Rock Quality Designation Values Research Articles

Geotechnical and Geophysical Investigations of Estimating Rock Mass Properties

Accurate assessments of the deformation and strength properties of rock masses are essential for analysing the foundations, subsurface excavation, and construction of slopes, however, identifying geotechnical properties, particularly weak rock masses, is challenging for geoscientists, geologists, and geotechnical investigators. To determine the rock mass properties, geophysical techniques, and rock mass classification methods are widely used. Although rock classification methodologies exist, they are rarely employed in foundation construction analysis due to being predominantly established for tunnelling purposes. This study aims to employ classification methods, and geophysical and geotechnical investigation approaches to provide realistic rock mass properties for a building foundation, compare results, and assess techniques' effectiveness; it also reviews current state-of-the-art methods for classification techniques and investigations. Results show that compressional and shear wave velocities increase with depth indicating a change from poor to satisfactory mechanical behaviour, and empirical equations provide highly valuable and satisfactory deformation modulus values when compared to field seismic values. Almost all classification methods provided reasonable results compared with the seismic results, however highly overestimated and underestimated outcomes were obtained from some empirical equations applying the Rock Quality Designation index and Rock Mass Rating system. Empirical equations that gave the practical deformation modulus outcomes are the Rock Mass Rating based equations and Q-based equations and Geological Strength Index-based equations. Overall, the Rock Quality Designation index underestimated the seismic result, whereas the Rock Mass Rating, Q, and Geological Strength Index systems produced good agreement and realistic results. To conclude, classification methods were effectively applied to a building foundation. Since the Rock Quality Designation is a key value and feature for other systems including Q, and Rock Mass Rating systems, it is an essential factor. Although the Q systems provided practical consequences, this system mainly relied on the Rock Quality Designation value not the compressive strength of the intact rock. The Rock Mass Rating and being the most feasible and essential systems due to detailed geotechnical observations as inputs more significantly the uniaxial compressive strength of the intact rock, and various factors including deformation modulus, strength, Poisson’s ratio, equivalent cohesion, and friction angle of rock mass as outcomes.

Geological-geotechnical investigations of the historical Diriyah urban zone in Riyadh, Saudi Arabia: an integrated approach

One hundred seventy geotechnical boreholes, two geo-seismic profiles of multichannel analyses of surface waves (MASW), and two geoelectrical cross-sections of electrical resistivity tomography (ERT) were carried out in the historical Diriyah urban zone. The borehole samples were analyzed for the geotechnical parameters of the standard penetration test (SPT N-value), the rock quality designation (RQD), and compressive strength (CS). Moreover, P- and S-wave velocities have been calculated. The measured SPT N-value has been corrected to amend the N60 parameter. According to the corrected N60, type, lithological, and soil profiles were divided into five distinct subsurface layers. It is indicated, for all recorded depths, these sediments become denser with depth where the values of N60 increase from 5 to 88 downward. According to grain size distribution, the major part of soil sites is composed of sand and varies from low to medium compressibility. The estimated RQD values range from 5 to 100 indicating rocks with significant variations in their geotechnical and physio-chemical characteristics. Furthermore, the compressive strength varies widely between 152.3 and 1,104.2 kg/cm2, which represents diverse geodynamic behavior under civil engineering constructions. Besides, the range of soil stiffness is indicated by the Shear-wave velocity (Vs) for soil layers, which varies from 171.3 to 456.2 m/s. According to the findings of the MASW profiles, the shear wave velocity is shown to be decreasing in MASW profile 1 between depths of 8 and 14 m, where the values range from 1,000 to 1,500 m/s, indicating a weak zone filled with secondary materials. But line 2 shows how the shear wave velocity decreased from depths of 5 m–11 m, where values ranged from 1,000 to 1,500 m/s, thus indicating a weak zone. The low resistivity zones in ERT cross-sections, which are found at the same depths in MASW profiles, are well associated with these findings. It can be concluded that there are a number of weak zones with varying depths, shapes, and dimensions found in the historical Diriyah urban zone’s near-surface deposits that civil engineers, urban planners, and urban developers should consider before undertaking any construction or developmental work involving civil engineering.

