Weak Rock Masses Research Articles

Simulating Crack Development and Failure Characteristic of Toppling Rock Slope under Seismic Loading on Lancang River in China

Abstract Toppling rock slopes, induced by rapid and continuous downcutting of Lancang River, are widely distributed in the mountainous area of southwest China. To investigate the instability mechanism of 1# toppling rock slope of Huangdeng Hydropower Station under seismic loading, particle flow code (PFC) is applied to simulate the dynamic response and failure mode. The study considers the particle characteristics of displacement, velocity, energy, and cracks. According to numerical results, the potential failure mechanism of toppling rock slope is identified: multisliding surfaces form at the interfaces between the highly and moderately toppled rock mass and between the highly/moderately and weak toppled-crept rock mass; intersecting faults cut rock mass at the toe, leading to shear-toppling deformation; tension cracks develop, penetrate, and coalesce in the weak toppled-crept rock mass, resulting in tension-toppling-bending deformation. During the 2 to 5 s of strong seismic intensity, crack increases sharply and energy of particles fluctuates greatly. The impacts of the amplitude of seismic loading and loading method in PFC are investigated. This study will provide a practically useful reference for seismic design of rock slopes.

Propose a viable stabilization method for slope in weak rock mass environment using numerical modelling: A case study from the cut slopes

The design of a stable slope in a rock mass environment is a quite complicated job due to the anisotropic behaviour of the rock mass. In this research work, the cut slopes at the Swat motorway in the weakest schist rock is numerically analyzed by the shear strength reduction (SSR) approach using the Finite Element-based 2D RS2 software. The slope is divided into two cases according to the nature of the rock. Each case of the cut slope is analyzed by two stabilization methods: 1) changing the characteristics of the slope 2) support system installation based on the Mohr-Coulomb (MCC) and Generalized Hoek and Brown (GHB) failure criteria in order to propose the most appropriate method for slope stabilization. The results obtained reveal that the Critical Strength Reduction Factor (CSRF) before applying the stabilization methods is 1.34 (MCC) and 1.04 (GHB) for Case-I and 1.21 (MCC) and 0.53 (GHB) for Case-II. CSRF for Case-I after changing the characteristics of the slope is observed to be 2.43 (MCC) and 2.33 (GHB), while for Case-II is 1.82 (MCC) and 1.26 (GHB), respectively. CSRF for Case-I after the support installation criteria is 1.59 (MCC) and 1.07 (GHB), while for Case-II is 1.65 (MCC) and 0.5 (GHB), respectively. Based on the comparative analysis, it is revealed that changing the characteristics of the slope method shows prominent results in both cases; therefore, this method can be effectively used in order to stabilize the slope in the weakest rock mass environment.

Construction of twin tunnel in weak and blocky rock mass by using arch supporting method

Design of tunnel excavation and support system in weathered and blocky rock mass is usually a principal challenge in tunneling. In this study, the excavation method of twin tunnel was newly taken into consideration in the condition of weak rock mass, the section size of tunnel was reasonably determined, and new supporting method was established. Firstly, the mechanical characteristics of the project rock mass were determined in each section of tunnel using in-situ point load test (PLT) and RocLab software. Then, design of twin tunnel excavation from mono-tunnel excavation in the weak parts of rock mass was proposed and section of twin tunnel joint was reasonably determined based on the safety and stability of the twin tunnel joint by FLAC3D. Finally, the sequential excavation method of combining top heading and benching with arch supporting method (ASM) was newly proposed and the validity of this method was determined by the numerical simulation modeling. Introducing ASM for the support system ensured the support strength in the excavation tunnel. Advanced excavation scheme and the support system have been successfully applied to ensure the safety and conveniency of the target tunnel construction, with about twice reduction of construction time and 1.5 times raising of stability factor. As a result, proposed method in this paper can be used to all manual tunnelling and underground structures in weak and blocky rock mass.

