Soft Rock Mass Research Articles

Long High-Performance Sustainable Bolt Technology for the Deep Coal Roadway Roof: A Case Study

High-efficiency maintenance and control of the deep coal roadway roof stability is a reliable guarantee for safe production and sustainable development of a coal mine. With belt haulage roadway 3108 in MenKeqing coal mine as the research background, in situ investigation, theoretical analysis, numerical simulation, and engineering practice were carried out to reveal the law of improving the bearing state of bolts by increasing the thickness of the roof anchoring layer. Also, the mechanism of the high-efficiency and long anchoring of the roof is revealed. Results show that increasing thickness of the roof anchorage layer could mobilize deep rock mass to participate in the bearing and promote the bolt to increase the resistance in a timely manner to limit the deformation of rock mass. Through the close link between shallow soft rock mass and deep stable rock mass, the deformation of the shallow rock mass is well controlled and so are the development and expansion of the roof separated fissures from shallow to deep. Long high-performance sustainable bolt technology for roof are proposed and carried out to control the stability of the deep roadway roof. Engineering practice indicates that deformations of roof could be efficiently controlled. The maximum deformations of the roof and sidewall-to-sidewall are 17 mm and 24 mm, respectively. No obvious separation fissures are found in the anchoring range of roof. This study provides a reference for roof stability control of deep roadway under similar conditions.

Tunnel lining load with consideration of the rheological properties of rock mass and concrete

Rheological processes in the rock mass for the stress-strain analysis are quite important when considering the construction of underground structures in soft rock masses, particularly in case of construction in several stages. In the analysis, it can be assumed that the reinforced concrete structure is slightly deformable in relation to the rock mass, and the rheological stress redistribution happens at the expense of the elements of rock mass. The basic elements of rheological models for certain types of rock mass and analysis of these models are presented in the first part of this paper. The second part is dedicated to the analysis of rheological processes in marl rock mass and the influence of these processes on the reinforced-concrete tunnel structure.

Base resistance of drilled shafts in soft rock using in situ load tests: A limit state approach

This paper presents a probabilistic limit state framework for the evaluation of the base resistance of drilled shafts in soft rock. In situ load tests and the Griffith fracture theory are used to evaluate the mode of failure for the rock mass under the drilled shaft base. The base resistance or the limit state in the context of this paper is defined by the contact pressure at which load induced vertical to subvertical cracks form and the contact pressure-displacement relationship passes into a steep and fairly straight tangent. The parameters affecting the base resistance are evaluated using an in situ load test database. A statistical approach using the maximum likelihood method is utilized for the development of the design model for the base resistance. Reliability analysis is used to calibrate the corresponding resistance factors. The in situ load tests and the Griffith fracture theory suggest that the soft rock mass underlying the drilled shaft base primarily fails by the formation of vertical to subvertical cracks. Field observations indicate that the displacements in the underlying rock mass are largely in the vertical direction and that the base displacements required to mobilize the proposed base resistance are generally less than 30 mm. Load test data show that the base resistance is chiefly related to the unconfined compressive strength of soft rock. The calculated resistance factors are found to slightly decrease with increase in the span length of the structure and increase with the increase in the foundation redundancy.

Influence of material spatial variability on slope stability in soft rock

Impacts of material spatial variability usually are not considered in traditional slope stability analyses. However, most geotechnical materials are not uniform inside a slope. The objective of the present study was to evaluate the impact of material spatial variability on slopes in soft rocks. The random field model was used to obtain the material spatial variability. The numerical analyses of slopes in soft rock mass were performed using a distinct element method based program UDEC. After that, Monte Carlo simulation, UDEC, and random field models were used together to analyze the impact of material variability and the slope stability. A series of numerical experiments were performed to understand the effect of joint spacing, joint angle, and size of scale fluctuation (δ). Also, failure types and mechanisms were synthesized and evaluated. The probability of failure became higher as the slope considered horizontal spatial variability of the material. The influence of the horizontal size of δ on slopes without joints was much higher than the slopes with joints. In addition, the slopes with joints yielded fewer failures as δ increased. The results of the analyses could be used for prevention of slope failure, future support system, and geotechnical exploration.