Mechanical properties of selected granitoids along the Karora hydropower project, Besham, Northern Pakistan: Insights from petrography and joint analysis

The stability of tunnel site depends upon the deformation behaviors of the surrounding rocks, including joint density, mineral composition, micro cracks, and grain boundaries. The aim of this study was to analyze the mechanical properties of the Besham Complex in Northern Pakistan in terms of petrography and joint analysis. The Besham complex is prone to deformation as it is bounded by the Main Mantle Thrust (MMT) in the north, the Thakot fault (Tf) in the east, and the Swat block in the west. In the study area, the deformation was observed at the mesoscopic scale, while alteration in grains and fractures were determined at a microscopic scale. Detailed petrographic study of the representative samples from each station revealed that the area is dominantly composed of granite, quartz syenite, quartz monzonite, granodiorite, quartz-rich granitoid, monzogranite, alkali feldspar granite, and quartz monzodiorite. Extensive field work was performed to determine the joint volume (JV), in which the number of joints per cubic meter was recorded for each joint set. The rock quality designation (RQD) was calculated through JV analysis. An RQD value <25% suggested that most rocks are very poor quality, while samples with RQD values between 25% and 50% are poor quality. The petrographic analysis revealed that the dominant contents of the rock samples are feldspar (55.6%), quartz (27%), and mica (17.4%). Petrographic control over surface joint volume was established, suggesting that abundant feldspar, coarse grain, anhedral to subhedral grain shape, microcracks, and alteration of minerals provide planes of weakness for joints and thus reduce the RQD value. Therefore, some serious precautions and supporting measurements, such as shotcrete, rock bolts, metal mesh, and steel arch supports, are essential to avoid severe geo-hazards during further excavation.

Characteristics and rippability conditions of near-surface lithologic units (Penang Island, Malaysia) derived from multimethod geotomographic models and geostatistics

The nature and rippability conditions (rock mass quality [RMQ]) of the near-surface lithologic units of Batu Maung, Penang Island, Malaysia, were investigated to develop sustainable groundwater and infrastructure. This study used a novel approach that combined seismic refraction tomography (SRT), electrical resistivity tomography (ERT), borehole drilling, and simple linear regression (SLR) analysis. The seismic P-wave velocity (Vp) and resistivity models revealed four distinctive units: residual soils (silty sand), very poor-to-good (weathered) granite, and the fresh granitic unit, including the fractured/faulted zones. The thicknesses of the residual and weathered materials ranged from a few centimeters to 15 m and 1.0–16 m, respectively. The developed empirical relationship effectively predicted rock quality designation (RQD) from Vp data through SLR analysis, with a prediction accuracy of 96% and a p-value<0.05. Also, the results from five key regression assumptions: linear relationship, multivariate normality, no multicollinearity, no autocorrelation, and homoscedasticity, suggested an accurate and statistically significant empirical relationship for use in granitic environments. The RMQ statistical model accurately classified the lithologic units beneath the area into Classes I–VI. Due to the low load-bearing of the rippable residual soils, very poor-to-fair weathered granitic rock masses, and fractured/faulted zones based on their Vp and resistivity values, and the steep slopes in the northern section of the study area, all intended infrastructures, particularly high-rise buildings and buildings with continuous footing foundations, should be piled to rest on the non-rippable fresh granitic units, with RQD and Vp values of >90% and >2100 m/s, respectively, in the central to the northcentral section in the study area. Conversely, the deep-weathered/fractured zones of depths >35 m, beneath Line 1, towards the central part of the area, with resistivity and Vp values of 100–900 Ω-m and <1900 m/s, respectively, were identified as potentially water-containing zones for sustainable groundwater abstraction.

Kajian Arah Tegasan dan Nilai RQD Berdasarkan Analisis Statistik Data Struktur Kekar

The research location is in Gorontalo City. The location is an area that has geological structural conditions of high complexity. The study used statistical analysis of the robustness data to determine the direction of the firmness and the RQD (rock quality designation) value. The research objectives consist of two, namely knowing the direction of the stress (the main force of tectonic formation) and knowing the RQD value based on the analysis of joint data. To achieve these two objectives, it is carried out with a research method in the form of an inductive method. This method combines the results of a literature review, previous research and field data, all of which are comprehensively studied, analyzed, and synthesized. Statistical analysis (fan diagram) on joint data on granite rocks in Gorontalo City. The results and discussion resulted in the direction of maximum stress (τ1) = N 15 o E and the direction of minimum stress (τ3) = N 75 o W. The rock RQD value is 40.60% so that the rock quality in this research area is classified as moderate. The results of the measurement of the joint spacing at this station are obtained from a distance of (5-10) cm, so it is included in the weak scale so that the joint space strongly supports the occurrence of landslides.

