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.     

Safe rapid drifting – Support selection

Current drift advance rates in mining fall short of expectations with advances in drilling and blasting technologies. Quick access to orebodies improves their net present value (NPV). This is critical for block cave mining where several kilometers of drift network is initially required at high capital cost. Many mining companies are now planning block caving because of its long-term low production cost. This paper critically reviews the developments in tunnelling and mine drift development rates with advances in drilling and explosives technologies. Current drift support practice during development is also critically reviewed together with the rock mass classification systems. These reviews show that, while drilling and explosive technologies have drastically improved since 1850, current drift advance rates in the Canadian metalliferrous mining industry have either remained stagnant or dropped below the1960 advance rate levels and in comparison to advance rates in civil tunnelling. It is also established that a major cause for this stagnation is the use of long-term support in good ground conditions where only temporary support is required near-face for worker safety in the short-term. Long-term support takes up 46% of development cycle time. The paper presents a methodology for drift support design for underground hard rock conditions typically found in current mining practice in the Canadian Shield and discusses the rationale for optimizing ground support systems installed near-face during drift development. An improved Q-system called Q-star (Q^*) that accounts for discontinuous joints (rock bridges) and roughness of short joints in the rock mass is developed to more reliably estimate the self-support capacities of rock masses. It is recommended that construction damage be accounted for in the rock mass rating for safe support selection during development. A procedure is developed for the adjustment of Q^* to account for construction damage. Perimeter blasting is recommended as pre-requisite for rapid drift development in order to minimize construction damage, reduce support demand and scaling and mucking times. A support matrix is presented based on rock mass quality and stress level for safe rapid drift development. Two case histories in active mines are presented to validate the procedure. The methodology is applicable to stress-induced damage and may not apply where complete relaxation occurs. While the procedures presented are focused on typical conditions in the Canadian Shield underground mines, they may be applicable in civil engineering tunneling and other underground mines where drill-and-blast is used as the excavation method.     

Classification and assessment of rock mass parameters in Choghart iron mine using P-wave velocity

Engineering rock mass classification,based on empirical relations between rock mass parameters and engineering applications,is commonly used in rock engineering and forms the basis for designing rock structures.The basic data required may be obtained from visual observation and laboratory or field tests.However,owing to the discontinuous and variable nature of rock masses,it is difficult for rock engineers to directly obtain the specific design parameters needed.As an alternative,the use of geophysical methods in geomechanics such as seismography may largely address this problem.In this study,25 seismic profiles with the total length of 543 m have been scanned to determine the geomechanical properties of the rock mass in blocks Ⅰ,Ⅲ and Ⅳ-2 of the Choghart iron mine.Moreover,rock joint measurements and sampling for laboratory tests were conducted.The results show that the rock mass rating(RMR) and Q values have a close relation with P-wave velocity parameters,including P-wave velocity in field(V_(PF)).P-wave velocity in the laboratory(V_(PL)) and the ratio of V_(PF) V_(PL)(i.e.K_p = V_(PF)/V_(PL).However,Q value,totally,has greater correlation coefficient and less error than the RMR,In addition,rock mass parameters including rock quality designation(RQD),uniaxial compressive strength(UCS),joint roughness coefficient(JRC) and Schmidt number(RN) show close relationship with P-wave velocity.An equation based on these parameters was obtained to estimate the P-wave velocity in the rock mass with a correlation coefficient of 91%.The velocities in two orthogonal directions and the results of joint study show that the wave velocity anisotropy in rock mass may be used as an efficient tool to assess the strong and weak directions in rock mass.     

Influences of the valley morphology and rock mass strength on tunnel convergence: With a case study of Khimti 1 headrace tunnel in Nepal

Underground structures are constructed at the bottom of the valley sides for various purposes and for different reasons. Hydropower projects and transport tunnels are some of the examples of such structures. In this paper, literatures on topographical effects on the in situ stresses in valley and fjord sides are reviewed. An attempt is made to correlate stress anisotropy problems with the valley side topography by using Phase² numerical modelling. Based on an underground construction case study, fifteen in situ stress measurements and the Phase² analysis, stress induced problems have been found to be influenced by the valley morphology. This influence can be monitored by the convergence measurement and by the stress measurement. In addition to the overburden height, the total valley height and the slope need to be considered in the assessment of the stress induced problem. The second aspect dealt with is the influence of the rock strength on the tunnel convergence. In the Khimti 1 headrace tunnel and 66 cases from 15 countries, it has been observed that the tunnel convergence is larger in the weaker rocks than in the stronger rocks though they may have similar Q-values. Rock type such as gneiss or phyllite (corresponding to the rock mass strength) is not considered in the Q-system but it has influence on the convergence that takes place in underground works. Thus, it also needs to be considered in the assessment of potential convergence of an underground structure.     