Load-displacement response of drilled shaft tip in soft rocks of sedimentary origin

A database of in situ drilled shaft and plate load tests in soft rock mass is developed. Analyses of the in situ load tests suggest that (i) the development of tip contact pressures is related to the rock mass shear strength, rock mass deformation modulus, and the vertical displacement of the drilled shaft tip, (ii) embedding the drilled shaft tip contributes to the development of punching shear failure mechanism in the underlying rock mass, however, the mobilized tip contact pressures do not significantly increase as the embedment depth increases unless the rock mass shear strength and deformational properties drastically increase with embedment depth, and (iii) the drilled shaft diameter does not significantly influence the development of the contact pressures, especially in large diameter drilled shafts. A framework for the analysis of the load-displacement response of the drilled shaft tip in soft rock mass is proposed using the in situ load tests. The proposed framework accounts for the observed nonlinear load-displacement response of soft rock mass and is a function of the initial normal stiffness and the initial yield pressure of the rock mass under the drilled shaft tip. Simple equations are developed for the initial normal stiffness and the initial yield pressure of soft sedimentary rocks. Additionally, in situ and laboratory test results are used to provide recommendations for the prediction of the parameters that are required in the proposed models for the initial normal stiffness and the initial yield pressure, namely the unconfined compressive strength and deformation modulus for the rock mass.

Coupled analytical solutions for deep-buried circular lined tunnels considering tunnel face advancement and soft rock rheology effects

For tunnelling in a deep soft rock mass, rock rheology and the advancing process significantly affect the induced pressures on the support system and the long-term safety of the tunnel. In this study, coupled analytical solutions, which take both the rock rheology and tunnel face advancement effects into consideration, are proposed to predict the mechanical behavior of deep-buried circular lined tunnels in a soft rock mass. To account for different rheological behaviors of rocks, five types of viscoelastic models are adopted in the theoretical derivations. The analytical solutions are validated by comparing the predicted results with corresponding existing solutions and those predicted by finite difference simulations. According to parameter degradation, the proposed solutions can be simplified to the classic viscoelastic solution without consideration of tunnel face advancement and the elastic solution with consideration of tunnel face advancement. Based on the proposed coupled solutions, the characteristics of convergent displacements of unlined tunnels with or without consideration of the tunnel face advancement, are studied. In addition, for lined tunnels, the influences of the initial stress release parameter, influence radius of the tunnel face, excavation rate and liner installation time on rock stress and displacement and support pressure are systematically investigated. Finally, the proposed solution is successfully applied to analyze the time-dependent behavior of the Lyon-Turin Base Tunnel, and good agreements are achieved between the theoretically predicted values and corresponding field monitoring data.

Study on deformation failure mechanism and support technology of deep soft rock roadway

The roadway support has always been a difficult problem in the mining engineering, especially in the deep soft rock roadway, whether it can provide endurable and effective support directly affects the safety production and sustainable development of the mine. In order to solve the supporting issues, it is necessary to make the deformation failure mechanism of deep soft rock roadway clear. This paper describes a case study of the deformation failure mechanism and support technology of deep roadway with soft rock mass in No.2 mine zone of Jinchuan mine that is located in Gansu Province, China. Based on a detailed field investigation and previous research experience, the modes, influencing factors, laws and mechanisms of deformation failure in the roadway are summarized. Then the primary combined supporting scheme “double layer bolt-mesh-shotcrete and U-shaped steel” is evaluated through numerical simulation calculation and onsite experiments. To make up for the shortcomings of the primary support, an improved combined supporting scheme “double layer long bolt-mesh-shotcrete and CFST (concrete filled steel tube)” is proposed. The results of numerical simulation, field monitoring and economic analysis show that the new support can control the deformation of the roadway effectively and has a high cost performance. Finally, several suggestions for supporting in deep soft rock roadway are put forward, aiming at serve as a reference for other engineering.