RQD calculation method based on 3D rock discontinuity network simulation in the dam foundation of Baihetan hydropower station

The original rock quality designation (RQD) index has several limitations, such as one-dimensional (1D) direction and a single threshold. For these reasons, a novel RQD calculation method is proposed by using 3D discontinuity network simulation. In this method, a 3D discontinuity network model (3D-DNM) of the rock mass is generated by Monte–Carlo analogy procedure, in accordance with the optimal probability models of the rock discontinuity’s geometrical elements. Then, the RQD values at different azimuth on the three orthogonal planes of the 3D-DNM can be analyzed statistically. Based on the analysis result, the novel index RQD t is put forward. In this index, the threshold corresponding to the maximum standard deviation of the RQD t is defined as the optimal threshold. The RQD t with an optimal threshold has the greatest capacity to reflect the heterogeneity and anisotropy of rock mass. To illustrate the effectiveness of the proposed method, we apply it on the foundation rock mass of Baihetan hydropower station on Jinsha River. Finally, the optimal threshold is determined as 0.6 m, and the quality and anisotropy of the dam foundation rock mass are fully demonstrated by the generalized RQD 0.6.

Geostatistical modeling of Rock Quality Designation (RQD) and geotechnical zoning accounting for directional dependence and scale effect

The characterization of rock masses is an essential component for the planning and development of engineering designs in rock mechanics and rock engineering. The Rock Quality Designation (RQD) is a widely used rock mass characterization system that is direction-dependent, i.e., the measurement of a core sample depends not only on the sample position but also on its orientation. This paper outlines the critical aspects of the determination of RQD and proposes a physically-based upscaling strategy from borehole samples to large blocks, based on block-averaging the RQD values corresponding to the same direction, then calculating the minimum value over all the directions. An anisotropy index indicating how much RQD varies between one direction and another is also derived. Using geostatistical simulation, our proposal allows interpolating and upscaling direction-dependent geotechnical variables like RQD at any place in the geographical space for any direction, avoiding directional biases. We illustrate this proposal by predicting RQD in a polymetallic deposit, achieving geotechnical zoning and comparing the results with those of the traditional approach where the directional dependence of RQD is ignored.

Characteristics Analysis of Generalized Rock Quality Designation (RQD) Based on Degree of Joint Development

Rock quality designation (RQD) is widely adopted as a fundamental tool in characterizing rock masses since it was devised in 1964. Since the conventional RQD calculation is limited due to its dependence on the selected threshold, previous research introduced generalized RQD to adequately reflect the anisotropy and scale effect of RQD. However, the influence of the joint development inside rock mass on generalized RQD remains unclear. The objective of this work is to investigate characteristics of the generalized RQD in view of different development degrees of discontinuities in rock mass, including spacing (density) and trace length. Three-dimensional fracture network modelling is employed to simulate the actual rock mass of open-pit iron mine in China. Virtual scanlines are set to obtain RQD values in different directions. The results primarily show that the generalized RQD should be introduced to calculate the RQD with different thresholds to fully reflect the anisotropy of rock mass. The optimal threshold can be obtained based on an anisotropic coefficient, which is defined by (RQDmax-RQDmin). It is also indicated that the fracture spacing has a great influence on both the anisotropy of RQD and the selection of the optimal threshold. The optimal threshold of the generalized RQD increases with the increase in the fracture spacing. In addition, the scale effect of RQD in different models is discussed by changing the length of the scanlines. The longer the scanlines we set, the more stable RQD value can be obtained in the model. It is recommended to fit much longer scanline to get realistic RQD in heavily fractured rock mass.

