Blocky Rock Mass Research Articles

Scaling rockburst hazard using the DDA and GSI methods

We examine the influence of rock mass quality, as scaled by the Geological Strength Index (GSI), on energy redistribution in tunnels driven through discontinuous rock masses. We assume that in blocky rock masses rockbursts develop asabrupt motion of finite rigid blocks along pre-existing discontinuities rather than by fracture of intact rock elements. We begin by formulating analytically the local energy density around a tunnel in continuous, homogenous, isotropic, linear-elastic medium and demonstrate the significance of the initial principal stress ratio on the result. We then introduce discontinuities into the rock mass and find analytically the peak acceleration of an ejected keyblock when it flies into the tunnel space, to demonstrate the viability of this mechanism asa potential rockbursting source. Using the numerical discontinuous deformation analysis (DDA) method we find the total kinetic energy released during rockbursting and validate our DDA results using monitored seismic energy emissions detected during an intensive rockburst event encountered while excavating one of the headrace tunnels at Jinping II hydroelectric project in China. Utilizing an analytical solution we published earlier for the redistribution of energy components due to tunneling, we explore the effect of rock mass quality as scaled by GSI on the elastic strain energy, dissipated energy, and kinetic energy. We find that the elastic strain energy and the energy dissipated by shear generally decrease with increasing GSI value. The kinetic energy of rockbursts however shows a more complicated behavior. It is low at low quality rock masses, peaks at GSI value of about 60, and decreases again with increasing rock mass quality. This result is supported by documented rockbursts during excavation of the deep tunnels of the Jinping II hydropower project, where the majority of rockbursts were recorded in tunnel segments with characteristic GSI values between 60 and 75.

Propagation of Pendulum Waves under Deep-Seated Cord Charge Blasting in Blocky Rock Mass

The object of the numerical study is the travel of pendulum waves in a blocky medium under nonstationary impact of deep-seated charge blasting on the surface of the expansion chamber. The blocky model is simulated by a two-dimensional lattice of masses connected by elastic springs along the axes and diagonals. The displacements and velocities of the masses at different half-space points are calculated using the finite-difference method.

Prediction of strength and deformability of an interlocked blocky rock mass using UDEC

The accurate prediction of strength and deformability characteristics of rock mass is a challenging issue. In practice, properties of a rock mass are often estimated from available empirical relationships based on the uniaxial compressive strength (UCS). However, UCS does not always give a good indication of in situ rock mass strength and deformability. The aim of this paper is to present a methodology to predict the strength and deformability of a jointed rock mass using UDEC (universal distinct element code). In the study, the rock mass is modelled as an assemblage of deformable blocks that can yield as an intact material and/or slide along predefined joints within the rock mass. A range of numerical simulations of uniaxial and triaxial tests was conducted on rock mass samples in order to predict the equivalent mechanical properties for the rock mass under different loading directions. Finally, results are compared with the deformability parameters obtained by analytical methods.

Features of Pendulum Waves under Deep-Seated Charge Blasting in Blocky Rock Mass

Features of Pendulum Waves under Deep-Seated Charge Blasting in Blocky Rock Mass

Effect of discontinuity stress shadows on hydraulic fracture re-orientation

Hydraulic fracturing has been applied to the cave mining industry as a pre-conditioning method to improve rock mass caveability and fragmentation sizes. In theory, the hydraulic fracture orientation is dictated by and is perpendicular to the minimum in-situ stress orientation. Such orientations will not always result in the creation of a blocky rock mass to aid caveability. The understanding of hydraulic fracture re-orientation induced by the stress shadow effect is important for either avoiding the undesirable interaction between multiple transverse hydraulic fractures or taking advantage of this phenomenon to create prescribed hydraulic fractures that result in the creation of blocky rock masses. In this paper, the existing knowledge of the stress shadow effect around a pre-existing discontinuity is reviewed. A 3D numerical modelling code, Rock Failure Process Analysis (RFPA) is used to investigate factors influencing hydraulic fracture re-orientation. It is established that hydraulic fractures can be forced to propagate in desired directions if advantage is taken of the stress shadow effect in combination with knowledge of rock mass homogeneity and site far field stress conditions. Both the differential stresses between σ2 and σ3 and between σ1 and σ3 affect the hydraulic fracture re-orientation geometry. The minimum far field stress magnitude has no direct impact on hydraulic fracture re-orientation. Rock mass homogeneity significantly influences how far a hydraulic fracture propagates along its predefined orientation from its initiation point before re-orientation.

