The correlation between RMR and Q systems in parts of Iran

One of the approaches for characterising rock masses with discontinuities due to the presence of engineered designs is to use rock mass classification. Thus far, many classification systems, including RMR, Q, and GSI, have been proposed in the literature. Their parameters are based on site investigations, such as surface/subsurface fracture studies and well coring, as well as laboratory experiments. When sufficient information is not available, the utilisation of several rock mass classification systems is useful to compile a more complete understanding of the composition and characteristics of a rock mass. Thus, many correlations have been drawn to relate different systems, especially between RMR and Q systems. In this study, the best correlation coefficient between RMR and Q systems was determined with the aim of suggesting a new potential correlation for various geotechnical activities in parts of Iran. To accomplish this aim, rock mass parameters for the RMR and Q systems were assessed by considering their values separately for more than 800 stations at 14 different sites and applying statistical procedures to the data. Finally, a new correlation was determined.     

Engineering geological appraisal of the Sulakyurt dam site,Turkey

This paper discusses the engineering geological investigations, diversion tunnel support design and slope stability assessment studies carried out at the Sulakyurt dam site, northeast of Ankara, Turkey. The Sulakyurt dam will be used for flood flow control and water storage for irrigation. Engineering geological mapping, discontinuity surveys, core drilling, water absorption and laboratory tests were undertaken. The RMR, Q and GSI approaches were used to estimate the rock mass quality, site characteristics, rock mass parameters and appropriate tunnel support elements. The results of kinematic and limit equilibrium analyses for the slopes on the right and left banks are reported.      

The ultrasonic P-wave velocity-stress relationship of rocks and its application

When a rock sample is extracted from an underground rock mass, it is subject to unloading, which will cause changes in the physical and mechanical properties. This article describes a laboratory experiment to determine the change of P-wave velocity of rock samples during a uniaxial compression test. It was found that the P-wave velocity vs. stress curves (V-S curves) of the rock samples could reflect three stages of bulk volume deformation commonly observed in a uniaxial compression test of rocks. When the applied stress was less than σ _c (about 0.25–0.33 of the uniaxial compressive strength), the P-wave velocities increased rapidly with the increase of stress; this part of the V-S curves could be fitted with a power function. When the stress was greater than σ _c, the P-wave velocities of rock samples increased more slowly and gradually approached the peak before decreasing dramatically near failure; the V-S curves above σ _c could be fitted with a polynomial function of the second degree. During the V-S experiment, it could be also observed that the increasing rate of P-wave velocity decreased dramatically when the applied stress reached the overburden stress. An unloading index was defined as the ratio of the P-wave velocity under in situ overburden stress to the P-wave velocity at free stress and could be calculated from the measured V-S curves. Based on the calculated unloading index, the calculation of the intactness index of rock mass could be modified, and then an improvement of the basic quality (BQ) classification method of rock masses, which is used widely in China, was made.     

Improved prediction of shear wave velocity for clastic sedimentary rocks using hybrid model with core data

Accurate measurement of acoustic velocities of sedimentary rocks is essential for prediction of rock elastic constants and well failure analysis during drilling operations. Direct measurement by advanced logging tools such as dipole sonic imager is not always possible. For older wells, such data are not available in most cases. Therefore, it is an alternate way to develop a reliable correlation to estimate the shear wave velocity from existing log and/or core data. The objective of this research is to investigate the nature of dependency of different reservoir parameters on the shear wave velocity (V_s) of clastic sedimentary rocks, and to identify the parameter/variable which shows the highest level of dependency. In the study, data-driven connectionist models are developed using machine learning approach of least square support vector machine (LSSVM). The coupled simulated annealing (CSA) approach is utilized to optimize the tuning and kernel parameters in the model development. The performance of the simulation-based model is evaluated using statistical parameters. It is found that the most dependency predictor variable is the compressional wave velocity, followed by the rock porosity, bulk density and shale volume in turn. A new correlation is developed to estimate V_s, which captures the most influential parameters of sedimentary rocks. The new correlation is verified and compared with existing models using measured data of sandstone, and it exhibits a minimal error and high correlation coefficient (R² = 0.96). The hybridized LSSVM-CSA connectionist model development strategy can be applied for further analysis to predict rock mechanical properties. Additionally, the improved correlation of V_s can be adopted to estimate rock elastic constants and conduct wellbore failure analysis for safe drilling and field development decisions, reducing the exploration costs.     

