Rock Mass Characterization Research Articles

Cross-scale rockmass characterization by high resolution seismic imaging around tunnels

Reflection seismic surveys are relevant to the detailed characterisation of crystalline bedrock. Relatively small-scale features, even single fractures, can demonstrably be detected. Integrated analysis and modelling of geological, hydrogeological and geophysical data is instrumental in the interpretation of reflection seismic data. This requires the inference of the nature of the reflectors, e.g. lithological contacts or deformation zones and resolving the orientation ambiguity of the reflectors. Brittle deformation zones and large fractures may be heterogeneous in their detailed structure, making their characterization a challenging task. Several examples of structure characterization ahead and around tunnels are presented, with focus on the safety assessment for engineering and mining applications. Seismic imaging techniques adapted for tunnels and galleries are presented with examples from high resolution seismic surveys carried out at ONKALO (Finland) and Äspö HRL (Sweden). Studies performed over a span of several years demonstrate that large fractures identified initially through geological mapping, can also be found, positioned and characterized by seismics at the small scale of tunnels and/or boreholes. We show that reflection seismic surveys are relevant to the detailed characterization of crystalline bedrock. The detection of some distinctive features, even large-scale, can be uncertain if the survey layout is spatially constrained. Combinations of borehole and tunnel measurements using measuring arrays with diverse orientations helped reduce the location ambiguities and are recommended to be used in mining and tunneling rock characterization surveys wherever possible.

Mechanical Properties and Energy Characteristics of Flawed Samples with Two Non-Parallel Flaws under Uniaxial Compression

Unequal flaws are widespread in rock engineering, in which the extensions induced by excavation and unloading are the main factors leading to engineering instability. Rock deformation and failure are essentially the results of energy transmission. In order to study the influence of flaw type and angle on the mechanical properties and energy characteristics of the flawed rock mass, uniaxial compression tests were conducted using particle flow numerical analysis software. The results show that the crack initiation strength and peak strength of the sample increase with the increase in the flaw angle. For smaller flaw angles, the sample shows obvious plastic deformation during uniaxial compression. For larger flaw angles, the sample shows elastic–brittle properties, whose storage energy is larger. The peak strength of samples with flaws of unequal length presents an obvious decrease compared with that of flawed samples with flaws of equal length, and the extent of the reduction is 6.15% on average. Unequal flaws decrease the ability of the flawed sample to absorb strain energy. Compared with flawed samples with equal flaws, flawed samples with unequal flaws decrease the boundary energy and elastic strain energy by 9.29% and 9.95% on average, respectively. Flaws of unequal length in this sample can weaken the performance of the flawed sample.

Numerical modelling and field monitoring study on large-span tunnelling using pretensioned bolt–cable combined support system

This study focused on a new support method, a pretensioned bolt–cable combined support system, for large-span tunnelling. The Badaling tunnel is a high-speed railway station tunnel that has a span of 32.7 m, a cross-sectional area of 494 m2, and a maximum depth of 102 m. The super large cross-section and unfavorable geological conditions around the tunnel resulted in considerable difficulties for tunnel construction. Because this tunnel was successfully excavated using the pretensioned bolt–cable support, the mechanical characteristics of this new support system are analyzed in this study. First, the necessity of a bolt–cable support for large-span tunnelling was analyzed using a numerical model. The deformation characteristics of rock mass in the large-span tunnel are discussed based on the modelling results of the tunnelling process. Second, based on field monitoring results, the mechanical behaviors of pretensioned rock bolts and anchor cables were analyzed. The anchoring forces of the rock bolts and anchor cables were determined by the pretension forces, loss of pretension forces, and passive support effects. The development of the anchoring force can be divided into three phases: rapid loss of the pretension force, a fluctuation phase, and a stable phase. Finally, The loading effect and mechanical responses of the primary lining were analyzed according to the monitoring results of the primary lining, which indicated that the rock mass was effectively reinforced by the pretensioned bolt–cable system and that the primary lining was safe.