A Review of Seismic Damage of Mountain Tunnels and Probable Failure Mechanisms

Severe cases of damages of mountain tunnels following 1995 Hyogoken-Nanbu (Japan), 1999 Chi-Chi (Taiwan), 2004 Mid-Niigata Prefecture (Japan) and 2008 Wenchuan (China) earthquakes have challenged the traditional belief of tunnel structures being seldom damaged in seismic events. These experiences are a reminder that seismic behaviour of mountain tunnels must be further studied in detail. Such investigations assume greater significance as more number of tunnels are being planned to be constructed to meet the infrastructural needs of mountainous regions all around the world. In this paper, seismic damages of mountain tunnels have been reviewed. Prominent failure patterns have been identified based on the case histories of damages. Damages in the form of cracking of tunnel lining, portal cracking, landslide induced failures, uplift of bottom pavement, failures of sidewalls, shearing failure of tunnel liner and spalling of concrete have been majorly observed. Based on the damage patterns and earthquake data, main factors leading to instabilities have been discussed. Probable failure mechanisms of mountain tunnels under seismic loading conditions have been explained. Seismic analyses of a circular lined tunnel in blocky rock mass have been carried out through discrete element based approach. The significant role of different seismic parameters like frequency, peak ground acceleration has been identified. Moreover, effect of tunnel depth on the seismic response of tunnels has been investigated. It is believed that the present study will help in advancing the present state of understanding with regard to the behavior of tunnels under seismic conditions.

Stochastic analysis of progressive failure of fractured rock masses containing non-persistent joint sets using key block analysis

A stochastic key block method is developed for the analysis of complex blocky rock masses containing non-persistent joint sets. A robust block generation program is developed to model the non-persistent discontinuities. Various uncertainties of geological and geometrical parameters of the discontinuities are considered and Monte Carlo simulations of key blocks are performed. Based on the present analysis, progressive failure of a rock mass can be evaluated in a stochastic manner and the statistics of the key blocks including the total number and volume, the maximum and mean volume, shape and failure mechanism, etc. can be assessed. This approach is applied to a hypothetical horseshoe shaped tunnel in a highly fractured rock mass. Three scenarios with varying mean discontinuity size are analyzed to consider size effect on the predicted blocks and key blocks. It is shown quantitatively that a persistent discontinuity network assumption causes over-fragmentation of predicted blocks, overestimation of key blocks, and underestimation of the largest key block volume compared with non-persistent ones. More realistic representation of the discontinuities by considering the non-persistence is important to give out more reliable failure estimation of fractured rock mass. In addition, a case study application to a slope at the right bank of the Jinping I hydropower station has been conducted. Key block statistics is also helpful in support design.

Integration of three dimensional discontinuous deformation analysis (DDA) with binocular photogrammetry for stability analysis of tunnels in blocky rockmass

This paper demonstrates and evaluates an integrated system coupling 3-dimensional binocular photogrammetry and discontinuous deformation analysis (DDA) for stability analysis of tunnels in blocky rockmass. 3D DDA is an effective numerical method for discontinuous and large displacement problems. In stability analysis of rockmass, especially for tunnels, one of the primary bottlenecks in 3D DDA is generating discontinuous models. For tunnel constructions requiring small interval time for blasting, traditional geotechnical data collection approaches such as hand-mapping and geologic compass are always lacking in efficiency and accuracy. 3D photogrammetry is an economical and efficient method for collecting geometric features and surface data. An integrated system connecting geometrical data acquisition and discontinuous numerical analysis automatically, is valid for improving safety in tunnel construction. This system includes photogrammetry module, modeling module and analysis module. In the photogrammetry module, binocular photogrammetry devices and image reconstruction technique are implemented for geometric data of rockmass. Then location, dip direction and dip angle of joints as well as other geometric information of tunnels are input into the modeling module to generate 3-dimensional discontinuous models. The analysis module using 3-dimensional DDA to evaluate the stability of the surrounding rock in tunnels. The integrated system is implemented in an engineering instance, Suocaopo Tunnel in Guizhou, China.

