Rock Mass Response Research Articles

Study on elastic P-wave propagation law in unfavorable geologic structures with discontinuous deformation analysis method

The propagation of elastic P-wave in unfavorable geologic structures is a complex process which contains reflex, transmission, and refraction. In this paper, the discontinuous deformation analysis (DDA) method is introduced, and the viscous boundary condition and stress wave input way are applied to enable the DDA method to simulate the seismic dynamic response of underground rock mass. The improved DDA program is used to analyze the propagation laws of elastic P-wave in joints and weak interlayer. The results show that (1) for a range of joint stiffness, the difference of transmission coefficient of joint surface between DDA calculation and theoretical solution is small; (2) the lower the joint stiffness, the more obvious the attenuation of the elastic P-wave is; (3) varied joint spacing and joint number play different roles on the attenuation of elastic P-wave; (4) the DDA calculation result is consistent with the theoretical solution for the transmission coefficient of interface; (5) the thickness and elastic modulus of weak interlayer will affect the interference effect of multiple transmission waves; (6) with the increasing weak interlayer thickness H (or a), the attenuation of the elastic P-wave is more obvious; and (7) the lower the wave impedance ratio of weak interlayer, the more obvious the attenuation of the elastic P-wave is.

The Safety Analysis for Underground Tunnel of Civil Engineering by the Stress and Deformation of Rock Mass Material

The Mohr-Coulomb criterion is used in this paper to describe the strength of rock mass in tunnel of civil engineering. The position of a stress point on this envelope is controlled by a non-associated flow rule for shear failure, and an associated rule for tension failure. Excavation of tunnel will lead to the redistribution of stress in rock mass, in order to compare the analytical and numerical solution of rock mass response to predict the safety degree of underground tunnel, to predict the safety degree of tunnel, some calculations are done by theoretical and numerical methods. The comparison results can give guidance for the real practice

Mine-by Experiment in a deep shaft in Neogene sedimentary rocks at Horonobe, Japan

The Japan Atomic Energy Agency (JAEA) has been implementing several research studies at the Horonobe Underground Research Laboratory (URL) in support of the Japanese R&D program for the geological disposal of High-Level Radioactive Waste (HLW). Understanding the development and properties of an EDZ (excavation disturbed or damaged zone), including the rock mechanical, hydrogeological and geochemical responses around excavated openings, is important to the design of plugs and sealing material and for performance assessment of geological disposal. The Mine-by Experiment described in this paper was carried out to investigate the development of an EDZ in one of the deep vertical shafts excavated in Neogene sedimentary rocks at the Horonobe URL site. The objective of this study is to obtain fundamental parameters of an EDZ in Neogene sedimentary rock, where case studies are few, in support of the geological disposal studies in Japan. Before and after shaft sinking, BTV (borehole television), borehole expansion tests, permeability tests and seismic velocity logging were carried out to determine initial conditions and changes in the mechanical and hydraulic properties. During shaft sinking, extensometers were installed in the rock mass to determine rock mass response to excavation. The variation of hydraulic properties was largest within one meter of the shaft wall and the variation in hydraulic conductivity was in the range of one order of magnitude.

Two-dimensional dynamic finite element simulation of rock blasting

In the present study, the two-dimensional blast model has been simulated using finite element software Abaqus/CAE. The John–Wilkins–Lee equation of state has been used to calculate the pressure caused by the release of the chemical energy of the explosive. Detonation point from center of hole has been defined for the traveling path of explosive energy. Elastoplastic dynamic failure constitutive with kinematic hardening model was adopted for rock mass responses under high explosive pressure to understand the mechanism of blast phenomena. In this model, it is assumed that failure of rock occurs under tensile failure when yield plastic stress exceeded to its static tensile strength. The hydrostatic pressure was used as a failure measure to model dynamic spall or a pressure cut off. Variation of detonation velocity has been measured in terms of simulation blast output energies index results.

