting discontinuity dip angle relative to the slope face. The effects of different geometrical parameters (e.g., slope angle and depth/height ratio) on bi-planar and ploughing failure are assessed using a sensitivity analysis approach. A “Damage Intensity” parameter is introduced and used to quantify damage in the numerical simulations using ELFEN.

How does the water–rock interaction of marly rocks affect its mechanical properties in the Three Gorges reservoir area, China?

Marly rock is a sensitive rock group of landslides in the Three Gorges reservoir area, China. It is composed predominantly of carbonate and clay minerals, water–rock interaction (WRI) of which could activate landslides in the reservoir area. To study the mechanism by which WRI affects the mechanical properties of marly rock, samples were collected from two boreholes (depth 301.78 and 307.14 m) and slope surface. Then, laboratory tests were designed to study the quantitative relationship between mineral contents and mechanical properties of intact rock, to analyze both change process of mineral composition and microstructure under short-term and long-term WRI. Finally, the change in mechanical properties and its effect on slope stability are suggested. This study indicates that the uniaxial compressive strength and Poisson ratio can be estimated by linear regression equations: (1) σ c = 8.959 × (C/Q) − 0.744 × CM + 58.516; (2) μ = 0.014 × (C/Q) − 0.001 × CM + 0.234. The chemical reactions of WRI mainly included dissolution and ion exchange. On the slope surface, dissolution mainly acts on calcite, illite, dolomite, feldspar and other minerals dissolved in water. Underground, both chemical reactions of dissolution and ion exchange easily approach equilibrium with long-term seepage. Small-size minerals and micropores damage the stable microstructure of marly rock. These changes of minerals and microstructure can trigger shallow slope failure and develop deep creep deformation along some crash zones in the reservoir shoreline.

Prefabricated Structures Surface Marble Rheological Tests and Constitutive Model Analysis

The mechanical properties of intact rock and rock containing structural plane are very different. From the diversion tunnel of Jinping deep rock site to retrieve the complete block of marble, after a high confining pressure triaxial compression simulation tectonic movements, the formation of structural plane, it represents the mechanical properties of the original rock. On the surface of the marble structure containing triaxial compression creep tests, the results showed: at low confining pressure, the weak marble surface as micro-damage accumulation, the emergence of non-uniform partial destruction, while at high confining pressure, creep curve better continuity and integrity; different confining pressures, marble initial rheology and stability both appear rheological phase, accelerated phase rheological obvious; different confining pressures, the same stage of the axial stress steady flow rate compared with the confining pressure increases, the axial steady state flow rate becomes smaller; marble under test showed the rheological properties, the use of Nishihara model can better demonstrate the rheological properties and determine the rheological parameters for other practical engineering reference.

Karst Sinkholes Stability Assessment in Cheria Area, NE Algeria

This research work deals with the problem of karst sinkhole collapse occurring in the last few years in Cheria area (NE Algeria). This newly revealed phenomenon is of a major constrain in land use planning and urbanization, it has become necessary to locate and assess the stability of these underground features before any planning operation. Several exploration methods for the localization of underground cavities have been considered. Geological survey, discontinuity analysis, resistivity survey [ground penetrating radar has not been used as most of the Mio-Plio-Quaternary filling deposit covering Eocene limestone contains clay layers which limits the applicability of the method (Roth et al. in Eng Geol 65:225–232, 2002)] and borehole drilling were undertaken in order to locate underground cavities and assess their depth, geometry, dimensions, etc. Laboratory testing and field work were also undertaken in order to determine both intact rock and rock mass properties. All the rock mechanics testing and measurement were undertaken according to the ISRM recommendations. It has been found that under imposed loading, the stability of the karst cavities depends on the geo-mechanical parameters (RMR, Rock Mass Rating; GSI, Geological Strength Index; E, Young modulus) of the host rock as well as the depth and dimensions of the gallery. It increases with RMR, GSI, E and depth and decreases as the cavity becomes wider. Furthermore, the calculation results show that a ratio (roof thickness to gallery width) of 0.3 and more indicate, a stable conditions. The results obtained in this work allow identifying and assessing the stability of underground karst cavities. The methodology followed in this paper can be taken as a road map in the establishment of a hazard map related to the studied phenomenon. This map will be a useful tool for the future urban extension planning in Cheria area.