Comparisons between centrifuge and numerical modeling results for slope toppling failure

This paper presents series studies on the toppling mechanism by centrifuge tests and numerical simulations. Two different discrete element methods, i.e., the continuum-based discrete element method (CDEM) and the discontinuous deformation analysis (DDA), are adopted. The modeling results show that both the methods can accurately capture the failure modes of the centrifuge tests, including three distinct zones and two failure surfaces. Comparisons are made between the physical test and numerical simulation results. The critical inclination angle of the tilting table where the slope models are fixed on can be moderately predicted by the two methods, with different degrees of precision. The error between the test results and the simulated results is within 1% for the slope models without rock-bridges by both CDEM and DDA. However, it is amplified for the staggered-joint models that simulate the rock-bridges. With DDA, the average error is about 5%, and the maximum error is up to 17%. While with CDEM, the errors for the aligned-joint models are ranged from 1% to 6%, and it is from 10% to 29% for the staggered- joint models. The two numerical methods show the capability in simulating toppling failure of blocky rock mass with and without rock-bridges. The model with rock-bridges which provides a certain bending resistance is more stable than the one without any rock-bridge. In addition, the two failure surfaces were observed, which is different from the common understanding that only one failure surface appears.

Rock slope stability assessment using finite element based modelling – examples from the Indian Himalayas

Numerical modelling of rock slides is a versatile approach to understand the failure mechanism and the dynamics of rock slopes. Finite element slope stability analysis of three rock slopes in Garhwal Himalaya, India has been carried out using a two dimensional plane strain approach. Two different modelling techniques have been attempted for this study. Firstly, the slope is represented as a continuum in which the effect of discontinuities is considered by reducing the properties and strength of intact rock to those of rock mass. The equivalent Mohr-Coulomb shear strength parameters of generalised Hoek-Brown (GHB) criterion and modified Mohr-Coulomb (MMC) criterion has been used for this continuum approach. Secondly, a combined continuum-interface numerical method has been attempted in which the discontinuities are represented as interface elements in between the rock walls. Two different joint shear strength models such as Barton-Bandis and Patton’s model are used for the interface elements. Shear strength reduction (SSR) analysis has been carried out using a finite element formulation provided in the PHASE2. For blocky or very blocky rock mass structure combined continuum-interface model is found to be the most suitable one, as this model is capable of simulating the actual field scenario.

Hydrofracture growth in a compressed quasi-regular blocky rock mass

Effect of irregular structure of a compressed blocky rock mass on propagation of a hydrofracture is analyzed in the limit case when biaxial compression field is hydrostatic.

A rational and realistic rock mass modelling strategy for the stability analysis of blocky rock mass

Due to the importance of a realistic rock mass model for the achievement of accurate and reliable results from numerical analysis of blocky rock mass, a probability based rock mass geometrical model is proposed in the present study. The rock mass geometry is built based on site investigation data and stochastic analysis of the discontinuity mapping results. The discontinuities and/or sets of them, either finite or infinite in size, planar or non-planar, convex or concave in shape, can be implemented in the three-dimensional rock mass model. Key block analysis and support design are then suggested to the stochastic rock mass models. To achieve a realistic stability analysis of the rock mass with the proposed probabilistic rock mass model, a few suggestions have also been made. These include the adoption of realistic constitutive models to intact rock and rock discontinuities, use of numerical model to technically handle numerical difficulties and development of a realistic modelling strategy to achieve realistic and affordable computational cost, etc.

Extended key block analysis for support design of blocky rock mass

An extended key block method for rock mass stability analysis is proposed by considering the force effect of key blocks in later batches to the first batch which is exposed at rock mass surfaces. Through a realistic numerical representation of a three-dimensional rock mass, different batches of key blocks are searched by means of a progressive failure process. A force transfer algorithm is developed to consider the interactions of the key blocks in different batches. The calculated sliding forces in the first batch which consider the later batches’ effect will lead to a better design of rock supports. After selection of a reinforcement scheme, the stability of the reinforced rock mass is re-assessed. Those larger instable blocks formed by inside blocks with anchored surface blocks are checked out. It is found that the two-step check of a reinforcement scheme, i.e. (1) safety check of the bolted individual blocks; (2) safety check of the bolted block groups is more effective. Results show that the proposed rock support design method is more realistic and rational for the reinforcement of blocky rock mass.

