Influences of changes in mechanical properties of an overcored sample on the far-field stress calculation

The concentration factors for the determination of far-field stresses have been calculated based on three-dimensional FLAC^3D simulated results. FLAC^3D is an explicit finite difference computer program. Three-dimensional numerical simulations have been conducted using FLAC^3D to determine the influence of altered overcored sample parameters on the far-field stress calculation. Both Young's modulus and Poisson's ratio are known to change during overcoring in highly stressed rock as a result of stress-relief damage. Modification factors have been proposed to take into account the influence of the reduction in Young's modulus on the far-field stress calculation. The influence of change in Poisson's ratio has also been analyzed. A method of determining a Young's modulus value suitable for use in the far-field stress calculation is described and the influence of reduction in Young's modulus of an overcored sample on the determination of the stress ratio (SR) in the RPR method is analyzed. Using the proposed modification, the in situ stresses for the Atomic Energy of Canada Limited's Underground Research Laboratory are re-calculated based on 21 deep doorstopper measurements. The re-calculation shows an increase in the trend of stress magnitudes versus depth in the URL.     

A new technique for repairing and controlling large-scale collapse in the main transportation shaft, Chengchao iron mine, China

A new technique, the ‘supporting funnel method’, is introduced in this paper for repairing and controlling large-scale collapse in the download junction of the tunnel and main transportation shaft. The key technical points include:

Construction of a box-sectioned beam across the collapsed junction floor, which serves as a supporting structure to recover the ore download function.

Construction of an inverse cone-shaped funnel shaft below the junction in the shaft space, which has multiple functions of ore storage, transportation, and control of the rock mass around the shaft.

Implementation of post-tensioned long bolts and high pressure grouting technique in construction.

Numerical analysis of the reinforced structures has been carried out using FLAC^3D. The study shows that the merits of the technique are: (i) it ensures that the system function of the transportation is fully and economically recovered and (ii) the stability of the whole system was obtained through the local reinforcement.     

Stability evaluation and optimal excavated design of rock slope at Antaibao open pit coal mine, China

Rock slope deformation stability for Antaibao open pit coal mine under mining was analyzed using finite difference technique (FLAC^3D). Optimal excavated scheme with relatively steeper slope angle of 47° instead of 30° was successfully implemented at the west wall on the geological section 73200 of the mine area, where the three-dimensional (3D) effect of nonlinear large deformation of the slope was taken into account. Physico-mechanical properties of the rock materials were obtained by laboratory tests conducted on samples cored from exploration drilling and rock blocks taken directly from the mine. A nonlinear Mohr–Coulomb material model with a tension cutoff was used in all present simulations. Nonlinear effect on excavated sequence was also discussed. It is demonstrated by dynamical response analysis that the action of earthquake magnitude VII (Richter) upon the relatively steeper excavated slope could not be more than that slope could bear.     

Study on time-dependent behavior and stability assessment of deep-buried tunnels based on internal state variable theory

The creep model based on thermodynamics with internal state variables theory can simulate complex time-dependent deformation of rock mass, describe process of energy dissipation of material system, and can be used to evaluate the long-term stability of underground structures quantitatively. In this paper, the creep model proposed by author is improved further and recast to be central difference equation. The redevelopment interface of FLAC^3D is used to develop a new calculation program, which is based on thermodynamics for vsico-plasticity (PTV-P). Program validation has been conducted by comparing the results from FLAC^3D and Matlab software under uniaxial compression condition. Then the developed program has been applied to analyze the time-dependent behavior of deep-buried double tunnels. The integral values of energy dissipation rate and its time derivative in domain can be calculated and are used to evaluate the long-term stability of tunnels quantitatively, and the evaluation criterion is also proposed. Moreover, the contour map of energy dissipation rate is used to exhibits the local non-equilibrium region clearly.     

