Empirical techniques for assessing rock mass condition

Due to complicated structures and discontinuities in surrounding rock mass, existing empirical failure criteria cannot meet the requirements of engineering practice such as tunnels. To improve estimation accuracy on the strength of rock mass with joints, a modified chart of the Geological Strength Index using Hoek–Brown criteria was further tested to estimate rock mass strength [Lin et al. (2014) Bull Eng Geol Environ 4(73):1245–1258], and, in this paper, new strength estimation equations for jointed rock mass were then modified based on a large dataset obtained from Chinese projects. Here, standard drilling time is first introduced and described in this study, and then used as a parameter to estimate rock strength. Different empirical formulas based on joint density, rock mass classification, Hoek–Brown criteria, and elastic wave velocity are thus used to estimate rock mass strength by using data from the Jiubao tunnel. The results estimated based on different empirical formulas were similar, indicating that the modified assessment method presented in this paper can be used to estimate rock mass strength under certain circumstances. Cross-correlation of different empirical methods provides significant confidence in predicted rock mass strength calculations.

Improved reliability in rock mass characterisation for underground support design – Discrete fracture network model and site observation

Empirical methods form the basis of design for ground support and reinforcement of development at the Challenger Gold Mine (CGM) in South Australia. In a general sense, the rock mass conditions at CGM are very good. In situ rock stress is low relative to most Australian conditions with near hydrostatic conditions at a depth of 1,000 m, and the rocks themselves are strong and competent. As a consequence of the generally favourable geotechnical conditions, the design of ground support and reinforcement for mine development areas is heavily reliant on the use of empirical methods. Detailed geological and geotechnical mapping is undertaken in all headings, and particularly at the site of proposed intersections. The rock mass is characterised using the Q-System, and various techniques are used to determine the appropriate assignment of the base parameters. As an example, rock quality designation (RQD) is interpreted from threedimensional contouring of diamond drill data and crosschecked against volumetric joint counts. Standard patterns for ground support and reinforcement are applied when the rock mass is characterised as ‘good’ or ‘fair’. Characterisation of the rock mass as ‘poor’ triggers site specific design to suit the prevailing conditions. All design is augmented by kinematic wedge analysis and geological observation. Currently, CGM ground support and reinforcement designs target a Factor of Safety (FS) of at least 1.5 in static conditions with an overriding dynamic capacity in excess of 5 kJ/m2. In recent years, considerable effort has been placed on optimising the empirical designs while maintaining the overall integrity of the process. This has led to a significant reduction in use of consumables through the optimised use of geotechnical engineering.

In open-pit excavations, overburden rock mass is disturbed by processes like blasting and mechanical excavation, leading to a reduction in mechanical properties. Accounting for this disturbance is essential for ensuring slope stability, optimizing costs, and maintaining feasibility. The Hoek–Brown failure criterion, a widely used empirical method in rock mechanics, incorporates the disturbance factor to reflect the reduction in rock mass strength after disturbance. This study reviews five approaches from the literature regarding the role of disturbance in rock mechanics, focusing on its impact on the factor of safety and the volume of rock mass above the potential failure surface. Additionally, an “S” shaped decay formulation was proposed as an alternative to existing equations. A key consideration is the transitional disturbance effect, which reflects the gradual change from a fully disturbed rock mass near the excavation surface to an undisturbed rock mass with increasing depth. Among the examined approaches, the “S” shaped decay equation, informed by insights from previous studies, appears to be the most realistic. One approach assumes the disturbance factor is highest at the surface due to the removal of blasted rock, leading to a fully disturbed rock mass in front of the excavation face. The disturbance then decreases with depth, transitioning to an undisturbed condition depending on the excavation method. Even when the rock mass is homogeneous and isotropic in joint properties, excavation induces anisotropy in mass strength, causing overall strength to increase with depth. This study also investigates the effect of anisotropic strength behavior resulting from the disturbance factor. For incorporating transitional disturbance in the design stage, both circular and combined failure mechanisms should be considered for a comprehensive understanding of slope stability.

Setting up simple estimating equations of TBM penetration rate using rock mass classification parameters