Dynamic response of rock mass induced by the transient release of in-situ stress

During the excavation of rock mass with the method of drill and blast, the release of in-situ stress, also known as excavation load on boundary, is traditionally assumed as a quasi-static process, so the inertia and all other dynamic responses induced by the release of in-situ stress can be ignored. After analyzing the process of release of in-situ stress induced by rock mass excavation with the method of drill and blast and determining the duration of the release of in-situ stress, it is found that the release of in-situ stress induced strain rate can reach a magnitude of 10−1–101/s or higher if the initial in-situ stress has a level of 20–50MPa, it implies that the release of in-situ stress is a transient process, and the transient character of the release of in-situ stress and correspondingly induced dynamic response of surrounding rock should be taken into account together with excavation. Problems such as the determination of redistributed in-situ stress corresponding to initiation sequences of millisecond delays, the way and path of the transient release of in-situ stress, transient release of in-situ stress induced vibration and comparison with blasting induced vibration are also discussed in this paper. Finally, a case of in-situ stress release induced vibration in Pubugou hydropower projects are presented as verifications.

Rock Mass Behavior Under Hydropower Embankment Dams: A Two-Dimensional Numerical Study

Sweden has more than 190 large hydropower dams, of which about 50 are pure embankment dams and over 100 are concrete/embankment dams. This paper presents results from conceptual analyses of the response of typical Swedish rock mass to the construction of a hydropower embankment dam and its first stages of operation. The aim is to identify locations and magnitudes of displacements that are occurring in the rock foundation and grout curtain after construction of the dam, the first filling of its water reservoir, and after one seasonal variation of the water table. Coupled hydro-mechanical analysis was conducted using the two-dimensional distinct element program UDEC. Series of the simulations have been performed and the results show that the first filling of the reservoir and variation of water table induce largest magnitudes of displacement, with the greatest values obtained from the two models with high differential horizontal stresses and smallest spacing of sub-vertical fractures. These results may help identifying the condition of the dam foundation and contribute to the development of proper maintenance measures, which guarantee the safety and functionality of the dam. Additionally, newly developed dams may use these results for the estimation of the possible response of the rock foundation to the construction.

Influence of a fault system on rock mass response to shaft excavation in soft sedimentary rock, Horonobe area, northern Japan

This paper focuses on the stress field that produced a fault system in the Horonobe area of Japan, and considers the relationship between the fault system and the rock mass response to shaft excavation in soft sedimentary rocks. In this area, hydraulic fracturing investigations in boreholes and tectonic plate movements indicate that the major horizontal principal stress is oriented E–W; however, during shaft excavation, the greatest convergence in the shaft was oriented NNE–SSW. Therefore, it is necessary to understand the influence of the fault system on the rock mass response to shaft excavation. We performed a fault-slip analysis of faults in the shaft wall and reconstructed the paleostress field that produced the fault system. The maximum horizontal principal stress was oriented mainly NNE–SSW, and the minimum horizontal principal stress was oriented mainly WNW–ESE. These directions are similar to the directions of maximum and minimum convergence in the shaft, respectively. The results show that a fault system can affect the rock mass response to shaft excavation in soft rock, and it is considered that the rock mass deformation is controlled by the fault system. In addition, it may be possible to assess and predict the rock mass behavior by focusing on paleostress field that produced the fault system.

Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression

The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling. Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing. To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures. All specimens were tested at their natural water content with loading approximately normal to the bedding. Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation. The unconfined compression strength of the tested specimens averaged 6.9 MPa. The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress–strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress.

The Responses of Jointed Rock Mass under Dynamic Loads

Joints can reflect and transmit stress waves, thus joints affect rock fracturing, and accordingly they could affect blasting efficiency. In this paper, numerical and experimental investigations have been implemented and the effect of joints on wave propagation and rock fracturing has been discussed. A dynamic numerical model of jointed rock mass has been established by using finite difference method. The simulation results show that as joints are parallel to wave propagation, the corresponding rock damage zones increase. Blasting experiments by using artificial joints have been carried out. Generally the experimental results agree with our numerical results.