Analysis of hydraulic conductivity of fractured groundwater flow media using artificial neural network back propagation

Groundwater flow in the Grasberg open-pit mine is governed by fractured flow media. Groundwater modeling in fractured media requires detailed hydraulic conductivity (K) value distribution to illustrate hydrogeological conditions of the Grasberg open-pit mine properly. The value of K is estimated by using the hydraulic conductivity (HC) system method based on rock quality designation (RQD), lithology permeability index (LPI), depth index (DI), and gouge content designation (GCD) data. Accordingly, all parameters must be available, but in this case, this information are only partially available. This paper proposes a method to solve this problem with artificial neural network (ANN) through a five approach scenario to find the K value with a model of incomplete empirical parameters. The artificial neural network back propagation (ANNBP) method is used to estimate the distributed K value based on the distributed observed RQD and LPI, as shown in the block model. This paper uses five optimum architectural schemes consisting of learning rate selection, momentum coefficient, number of nodes, number of hidden layers, activation method (sigmoid, tangent hyperbolic and Gaussian). Verification of the results of the K value is approached by the statistical approaches determination coefficient (R2), correlation coefficient (R), standard error of estimate (SEE), root-mean-squared (RMS), and normal root mean square (NRMS). ANNBP modeling used data for training and testing based on packer test and slug test as many as 49 points. Through five scenarios, it was found that scenario with learning rates of 10–5, momentum coefficient 0.1, number of nodes 10, number of hidden layer 5, with hyperbolic tangent activation methods is the most optimum result with R2 = 0.822, the accuracy of predicting/validating results is expressed by using R2 = 0.863, with R = 0.929, and the error percentage = 3.67%. The K value prediction results are used as an input for groundwater modeling. The modeling results show a significant correlation with high validity between the data extracted from the model and field observation. In a 3D model, the K value distribution is similar to the field RQD data distribution. Furthermore, the K value distribution results are clustered in order to understand the relationship between geological as well as geotechnical data. The highest K values are found in volcanic breccia rocks (DLMVB), volcanic sediments (DLMVS), volcanic andesite (DLMVA) and quaternary glacial till. The geological condition is confirmed by the average rock RQD value between 12.4 and 39.6%.

An Assessment on Permeability and Grout Take of Limestone: A Case Study at Mut Dam, Karaman, Turkey

The main purposes of the present study are to evaluate pilot grouting and to develop regression equations for prediction of grout intake. There are no permeability problems with the sandstone-siltstone-claystone alternations and basement clayey limestone at the dam site. Karstic limestone block is permeable due to karstification and heavy discontinuities. For the purpose of the study, Q system, geological strength index (GSI), secondary permeability index (SPI), joint spacing (JSP), joint apertures (Ap), Lugeon (Lu), and the permeability coefficient (k) were determined. Karstic limestone block rock mass properties correlated with grouting material amount. A series of simple and multiple nonlinear regression analyses was performed between grout take material amount (Gt) and average values of these rock mass properties. Significant determination coefficients were determined. Prediction capacity of the empirical equations were also examined with root mean square error (RMSE), values account for (VAF), mean absolute percentage error (MAPE), and prediction error evaluations. Considering simple regression analyses, the equation derived with Gt-SPI gives the best performance. The best prediction is determined with the equation derived with rock quality designation values (RQD), SPI, and joint aperture as input parameters with the multiple nonlinear regression analysis, in addition to this, other empirical equations also provide acceptable results.

Uncertainty quantification of structural and geotechnical parameter by geostatistical simulations applied to a stability analysis case study with limited exploration data

There are many uncertain factors impacting stability analyses of rock structures. Quantification of these factors significantly impact the engineering decision making process. Some of these factors are spatially distributed within the geological domain, and the quantification of the spatial uncertainty is beneficial for stability analysis of the structure. This paper proposes an integrated methodology for a stability analysis of rock structure by incorporating spatially varying factors. This research models two key factors: one categorical structural parameter (fault zones) and one continuous geotechnical parameter (rock quality designation (RQD)). Two geostatistical simulation algorithms i.e., sequential indicator simulation and sequential Gaussian simulation are applied to quantify the spatial uncertainty by generating multiple equiprobable simulation maps of fault zones and RQD values. A slope stability analysis is performed by modeling the spatially varying fault zones and RQD. The proposed method is validated using a synthetic data set. The results from the synthetic data show the stability value of the synthetic data closely match with stability value calculated from simulated data. Finally, the proposed method is applied to a slope stability analysis for an open pit mine design where limited exploration data is available. Geostatistically simulated RQD maps from the case study mine are analyzed to determine a safe slope angle of 44° for the open pit mining operation. The proposed method can be applied for quantifying other geotechnical risk and integrated within any stability analysis framework.