Effects of confinement on rock mass modulus: A synthetic rock mass modelling (SRM) study

The main objective of this paper is to examine the influence of the applied confining stress on the rock mass modulus of moderately jointed rocks (well interlocked undisturbed rock mass with blocks formed by three or less intersecting joints). A synthetic rock mass modelling (SRM) approach is employed to determine the mechanical properties of the rock mass. In this approach, the intact body of rock is represented by the discrete element method (DEM)-Voronoi grains with the ability of simulating the initiation and propagation of microcracks within the intact part of the model. The geometry of the pre-existing joints is generated by employing discrete fracture network (DFN) modelling based on field joint data collected from the Brockville Tunnel using LiDAR scanning. The geometrical characteristics of the simulated joints at a representative sample size are first validated against the field data, and then used to measure the rock quality designation (RQD), joint spacing, areal fracture intensity (P₂₁), and block volumes. These geometrical quantities are used to quantitatively determine a representative range of the geological strength index (GSI). The results show that estimating the GSI using the RQD tends to make a closer estimate of the degree of blockiness that leads to GSI values corresponding to those obtained from direct visual observations of the rock mass conditions in the field. The use of joint spacing and block volume in order to quantify the GSI value range for the studied rock mass suggests a lower range compared to that evaluated in situ. Based on numerical modelling results and laboratory data of rock testing reported in the literature, a semi-empirical equation is proposed that relates the rock mass modulus to confinement as a function of the areal fracture intensity and joint stiffness.     

Reply to Discussion on “Empirical methods for determining shaft bearing capacity of semi-deep foundations socketed in rocks”

Semi-deep foundations socketed in rocks are considered to be a viable option for the foundations in the presence of heavy loads imposed by high-rise buildings and special structures, due to the low settlement and high bearing capacity. In this study, the unconfined compressive strength (UCS) and rock mass cuttability index (RMCI) have been applied to investigating the shaft bearing capacity. For this purpose, a comprehensive database of 178 full-scale load tests is compiled by adding a data set (n = 72) collected by Arioglu et al. (2007) to the data set (n = 106) presented in Rezazadeh and Eslami (2017). Using the database, the applicability and accuracy of the existing empirical methods are evaluated and new relations are derived between the shaft bearing capacity and UCS/RMCI. Moreover, a general equation in case of unknown rock types is proposed and it is verified by another set of data (series 3 in Rezazadeh and Eslami (2017)). Since rock-socketed shafts are supported by rock mass (not intact rock), a reduction factor for the compressive strength is suggested and verified in which the effect of discontinuities is considered using the modified UCS, based upon RMR and RQD to consider the effect of the rock mass properties.     

A geo-engineering classification for rocks and rock masses

In this article, an attempt is made to assess the reliability of predicting the uniaxial compressive strength and the corresponding modulus of a rock mass by current approaches. These two basic engineering properties, when estimated from rock mass rating (RMR), Q and geological strength index (GSI), indicate hardly any change in the modulus ratio with the change in the quality of the rock mass from very good to very poor. However, the modulus ratio obtained from the relations involving the joint factor, J_f, indicate a definite decrease in the modulus ratio with a decrease in the quality of the rock mass. The strength and modulus in the unconfined and confined states, the modulus ratio and failure strain in the unconfined case were linked to J_f in earlier publications based on a large experimental database. Some of these relations were adopted to verify the response of jointed test specimens, the response of the rock mass during excavations for mining and civil underground chambers, in establishing ground reaction curves including the extent of the broken zone, and the bearing capacity of shallow foundations.The joint factor is now linked to RMR, Q and GSI. The prediction of compressive strength and modulus of the rock mass appears to be more suitable. For classifying the rock, based on these properties, the Deere and Miller engineering classification, applicable to intact rocks, has been suitably modified and adopted. The results of different modes of failure of jointed specimens establish definite trends of changes in the modulus ratio originating from the intact rock value on the modified Deere and Miller plot. A geo-engineering classification is evolved by considering strength, modulus, quantifiable weathering index and lithological aspects of the rock.     

Some new Q-value correlations to assist in site characterisation and tunnel design

The rock mass quality Q-value was originally developed to assist in the empirical design of tunnel and cavern reinforcement and support, but it has been used for several other tasks in rock engineering in recent years. This paper explores the application of Q and its six component parameters, for prediction, correlation and extrapolation of site investigation data, and for obtaining first estimates of some input data for both jointed distinct element and continuum-approximation modelling. Parameters explored here include P-wave velocity, static modulus of deformation, support pressure, tunnel deformation, Lugeon-value, and the possible cohesive and frictional strength of rock masses, undisturbed, or as affected by underground excavation. The effect of depth or stress level, and anisotropic strength, structure and stress are each addressed, and practical solutions suggested. The paper concludes with an evaluation of the potential improvements in rock mass properties and reduced support needs that can be expected from state-of-the-art pre-injection with fine, cementicious multi-grouts, based on measurements of permeability tensor principal value rotations and reductions, caused by grout penetration of the least favourable joint sets. Several slightly improved Q-parameter ratings form the basis of the predicted improvements in general rock mass properties that can be achieved by pre-grouting.     