Correlations Developed for Estimation of Hydraulic Parameters of Rough Fractures Through the Simulation of JRC Flow Channels

The hydro-mechanical response of fractured rock masses is complex, due partly to the presence of fractures at different scales. Surface morphology has a significant influence on fluid flow behaviour of a fracture. Different empirical correlations and statistical models have been proposed to estimate the equivalent hydraulic aperture and determine the pressure drop along a fracture. However, the existing models suffer from not being adequately generalised to be applicable to a wide range of real fracture surfaces. To incorporate the effect of profile roughness in the hydro-mechanical behaviour of fractured rock masses, the joint roughness coefficient (JRC) is the most widely used empirical approach. However, the average JRC of two fracture walls in fluid flow analysis, as is a common practice, appears to be inappropriate. It will be shown how different combinations of pairs of JRCs could lead to a similar JRC value. Also, changing the position of the top and bottom walls of a fracture can significantly change the hydraulic response of the fracture while the average JRC is identical in both cases. In this paper, correlations are developed which are based on the simulation of JRCs using estimated fluid flow parameters of 2D fractures can be estimated. In order to widen the application range of the correlations, JRC flow channels were generated: these are 2D channels with their top and bottom walls being made from two of the JRC profiles. To estimate the JRC of linear profiles, a correlation developed between JRC and a newly developed Riemannian roughness parameter, D R1, is proposed. Considering ten JRC profiles, a total of 100 JRC flow channels were generated. In order to only investigate the effect of surface roughness on fluid flow, the minimum closure between the top and bottom walls of JRC flow channels were considered to be constant. Three cases with minimum closures of 0.01, 0.05 and 0.10 cm were considered in this study. All JRC flow channels were subjected to fluid analysis using FLUENT software. Based on these results, correlations were developed between the geometrical and hydraulic properties of flow channels. Analysis of several real fractures demonstrated the applicability of these correlations.

Analysis of natural rock slope deformations under temperature variation: A case from a cool temperate region in Japan

In this paper, natural rock slope deformation across fractures predominantly in a chert rock mass was monitored using six surface fracture displacement sensors, and the deformations arising from thermal stresses were predicted using (5 m × 5 m) two-dimensional (2D) finite element (FE) plane strain analysis coupled with a model for rock mass expansion due to freezing of pore water. A new and simple method to minimize displacement proportional to temperature (due to thermal response of chert rock mass and sensor) was proposed. By applying the method, the corrected displacement, u ′, can be well recognized. Under u ′, clear rock mass movement, which could be related to fracture growth, was observed. In addition, progressive fracture opening and closure were noted. Results from this study indicate insignificant influences of weather conditions on fracture/rock mass movement. Furthermore, under numerical analysis (FE), in the rock mass model (with 1-m deep fracture), tensile stresses that were large enough to induce fracture growth appeared at the fracture tip when temperature lowered. And in the rock slope model (with 1-m deep fracture), small tensile stresses, which were sufficient to cause fracture growth along the planes of weakness, were observed. This research suggests that freezing effects on deformation of chert rock mass are insignificant, and we tentatively suggest that thermal fatigue predominantly caused the permanent fracture deformations.

Numerical modeling of coupled thermo-mechanical response of a rock pillar

Understanding the rock mass response to excavation and thermal loading and improving the capability of the numerical models for simulating the progressive failure process of brittle rocks are important for safety assessment and optimization design of nuclear waste repositories. The international cooperative DECOVALEX-2011 project provides a platform for development, validation and comparison of numerical models, in which the Äspö pillar stability experiment (APSE) was selected as the modeling target for Task B. This paper presents the modeling results of Wuhan University (WHU) team for stages 1 and 2 of Task B by using a coupled thermo-mechanical model within the framework of continuum mechanics. The rock mass response to excavation is modeled with linear elastic, elastoplastic and brittle-plastic models, while the response to heating is modeled with a coupled thermo-elastic model. The capabilities and limitations of the model for representation of the thermo- mechanical responses of the rock pillar are discussed by comparing the modeling results with experimental observations. The results may provide a helpful reference for the stability and safety assessment of the hard granite host rock in China’s Beishan preselected area for high-level radioactive waste disposal.