A New Rock Mass Quality Rating System: Rock Mass Quality Rating (RMQR) and Its Application to the Estimation of Geomechanical Characteristics of Rock Masses

The qualitative description of rock masses by means of classification systems and subsequent correlation to establish engineering quantities or design parameters has become one of the most challenging topics in rock engineering. Many rock mass classification systems have been proposed for rock masses with the consideration of a particular rock structure and/or specific purposes. Therefore, direct utilization of these systems, in their original form, for the characterization of complex rock mass conditions is not always possible. This is probably one of the reasons why rock engineers continue to develop new systems or modify and extend current ones. The recent tendency is to obtain rock mass properties from the utilization of properties of intact rock and rock classification indexes, which have some drawbacks. In this study, it is aimed to propose a new rock mass quality rating system designated as Rock Mass Quality Rating (RMQR). This new rock mass rating system is used to estimate the geomechanical properties of rock masses. In the first part of this paper, the input parameters of RMQR and their ratings are given and discussed. In the second part, the unified formula proposed by the first author is adopted for the new rock mass rating system for estimating the rock mass properties and compared with the results of the in situ tests carried out in Japan and those estimated from some empirical relationships developed by other investigators, and the outcomes of these studies are presented and discussed.

Investigation of Grain Size Effects on Micro/Macro-Mechanical Properties of Intact Rock Using Voronoi Element—Discrete Element Method Approach

The effects of grains and sampling sizes on macro/micro-mechanical properties and the representative elementary volume of intact rocks were determined using a Voronoi element-discrete element method approach. Voronoi element was used for generation of grain assembles of rock matrices and the discrete element method was used as the numerical tool. Micro-mechanical parameters of Voronoi models were determined using back-calculation techniques through numerical calibrations. A series of two dimensional Voronoi element with different grain size distributions for calcite based marble intact rocks of Neiriz and Baghat from two quarries in Iran with varying model sizes were generated and numerical uniaxial compression tests were performed to calculate the equivalent macro-mechanical properties of intact rocks using the universal distinct element code. The results show high size-dependency of mechanical properties of intact rocks. The side lengths of 10 cm and 6 cm were determined as the representative elementary volumes for intact rocks of Neiriz and Baghat with grain sizes of 2 mm and 4 mm, respectively. The results also show that the model scale has more significant impact on macro-mechanical properties than the micro-structure pattern.

Influence of fracture orientation on excavatability of soft sedimentary rock using a hydraulic impact hammer: A case study in the Horonobe Underground Research Laboratory

In the Japanese deep geological disposal project for high-level radioactive waste, the ease of excavation of a potential host rock is a key issue for the economics of project implementation. This is because it is estimated that about 300-km of disposal drifts will need to be excavated for the disposal of 40,000 waste packages deep underground. In this study, the influence not only of the mechanical properties of intact host rock, but also fracture characteristics and their effect on the practical excavation rate of drifts using a hydraulic impact hammer were analyzed. The drifts were excavated through soft sedimentary rock in the Horonobe Underground Research Laboratory. Fracture mapping was conducted at the excavation face before every excavation round for the evaluation of fracture characteristics. Equotip hardness testing was also conducted to estimate the strength of intact rock. The results indicate that the excavation rate of the drift excavated in the direction perpendicular to natural fractures was faster than the excavation of the drift excavated in the direction parallel to natural fractures. In addition, the drift excavated in the direction perpendicular to natural fractures had a good correlation with fracture density. This result suggests that the preferred direction of disposal drifts should be perpendicular to the major natural fractures in the host rock to optimize excavatability in any deep geological disposal project.