Dynamic response of underground gas storage salt cavern under seismic loads

A dynamic elastoplastic damage constitutive model is proposed based on the failure characteristic of rock salt under seismic loads. The coding of the proposed model is achieved by the embedded FISH (short for FLACish) language of FLAC^3D (Fast Lagrangian Analysis of Continua). Numerical models of bedded salt cavern gas storage facilities in China are developed by using FLAC^3D, and the proposed constitutive model is used in the simulations. The effects of seismic input angle, seismic acceleration, seismic moment, types of seismic waves, and gas pressure on the dynamic response, stress, displacement, plastic zone, and safety factor (SF) of rock masses that surround salt cavern gas storage facilities are studied. Results show that the seismic wave perpendicular to the surface poses the greatest risk to the safety of the cavern. With an increase in seismic acceleration, the cavern’s SF decreases and that of the lower structure of the cavern decreases more than that of the upper section. Plastic zones propagate from the cavern’s internal surface to the pillar, and then to the pillar and floor along the right and left corners of the cavern bottom. Higher internal gas pressure improves cavern safety. The acceleration and duration of seismic waves are critical factors in ensuring the safety of the cavern. The SF of the cavern’s lower structure is more sensitive to changes in seismic parameters than that of the other locations, which makes the cavern bottom more likely to be destroyed during an earthquake. Therefore, the lower structure should be the study target in the seismic design for a salt cavern gas storage facility. Results have been used in the seismic design of salt cavern gas storage facilities in China.     

Experimental study on face instability of shield tunnel in sand

Face stability is critical for ground settlement and construction safety control in shield tunneling. In this paper, a series of 3D large-scale model tests with a tunnel of 1 m diameter were conducted in dry sand for various cover-to-diameter ratios C/D = 0.5, 1, and 2 (i.e., relative depth; C is the cover depth and D is the diameter of tunnel). Each test provided a measurement of the support pressure and the ground settlement with the advance of face displacement. The evolution of soil arching during face failure was investigated by monitoring the redistribution of earth pressure in front of the face in the test case of C/D = 2. In the displacement-controlled face failure tests in the medium density sands, the support pressure dropped steeply to the minimum value, then increased to a steady state with the continuing increase in the face displacement. Relationships between the support pressure and face displacement for various cover depths were also verified by the numerical analysis using the finite difference program, FLAC^3D (Itasca, 2005). The limit support pressure increases with the increase of the relative depth C/D and then tends to be constant. A significant rotation of principal stress axes in the upward arches in the soil during face failure was found in the tests. A two-stage failure pattern is proposed based on the observation of earth pressure. The theoretical and empirical formulas for estimating limit support pressure were verified by the tests results.     

植筋梁中植筋端头加设箍筋对梁受力性能的影响

为研究植筋梁中植筋端头加设箍筋对梁受力性能的影响,对3根植筋深度为20D、25D、30D(D为植筋的直径)的植筋端头未加设箍筋的植筋梁进行静载试验,对3根植筋深度为20D、25D、30D的未加设箍筋的植筋梁和3根植筋深度为20D、25D、30D的加设箍筋的植筋梁进行循环荷载试验,并用ANSYS模拟植筋深度为20D、25D、30D的植筋梁进行静力受弯力学性能的模拟分析,绘制跨中荷载挠度曲线,研究植筋随荷载的应力变化,进而反映有无加设箍筋对梁受力性能的影响,并整理了植筋端头加设箍筋的端头应变和未加设箍筋植筋端头应变的关系。循环荷载试验及有限元分析表明:在植筋端头加设箍筋可以缓解植筋端头应力集中现象,并提高植筋梁的延性、开裂荷载、极限荷载,降低了植筋应变、残余变形。     

Simulation of excavations in jointed rock masses using a practical equivalent continuum approach