Deterioration laws of Hoek-Brown parameters in freeze–thaw multi-fractured rock mass

The Hoek-Brown criterion can better depict the nonlinear failure characteristics of rock mass, which has been widely used in the fields of large-scaled slopes, deep-buried tunnels, foundations in complex geological conditions, hydropower dams, and energy mining. In view of the previous studies mostly focused on the property reduction of freeze–thaw rock, the deterioration study of Hoek-Brown parameters for freeze–thaw multi-fractured rock mass was investigated in this study. Firstly, Monte-Carlo method was used to generate two-dimensional discrete fracture network, according to which the multi-fractured rock-like samples were prepared. Freeze-thaw cycle experiments were subsequently designed and executed to observe the apparent damage of freeze–thaw effect on the samples. Then, laboratory direct shear tests on freeze–thaw samples under multistage normal loading were carried out. The shear performances of freeze–thaw rock mass were analyzed based on the shear stress-shear displacement curves of samples with different freeze-thawed cycles. Besides, considering the deficiencies of the existing Hoek-Brown parameter estimation methods such as complicated sample preparation and strong subjectivity, a new estimation method based on shear test was proposed by deducing the shear expression of Hoek-Brown criterion, which was programmed in MATLAB and verified by direct shear test results. On this basis, the values of Hoek-Brown parameter of those samples under different freeze–thaw cycles were determined, and the variation law with freeze–thaw cycles was summarized. It turned out that the values of GSI and mi both decrease with the increase of freeze–thaw cycles, and the decreasing rate is also lowering, according with the negative exponential function model.

A New Methodology for Rockfall Hazard Assessment in Rocky Slopes

This article presents an approach to rockfall hazard assessment for rocky slopes based on a previously published rockfall hazard methodology. The original method is appropriate to high alpine rocky slopes exposed to large scale deformations. It evaluates the parameters related to the geomechanical characterization of rock mass, indications of activity, external influences and event intensity. The original methodology was modified to consider different contexts, including geological, climatic and social environments. Parameters related to external influences were modified; the geometry and characteristics of the slope and the catchment area were introduced. The original methodology and the new proposal were applied to two urban slopes and one railway slope in order to test and compare the methods. The original proposal could not represent the rockfall conditions of these slopes. The new proposal was validated using two mine slopes, whose conditions of stability are known. The results of the analyses with the urban slope and the railway slope were coherent with the situation observed at the field. The validation in the mine slopes showed that this approach is applicable in several situations, being able to determine how hazardous a slope is in relation to rockfall events.

Analysis of explosive energy distribution at Pit 7 West PT. Makmur Mandiri Utama Binungan Suaran - Berau, East Kalimantan Province

Blasting geometry and blasting material filling are closely related to the rock mass characteristics and the geological conditions to obtain ideal fragmentation. Blastability Index analysis, including Description of Rock Mass, Combined Plane Spacing, Combined Plane Orientation, Specific Gravity Influencey, and Hardness, are the alternative geometry experiment conducted to overcome the problem of rock fragmentation so that the speed of excavation equipment can increase according to the productivity of Komatsu PC2000 plan at PT. BUMA Jobsite BINSUA. Furthermore, the actual rock values obtained from blasting location and alternative geometry recommendations using R.L.Ash theory combined with Vertical Energy Distribution theory. In the C2 layer with a rock factor value of 5.95, the recommended load is 7.2 m, space is 8.3 m, and the VED explosive power is 48%. In layer D2 the rock factor value is 6.89 with a load of 7.5 m, space of 8.3 m, and 55% VED explosive charge. While in the DU layer, the rock factor value is 6.39 with a load of 7.3 m, 8.4 m space, and 51% VED filling of explosives. Prediction of blasting fragmentation analysis using Kuz-ram theory obtained fragmentation > 100 cm, namely 14.99% for the C2 layer, 14.84% for the D2 layer, and 14.82% for the DU layer.

Nonlinear time series analysis of limestone rock failure process

Nonlinear analysis of the rock mass failure process is a powerful tool for the study of instability and prevention of disaster in rock mass engineering. Nonlinear analysis has rapidly developed in recent decades. We propose a new method to determine the fractal characteristics of the failure process using time series analysis based on an iterative cycle division of the box dimension of fractal theory. We use AE technology to carry out time series analysis of fracture sources in the failure process of rock mass. We show that the disordered failure of rock mass exhibits notable fractal characteristics in the time series. The advantage of this method is that only the time and frequency of AE monitoring are used, resulting in a better identification and classification of damage, compared with traditional natural sequence frequency distribution. In addition, MATLAB numerical simulation analysis shows that these time series fractal dimensions can accurately characterize the failure characteristics of rock mass. In brief, we show the potential for future orderly time analyses of the complex and disorderly process of rock mass failure.