ABSTRACT: The main classification systems that estimate rock mass properties include variants of the Rock Mass Rating (RMR) System, Rock Quality Designation (RQD), the Norwegian Tunneling Index (Q) (Barton), and the Geological Strength Index (GSI). The RMR system is based on intact strength, Rock quality designation, fracture frequency, and discontinuity condition. Typical distributions for these parameters may be approximated by normal, lognormal, or Weibull distributions and discrete distributions for fracture conditions. Water and pore pressure are sometimes included but should generally be treated as a boundary condition. RQD is a single-parameter system describing the percentage of sound rock core with a length greater than 10 cm. The Barton system has three main terms to describe a relative measure of the block size, an indicator of the inter-block shear strength, and a third quotient to describe "active stresses." One of the primary findings of the statistical study is that two parameter systems, such as blockiness and joint condition, are better differentiators between rock mass classes; however, these systems still smear parameters. For example, a GSI range of 25-55 each indicates anything from blocky to disintegrated rock mass conditions. As a result, strength estimators based on GSI are not reliable for estimating rock mass strength and, in turn, the factor of safety. RMR and GSI results result in values on a normal distribution scaled from 0-100, typically centered on 50. The models are insufficient to describe the tail end of the distribution where rock mass failure typically occurs. The Q system significantly improves when the terms are derived independently but suffers from the same normalization issue when calculated from an RMR value. The surprising finding is that single-parameter systems, including Fracture Frequency and Volumetric joint count, are better estimators (and differentiators) or properties across the range of rock mass conditions when looked at individually. Always understand the range of the rock mass parameters used and how that can impact the design. The paper will evaluate the need to develop strength estimators based on fracture frequency and volumetric joint count. Rock mass strength estimates should be developed from the underlying input data for classification rather than from normalized sums of multiple parameters.

Purpose. Empirical and numerical methods play a vital role in assessing rock mass behaviour quantitatively and qualitatively to design underground structures/caverns and support systems. This research aims to assess and evaluate the rock mass be-haviour for safe, stable, efficient, and economical design of support system for underground structures especially tunnels in diverse rock mass conditions. Methods. In this research, such empirical design methods as Rock Mass Rating (RMR), Q-system and GSI were used to characterize and classify the rock mass environment along the tunnel for the preliminary design of twin tunnels and support systems. The geomechanical parameters, Hoek-Brown failure criterion, and its variants for assessing rock mass behaviour were optimized using multiple regression of Stewart, generalized and globalized variant of nonlinear regression method. The rock mass was classified for the selected section A-A. The excavation method and support system for the said section were designed based on the results obtained from empirical modelling. 2D elasto-plastic finite element method (FEM) was used for numerical analysis of rock mass behaviour and performance of the designed supports in section A-A. Findings. The major rock type encountered in the diversion scheme comprises gabbronorite (GN) and Ultramafic Association (UMA). Based on the quantification of RMR, Q-system, and GSI, section A-A’s rock mass ranges from very poor to poor. From the numerical analysis for the said rock mass environment both RMR and Q system support recommendations are equally efficient to support the rock mass surrounding the tunnel. However, keeping in view the yield zone, especially in the crown, the rock bolt’s length should not be less than 5 meters. Based on the analysis of results, both the tunnels are at a safe distance from each other. Originality. In this research, the design input parameters for numerical modeling were optimized by using different techniques to eliminate the chances of error in evaluating rock mass behaviour and designing an optimum support system in the said rock mass environment. Practical implications. The assessment of rock mass behaviour and the design of optimum support systems in heterogenous conditions is quite challenging and requires thorough investigation through different design techniques. This research provides a refined meth-od to be used for the safe, stable, and economical design of tunnels.

A fractured rock mass is divided by the fracture sets into mixed shapes of blocks and wedges. The geotechnical parameters of the rock mass are obviously dominated by the presence and the spatial distribution of fracture sets. In rock engineering, the determination of geotechnical parameters of rock masses is a key issue for the prediction of expected behaviour of the fractured rock mass. Several measurements and empirical methods can be used to investigate the characteristics of the fractures including its spacing and orientation. The goal of this paper is to find the possible connection between the calculated fractal dimension (D) and the derived values originating from several rock mass classification systems (e.g. RQD, RMR, Q, GSI). Principally, the derivation of fractal dimensions and the rock mass classification systems are based on empirical methods. For our present investigation the fractures visible in scores of pictures taken by automatic photo robot of the tunnel faces were used at the low and intermediate radioactive waste repository located at the Bataapati site in Southern Hungary. The BENOIT software was used to calculate the single fractal dimension according to the box-dimension methodology and the rock mass classification values were evaluated with the JointMetriX3D (ShapeMetriX3D) software. Our results show that some connection between geotechnical parameters and fractal dimension values can be obtained by the applied methodology. Using these empirical relationships, it is possible to carry out more safety construction and calculations in the radioactive waste repository in Bataapati.

Effect of the rock material strength on the RMR value and tunnel support designs were investigated within this study including site works, analytical and numerical analyses. It was found that rock material strength effect is quite limited in the RMR method to determine an accurate rock mass class to design tunnel support. Since the limitation, rock mass classes are evaluated to be usually misleading and supports designed in accordance with the RMR value are insufficient for tunnels excavated in rock masses with low strength values of rock materials. Totally, five different tunnels in Turkey have been supported using a new strength adjustment factor calculated in consideration of the in-situ stress and the uniaxial compressive strength values of rock materials. As confirmed by the field applications, analytical and numerical analyses, a newly modified RMR value (RMRus) was suggested to be used in tunnel support design works.