A load-transfer function for the side resistance of drilled shafts in soft rock

The shear stress and shear displacement relationship for the rock socket sidewalls is required for the calculation of the drilled shaft butt settlement under the service loads. This paper first introduces a comprehensive database of in situ axial load tests on drilled shafts, anchors and plugs that are embedded in different soft rock formations. The database includes measurements of (i) the initial shear stiffness, (ii) the peak shear stress and (iii) the post-peak reduction in shear stresses for the socket sidewalls. In addition to the load test results, information on soft rock mass mechanical properties and rock socket geometry is also included. It is found that (i) the initial shear stiffness is directly related to the deformation modulus of the soft rock mass and inversely proportional to the rock socket diameter and length, (ii) the mobilized peak shear stress is related to the drained friction angle of the rock mass and normal stress on the socket sidewalls at failure. The rock mass friction angle is related to the rock type and the geological strength index, and the normal stress at failure is directly related to deformation modulus of rock mass and inversely to the product of rock socket length and diameter, and (iii) the post-peak brittleness is related to the soft rock type and the post-peak shear displacement. An empirical framework for the prediction of the load-transfer function for side resistance of sockets in soft sedimentary and fine-grained rock is developed using the load test database introduced herein. The proposed framework accounts for the socket geometrical characteristics, and the rock mass engineering properties. The pre-peak range in the load-transfer function is modeled using a hyperbolic function, and the post-peak range is modeled using a brittleness index to account for the reduction in shear stresses with post-peak displacement.

Stability Analysis and Support Control for a Jointed Soft Rock Roadway Considering Different Lateral Stresses

The stability of coal mine roadways in jointed soft rock masses is an important issue for safe and efficient mining production in underground mines. Taking a main haulageway of a coal mine in Guizhou Province in China as the research background, the instability failure caused by large deformation of a roadway in a cracked rock mass was analysed, and its support control measures were put forward. First, the stability of the surrounding rock was evaluated by field investigation on the failure characteristics and statistics for the development of joints and fissures in the surrounding rock. Second, the corresponding numerical model was established by using FLAC3D software, and the influence of the lateral pressure coefficient λ on the plastic zone and principal stress were analysed quantitatively. In addition, the evolution process of the plastic zone over time was summarized. Furthermore, the key points of the reinforcement plan and support parameter design were provided on the basis of the technical approach for deformation control of the surrounding rock of the roadway. The original supporting scheme was optimized step by step, and the combined supporting scheme with “short bolt, long anchor cable and two-step grouting shell” serving as the main concept was put forward. Finally, the results of numerical calculation and field practice showed that the optimized scheme could effectively restrain the local deformation in the plastic zone of the surrounding rock of the roadway. The overall deformation was in a controllable range. In addition, the reinforced anchor cable was anchored to the deep part of the surrounding rock, ensuring the safety and stability of the roadway.

Reliability analysis for seismic stability of tunnel faces in soft rock masses based on a 3D stochastic collapse model

A new horn failure mechanism was constructed for tunnel faces in the soft rock mass by means of the logarithmic spiral curve. The seismic action was incorporated into the horn failure mechanism using the pseudo-static method. Considering the randomness of rock mass parameters and loads, a three-dimensional (3D) stochastic collapse model was established. Reliability analysis of seismic stability of tunnel faces was presented via the kinematical approach and the response surface method. The results show that, the reliability of tunnel faces is significantly affected by the supporting pressure, geological strength index, uniaxial compressive strength, rock bulk density and seismic forces. It is worth noting that, if the effect of seismic force was not considered, the stability of tunnel faces would be obviously overestimated. However, the correlation between horizontal and vertical seismic forces can be ignored under the condition of low calculation accuracy.