A simple practical equivalent continuum numerical model previously presented by Sitharam et al. (Int. J. Rock Mech. Min. Sci. 38 (2001) 437) for simulating the behaviour of jointed rock mass has been incorporated in the commercial finite difference programme fast Lagrangian analysis of continua (FLAC). This model estimates the properties of jointed rock mass from the properties of intact rock and a joint factor (J_f), which is the integration of the properties of joints to take care of the effects of frequency, orientation and strength of joint. A FISH function has been written in FLAC specially for modelling jointed rocks. This paper verifies the validity of this model for three different field case studies, namely two large power station caverns, one in Japan and the other in Himalayas and Kiirunavara mine in Sweden. Sequential excavation was simulated in the analysis by assigning null model available in FLAC to the excavated rock mass in each stage. The settlement and failure observations reported from field studies for these different cases were compared with the predicted observations from the numerical analysis in this study. The results of numerical modelling applied to these different cases are systematically analysed to investigate the efficiency of the numerical model in estimating the deformations and stress distribution around the excavations. Results indicated that the model is capable of predicting the settlements and failure observations made in field fairly well. Results from this study confirmed the effectiveness of the practical equivalent continuum approach and the joint factor model used together for solving various problems involving excavations in jointed rocks.     

Understanding the failure mechanism of a large underground cavern in steeply dipping layered rock mass using an enhanced ubiquitous-joint model

To understand the mechanical behavior and failure mechanism of the surrounding layered rock masses during the excavation of underground caverns for a hydropower station, an enhanced equivalent continuum model based on the ubiquitous-joint concept is developed and compiled as a plugin DLL file in FLAC^3D. This model is then applied to analyze two engineering geological issues arisen during the excavations of a large underground powerhouse. Both cases are the typical responses controlled primarily by the internal structure of layered rock mass. The first case is mainly concerned with the continuous increase in the displacement of the upstream sidewall after removal of an auxiliary tunnel crown. Numerical simulation reveals the mixed shear-tensile fractures developed along bedding planes. The preserved crown thickness can affect the maximum displacement of sidewall. Failure region will progressively deepen into the interior if no effective reinforcements are adopted. The second case exhibits as the gradual cracking or slabbing of the shotcrete at downstream roof. Stress concentration is the main cause of shotcrete cracking as shown by numerical simulation. The degree of stress concentration can be influenced largely by the angle between rock strata and cavern axis. This degree also varies with the initial stress level and with excavation process, which is confirmed by field monitoring data. The validity and capability of the newly developed model are thus verified by the actual engineering issues.

     

Stress evolution with time and space during mining of a coal seam

Mining of the upper protective coal seam is widely practiced in China for coal mine safety, but relief gas may present a new risk of blasting. To control the relief gas effectively, a strain-soften model was built by FLAC^3D software to investigate the stress evolution during the process of mining the upper protective coal seam. The results show that the abutment stress changes rapidly within 10 m in front of the coal face, and the maximum abutment stress is approximately twice the original when the coal seam is mined 20–30 m. The abutment stress should break the rock mass and cause the gas to flow easily. In the stable mining period, the change trends of the x-stress and z-stress are different, and these should also pre-break the rock mass. The stress distributions of the rock mass at different distances under the protective coal seam are different, especially near the coal face, which should greatly affect the gas flow when the space of the protective and protected coal seams change over a large range. The relief angle also changes over a large range, increasing to a maximum approximately 30 m behind the coal face, and it decreases gradually when it is far away from the protective coal seam. The results are helpful for designing the coal face of protected coal seams and borehole layouts to control the relief gas.     

Physical and particle flow modeling of jointed rock block behavior under uniaxial loading

Laboratory experiments and numerical simulations, using Particle Flow Code (PFC^3D ), were performed to study the behavior of jointed blocks of model material under uniaxial loading. The effect of joint geometry parameters on the uniaxial compressive strength of jointed blocks was investigated and this paper presents the results of the experiments and numerical simulations. The fracture tensor component in a given direction is used to quantify the combined directional effect of joint geometry parameters including joint density, orientation and size distributions, and the number of joint sets. The variation of the uniaxial compressive strength of the jointed blocks of the model material with the fracture tensor component was investigated. Both the laboratory experiments and the numerical simulations showed that the uniaxial block strength decreases in a nonlinear manner with increasing values of the fracture tensor component. It was observed that joint geometry configuration controls the mode of failure of the jointed blocks and three modes of failure were identified, namely (a) tensile splitting through the intact material, (b) failure by sliding along the joint plane and/or by displacement normal to the joint plane and, (c) mixed mode failure involving both the failure mechanisms in (a) and (b). It has also been shown that with careful parameter calibration procedures, PFC^3D could be used to model the strength behavior of jointed blocks of rock.     