On the influence of rock mass quality on the quality of blasting work in tunnel driving

The complex structure of the columnar jointed rock mass (CJRM) brings difficulty in determining its field mechanical parameters, and a reliable estimation of the strength and deformation of CJRM is crucial for engineering safety. Uniaxial compression tests were conducted on artificial CJRM specimens with columnar structures of quadrangular, pentagonal and hexagonal prisms to model the mechanical behaviour of the CJRM. The strength and deformation anisotropy were compared and analyzed on the basis of the results by combining the structural features of the three models. The failure modes and mechanisms were summarized in accordance with the failure processes and final appearances of artificial specimens. The empirical relations of predicting the field mechanical parameters were derived on the basis of the joint factor approach, the empirical gradient method and the rock quality designation (RQD). For the CJRM of the Baihetan Project, the proposed empirical correlations were used to estimate the field unaxial compression strength and elastic modulus, and the predicted values were compared with those obtained from the field tests and other empirical methods. Results revealed that the estimated values calculated by the proposed empirical equations are in good agreement with the field test results, and the three empirical relations can predict the mechanical parameters of the Baihetan CJRM effectively. The empirical relation based on RQD is extremely valuable when RQD is the only available information about discontinuities in site investigations.

Real-time perception of rock–machine interaction information in TBM tunnelling using muck image analysis

The limestone of the South China Lower Permian Qixia & Maokou Group usually features high purity, high strength, and poor weathering, while the limestone of the Qixia & Maokou Group at margin of the Chuan-Dian Paleouplife of southeast China is characterized by obvious strength weakening. The weakening of limestone strength leads not only to excessive errors in the estimation of useful material reserves in the limestone quarry, but also increases the engineering slope support cost in the limestone distribution area on the eastern margin of the Kangdian Paleouplife. In order to ascertain the cause mechanism of the strength weakening of the limestone in the stockyard, we take the stockyard in the dry valley as an anatomical point. The strength weakening of limestone is comprehensively analyzed by means of drilling, large-section excavation, site investigation, sampling, and physical mechanics tests. The main findings are as follows: (1) The limestone area of the Qixia & Maokou Formation near the Kangdian Paleouplife generally contains terrigenous fine-grained materials and mud iron; (2) The weakened rock masses are divided into two types of weakening: deep dissolution type and deep strong weathering type. The uniaxial compressive strength of the deep weakened rock mass is only 2.5-25 MPa, which is 70%-90% lower than that of the fresh rock mass. Compared with the natural state, the compressive strength of the saturated rock mass is reduced by 60%-90%. The deep weakened rock mass has the characteristics of weakened strength and is softened by water; and (3) The dissolution type rock mass is weakened as a result of the dissolution of the high-level exposed broken spring along the east-west fracture zone; the formation mechanism of the strong weathering rock mass weakening is that under the conditions of tectonic shear, terrigenous muddy interlayer and dry-heating condition. The buried depth of groundwater is large, the hydrodynamic force is weak, and the coupling of water and rock under long-term infiltration environments makes the limestone body softened and argilized in situ, leading to a significant decrease in the strength of the rock mass.

Determining stability, quantifying planned dilution, and estimating the potential dilution associated with hangingwall overbreak are critical in the process of stope design in sublevel open stoping mines. To satisfy these objectives, empirical stability graphs and numerical modelling are currently used in the mining industry. Empirical methods are limited to the database used to calibrate them. In the case of numerical modelling, some of the available criteria used to evaluate hangingwall overbreak do not include the intermediate principal stress around the stope and/or the rock mass geotechnical characteristics. In this study, a new criterion for numerical modelling is proposed to estimate the hangingwall overbreak in open stopes. This new criterion includes the intermediate principal stress around the stope and the rock mass geotechnical characteristics. To develop the criterion, several open stope numerical models are simulated considering different geometrical and geotechnical conditions. The criterion is calibrated to reproduce empirical case histories of hangingwall overbreak. Next, the criterion is verified with case histories of hangingwall overbreak that presented different conditions used to calibrate the criterion. The proposed criterion establish a significant influence and relationship between rock quality and the minimum and intermediate principal stresses on hangingwall overbreak. The criterion offers sufficient flexibility for application to a wide range of geometries, in situ stress conditions, and depth and rock mass properties.

This work explains the development of a so-called rock mass system performance map, that provides a backdrop to assist in performance interpretation. The rock mass system performance maps presented in this paper are 2D graphical devices prepared using Sammon mappings, Learning Vector Quantisation, Self-Organising Topological Maps and combinations of these mathematical techniques. Using supervised or unsupervised learning methods, these algorithms project and partition high-dimensional vector spaces, of a dimension equal to the number of environmental and rock mass condition parameters considered, into 2D categories of rock mass condition. In providing 2D renderings, the techniques aim to preserve properties of the characterising vector space such as adjacency of states and topology. The condition of a rock mass can be ‘plotted’ on that map by identifying its k-nearest neighbours and interpreted relative to stability metric categories. Should the defining vector space include rock mass properties or environmental parameters that are repeatedly measured, or possibly continuously monitored, then the rock mass condition marker traces out a performance trajectory across the performance map. An example of rock mass performance map synthesis and use is presented.