Terrestrial Photogrammetry and Numerical Modelling for the Stability Analysis of Rock Slopes in High Mountain Areas: Aiguilles Marbrées case

Several high-altitude slope instability phenomena, involving rock blocks of different volumes, have been observed in recent years. The increase in these phenomena could be correlated to climatic variations and to a general increase in temperature that has induced both ice melting with consequent water seepage and glacial lowering, with a consequent loss of support of the rock face. The degradation of the high-altitude thermal layer, which is known as “permafrost”, can determine the formation of highly fractured rock slopes where instabilities can concentrate. The present research has developed a methodology to improve the understanding and assessment of rock slope stability conditions in high mountain environments where access is difficult. The observed instabilities are controlled by the presence of discontinuities that can determine block detachments. Consequently, a detailed survey of the rock faces is necessary, both in terms of topography and geological structure, and in order to locate the discontinuities on the slope to obtain a better geometric reconstruction and subsequent stability analysis of the blocky rock mass. Photogrammetric surveys performed at different times allow the geostructure of the rock mass to be determined and the rock block volumes and detachment mechanisms to be estimated, in order to assess the stability conditions and potential triggering mechanisms. Photogrammetric surveys facilitate both the characterisation of the rock mass and the monitoring of slope instabilities over time. The methodology has been applied in a case study pertaining to the North Face of Aiguilles Marbrées in the Mont Blanc massif, which suffers from frequent instability phenomena. A slope failure that occurred in 2007 has been back-analysed using both the limit equilibrium method (LEM) and 3D distinct element modelling (DEM). The method has been supported and validated with traditional in situ surveys and measurements of the discontinuity orientation and other rock mass features.

Modeling of Ultimate Bearing Capacity of Shallow Foundations resting on Jointed Rock Mass

Ultimate bearing capacity of jointed rock mass specimens, studied by distinct element software UDEC which allows explicit modelling of blocks of rock, is presented in this paper. Since the other numerical methods like FEM considers the rock mass as continuum, their applicability to anisotropic blocky rock masses remains doubtful. A series of numerical tests has been conducted, by developing a programme using fish functions, on rock mass having two orthogonal joint set with the joint orientation from 0° to 90°. Simple models have been assigned to rock material and joints. An experimental study was conducted by Bindlish (Bearing capacity of strip footings on jointed rocks, 2007) wherein a rigid footing placed on the top surface of the jointed rock mass was loaded up to the failure. The effect of the joint orientation and interlocking of rock mass on the ultimate bearing capacity of the rock mass was studied. The numerical findings by distinct element software UDEC on the ultimate bearing capacity of the jointed rock mass have been found to be close to the experimental results obtained by the author.

Analysis and prediction of TBM performance in blocky rock conditions at the Lötschberg Base Tunnel

This paper focuses on the analysis of the TBM performance recorded during the excavation of the Lötschberg Base Tunnel. The southern part of the tunnel was excavated by two gripper TBMs, partly through blocky rock masses at great depth. The jointed nature of the blocky rock mass posed serious problems concerning the stability of the excavation face. A detailed analysis has been carried out to obtain a relationship between the rock mass conditions and the TBM performance, using the Field Penetration Index (FPI). In blocky rock conditions, the FPI is defined as the ratio between the applied thrust force and the actual penetration rate. A database of the TBM parameters and the geological/geotechnical conditions for 160 sections along the tunnel has been established. The analysis reveals a relationship between the FPI and two rock mass parameters: the volumetric joint count (Jv) and the intact rock uniaxial compressive strength (UCS). Through a multivariate regression analysis, a prediction model for FPI in blocky rock conditions (FPIblocky) is then introduced. Finally, other TBM performance parameters such as the penetration rate, the net advance rate and the total advance rate are evaluated using FPIblocky.