Weak rock mass characterization for determination of support system of the DK99-DK100 Jakarta – Bandung speed railway tunnel, Indonesia

Rock mass characterization is a crucial factor in determining a safe design of a tunnel support system. This research is carried out in constructing tunnel DK 99 – DK 100 high-speed railway Jakarta-Bandung, Indonesia. The research includes determining surface and subsurface quality of rock masses based on rock mass classifications of Geological Strength Index (GSI) and the Basic Quality (BQ-System). Both rock mass classifications were then correlated to the Rock Mass Rating (RMR) for the tunnel support system determination. Another support system determination based on the Japanese Society of Civil Engineers (JSCE, 2007) method was also compared. The result showed that the rock masses at the tunnel elevation had very poor to poor quality according to GSI, RMR, and BQ. The rock masses were classified as category class E and DII according to the JSCE competence factor. The recommended tunnel support system based on the RMR are shotcrete, rockbolt, steel set, while those based on the JSCE are shotcrete, rockbolt, steel support, and lining. This paper is expected to provide a better understanding of tunnel construction in weak rock masses, particularly in Indonesia.

Time-Dependent Solutions for Lined Circular Tunnels Considering Rockbolts Reinforcement and Face Advancement Effects

During deep excavation in weak rock masses, the time-dependent deformation of the rock and longitudinal advancement process have significant influences on the tunnel performance. Accordingly, a combined support system consisting of rockbolts and linings is commonly used in such tunnels to ensure their stability. However, there are no well-established solutions for quickly and accurately evaluating the safety of those support systems. To address this problem, mechanical model of a circular tunnel with a rockbolt–lining combined system is analytically built in this study. Then mathematical formulas for the stresses and displacements around the tunnel are derived, considering the time-dependent behaviors of the rocks and tunnel face advancement effect. Furthermore, the analytical formula proposed in this study is compared with existing solutions and numerical results obtained using the finite-element software Abaqus, and a good agreement is achieved. Finally, a parametric investigation is conducted regarding the influences on the tunnel’s mechanical performance. The results show that the installation time (t0) and distribution density of rockbolts (SθSz) and the installation time of the lining (t1) have a significant influence on the deformation of the surrounding rock. Although the installation times of bolt and lining are positively related to the deformation suppression effect of the surrounding rock, they are negatively related to the stress of the bolt and lining. It is found that the closer the installation time of bolt and lining, the greater the stress of lining; otherwise, the greater the stress of bolts. Furthermore, the initial release coefficient (m) and the ratio of tunnel excavation rate to influence radius can influence the deformation rate of surrounding rock. Finally, the study proposes a new method for quickly evaluating the viscoelastic deformation of a tunnel.

Design aspects of segmental tunnel lining in difficult rock conditions

Despite plenty of research studies in design of segmental tunnel lining, there have still been serious concerns in damage mechanism of segmental lining in weak rock masses. With significant increasing of mechanized tunnelling practically in all kind of ground, including highly weathered rocks, severely jointed rocks, shear, and fault zones, squeezing, and swelling prone conditions, the need to investigate the mechanical behaviour and the damage mechanism of segmental tunnel lining is unavoidable. Any misjudging of those adverse geo-conditions may often cause an irreversible economical loss in the tunnelling project. This paper deals with the significant design aspects of segmental lining for such difficult rock mass conditions which can cause lining deformation and failure in segment in terms of either compression or tension as well as in the segment joints. Further, some critical design aspects extracted from the tunnel T26 of Ankara – Eskisehir high speed railway are presented and analysed.

STUDY ON STABILITY CONTROL OF COAL MINE TUNNEL EXCAVATED IN WEAK ROCK MASS IN INDONESIA

This paper focuses on the stability analysis and support design of the coal mine tunnel excavated in weak rock mass in an Indonesian underground coal mine through numerical simulations using the FLAC3D software. The PT Gerbang Daya Mandiri (GDM) coal mine situated in Indonesia was selected as a mine site in this study. According to the results of a series of numerical simulations, the stability of the mine tunnel decreases by increasing the depth and stress ratio. Ground control problems, for example falling roof, sidewall collapse, and floor heave are expected unless an appropriate support system is anticipated. Three support systems, including friction rockbolt, steel arch, and shotcrete are discussed as methods to stabilize the roof and sidewalls of the mine tunnel. From the simulated results, the steel arch is considered to be the most effective support method when compared with other support systems. The steel arch which is installed with closer space and larger crosssection delivers a better stability control to the roof and sidewalls of the mine tunnel. Although the stability of the roof and sidewalls of the mine tunnel can be maintained effectively by the steel arch support, the occurrence of floor heave is expected when the mining depth is increased. To control the floor stability of the mine tunnel, three techniques by applying cablebolt, invert-arch floor, and grooving method are therefore investigated and discussed. Based on simulated results, the heaving of the floor is well controlled after the cablebolt, invert-arch floor, and grooving methods are applied. Nevertheless, it is found that controlling the floor heave by cablebolt support could be the most suitable method comparing with other support systems in terms of the installation process, providing flat and safe working conditions of the floor, and economy. Additionally, the cablebolt with closer row space and longer length works more effectively to control the heaving problem of the floor. Keyword