Modification of the rock mass rating system (RMRmbi) considering three-dimensional rock block size

Traditional measurements of block size (or degree of jointing), such as the rock quality designation (RQD), are questionable due to certain theoretical limitations, including orientation bias and their weakness in considering the joint persistence and three-dimensional shapes of block sizes. This may lead to inaccurate characterizations of rock mass structures and unreliable classification of rock mass qualities. The modified blockiness index (MBi) is a three-dimensional measurement of block size which was developed to overcome these problems. In this study, correlations between MBi and several traditional block size measurements were assessed; based on the MBi, the rock mass rating (RMR) system was modified, and this version was termed RMRmbi. In the first part of this work, multiple simulated experiments were conducted using the GeneralBlock software program and 3DEC (three-dimensional distinct element code), and a large volume of MBi, RQD, joint frequency (JF) and volumetric joint count (Jv) values (artificial data sets) were obtained; subsequently, the correlations between MBi and RQD, JF and Jv were assessed. In the second part, the combined use of RQD and JF in the RMR system was replaced with MBi, and hence, the RMRmbi system was developed; based on the artificial data sets, the viability of RMRmbi was preliminarily supported. At the end of this study, the correlations between MBi and RQD, JF and Jv were verified based on actual data; the RMRmbi was applied to real cases, and a comparison between RMRmbi and RMR was conducted. The results showed that (i) the MBi can capture the influence of joint persistence; (ii) the RMRmbi system can overcome the theoretical limitations caused by RQD and JF; and (iii) simulated experiments showed that the RMRmbi values are more reliable, thus validating the accuracy of RMRmbi and revealing its great potential for future application.

Correlation of P-wave Velocity with Rock Quality Designation (RQD) in Volcanic Rocks

One important part of rock mass investigation is the geomechanical assessment in terms of rock mass classification systems. Rock mass classification is one of themost efficient methods in rock mechanics to provide a basic understanding of rock masscharacterization. Rock mass properties can be determined by a seismic refraction surveyas an indirect geophysical assessment. In this study, the P-wave velocity from seismicrefraction was compared with the Rock Quality Designation (RQD) from the boreholes.The empirical correlation between the RQD and the P-wave velocity was found by usingthe linear regression analysis. The RQD value estimated from the P-wave velocity can beapplied for tropical environment study with geological conditions of volcanic rocks. This study helps to estimate and predict the subsurface rock quality, to reduce investigation costs, and to improve understanding of subsurface rock quality.

Probabilistic Of Slope Failure Grasberg Open Pit Mining PT Freeport Indonesia

The stability of a slope in mining activities is highly influenced by geology, especially thenature of rocks and geological structures. Slope stability analysis will be faced with severalproblems in the uncertainty of rock properties and rock mass. Slope stability analysis usingprobabilistic methods offers a more systematic way of treating conditions of uncertainty andprovides other alternatives to the value factor approach to security regarding information onthe probability of a slope failure.Grasberg Open Mine rock type classification based ongeotechnical parameters or Geotechnical Rock Code (GTRCK) classifies rocks based onrock type, rock mechanical properties, hydrothermal alteration type, clay content and RockQuality Designation (RQD) to 46 types of GTRCK. The GTRCK type which has a low rockmass strength value is a change in intrusive rock minerals and a low RQD value such as alot of clay material is exposed in the Northeast and Southwest of the Grasberg Open PitMine.The overall slope probabilistic modeling of the Grasberg mine uses a cross section of220o (northwest) with the effect of a 0.02g seismic factor which is the maximum criterionaccording to the probability of slope failure received with the average FK 1.13 and PK 0%.

Selection of Optimal Threshold of Generalised Rock Quality Designation Based on Modified Blockiness Index

Rock quality designation (RQD) is a critical index for quantifying the degree of rock mass jointing; it is widely used for evaluating the qualities and stabilities of engineering rock masses. However, the use of traditional RQD may yield inaccurate assessments because only core pieces longer than 100 mm are counted. To enhance the utility of RQD, generalised RQD was introduced. Based on the modified blockiness index (MBi), the determination of the optimal threshold of generalised RQD was performed. In this work, 35 types of hypothetical three-dimensional joint network models were constructed, and their generalised RQD values (with different thresholds) and MBi values were measured. The correlation between the standard ratings of MBi and RQD was assessed; based on this correlation, the theoretical RQD values of the 35 models were derived. The reasonable thresholds of the generalised RQD were determined according to the theoretical RQD values, and the optimal threshold of generalised RQD was obtained using the variation coefficient and anisotropy index of the jointing degree. The discrepancy between the results produced using traditional and generalised RQDs was discussed. Finally, an actual case study was conducted, and the results indicate that the generalised RQD associated with the optimal threshold determined in this study can properly quantify the degree of jointing of a given rock mass.