A fatigue constitutive model for rock masses based on cross-applications of rheological theory

In geotechnical engineering construction and underground ore mining, periodic cyclic fatigue loading is a major cause of rock mass instability and failure. With a constitutive model, the fatigue characteristics of rock masses can be studied effectively. Based on the similarities between rock mass rheology and cyclic fatigue loading, this paper applies rheological theory to study the fatigue characteristics of rock masses. Two constitutive models that describe different stages of nonlinear mechanical behaviour during the fatigue process were proposed, and a fatigue constitutive model that accurately describes the mechanical properties of rock during different fatigue stages was established after combining the two models. A sensitivity analysis showed that the model parameters EK , γ and δ can effectively reflect the deformation and damage evolution characteristics of the rock mass during the three fatigue stages. By comparing the laboratory splitting and three-point bending fatigue experiment results with the calculated model values, the rationality, reliability, and application scope of the model were verified. Moreover, based on the model parameter inversion results, the evolution mechanism of the rock mechanical properties during the different fatigue stages can be accurately understood.

A Geotechnical Perspective on a Complex Geological Environment in a High-Speed Railway Tunnel Excavation (A Case Study from Türkiye)

The T26 tunnel was designed within the scope of the Ankara-Istanbul high-speed railway in accordance with the speed of 250 km/h. Some serious problems and excessive deformations were encountered during the excavation works. The deformations in the tunnel caused subsidence on the surface and the Tunnel Boring Machine (TBM) became stuck; therefore, tunnel excavation works were suspended. Design works for re-excavation in the T26 tunnel and extracting of the TBM were carried out and the tunnel was re-designed by the New Austrian Tunneling Method (NATM) system. The main purposes of the present study are to describe the problems encountered during the T26 tunnel and to discuss the sources of the problems. The advantages and disadvantages of TBM and NATM methods for the tunnel having difficult ground conditions were discussed. Critical points needing to be considered for the tunnels excavated with TBM through weak ground conditions and the effect of the TBM selection process were discussed. Considering the complex geological and geotechnical structure of the tunnel route, it is possible to say that the T26 case is an interesting case for tunnel engineering. Along the tunnel route, landslides, high seismic activity, groundwater conditions, and extremely weak rock mass features coexist. Therefore, the tunnel route is a very complex environment. However, due to the geometric limitations of the high-speed railways, relocation of the route is not possible. The experiences gained from tunnel excavations under difficult conditions are capable of bringing new horizons to future tunneling studies.

Influence of multi-perforations hydraulic fracturing on stress and fracture characteristics of hard rock mass under excavation condition

Performing multi-perforations hydraulic fracturing (MPHF) on the rock mass is a highly effective method for increasing the fracture network, in this paper, MPHF technology was considered to assist mechanical excavation for hard rock mass in underground engineering. To end this, the influences of MPHF on the stress and fracture characteristics of hard rock mass under excavation conditions were investigated theoretically and numerically by considering different perforation spacings, perforation heights, and injection pressures. Theoretical analyses were performed based on the stress field theory of hydraulic fracturing and analytical solutions for excavation stress redistribution. A numerical model was established using a flow-coupled discrete element method, and it was verified using theoretical solutions. The results show that the local maximum principal stresses are in the horizontal direction under different MPHF parameters. The local stress differences decrease as the increasing of perforation spacing and the decreasing of perforation height and injection pressure. With an increase in perforation spacing, fewer horizontal fracture trajectories were generated between the perforation zones, and the number of microcracks initially increased and then decreased, indicating there is an optimal perforation spacing in MPHF under excavation. Moreover, the increase in perforation height, injection pressure, and injection time promote the propagation of crack trajectories and increase the number of microcracks. However, in practice, they should not increase without limits owing to efficiency and economic considerations. Therefore, when using MPHF for preconditioning the rock mass to assist mechanical excavation in the hard rock mass, the project economics and performance can be enhanced by careful selection of appropriate MPHF parameters.