Experimental Study on the Brittle-Ductile Response of a Heterogeneous Soft Coal Rock Mass under Multifactor Coupling

After a gas drainage event causes different degrees of initial porosity in the coal seam, the heterogeneity of the coal mass becomes much more obvious. In this paper, soft coal testing samples with different degrees of heterogeneity were prepared first by a new special experimental research method using hydrogen peroxide in an alkaline medium to generate oxygen. Then, a series of mechanical tests on the soft coal mass samples were carried out under multiple factor coupling conditions of different heterogeneities and confining pressures. The results show that with a low strength, the ductility failure characteristic and a kind of rheology similar to that for soft rock flow were reflected for the soft coal; i.e., the stress-strain curve of the coal mass had no apparent peak strain and residual strength. An interesting phenomenon was found in the test process: there was an upwardly convex critical phase, called the brittle-ductile failure transition critical phase, for the heterogeneous soft coal mass between the initial elastic compression phase and the ductile failure transition phase in the stress-strain curve of the coal mass. An evolution of the brittle-ductile modulus coefficient of the soft coal was developed to analyze the effect of the internal factor (degree of heterogeneity) and external factors (confining pressure) on the transition state of the brittle-ductile failure of soft coal. Further analysis shows that the internal factor (heterogeneity) was also one of the essential factors causing the brittle-ductile transition of soft coal.

Stress Field Distribution and Deformation Law of Large Deformation Tunnel Excavation in Soft Rock Mass

The problem of large deformation is very prominent in deep-buried tunnel excavation in soft rock, which brings serious potential safety hazards and economic losses to projects. Knowledge of the stress field distribution and deformation law is the key to ensuring rational design and safe construction in large deformation tunnels of soft rock. As described in this paper, theoretical analysis, numerical simulation and field monitoring were employed to investigate the surrounding rock stress and displacement state in the Dongsong hydropower station in Sichuan Province, China. The results show that the short-bench construction method can effectively control the deformation of surrounding rock and range of the plastic zone. In order to reserve enough working space, the optimum bench length in the actual construction was 10 to 14 m. The peripheral displacement and plastic radius decreased with the increase of tunnel support strength and the advance of supporting time. The displacement can be effectively controlled by applying the second lining in time at a position about twice the diameter of the hole (16 m) from the working face. A reasonable reserved deformation should be adopted to avoid secondary expanding excavation. The values of different positions in the tunnel laterally and longitudinally may be different, and adjustments are needed according to the actual situation.

Slip perturbation during fault reactivation by a fluid injection

Slip orientation inferred from fault striae or focal mechanism datasets is commonly used in stress inversion methods based on the Wallace-Bott hypothesis. The hypothesis postulates that slip on a fault plane is collinear with the orientation of the resolved shear stress. It is valid for a single planar fault subjected to a homogeneous far-field stress. However, the experimental displacement data from an induced fault reactivation experiment, conducted in the Mont Terri rock laboratory, Switzerland, indicated multiple triggered slip orientations, thereby preventing application of the above inversion method. We present numerical and analytical results of slip on a reactivated fracture with a non-uniform fluid pressure distribution. Using these models, we evaluate the reasons for the inconsistency of our observations and the traditional Wallace-Bott hypothesis and test the physical effects of various parameters on fault slip. In the fully coupled hydromechanical numerical model (three-dimensional distinct element method), fluid pressure at a point on the fault surface is increased stepwise (assumed planar and singular) until shear reactivation of the fault is induced. We studied two different models with high and low fault plane stiffness to represent hard and soft rock masses, respectively. The model shows that high fault stiffness preserves the planarity of the fault plane, while low fault stiffness permits dilation and morphological changes of the fracture related to fluid pressure diffusion. The highest slip perturbation was observed in the low stiffness model due to the change of the fracture shape, controlled by the non-uniform pressure distribution. The Eshelby analytical solution confirmed that the more the fracture is dilated, the more the corresponding resolved shear stress is perturbed. Additionally, when compared to dilation and fault aperture, the friction angle has the most influence on the angular difference between geomechanical slip vectors and resolved shear stress.