Integration of three-dimensional laser scanning with discontinuum modelling for stability analysis of tunnels in blocky rockmasses

This paper demonstrates and evaluates 3-dimensional laser scanning (Lidar), in conjunction with block modelling, as a tool for underground rockmass characterization and stability analysis. As a drill-and-blast tunnel advances, Lidar scanning allows for the documentation of the rockmass by collecting creating millions of rock surface point locations in space creating geometric scenes. Databases of structural geology data (joints) can be created by interpreting the “virtual” rockmass. These databases, which include the location of each measurement, can be much more extensive than what is obtained by hand-mapping in traditional geotechnical data collection. The advantages and challenges of Lidar data for underground rockmass evaluation are discussed. The joint database can be subsequently be used in discontinuum modelling in order to evaluate structurally-controlled failure in blocky rockmasses, although not without a number of critical caveats addressed here. These joint system models, either statistically generated or discretely represented, can be far more representative block models than previously possible due to joint position information and a wealth of joint measurements, although a number of pitfalls can occur. The authors present a workflow from data collection and analysis to design outputs for integrating Lidar-derived point-cloud data into rockmass stability modelling.

Quantitative Measurements of Fracture Aperture and Directional Roughness from Rock Cores

The hydro-mechanical behavior of a blocky rock mass near the surface and at shallow depths is more dependent on the characteristics of the system of discontinuities within the rock mass than the characteristics of the intact rock. Discontinuities represent planes of weakness and conduits of enhanced hydraulic conductivity relative to the intact rock. The spatial aperture distribution and roughness of these fractures can have a significant influence on their hydromechanical behavior. In terms of mechanical behavior, the aperture distribution and roughness directly affect the spatial distribution and inclination of contact areas, which in turn influence the stress distribution, deformation, and asperity damage, under normal and shear loading (e.g., Re and Scavia 1999; Gentier et al. 2000; Grasselli and Egger 2003). In terms of hydraulic behavior, the spatial aperture distribution and roughness directly affect the tortuosity and connectivity of flow paths, which in turn influence the hydraulic transmissivity of the fracture (Zimmerman and Bodvarsson 1996; Berkowitz 2002). The evaluation of fracture roughness requires measurements or observations of surface topography coupled with some empirical (e.g., Barton and Choubey 1977), statistical (e.g., Reeves 1985; Maerz et al. 1990) or fractal (e.g., Seidel and Haberfield 1995; Kulatilake et al. 2006) analysis methodologies, which yield one or more roughness parameters. Several contact and non-contact tools and techniques have been used to measure surface topography of discontinuity surfaces in rock. Contact techniques include:

Study on Artificial Neural Network Method for Ground Subsidence Prediction of Metal Mine

Metal mine with fractured blocky rock mass is much different from coal mine, which shows discontinuity and irregularity, meanwhile large differences also exist in stratum structure, geological condition, ore body shape and mining methods, so the influential factors in metal mines is more complex and volatile. The research on theory and application of ground subsidence has not reached a mature stage at present. So this paper focused on the study of this issue based on neural network with its characteristic that it has fast learning speed and could approach to any non-liner mapping, which are adapted to the complex environment in metal mine. The time series prediction model was established, which is based on the measured data of the roof subsidence in the goaf of metal mine, and the tested sample data were trained and tested by many times. Finally the predicted value of the neural network was compared with the measured value by automatic optical level, which showed that the prediction model achieved good accuracy, and could be accepted in the engineering application. This method could fill the gap of incomplete monitoring data in mine ground subsidence, and provide a reference for production in the metal mine.

Stability of shallow karstic caverns in blocky rock masses

The limiting relationship is explored between underground opening span and required cover height for stability in blocky rock masses characterized by a network of horizontal bedding planes and vertical joints. Understanding this relationship is crucial for the design of mining excavations in karstic terrain as typically encountered in carbonate rock masses. We perform numerical analysis of multiple roof spans vs. cover height geometries using the DDA method to obtain the boundary curve between stable and unstable opening geometries. Our results indicate that for cavern spans of up to 18 m a low cover height vs. opening span ratio of h/B=0.33 is sufficient for stability. For spans greater than 18 m the demand for cover height rapidly increases and it appears to stabilize at h/B=1.0 for B=26 m and above. To validate our numerical analysis results, a unique case study is analyzed wherein a 40 m span karstic cavern, the Ayalon cave, has been preserved below an active open pit mine in central Israel with cover height of only 30 m, thus rendering the cave marginally stable according to our model prediction. Indeed there is ample evidence of partial collapse of the roof in the cave. The predictive capability of our model is further confirmed using two additional case studies in blocky rock masses, each of which possesses very different mechanical parameters such as intact rock strength, density, and deformability, suggesting that our model predictions remain valid as long the rock mass maintains a “blocky” structural configuration.