Probabilistic stability assessment of tunnel-support system considering spatial variability in weak rock mass

Analysis of tunnel-support system stability with uncertain rock mass properties is conducted in this paper. A horseshoe shaped tunnel driven in weak rock mass is analyzed through deterministic, random variable and random field approaches. Performance of the tunnel in probabilistic analysis was assessed by defining three limit states which ensure that tunnel convergence remained below a safe threshold level, the rock bolts installed are embedded sufficiently beyond the depth of yielded rock mass and load induced on liner support does not exceed its capacity. Both unsupported and supported tunnels were analyzed with deterministic, random variable and random field approaches. Fourier series method was applied to discretize the random fields, and random finite difference analysis was conducted using Monte Carlo simulations. Scale of fluctuation (SOF) for isotropic random fields and horizontal and vertical SOF ratios for anisotropic random fields were varied to study their effect on performance of the tunnel and failure mechanisms involved. It was found that SOF significantly influences the output statistics and Pf of the limit states. It was observed that, random variable approach underestimates the performance of the tunnel-support system; however, it can be adopted as conservative option in absence of data required for random field characterization.

On the Use and Interpretation of In Situ Load Tests in Weak Rock Masses

Sockets are often constructed into weak rock mass to improve their axial response to applied compressive or tensile loads. This paper first establishes two separate load test databases for the side and base resistances in in situ socket load tests in weak rocks. The databases include the rock socket geometry, rock socket load–displacement response and the rock mass properties. The paper then reviews the available models for the prediction of (i) the axial resistance (compression or uplift), and (ii) axial deformation. The databases are used to evaluate the existing predictive models for axial resistance and axial deformation, and to evaluate the behavior of drilled shafts in weak rock mass. The load test databases are also used to evaluate the effect of rock mass variability on the reliability of foundations in weak rock. Based on the results of the analysis presented, the applicability of the existing models and approaches to the study of socket behavior in weak rocks are discussed.

A Solution to the Time-Dependent Stress Distribution in Suborbicular Backfilled Stope Interaction with Creeping Rock

The creep behavior of deep weak rock masses is important due to an underground opening. Appreciating the nature and source of these deformations requires the knowledge of rock mass and ground support interaction. The theoretical solution of the backfill’s internal stresses needs to consider the time-dependent effect. In the present study, the coupling interaction between the creep behavior of the nearby rock material and the internal stresses in the backfilled stope is considered and the interaction characteristics are given analytically. A solution is then proposed regarding the time-dependent stress distribution in suborbicular backfilled stope interaction with creeping rock. Besides, the correctness of the theoretical solution is verified by numerical simulation, while influential parameters such as stope buried depth, lateral pressure coefficient, horizontal stress ratio, creep time of surrounding rock mass, delay time of the backfill, and Young’s modulus are thoroughly discussed. Research shows that when the stope buried depth becomes large as well as the rheological effect of the nearby rock materials becomes significant, the stress distribution in the backfill material exceeds its self-weight stress and presents significant time-dependent characteristics. The delayed backfilling weakens the backfill’s ground support effect on the nearby rock material. Hence, timely and multipoint simultaneous backfilling is needed for a stope with significant rheological deformation of surrounding rock mass. Lastly, this work will offer useful knowledge while designing the backfill materials for underground mines.