Rock quality designation (RQD): time to rest in peace

Rock quality designation (RQD) was introduced by Don Deere in the mid-1960s as a means of using diamond core to classify rock for engineering purposes. Subsequently, it was incorporated into the rock mass rating (RMR) and Q-system classification methods that, worldwide, now play substantial roles in rock mechanics design, whether for tunnels, foundations, rock slopes or rock excavation. It is shown that a key facet of the definition of RQD is ignored in many parts of the world, and it is noted that there are several inherent limitations to the use of RQD. Based on mapping of rock formations by 17 independent professionals at different locations in Australia and South Africa, it is shown that differences in assessed RQD values result in significant errors in computed RMR and Q ratings, and also in geological strength index (GSI) and mining rock mass rating (MRMR). The introduction of a look-up chart for assessing GSI has effectively removed the need to measure, or estimate, RQD. It has been found that GSI values derived from the look-up chart are as valid as those derived by calculation from the original component parameters, and are satisfactorily consistent between professionals from diverse backgrounds. The look-up charts provide a quick and appropriate means of assessing GSI from exposures. GSI is, in turn, a useful rock mass strength index; one new application is presented for assessing potential erosion of unlined spillways in rock. Incorporation of RQD within the RMR and Q classification systems was a matter of historical development, and its incorporation into rock mass classifications is no longer necessary.

Geology and geotechnical evaluation of Doha rock formations

A generalised ground model with geotechnical characterisation has been developed for the city of Doha, in Qatar, combining a wide range of ground investigation data and observations during the construction of projects that WS Atkins &amp; Partners Overseas were involved with. The model proposes a geotechnical characterisation and ranges of parameters for the main units found at depths relevant to civil engineering activities. A geotechnical classification system is proposed to differentiate the Simsima Limestone subunits encountered on site, based on rock quality designation (RQD) values and weathering degree. The proposed classification system and the relevant geotechnical parameters have been validated during construction. The construction of all excavations, temporary retaining systems and tunnel excavations that rely on the information presented in this paper has been completed successfully. This confirmed a reasonable agreement between the theory presented in the paper and the practical conditions encountered on site. Best site investigation practices are discussed with highlights on difficulties encountered on site and how they were resolved. RQD and rock mass rating values were found to produce a good representation of the ground conditions while the applicability of the geological strength index classification is believed to be limited.

Application of alternative methods for determination of rock quality designation (RQD) index: a case study from the Rožná I uranium mine, Strážek Moldanubicum, Bohemian Massif, Czech Republic

A comparison of rock quality designation (RQD) parameters obtained by drill core analysis and the RQD determined using alternative methods is presented using metamorphic rocks such as migmatized gneisses, migmatites, and amphibolites. Methods of borehole–wall imaging using high-resolution acoustic logging, optical televiewer, and simple video inspection as well as the structural analysis of exploration drift walls oriented subparallel to the analysed boreholes are used for alternatively evaluated RQD values. Results show that, in most cases, the RQD index measured directly on a drill core section reaches lower values (about 10%–30%) than the RQD evaluated by alternative methods. Significant differences in determination of the rock mass failure degree using classical and alternative methods may occur, especially in sections with higher frequency of the metamorphic foliation planes created by biotite-rich bands or by dark-coloured melanosome layers. The nature of biotite itself is favourable for the creation of pre-disposed jointing planes, in which the increased drill core disintegration caused by drilling technology or core handling can occur. Results show it is highly appropriate to make these comparisons on a larger scale and thus verify whether the observed differences affect the qualitative classification of the rock mass.