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (V_S1) and perpendicular (V_S2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.     

Rock quality classification and stability evaluation of undersea deposit based on M-IRMR

The mine improvement of rock mass rating (M-IRMR) evaluation method was put forward based on the existing theory of the rock quality classification and the stability evaluation in the undersea deposit of Sanshandao Gold Mine, China. The M-IRMR evaluation method includes 9 evaluation indexes which are rock compressive strength, rock quality index RQD, joint spacing, joint state, groundwater state, joint direction, ground stress, blasting vibration, and exposed area, respectively. During the evaluation process, according to the special features of the undersea deposit, the rock mass rating (RMR) method was used as the foundation, and the four geological parameters (i.e., rock compressive strength, rock quality index, joint spacing, and ground stress) were modified. The computing methods of the two engineering factors (blasting vibration and exposed area) were presented. The M-IRMR rock quality classification and stability evaluation method was applied in the level of −420 to −690 m in the undersea deposit of Sanshandao Gold Mine and the classification results were consistent with the actual situations, which can provide a scientific basis for choosing the suitable mining method and stope support system of the undersea deposit.     

Investigation of block foundations resting on soil–rock and rock–rock media under coupled vibrations

In the present study,the dynamic response of block foundations of different equivalent radius to mass(R_o/m) ratios under coupled vibrations is investigated for various homogeneous and layered systems.The frequency-dependent stiffness and damping of foundation resting on homogeneous soils and rocks are determined using the half-space theory.The dynamic response characteristics of foundation resting on the layered system considering rock-rock combination are evaluated using finite element program with transmitting boundaries.Frequencies versus amplitude responses of block foundation are obtained for both translational and rotational motion.A new methodology is proposed for determination of dynamic response of block foundations resting on soil-rock and weathered rock-rock system in the form of equations and graphs.The variations of dimensionless natural frequency and dimensionless resonant amplitude with shear wave velocity ratio are investigated for different thicknesses of top soil/weathered rock layer.The dynamic behaviors of block foundations are also analyzed for different rock-rock systems by considering sandstone,shale and limestone underlain by basalt.The variations of stiffness,damping and amplitudes of block foundations with frequency are shown in this study for various rock—rock combinations.In the analysis,two resonant peaks are observed at two different frequencies for both translational and rotational motion.It is observed that the dimensionless resonant amplitudes decrease and natural frequencies increase with increase in shear wave velocity ratio.Finally,the parametric study is performed for block foundations with dimensions of 4 m × 3 m × 2 m and 8m×5m×2m by using generalized graphs.The variations of natural frequency and peak displacement amplitude are also studied for different top layer thicknesses and eccentric moments.     

Numerical simulations of rock mass damage induced by underground explosion

The damage prediction of rock mass under blast loads induced by accidental explosions, rock bursts or weapon attacks is crucial in rock engineering. In this paper, parametric studies are conducted to evaluate the effect of loading density, rock mass rating (RMR) and weight of charge on the rock mass damage induced by underground explosions. The numerical simulations are carried out based on the transient dynamic finite element program ANSYS-LSDYNA. The numerical model was calibrated against the data obtained from a field blast test. A fully coupled numerical analysis, incorporating the explosion process, has been performed, where the large deformation zone near the charge is solved by the Arbitrary Lagrange–Euler (ALE) method. The deformable modulus and compressive strength of rock mass of granite are estimated by the RMR system. The peak particle velocity (PPV) damage criterion and the plastic strain criterion were adopted to study the damage zone around the charge hole, and an empirical formula considering the effects of loading density, RMR and weight of charge was obtained to estimate the damage zone in granite based on the numerical results.     

Discussion on “Empirical methods for determining shaft bearing capacity of semi-deep foundations socketed in rocks”[J Rock Mech Geotech Eng 6(2017)1140-151]

A new comprehensive set of data(n = 178) is compiled by adding a data set(n = 72) collected by Arioglu et al.(2007) to the data set(n = 106) presented in Rezazadeh and Eslami(2017). Then, the compiled data set is evaluated regardless of the variation in lithology/strength. The proposed empirical equation in this study comprises a wider range of uniaxial compressive strength(UCS)(0.15 MPa σ_(rc) 156 MPa) and various rock types. Rock mass cuttability index(RMCI) is correlated with shaft resistance(r_s) to predict the shaft resistance of rock-socketed piles. The prediction capacity of the RMCI versus r_s equation is also found to be in a fair good agreement with the presented data in Rezazadeh and Eslami(2017). Since the RMCI is a promising parameter in the prediction of shaft resistance, the researchers in the rock-socketed pile design area should consider this parameter in the further investigations.     