Field Testing of Blast – Induced Ground Excitation due to Explosion in Rock Mass

Abstract This paper presents results of an experimental investigation to study the effect of blast wave propagation in rock mass due to explosions. The aim of this study is to observe the dynamic response of rock mass under blasting vibrations and the effect of different geological conditions of rock mass on the wave propagation of the explosion. A series of field experiments were conducted including different variables such as, charge weight, horizontal charge–target distance and target location below the ground surface. The main results show that the geological conditions of rock mass such as, joints, fractures, cracks and cavities have a direct effect on blast wave propagation and the value of pressure on rock mass due to explosion. The effect of rock joints on blast wave propagation can be used to assess the quality of rock mass and hence can be utilized to protect underground structures from blast-induced damage.

The analysis of radial displacements occurring near the face of a circular opening in weak rock mass

The effect of opening size, rock properties and stress conditions on the radial displacements ahead and behind of the advancing face of a circular opening is one of the most important topics in civil and mining engineering. In this paper we develop a prediction system for radial displacements around the face of an opening by combining rock mass rating classification system, a three dimensional finite difference code and multiple regression modeling. Predictive equations and response surfaces for rock mass response are proposed. By using these equations and response surfaces, it is possible to assess radial displacements around the face and to construct the longitudinal deformation profile of a circular opening with different diameters excavated in weak rock mass under different stress conditions. The predictive equations were validated by means of statistical tests and analytical solution. Proposed response surfaces and equations can provide the basis of guiding the design and construction of underground openings.

Effective grouting area of jointed slope and stress deformation responses by numerical analysis with FLAC3D

To study the grouting reinforcement mechanism in jointed rock slope, first, the theoretical deduction was done to calculate the critical length of slipping if the slope angle is larger than that of joint inclination; Second, the numerical calculation model was founded by FLAC3D, so as to find the stress and deformation responses of rock mass in the state before and after grouting. the analysis results show that the range between the boundary of critical slipping block and the joint plane that passes the slope toe is the effective grouting area (EGA). After excavation, large deformation occurs along the joint plane. After grouting, the displacements of rock particles become uniform and continuous, and large deformations along the joint plane are controlled; the dynamic displacement can reflect the deformation response of slope during excavation in the state before and after grouting, as well as the shear location of potential slip plane. After grouting, the dynamic displacement of each monitoring point reaches the peak value with very few time steps, which indicate that the parameters of the joint plane, such as strength and stiffness, are improved; the stress field becomes uniform. Tensile area reduces gradually; whole stability of the slope and its ability to resist tensile and shear stress are improved greatly.

Source Mechanisms of Mine-Related Seismicity, Savuka Mine, South Africa

We report full moment tensor solutions for 76 mine tremors with moment magnitudes (Mw) between 0.5 and 2.6 recorded by a network of 20 high-frequency geophones in a deep gold mine in South Africa. Source mechanisms convey important information on how in-mine stresses are relaxed, and understanding the nature of such mechanisms is essential for improving our assessment of rock mass response to mining. Our approach has consisted of minimizing the L2 norm of the difference between observed and predicted P, SV, and SH spectral amplitudes, with visually assigned polarities, to constrain all six independent components of the seismic moment tensor. Our results reveal the largest principal stresses in the mine are com- pressive, oriented near vertically, and relaxed through a mix of volumetric closure and normal faulting, consistent with a gravity-driven closure of the mined-out areas. Pre- vious moment tensor studies in deep mines had suggested that the distribution of seis- mic sources in terms of the volumetric-shear mix was bimodal. A bimodal distribution is compatible with our moment tensor solutions only for moment magnitudes above 2.2. Events in the 0:5 <Mw <2:2 moment magnitude range display a continuous distribution of their volumetric-shear mix. Online Material: Focal parameters for mine tremors at Savuka.