Characteristics of the In Situ Stress Field and Engineering Effect along the Lijiang to Shangri-La Railway on the Southeastern Tibetan Plateau, China

This work aims to characterize the in situ stress field along the Lijiang to Shangri-La railway and identify possible engineering geological problems when constructing tunnels along this railway on the margin of the Tibetan Plateau. The in situ stress measured at 76 points in 12 boreholes by the hydraulic fracturing method was analysed. A rose diagram of the maximum principal stress direction was plotted based on the measured in situ stress data. The results show that the measured in situ stress is mainly horizontal stress, corresponding to a strike-slip fault-related tectonic stress field with a moderate to high in situ stress value. The main stress values have a good linear relationship with the burial depth, and the maximum horizontal principal stress (σH) increases by 1.1–8.8 MPa per 100 m, with an average gradient value of 3.6 MPa per 100 m. The maximum and minimum horizontal principal stresses and the stress differences increase with depth, and the lateral pressure coefficient (σH/ σ v ) is generally 1–1.5. The ratio of the maximum and minimum effective stresses is less than the threshold at which faulting occurs, resulting in faults that are relatively stable at present. The direction of the maximum horizontal principal stress is oriented at a small angle to the axial direction of the deeply buried tunnel along the railway line; therefore, the tunnel sidewalls could readily deform during the construction process. Rock bursts are expected to occur in strong rock masses with high risk grades, whereas slight to moderate deformation of the rock surrounding the tunnel is expected to occur in weak rock masses. This study has significance for understanding the regional fault activity and issues related to the construction of deeply buried tunnels along the Lijiang to Shangri-La railway.

WITHDRAWN: Portrayal of contact and consolidation grouting surrounding HRT lining: A study

Abstract Construction of tunnels through naturally fragile, weak and sheared rock masses is a challenging task for engineers, planners, designers and geologists. The presence of discontinuities like folds, faults, voids, number of shear zones, highly changing geological conditions with thick vegetative cover and over burden pose threat to construction activities in frail and fragile rock mass. These discontinuities get further separated and more widened as the excavation of tunnels involve drilling and blasting operations and do not allow the rock mass to behave monolithically. Proper grouting of the surrounding rock mass around the opening ensures the monolithic behavior of the rock mass. Contact grouting or back fill is required for filling of spaces between concrete lining or steel lining and shattered surface of the parent rock. Consolidation grouting in tunnels is essential to reduce the co-efficient of permeability and for strengthening the rock mass by filling up the open joint cracks. This paper deals with the proper grouting of the surrounding shattered, sheared and fragile rock mass around the power tunnel lining to ensure its monolithic behavior.

Modeling of the effects of weakness planes in rock masses on the stability of tunnels using an equivalent continuum and damage model

This paper proposes an equivalent continuum model to describe the mechanical behavior of transversely isotropic rocks. In this model, the transverse isotropy of deformation and strength was achieved based on the Mohr–Coulomb and maximum tensile-stress criteria, and the damage was captured by adopting a statistical damage evolution rule. The application of the model is verified through numerical simulation of conventional triaxial tests. The model is then used to reveal the non-uniform mechanical response of the surrounding rock and the secondary lining for a tunnel situated in a weak layered rock mass. The results show that: (1) The proposed model can capture the transverse isotropy in deformation and strength of rocks, and the proposed damage formulation can represent the deterioration and degree of failure of rocks; (2) The fracturing pattern, failure strength and stress–strain curves obtained from the proposed model agree well with test results for three typical rocks with different directional variations in strength; (3) The damage distribution based on the proposed model can identify the failure of layered rock mass; and (4) The damage zones of the surrounding rock and the loads on the secondary lining after tunnel excavation show distinctly asymmetric behavior, that is, the damaged zones are concentrated in the tunnel direction normal to the weak planes, and the positive bending moment and larger axial force are parallel and vertical to the weak planes, respectively.

Geomechanical model test for analysis of surrounding rock behaviours in composite strata

Due to the large differences in physico-mechanical properties of composite strata, jamming, head sinking and other serious consequences occur frequently during tunnel boring machine (TBM) excavation. To analyse the stability of surrounding rocks in composite strata under the disturbance of TBM excavation, a geomechanical model test was carried out based on the Lanzhou water supply project. The evolution patterns and distribution characteristics of the strain, stress, and tunnel deformation and fracturing were analysed. The results showed that during TBM excavation in the horizontal composite formations (with upper soft and lower hard layers and with upper hard and lower soft layers), a significant difference in response to the surrounding rocks can be observed. As the strength ratio of the surrounding rocks decreases, the ratio of the maximum strain of the hard rock mass to that of the relatively soft rock mass gradually decreases. The radial stress of the relatively soft rock mass is smaller than that of the hard rock mass in both types of composite strata, indicating that the weak rock mass in the composite formation results in the difference in the mechanical behaviours of the surrounding rocks. The displacement field of the surrounding rocks obtained by the digital speckle correlation method (DSCM) and the macro-fracture morphology after tunnel excavation visually reflected the deformation difference of the composite rock mass. Finally, some suggestions and measures were provided for TBM excavation in composite strata, such as advance geological forecasting and effective monitoring of weak rock masses.