Seepage characteristics of fractured rock mass with non-equal width filling under cyclic loading

Fractured rock mass with non-equal width filling often appears in the tunel engineering, the non-equal width ratio is the ratio of the filling width at the seepage inlet to the outlet. To study the seepage characteristics of FFRM (filling fracture rock mass) in the surrounding rock of the Gaoligongshan railway tunnel, the samples of FFRM with different non-equal width ratios were prepared indoors. The permeability coefficient formula of the non-equal width ratio FFRM was deduced for the permeability measurement. Then the triaxial seepage apparatus was used to conduct high and low confining pressure test. Results show that the permeability coefficient of FFRM with non-equal width ratios is a function of multi-parameter interaction, such as seepage outflow, inlet width and filling porosity. The permeability and confining pressure at LCP (low confining pressure) and HCP (high confining pressure) is the power and translation exponential function, respectively. While the relationship between the permeability and non-equal width ratio under confining pressure-seepage condition is the quadratic function. After the first cycle, the permeability loss of the samples under HCP is larger than that of the LCP, and the recovery degree is less than that of the LCP. The SSC (stress sensitivity coefficient) decreases with the increase of the confining pressure, the recovery degree is poor after the first cycle unloading. The trend of the subsequent cycle is almost consistent and the recovery degree is good, which is more obvious under the action of the HCP. The conclusions obtained in this study provide a reference for the stability evaluation of the surrounding rock in the underground engineering.

Failure Behaviour of Jointed Rock Masses with 3D Nonpenetrating Joints under Uniaxial Compression: Insights from Discrete Element Method Modelling

It is well known that joints or fissures have an important effect on the failure mechanism of natural rocks. Previously, many numerical and experimental papers have been carried out to study the strength anisotropy and failure characteristics of jointed rocks. However, few studies have been carried out on the failure mechanism of nonpersistent jointed rock masses with different persistence, especially for nonpersistent joints in three dimensions. In the present study, the failure characteristics of a 3D nonpersistent jointed rock mass with different inclinations (θ) and persistence (K) are studied by numerical simulation. For the 3D digital elevation model (DEM), the linear parallel bond model (LPBM) and smooth-joint model (S-J) were used to model the rock-like material and joint interface, respectively. The connections between the geometric parameters of joints and peak strength are revealed. For the peak strength, the joint persistence only plays a minor role in specimens with inclinations of 0° and 90°, and its influence on strength is mainly reflected in the specimens with shear failure (θ = 45°, 60°, and 75°). Based on microcrack accumulation and evolution, four typical failure processes (shear failure, split failure, mixed failure, and intact failure) are analysed from the micro perspective. The shear stress evolution process on the 3D nonpersistent joint of the specimen with different inclinations under K1 = 0.42 was monitored by the measurement circle, and it was found that the distribution of shear stress inside the rock bridge is related to the failure mode of the specimen. For the specimens with θ = 0° and 90°, the shear stress had little change, indicating that there is slight shear slip behaviour on the joint surface. When the inclination is 45°, 60°, and 75°, the shear stress changes obviously during loading, indicating that the shear action is strong in this failure mode.

Research on Seismic Wave Quality Factor of Marble Jointed Rock Mass under SHPB Impact

In order to quantitatively describe the energy dissipation law of jointed rock mass, we obtained the jointed cores in laboratory conditions using marble from the roof and floor of Jinchanghe Lead–zinc mine in Baoshan. The dissipative degree of stress wave in marble jointed rock mass is measured by introducing quality factor Qs parameter. Based on the experimental principle of Split Hopkinson Pressure rod loading device (SHPB), we proposed a three-wave energy method of incident wave, reflected wave and projected wave for calculating jointed rock samples’ quality factor Qs based on stress wave energy. Using the SHPB test system for multiple specimens taken from the same piece of rock mass shock compression experiment, the three groups of specimens under different loading conditions gained incident wave and reflected wave and transmission wave experimental data, using the method of stress wave energy to deal with stress wave data, and calculate the joint sample maximum storage energy, dissipation energy and Qs quality factors. The results show that: ① The non-destructive breaking time–history strain of Dali rock mass under impact load is obtained by SHPB dynamic test system; combined with the deformation energy and dissipation energy calculation principle of quality factor, six groups of Qs experimental values are obtained. Although the Qs experimental values are discrete, the overall deviation is small with an average of 43.07. ② AUTODYN-Code was used to simulate the damage and fracture characteristics of rock mass with different quality factors under explosive dynamic load. The results showed that the radius of rock mass compression shear damage area gradually increased with the increase in porosity, but it was not obvious.