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 evaluation and preliminary support design for the metro extension tunnel,Ankara, Turkey

The paper reports an assessment of the engineering geological characteristics of the rock mass to be encountered between Mecidiye and Gazino stations on the new extension of the Ankara metro and the determination of appropriate support and excavation methods. The rock mass quality was estimated using the rock mass rating (RMR), geological strength index (GSI) and rock mass quality (Q) systems and the tunnel divided into sections. The RMR, Q and NATM systems were used to determine the support and excavation methods in these areas. The deformations and stress concentrations around each tunnel section were investigated and the interaction of the support systems with the rock mass was analyzed using finite element software. It is concluded that rock mass classification systems should be used in tandem with numerical tools, although it is emphasized that the estimation of rock mass properties is not an exact science and both rock properties and numerical models should be refined based on observations and the results of instrumentation installed during the construction of a tunnel.      

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.      

Measurements of and correlations between block size and rock quality designation (RQD)

Various measurements of the block size or degree of jointing (i.e. density of joints, RQD, block volume, joint spacing) are described. It is concluded that the RQD measurements are encumbered with several limitations and that this parameter should be applied with care. These limitations influence the engineering results where RQD is applied in classification systems, numerical modelling and other engineering assessments.The three-dimensional block volume (V_b) and the volumetric joint count (J_v) measurements give much better characterizations of the block size. As the block size forms an important input to most rock engineering calculations and estimates, it is important to select the most appropriate method to measure this parameter.Correlations between various measurements of block size have been presented. It turned out difficult to find any reliable correlation between RQD and other block size measurements. An adjusted, better equation between RQD and J_v than the existing is presented, though still with several limitations.More efforts should be made to improve the understanding on how to best measure the block size in the various types of exposures and patterns of jointing.     

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.     

Extending the Q system's prediction of support in tunnels employing fuzzy logic and extra parameters

Rock mass classifications predict support measures according to expert rules by rating rock mass and taking into account the span of the opening. A similar procedure is adopted, in this work, and computerized using statistics and fuzzy logic. Fuzzy expert systems are trained with data of previously constructed underground openings. Using subtractive clustering the systems have the intelligence to pick up the relations between input and output and define the rules that represent the system's behavior automatically. These systems are found to predict support to be used more successfully than the Q system. With the introduction of extra input variables, which are important in numerical analysis, such as depth and intact rock strength, an extended fuzzy system is developed. This system is suggested for preliminary use as it is able to predict support even better.     

Determining the damping factor of sedimentary rocks required for seismically designed structures

Relationships for the shear damping ratio D_s (a function of shear quality factor Q_s) and modulus decay curve G_seis/G_max as a function of shear strain for mudrocks have been developed. Field experiments to determine damping ratio and elastic moduli should be performed at frequencies as close as possible to the bandwidth (0–100 Hz) of interest for building design. Estimates of these parameters made from extrapolating laboratory core (MHz) or wireline (kHz) data to lower frequencies can be highly unreliable. Field estimates of damping ratio are very dependent on, and often totally dominated by, the effects of scattering of P and S waves by inhomogeneities in the rock mass structure. This is, particularly, the case when open fractures or cracks are present in near-surface rocks.     

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.     

The joint intersection probability

In this paper a practical method to apply block theory is presented. Block theory provides the removable joint pyramids from a given free surface regardless of the number of joints in any joint intersection. While robust, the application of the theory in real practice is hampered by the large outcome space of possibly removable joint pyramids consisting of k mutually exclusive joints in a rock mass consisting of m joint sets. In this paper, we prove that the probability that k is greater than three in a three-dimensional space is zero. Consequently, only tetrahedral blocks need to be considered in the stability analysis for the analyzed free surface. The outcome space of theoretically removable joint pyramids can be further reduced by considering “safe” joint intersections, which consist of at least one line of intersection which is sub-parallel to the free surface. The block failure likelihood of the remaining joint intersections is proportional to two independent parameters: (1) the joint intersection probability and (2) the block instability parameter. We develop here a rigorous joint intersection probability expression based on simple frequency probability considerations which predicts that the probability for x in the rock mass to fall in joint intersection L_i,j,k is inversely proportional to the volume of the parallelepiped formed by joints i,j,k with mean spacing values x_i, x_j, x_k:Using the joint intersection probability and the instability parameter associated with each removable JP the critical key blocks of the excavation can be determined. In a brittle rock mass only the critical key blocks will require reinforcement. The paper concludes with a practical example which demonstrates the application of the concepts.     

Effect of rock mass quality on construction time in a road tunnel

On the basis of the existing NTNU (Norwegian University of Science and Technology) advance rate model and field experiences, an estimation model which can analyze construction time for a broad range of works related to tunnel construction by drill and blast has been established. The model includes the time spent for excavation, rock support, various installations in the tunnel, and site preparation. The model is developed as a spreadsheet. Furthermore, by the use of the model, analyses on various effects of rock mass quality on construction time and advance rate for four Q-values of 0.01, 0.1, 1.0 and 10.0 on seven sizes of road tunnels applied in Norway were made. The results show that construction time increases up to 30–40% with cross sectional area varying from T5 (35.2 m²) to T12 (86.9 m²) under the assumption that the same Q-value is applicable to the entire tunnel length. Standard advance rate considering the effect of rock mass quality may be about 50% lower for Q = 0.01 than in the case of not considering the same effect in a 3 km tunnel. The gap between the two standard advance rates is gradually decreasing with increased Q-value.     