Resonance features of rock slope with anti-dip weak interlayer under seismic actions

Resonance avoidance is an important topic in engineering seismic design. However, there is little report on the resonance features of rock slopes with anti-dip weak rock layers under seismic actions. Through finite-element method, this paper carries out modal analysis and harmonic response analysis on a slope, captures the natural frequencies and vibration modes of the slope, and plots the resonance curves about displacement frequency relationships. The results show that slope resonance is greatly affected by damping ratio: the greater the damping ratio, the lower the resonance peak; the inverse is also true. This means that weak and broken rock mass is capable of absorbing shocks, but not necessarily easy to be damaged. On the contrary, broken rock mass has poor mechanical performance and is prone to damage under small vibration. Besides, the resonance effect of the slope is mainly excited by the natural frequencies of the first three orders; the vertical resonance displacement peaked at about 1.1 Hz, while the horizontal resonance displacement peaked at about 1.38 Hz. From the vibration modes and frequency response curves, the resonance peak of the slope is amplified in the vertical direction and on the free face, and the maximum resonance displacement appears on the surface of the slope, indicating that the strongest resonance occurs on slope surface. This research fully clarifies the stability and dynamic response law of rock slopes with anti-dip weak interlayer under seismic actions, laying a solid basis for the mic fortification, landslide prediction, and landslide control of slopes.

Longwall top coal caving design for thick coal seam in very poor strength surrounding strata

Alpu lignite field is an important coal deposit with nearly 2 billion tons of coal resources located in the middle of Turkey. The mine deposit consists of three main seams. The thickness of two of them vary from 4 to 30 m. The surrounding rock mass is very poor in terms of strength. The high clay content and weak rock mass make mechanized mining difficult. In this research, applicability of the longwall top coal caving method was investigated. The very weak strength behavior of the coal and the surrounding strata increases the importance of research in the mine site in terms of ground control. The aim is to design the mechanized longwall mine based on ground control principles. First of all, classification of the roof, coal, inter-burden, and floor strata were classified based on geotechnical aspects. Then, cavability index, shield, and floor bearing capacity were investigated. Different methods were applied to understand the limitations of a mechanized system that is very critical due to the very low strength strata. According to the main results, roof strata was classified as immediately caving while mining height was calculated as 5–6 m. Finally, the relations among geotechnical characterizations of roof and floor strata, cutting and caving heights, and required shield capacity were presented based on analytical and numerical applications. The proposed approach can be used as a ground control method for the applicability as well as the limitations of mechanized longwall mining design in weak strata conditions.

Stability of a Rock Tunnel Passing through Talus‐Like Formations: A Case Study in Southwestern China

Tayi tunnel is one of the component tunnels in the Jian‐Ge‐Yuan Highway Project located in Yunnan Province, southeast of China. It mainly passes through talus‐like formations comprised of rock blocks of diverse sizes and weak interlayers with clayey soils with different fractions. Such a special composition leads to the loose and fractured structure of talus‐like formations, which is highly sensitive to the excavation perturbation. Therefore, Tayi tunnel has become the controlled pot of the whole highway project as the construction speed has to be slowed down to reduce the deformation of surrounding talus‐like rock mass. To better understand the tunnel‐induced ground response and the interaction between the surrounding rock mass and tunnel lining, a comprehensive in situ monitoring program was set up. The in situ monitoring contents included the surrounding rock pressure on the primary lining, the primary lining deformation, and the stress of steel arches. Based on the monitoring data, the temporal and the long‐term spatial characteristics of mechanical behavior of surrounding rock mass and lining structure due to the excavation process were analyzed and discussed. It is found that the excavation of lower benches released the surrounding rock pressure around upper benches, resulting in the decrease of the surrounding rock pressure on the primary lining and the stress of steel arches. In addition, the monitoring data revealed that the primary lining sustained bias pressure from the surrounding rock mass, which thereby caused unsymmetrical deformation of the primary lining, in accordance with the monitored displacement data. A dynamically adaptive support system was implemented to strengthen the bearing capacity of the lining system especially in the region of an extremely weak rock mass. After such treatment, the deformation of the primary lining has been well controlled and the construction speed has been considerably enhanced.