Shear behavior of sandstone fractures after exposure to CO2 environment

CCS (Carbon Capture and Storage) is a promising technique for greenhouse effect mitigation by geological sequestration of CO2. Deep saline aquifers are considered the major storage reservoirs, mainly due to their abundance on the earth’s crust. Even though rock joint sets (fractures) are critical characteristics of rockmass (both for their mechanical and hydrological behavior), they are not simulated in detail in reservoir scale models for CO2 storage. That's due to significant demands for computational power and model design complexity. Advances in discrete medium models and discrete fracture network representation for models/reservoirs of considerable size will allow the usage of jointed rock as a discrete medium, for assessing reservoir hydromechanically. Hence, assessments of the performance under CO2 exposure of rock fractures inside CO2 environment will be necessary. In this study, we tested planar and rough, induced fractured sandstone specimens for shear mechanical behavior, under reservoir conditions. Results revealed a friction angle reduction (−23%) of exposed rough fractures, compared to unexposed ones. The moderate fracture roughness, even though still present, did not contribute as much to shear strength under effective normal stresses up to 1.5 MPa, with rough fractures behaving likewise to planar ones. Gouge production on fracture surfaces contributes to shear strength loss, calcite dissolution is considered the major reason for such behavior. We also observed inadequacy for shear resistance on planar exposed specimens when normal stress exceeds a critical value.

Investigation on the Deformation of Segment Linings in Cross-Fault Tunnel Considering the Creep Behavior of Surrounding Rock during Construction-Operation Period

Time-dependent deformation of surrounding rocks during construction and service directly affects the structural safety of hydraulic tunnels. Specifically, the creep effect on the cross-fault diversion tunnels is particularly significant. In recent years, a variety of research has focused on the impact of creep on operating cross-fault diversion tunnels. However, there is a lack of consideration for the creep phenomenon during the construction-operation period. This study proposes a simulation method of segment linings deformation of cross-fault diversion tunnel combining deformation monitoring, creep constitutive identification, and creep during construction operation. According to the measured time-displacement data of the surrounding rock, the suitable creep constitutive model is selected by a regression-comparison method. Subsequently, the three-dimensional numerical model of the cross-fault tunnel is established, with the tunnel excavation and long-term creep calculation carried out under the creep effect of the surrounding rock. In addition, the intelligent inversion model was adopted to obtain the basic parameters of the surrounding rock. The results show that fault directly affects the rock mass characteristics near the fault. The radius of influence is about one time the tunnel diameter from the fault. The vertical deformation of segment linings of the vault and inverted arch tends to be subsidence when the fault is above the tunnel; however, the trend is inverse when the fault is under the tunnel. This work gives targeted engineering suggestions and provides a scientific basis for designing and constructing diversion tunnels under complicated geological structures.

Experimental Study on Grouting Seepage Characteristics of Single-Fractured Rock Masses with Different Inclination Angles under Three-Dimensional Stress

In underground engineering, the fracture dip angle and three-dimensional stress have great influence on the permeability and mechanical properties of fractured rock mass. Firstly, single-fracture samples with similar three-dimensional fractal dimensions and different dip angles were selected by using CT scanning technology. Then, the grouting material seepage tests under different dip angles and different three-dimensional stresses were carried out by self-developed single-fracture stress-seepage coupling true triaxial test system. The experimental data of stress-strain curve, grouting pressure, and grouting material flow rate of single-fracture specimen under different fracture dip angles and three-dimensional stress were obtained. The results show that the three-dimensional fractal dimension can be used as an index to measure the influence of fracture factors on grouting seepage test. The third principal stress σ 3 had a greater influence on the fracture width than the first principal stress σ 1 . The larger the fracture inclination, the lower the strength of the single-fracture specimen. As the third principal stress σ 3 increases, the grouting pressure increased while the grouting flow decreased. With the increase of the fracture inclination angle, the influence of σ 1 on the hydraulic conductivity became larger, while the influence of σ 3 on the hydraulic conductivity became smaller. Moreover, the expression of hydraulic conductivity of single-fracture specimen with different dip angles under three-dimensional stress was obtained by nonlinear fitting of hydraulic conductivity data.