Modeling of a jointed rock mass under triaxial conditions

One of the most important aspects of designing a structure on or in a rock mass is based on the strength response of a jointed rock mass. Understanding this important aspect, the present study was undertaken to understand the strength response of a jointed rock mass with the help of a finite difference package FLAC3D. In the present work, an attempt has been made to understand the effect of discontinuity angle on the failure mode and strength of the rock mass. For this purpose, stress and displacement in the model were studied and various stress–strain histories were recorded at constant strain loading rate. Rock discontinuity plays a critical and vital role to understand physico-mechanical characteristics of a rock mass. It has wider application in the rock excavation engineering, e.g., caverns, tunnels, slope stability, dams, etc. Simulated rock results are compared with the analytically calculated results of the jointed rock mass and found in good agreement.

The Äspö Pillar Stability Experiment: Part II—Rock mass response to coupled excavation-induced and thermal-induced stresses

A 1-m-thick pillar was subject to coupled excavation- and thermal-induced stresses to induce brittle rock mass yielding. The yielding strength of the heterogeneous and fractured rock mass consisting of Äspö diorite was evaluated at eighteen discrete locations using data from the displacement, acoustic emission, and thermal monitoring systems. The average rock mass yielding strength was determined to be 0.59 of the uniaxial compressive strength. The onset of dilation in uniaxial laboratory tests, determined from strain gauge data, was found to occur at approximately 0.45 of the uniaxial compressive strength. It was shown that that the onset of acoustic emission events in situ also occurred when the tangential stress exceeded 0.43 of the uniaxial compressive strength. For sites with absence of in situ data it is recommended that this lower-bound value determined from laboratory data may be used for assessing the in situ rock mass yielding strength. Visual observation and displacement monitoring showed that extent of rock mass yielding is sensitive to small changes in the tangential stress magnitudes. It was determined using three-dimensional modelling that changes in the tangential stress magnitude of approximately 1 MPa was sufficient to cause yielding of the pillar to propagate in what appeared to be intact rock. Observations suggest that without this small stress change yielding of the rock mass would not occur. In other words, there appeared to be a well defined boundary, and if the stresses reached this boundary yielding was observed. However, if stresses were only slightly below this boundary yielding or time-dependant processes were not observed over the monitoring period used in the experiment.

Fracture dilation during grouting

Sealing underground excavations from ingress of water constitutes a large part of both the cost and the risk for many infrastructure projects. In this paper we present a mechanical model for the rock mass response when grouting hard jointed rock. The model predicts a stiff and a non-stiff behaviour and a transition between them that is dependent on the relationship between the grouting pressure and the in situ stress conditions. The predictions are consistent with previously published measurements and explain grouting behaviour that has been difficult to model with previous methods.

Numerical Analyses of the Influence of Blast-Induced Damaged Rock Around Shallow Tunnels in Brittle Rock

Most of the railway tunnels in Sweden are shallow-seated (<20 m of rock cover) and are located in hard brittle rock masses. The majority of these tunnels are excavated by drilling and blasting, which, consequently, result in the development of a blast-induced damaged zone around the tunnel boundary. Theoretically, the presence of this zone, with its reduced strength and stiffness, will affect the overall performance of the tunnel, as well as its construction and maintenance. The Swedish Railroad Administration, therefore, uses a set of guidelines based on peak particle velocity models and perimeter blasting to regulate the extent of damage due to blasting. However, the real effects of the damage caused by blasting around a shallow tunnel and their criticality to the overall performance of the tunnel are yet to be quantified and, therefore, remain the subject of research and investigation. This paper presents a numerical parametric study of blast-induced damage in rock. By varying the strength and stiffness of the blast-induced damaged zone and other relevant parameters, the near-field rock mass response was evaluated in terms of the effects on induced boundary stresses and ground deformation. The continuum method of numerical analysis was used. The input parameters, particularly those relating to strength and stiffness, were estimated using a systematic approach related to the fact that, at shallow depths, the stress and geologic conditions may be highly anisotropic. Due to the lack of data on the post-failure characteristics of the rock mass, the traditional Mohr–Coulomb yield criterion was assumed and used. The results clearly indicate that, as expected, the presence of the blast-induced damage zone does affect the behaviour of the boundary stresses and ground deformation. Potential failure types occurring around the tunnel boundary and their mechanisms have also been identified.