Departure of soft rock mass from undrained response in drilled shaft and plate load tests

The average degree of consolidation of soft rock mass in the immediate vicinity of the base of drilled shaft or plate load tests was evaluated using the Biot Three-Dimensional Consolidation Theory. The compressibility parameters of soft rock mass were estimated using the contact pressure and displacement relationships obtained from the in situ drilled shaft and plate load tests. It was found that typical drilled shaft and plate load tests on soft rocks are drained to partially drained, with the average degree of consolidation dependent on the rock type and weathering condition, foundation diameter, and the loading stage. It was also determined that drained behavior is observed under service conditions due to the low compressibility of soft rocks in the early stages of loading.

Theoretical and numerical study on reinforcing effect of rock-bolt through composite soft rock-mass

Anchoring mechanism and failure characteristics of composite soft rock with weak interface usually exhibit remarkable difference from those in single rock mass. In order to fully understand the reinforcement mechanism of composite soft roof in western mining area of China, a mechanical model of composite soft rock with weak interface and rock bolt which considering the transverse shear sliding between different rock layers was established firstly. The anchoring effect was quantified by a factor defined as anchoring effect coefficient and its evolution equation was further deduced based on the deformation relationship and homogenized distribution assumption of stress acting on composite structure. Meanwhile, the numerical simulation model of composite soft rock with shear joint was prompted by finite element method. Then detailed analysis were carried out for the deformation features, stress distribution and failure behavior of rock mass and rock bolt near the joint under transverse load. The theoretical result indicates that the anchoring effect of rock-bolt through weak joint changes with the working status of rock mass and closely relates with the physical and geometric parameters of rock mass and rock bolt. From the numerical results, the bending deformation of rock bolt accurately characterized by Doseresp model is mainly concentrated between two plastic hinges near the shear joint. The maximum tensile and compression stresses distribute in the plastic hinge. However, the maximum shear stress appears at the positions of joint surface. The failure zones of composite rock are produced firstly at the joint surface due to the reaction of rock bolt. The above results laid a theoretical and computational foundation for further study of anchorage failure in composite soft rock.

Characterization of Weathered Soft Rock Mass by Index Tests

The understanding of geotechnical and geomechanical rock mass behavior is challenging, mainly regarding weathered parts, since they may trigger stability issues. Soft Rocks, as phyllite, are known to enhance these problems. In this case, a road cut on a highway between the cities of Ouro Preto and Mariana (MG – Brazil) was studied, showing a particular weathering zone with changing conditions. After morphological description and geological fragmentation (using geological hammer, the Schmidt hammer and a switchblade) of the weathering zone, tests were done on rock matrix and rock mass in order to identify the discontinuity features. Physical properties were determined by physical index, using the point load test and slake durability test. The results permit to define the weathering zone, showing some huge anisotropy and heterogeneity in the rock properties.

Optimal pre-conditioning and support designs of floor heave in deep roadways

In order to reduce deformation of roadway floor heave in deep underground soft rockmass, four support design patterns were analyzed using the Fast Lagrangian Analysis of Continua (FLAC)3D, including the traditional bolting (Design 1), the bolting with the backbreak in floor (Design 2), the full anchorage bolting with the backbreak in floor (Design 3) and the full anchorage bolting with the bolt-grouting backbreak in floor (Design 4). Results show that the design pattern 4, the full anchorage bolting with the bolt-grouting backbreak in floor, was the best one to reduce the deformation and failure of the roadway, the floor deformation was reduced at 88.38% than the design 1, and these parameters, maximum vertical stress, maximum horizontal displacement and maximum horizontal stress, were greater than 1.69%, 5.96% and 9.97%. However, it was perfectly acceptable with the floor heave results. The optimized design pattern 4 provided a meaningful and reliable support for the roadway in deep underground coal mine.