Stress–strain–electrical resistance effects and associated state equations for uniaxial rock compression

This paper reports stress–strain–electric resistance experiments for diabase, limestone and marble containing NaCl solution during the whole process of uniaxial compression. We obtained the complete testing data for the stress–strain curve and the associated electrical resistance–strain curve. The change caused by internal cracking of the rock causes the corresponding variation of rock electrical resistance. There is a minimum value for all the electric resistance–strain curves, corresponding to the cracking stress σ_c or the initial cohesion c_i. Based on the experimental results and stochastic property analyses of the rock fracture variation, we put forward a group of state equations for rock in sections to express the characteristics of rock during the whole process of uniaxial compression. The three variables, stress, strain and electrical resistance, together with data-fitted parameters, α₁,α₂ and β, are contained in the equations. The equations are used to express the inelastic response which intensifies with the propagation of cracking.     

A quantitative comparison of strength criteria for hard rock masses

Knowledge of the rock mass strength is important for the design of all types of underground excavations. A frequently applied approach for estimation of the rock mass strength is through an empirical failure criterion, often in conjunction with rock mass classification/characterisation systems. This paper presents a review of existing methods to estimate the rock mass strength using empirical failure criteria and classification/characterisation systems—in this study, commonly denoted as estimation methods. A literature review of existing methods is presented, after which a set of methods were selected for further studies. The selected methods were used in three case studies, to investigate their robustness and quantitatively compare the advantages and disadvantages of each method. A Round Robin test was used in two of the cases. The case studies revealed that the N, Yudhbir-RMR₇₆, RMi, Q-, and Hoek–Brown-GSI methods, appeared to yield a reasonable agreement with the measured strengths. These methods are thus considered the best candidates for realistic strength estimation, provided that care is taken when choosing values for each of the included parameters in each method. This study has also clearly shown the limits of presently available strength estimation methods for rock masses and further work is required to develop more precise, practical, and easy-to-use methods for determining the rock mass strength. This should be based on the mechanical behaviour and characteristics of the rock mass, which implies that parameters that consider the strength of intact rock, block size and shape, joint strength, and physical scale, are required.     

Influence of intermediate principal stress on rock fracturing and strength near excavation boundaries—Insight from numerical modeling

The influence of the intermediate principal stress on rock fracturing and strength near excavation boundaries is studied using a FEM/DEM combined numerical tool. A loading condition of σ₃=0 and σ₁≠0, and σ₂≠0 exists at the tunnel boundary, where σ₁, σ₂, and σ₃, are the maximum, intermediate, and minimum principal stress components, respectively. The numerical study is based on sample loading testing that follows this type of boundary stress condition. It is seen from the simulation results that the generation of tunnel surface parallel fractures and microcracks is attributed to material heterogeneity and the existence of relatively high intermediate principal stress (σ₂), as well as zero to low minimum principal stress (σ₃) confinement. A high intermediate principal stress confines the rock in such a way that microcracks and fractures can only be developed in the direction parallel to σ₁ and σ₂. Stress-induced fracturing and microcracking in this fashion can lead to onion-skin fractures, spalling, and slabbing in shallow ground near the opening and surface parallel microcracks further away from the opening, leading to anisotropic behavior of the rock. Hence, consideration of the effect of the intermediate principal stress on rock behavior should focus on the stress-induced anisotropic strength and deformation behavior of the rocks. It is also found that the intermediate principal stress has limited influence on the peak strength of the rock near the excavation boundary.     

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.     

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 and assessment of pre-grouted rock mass

Pre-grouting is a technique for reducing water ingress into tunnels and caverns by grouting fractures and joints prior to excavation. This study investigates pre-grouted rock mass to evaluate grout penetration in fractures and transmissivity of water in the rock mass surrounding the built tunnel, with the use for core drilling, OTV, high-precision water injection tests and core logging. The study was performed in three tunnel localities, in tunnels excavated in connection with the Follo Line project in Norway, where pre-grouting was performed using cement-based grouts. It was found less cement than expected in fractures with small apertures, compared with results of grout penetrability in laboratory studies of similar grouts. Further, it was found that fractures in coarse-grained rock types had rougher fracture surfaces and higher hydraulic apertures, than fractures in fine-grained rock types. It was also found that fractures with smoother surfaces had smaller hydraulic apertures in general. Hydraulic jacking was evidenced during the pre-grouting in this area, which is likely to have contributed to unnecessary high grout consumption.