Deformation and mechanical characteristics of tunneling in squeezing ground: A case study of the west section of the Tawarazaka Tunnel in Japan

Ground squeezing induced by tunnel excavation is a common phenomenon in weak and jointed rock masses. The prediction of convergence during tunnel construction in squeezing ground is important for the selection of temporary support and excavation methods. In the present study, the main factors that lead to ground squeezing are discussed based on a case study of the west section (WS) of the Tawarazaka Tunnel. A systematic analysis of convergences measured on site is presented, and numerical modeling using the finite difference method (FDM) is performed. The results show that ground squeezing is prone to occur in Paleogene mudstone formations, especially when slickensides are strongly developed. Ground squeezing is affected by the initial horizontal stress ratio, and a larger initial horizontal stress ratio lead to a larger convergence ratio, which is defined as the ratio of horizontal convergence to vertical convergence. Tunnel convergence values linearly increase with the decrement of bedding joint stiffness, and tunnel convergence increases with the increment of internal friction angle of bedding joints. Ground squeezing in the WS is mainly affected by the modulus values of nearby rocks and the existence of bedding joints. For the WS of the Tawarazaka Tunnel, the convergence ratio tends to stabilize when a critical overburden depth of 100 m is exceeded.

The support load analysis of deep-buried composite lining tunnel in rheological rock mass

The loads on primary support and secondary lining in composite lining tunnel are one of the key problems in support design. The primary support and secondary lining which as the main bearing structure determines the long-term operation safety of tunnels. In this study, three kinds of tunnel support conditions are established and the rheological model, stress releasing coefficient and additional radial body force of bolts are considered. Also, the analytical formulas of displacement on the primary support and secondary lining are derived. According to the relationship between displacement difference and support stiffness, the support loads are obtained. The calculated displacements and loads of the support structure are basically consistent with the field measurement, which verifies the reliability of the theoretical method. In addition, the influences of support parameters on the support load and bearing ratio are studied. Finally, the variation of the time-history curves of the measured support loads are discussed. The results show that the installation time of primary support, installation time of secondary lining, bolt length, thickness of the secondary lining, cohesion and rheology all may influence the support load. Meanwhile, the thickness of the secondary lining and the installation time of secondary lining have greater impact to the load bearing ratio. The combination of the weak rock mass, the early installation of the secondary lining, and the insufficient thickness of the secondary lining will make the load ratio of the secondary lining significantly higher. The stress paths of support loads are different depending on whether the support structure is in tension or in compression.

Structural Mechanical Characteristics and Instability Law of Roof Key Block Breaking in Gob-Side Roadway

The influence of mining on the upper section of working face leads to the fracture of the lateral key block of the roof. From the goaf to the coal body, a group of “left-middle-right” key blocks are formed. According to the three different spatial position structure relations formed by roadway and broken key block in practical engineering, the mechanical causes of broken structure of key block in roof of roadway along goaf are analyzed. FLAC3D is used to simulate and analyze the deformation characteristics and stress state of key block structure model before and after roadway excavation, and the mechanical characteristics and instability mechanism of key block sliding and breaking under three spatial structure modes are obtained. With the help of the mathematical model of material mechanics, the structural mechanical behavior of key block model of roof before and after roadway excavation and the temporal and spatial evolution law of unloading and breaking are studied. The results show that the complex influence factors of mining disturbance and low strength of the weak rock mass will weaken the internal balance of “masonry beam” structure. When the roadway is located below the fracture line of the key block, the middle key block will rotate and lose stability with the side hinge joint of the goaf as the axis; when the roadway is located in the fracture line of the key block, it is easy for the middle key block to slide and lose stability; when the roadway is located outside the fracture line of the key block, the middle key block is in the state of complete collapse, the mechanical transmission mechanism of the surrounding rock in the vertical direction is weakened, and the surrounding rock is the most stable.