Longitudinal deformation profile of a tunnel in weak rock mass by using the back analysis method

Analysis of the rock mass deformation behavior is a very important aspect of the safety assessment for tunnel construction in weak rock mass. In this paper, the deformation characteristics of a soft rock mass tunnel using three beaches construction method were investigated, which include the crown settlement and horizontal displacement and have 9 sections with 3 different construction schemes. The optimized construction schemes by decreasing the beaches length and changing the geologist of primary support were proposed. Then, applying the displacement back analysis method to calculate the rock mass parameters, double parameters were analyzed by using the golden section method. Results show that the tunnel deformations were affected by the elastic modulus E and the lateral pressure coefficient λ of rock mass, and the change of E has greater influence than λ on the tunnel deformation. The change of λ has greater influence on the crown settlement than that on the horizontal displacement. Furthermore, the regularity and characteristics of longitudinal deformation profile (LDP) in a weak rock mass tunnel was studied by utilizing the Fast Lagrangian Analysis of Continua (FLAC), and the LDP of the three long-beach construction scheme and the three short-beach construction scheme were compared. The results show that the complete displacements of tunnel under the three short-beach construction scheme condition by decreasing the lengths of the middle and lower benches are smaller than that under the three short-beach construction scheme condition, however the pre-deformation of the tunnel deformation under this two construction scheme conditions is nearly the same. The extrusion deformation at the tunnel face of the three short-beach construction scheme is larger than that of the three long-beach construction scheme. Therefore, increasing the area of the core soil is a feasible measure to control the extrusion deformation on the tunnel face. Finally, the tunnel optimized construction scheme was verified benefit the tunnel stability. The measures of decreasing the length of middle and lower bench and closing the invert early and immediately will benefit the tunnel stability.

Analysis of the Stress Distribution in Inclined Backfilled Stopes Using Closed-form Solutions and Numerical Simulations

Backfilling is often used in underground mines to ensure stope stability and workers safety. Evaluating the stress state in the fill material and surrounding rock mass is a critical step for the design of backfilled stopes. The majority of analytical (closed-form) solutions to obtain the stresses have been developed for vertical openings. In reality, most mine stopes have inclined walls. Previous studies have shown that in such cases, the stresses developing along the hanging wall and footwall can be quite different. Recent investigations have also indicated that the stress transfer between the relatively soft backfill and stiff rock mass is typically not as well developed in inclined stopes, compared with vertical openings. In this paper, the authors first recall analytical solutions that have been proposed for evaluating the stresses in backfilled stopes with vertical and inclined walls. Numerical simulations are then used to assess the interactions between the backfill and rock mass. The influence of backfill properties and stope geometry (in terms of height, width and inclination) is examined. The stresses obtained from existing solutions and new simulations are then compared and discussed. This comparison points to significant differences, indicating that an alternate formulation is required to properly assess the stress state in inclined stopes.

Applicability of geomechanical classifications for estimation of strength properties in Brazilian rock masses.

Many authors have been proposed several correlation equations between geomechanical classifications and strength parameters. However, these correlation equations have been based in rock masses with different characteristics when compared to Brazilian rock masses. This paper aims to study the applicability of the geomechanical classifications to obtain strength parameters of three Brazilian rock masses. Four classification systems have been used; the Rock Mass Rating (RMR), the Rock Mass Quality (Q), the Geological Strength Index (GSI) and the Rock Mass Index (RMi). A strong rock mass and two soft rock masses with different degrees of weathering located in the cities of Ouro Preto and Mariana, Brazil; were selected for the study. Correlation equations were used to estimate the strength properties of these rock masses. However, such correlations do not always provide compatible results with the rock mass behavior. For the calibration of the strength values obtained through the use of classification systems, ​​stability analyses of failures in these rock masses have been done. After calibration of these parameters, the applicability of the various correlation equations found in the literature have been discussed. According to the results presented in this paper, some of these equations are not suitable for the studied rock masses.