工程岩体的超声波分类及强度预测

针对目前在岩石工程中广泛采用的隧洞围岩分类Ｑ系统和ＲＭＲ工程岩体分类,利用统计结果将岩体的超声波速与岩质量指数Ｑ和岩体分类指数ＲＭＲ联系起来,建立了由岩体超声波速估计岩体分类指数ＲＭＲ的计算公式,通过测定岩体的超声波可对岩体进行分类。为了预测岩体的变形及强度参数,引入Ｈｏｅｋ－Ｂｒｏｗｎ准则,给出了岩体变形及强度参数的预测方法。在三峡工程永久船闸高边坡岩体的应用中,超声波预测的力学参数与ＲＭＲ分类     

Assessing the ability of rock masses to support block breakage at the TBM cutter face

In order for tunnel boring machines to efficiently cut or break rock, it is necessary that the block of rock in contact with the cutter be adequately supported by the surrounding rock mass. This support is provided by the interlocking of blocks and the friction of the surfaces. If blocks are inadequately supported or become free without breakage the result can be jamming at the TBM face. Such blocky ground conditions are typically assessed according to the spacing and orientation of discontinuities (including joints) within the rock mass, typically using a rock mass classification system. In laboratory tests on cuttability or abrasivity of rocks, test samples are typically supported securely in a frame or jig. Numerical models of rock breakage also assume boundary conditions in which the sample is completely supported. Therefore the applicability of the results from laboratory and numerical studies depends on the same degree of support of blocks in the ground. The conditions required to adequately support a block for breakage are investigated and related to rock mass parameters, in particular, the three-dimensional patterns of discontinuities. A rock mass can be capable of providing adequate support to a block of rock such that the cuttability is adequately described by conventional methods. However, there are some rock mass conditions where support of blocks is not well developed, potentially resulting in otherwise unexpected poor TBM progress or jamming of TBM with loose blocks. Three-dimensional discontinuity patterns can be assessed using stereographic methods or borehole (α–β) methods. It is proposed that problematic conditions may occur where: two or more oblique (α between 20° and 70°) discontinuity sets are present (and over-represented relative to a uniform distribution); one or more of these discontinuity sets are dipping into the opening (β = 180° ± 90°) and additional discontinuities (in sets or randomly oriented) are present to form complete tetrahedral wedge blocks.     

Seismic bearing capacity of a strip footing on rock media

The bearing capacity factors for a rough strip footing placed on rock media, which is subjected to pseudo-static horizontal earthquake body forces, have been determined using the lower bound finite element limit analysis in conjunction with the power cone programming (PCP). The rock mass is assumed to follow the generalized Hoek-Brown (GHB) yield criterion. No assumption needs to be made to smoothen the GHB yield criterion and the convergence is found to achieve quite rapidly while performing the optimization with the usage of the PCP. While incorporating the variation in horizontal earthquake acceleration coefficient (k_h), the effect of changes in unit weight of rock mass (γ), ground surcharge pressure ($q_0$) and the associated GHB material shear strength parameters (geological strength index (GSI), yield parameter (m_i), uniaxial compressive strength (σ_ci)) on the bearing capacity factors has been thoroughly assessed. Non-dimensional charts have been developed for design purpose. The accuracy of the present analysis has been duly checked by comparing the obtained results with the different solutions reported in the literature. The failure patterns have also been examined in detail.     

Influence of rock mass fracturing on the net penetration rates of hard rock TBMs

Penetration rates during excavation using hard rock tunnel boring machines (TBMs) are significantly influenced by the degree of fracturing of the rock mass. In the NTNU prediction model for hard rock TBM performance and costs, the rock mass fracturing factor (k_s) is used to include the influence of rock mass fractures. The rock mass fracturing factor depends on the degree of fracturing, fracture type, fracture spacing, and the angle between fracture systems and the tunnel axis. In order to validate the relationship between the degree of fracturing and the net penetration rate of hard rock TBMs, field work has been carried out, consisting of geological back-mapping and analysis of performance data from a TBM tunnel. The rock mass influence on hard rock TBM performance prediction is taken into account in the NTNU model. Different correlations between net penetration rate and the fracturing factor (k_s) have been identified for a variety of k_s values.