Experimental investigation and empirical model on damping properties of rock under multistage cyclic loading

The dynamic shear modulus and damping ratio of rock are the main indicators that reflect the dynamic characteristics of rock masses under seismic and dynamic action. The multistage cyclic triaxial compression test is performed on granite and red sandstone under different confining pressures with two stress paths. To reasonably describe the dynamic characteristics of rock, a modified Hardin hyperbolic model is proposed to describe the backbone curve. The relationship between the dynamic shear modulus, damping ratio and cyclic shear strain is analysed under different confining pressure conditions with variable and same amplitude. Furthermore, an empirical formula is established for estimating the dynamic shear modulus and damping ratio. The obtained results show that with increasing shear strain, the damping ratio of red sandstone and granite decreases and then increases under the condition of variable amplitude. When the shear strain is constant, the damping ratio of red sandstone and granite is less affected by the confining pressure. Under the same amplitude conditions, as the shear strain increases, the dynamic shear modulus under different confining pressures shows an increasing trend. The two established empirical formulas can be used to predict the dynamic shear modulus and damping ratio under multistage cyclic loading. This has important reference significance for studying the dynamic characteristics of rock.

Analysis of the Deformation Characteristics of the Surrounding Rock Mass of a Deep Tunnel During Excavation Through a Fracture Zone

Analysis of the Deformation Characteristics of the Surrounding Rock Mass of a Deep Tunnel During Excavation Through a Fracture Zone

Upscaling the Mechanical Properties of a Fractured Rock Mass Using the Lattice-Spring-Based Synthetic Rock Mass (LS-SRM) Modeling Approach—Comparison of Discontinuum, Continuum and Empirical Approaches

A numerical characterization of a fractured rock mass and its mechanical behavior using a discontinuum approach was carried out utilizing lattice-spring-based synthetic rock mass (LS-SRM) models. First, LS-SRM models on a laboratory scale were created to reproduce standard rock mechanical tests on Triassic sandstone samples from a quarry in Germany. Subsequently, the intact rock properties were upscaled to an element volume representative for geotechnical applications, recalibrated and combined with a Discrete Fracture Network (DFN) model. The resulting fractured rock mass properties are compared to predictions from empirical relationships based on rock mass classification schemes and the DFN-Oda-Geomechanics approach. Modeling results reveal a significant reduction in the strength of the fractured rock mass compared to the intact rock, showing a high agreement with empirically calculated values. Results for the deformation modulus reveal a significant reduction induced by the fracture network and a good agreement compared to the results obtained by other approaches. It is shown that the LS-SRM allows analyzing the complex mechanical behavior during failure of rock masses, including crack initiation, propagation and coalescence. The resulting rock mass properties are key parameters for a wide range of geotechnical applications and can be used for large-scale numerical modeling as well.

Study on the slope dynamic stability considering the progressive failure of the slip surface under earthquake

The strength of a rock-soil mass shows complex and obvious weakening characteristics under seismic dynamic load. The previous stability analysis methods of a seismic slope do not fully depict the attenuation law of geotechnical materials and cannot truly reflect the stable state of a slope under earthquake action. Based on the theoretical analysis of the progressive failure mechanism and the evolution law of a seismic slope, the adverse effect of progressive failure on slope stability is clarified. According to the progressive failure process of a slope under dynamic load, the strain-softening model and vibration deterioration model are introduced to represent the attenuation law of rock strength parameters, and a calculation method of seismic slope stability coupled with vibration disturbance and progressive failure is proposed. The method considers the strength parameter characteristics of a rock-soil mass in different stages and is combined with the vector sum method to obtain the time-history curve of the slope safety factor under earthquake action, which makes the evaluation result of slope stability more accurate and reliable. The numerical examples show that this method can effectively reflect the dynamic stability of seismic slopes, and solve the problem that the traditional calculation methods are difficult to characterize the strength attenuation characteristics of rock and soil mass. If these characteristics are not considered, the calculation results will be unsafe.