A New Method for Improving the RQD Determination of Rock Core in Borehole

Rock mass soundness is typically measured by engineers using the rock quality designation (RQD) combined with their own experience (Choi and Park 2004). Many researchers have established empirical models to estimate mechanical properties from the RQD (Zhang and Einstein 2004; Jiang et al. 2009), rock mass rating (RMR) (Bieniawski 1978; Nicholson and Bieniawski 1990; Chun et al 2009; Hoek and Brown 1997), or Q methods (Barton et al. 1974; Barton 1983, 2002) and the cumulative core index (Sen 1990). Considering its worldwide acceptance and usage in major rock mass classifications such as Q and RMR, RQD is among the practical useful parameters, especially when the geological strength index (GSI) is concluded from the RMR. Applications of RQD have also been quickly extended to other areas of rock mechanics, so that it is now among the fundamental parameters of geotechnical engineering (Hoek and Brown 1980; Hoek and Bray 1981). The RQD index is affected by a number of well-known factors and limitations; For example, it yields different values for a given location when samples are extracted from cores with different drilling orientations. Moreover, factors such as the quality of drilling and logging, rock strength and core size, water conditions, joint apertures, alteration, and roughness affect RQD values. Another significant limitation of the RQD definition is its dependence on the selected threshold length of unbroken rocks (Priest and Hudson 1976; Harrison 1999; Hack 2002; Chen et al. 2005). Therefore, RQD values for the same core typically vary for different threshold lengths. In practice, a familiar observation associated with this drawback is that RQD values tend to be either high or low (often above 70 % or below 10–20 %) in most rock engineering projects. Some values (e.g., between 40 and 60 %) are less frequently encountered, due to the customarily and universally adopted, but very arbitrarily selected, threshold value of 10 cm (for NX cores) in the assessment of RQD (Harrison 1999). To obtain a wider range of RQD values, Harrison (1999) proposed a technique for determining the optimal threshold length. However, this approach is only appropriate for a particular rock mass. Besides, this technique requires the determination of the minimum and maximum values of the discontinuity frequency in the rock mass, which generally means that the simplicity of the original RQD is lost. It is concluded that, since RQD measurements are encumbered by several limitations, they should be used with caution. Limitations of the RQD index influence engineering results when applied in classification systems, numerical modeling, and other engineering assessments (Palmstrom, 2005). Thiswork aims to describe the deficiencies of the RQD and propose a new, straightforward method for calculation of borehole RQD. This method also enables removal of the effect of joint orientation and piece length on RQD values.

Practical Investigations on Use of Weighted Joint Density to Decrease the Limitations of RQD Measurements

Various techniques have been established in geotechnical engineering to determine the quality of rock drilled out from a borehole. These simple and inexpensive methods include total core recovery (TCR), solid core recovery (SCR), and rock quality designation (RQD). Among these, RQD provides a general indication of rock mass quality and is widely used as an index to evaluate rock quality by expressing the degree of fracturing in drill cores. The RQD was modified from the TCR method by Deere (1963) to indirectly measure the number of fractures and amount of softening or/and alteration in rock mass for engineering applications. Its simplicity, low cost, and reproducibility generally resulted in quick development of the RQD for a wide variety of rock engineering applications, such as tunnels, mining engineering, and large caverns. Nowadays, this method is considered as a standard method in drill core logging and forms a basic element of the most widely used rock mass classification systems, such as the rock mass rating (RMR) and Q-system (Bieniawski 1989; Barton 1995). However, a number of limitations such as dependence on borehole orientation and the selected threshold value for the minimum intact core length restrict the consistency of the measured values. Accordingly, several methods have been established to overcome these limitations, such as the corrected rock quality designation (RQDc) (Li et al. 2009), volumetric joint density (Jv) (Palmstrom 1982, 1985, 1986; Sen and Eissa 1992), cumulative core index (Sen 1990), and weighted joint density (WJD) (Palmstrom 1995, 2005). Although such new methods have been developed, the RQD is still used with the initial definition, without correction, in many geotechnical engineering applications, e.g., rock mass classification, rock mass strength, and modulus of elasticity. This paper reviews the conventional RQD measurement techniques by Deere (1963) and RQD measurements from the WJD (hereafter called RQDWJD) by Palmstrom (2005) using rock cores drilled out from directional boreholes at the Bakhtiary Dam site. In this way, the RQD values from different methods were compared, and a new modification is proposed, called RQDM-WJD.