Numerical simulation of a jointed rock block mechanical behavior adjacent to an underground excavation and comparison with physical model test results

Mechanical behavior of a jointed rock mass with non-persistent joints located adjacent to a free surface on the wall of an excavation was simulated under without and with support stress on the free surface using approximately 0.5 m cubical synthetic jointed rock blocks having 9 non-persistent joints of length 0.5 m, width 0.1 m and a certain orientation arranged in an en echelon and a symmetrical pattern using PFC^3D software package. The joint orientation was changed from one block to another to study the effect of joint orientation on strength, deformability and failure modes of the jointed blocks. First the micro-mechanical parameters of the PFC^3D model were calibrated using the macro mechanical properties of the synthetic intact standard cylindrical specimens and macro mechanical properties of a limited number of physical experiments performed on synthetic jointed rock blocks of approximately 0.5 m cubes. Under no support stress, the synthetic jointed rock blocks exhibited the same three failure modes: (a) intact rock failure, (b) step-path failure and (c) planar failure under both physical experiments and numerical simulations for different orientations. The jointed blocks which failed under intact rock failure mode and planar or step-path failure mode produced high and low jointed block strengths, respectively. Three phases of convergence of free surface were discovered. The joint orientation and support stress played important roles on convergence magnitude. The average increment of jointed block strength turned out to be about 10, 7.9 and 6.6 times the support stress when support stresses of 0.06 MPa, 0.20 MPa and 0.40 MPa were applied, respectively. The modeling results offer some guideline in support design for underground excavations.     

Study of failure and scale effects in rocks under uniaxial compression using 3D cellular automata

A numerical model for simulating the rock fracturing process has been developed and implemented in the three-dimensional code EPCA^3D, which is based on the earlier EPCA^2D (elasto-plastic cellular automaton). EPCA^3D has the ability to simulate the initiation, propagation and coalescence of cracks in the failure processes of rocks. Using this code, we studied the failure processes of simulated rock specimens with different sizes and shapes in three-dimensional space. It is concluded that the ‘scale effect’ is not directly related to the heterogeneity, if the rock specimens have the same homogeneity. If the platen-rock elastic mismatch effect in physical testing is not considered in the numerical simulations, there is almost no scale effect for rock specimens with different sizes and shapes. However, by considering the platen-rock interaction in the simulation of the rock failure process with the EPCA^3D code, the modeling results reproduced the well-known phenomenon of scale effects found in physical experiments.     

A new rheological model and its application in mountain tunnelling

The time-dependent features of soft rock, named rheology generally, should be taken into account in the long-term design and maintenance of mountain tunnels. Based on the classic Burger-MC rheological model, a Burger-deterioration rheological model is proposed in this paper and is implemented in the numerical codes FLAC^3D. A deterioration threshold and two deterioration ratios are introduced in this model to consider the time-dependent strength deterioration aspect of the rock mass. The proposed model is applied to an engineering instance (Ureshino Tunnel Line I, Nagasaki, Japan) to account for the delayed deformations that occurred after its completion since November 1992. The delayed crown settlement and invert upheaval computed from simulations are featured by an exponential characteristic and a stair-typed characteristic, respectively, which agree well with the in-site monitoring data qualitatively. In addition, the realistic rheological parameters involved in the proposed model can be back-analyzed from the in-site monitoring data.     

The mechanical behaviour of synthetic sandstone with varying brittle cement content

The purpose of this work was to investigate the influence of cement on the mechanical behaviour of granular rocks. Following the technique described in den Brok et al. [den Brok, S. W. J., David, C. and Bernabé, Y., Preparation of synthetic sandstones with variable cementation for studying the physical properties of granular rocks. C. R. Acad. Sci., 1997, 325, 487–492], two blocks of synthetic sandstones with different cement content were prepared for mechanical testing under hydrostatic and triaxial conditions. The results of the mechanical tests show that the behaviour of the synthetic rocks compares well with that of natural sandstones. Increasing the amount of cement from 3 to 5% in volume had important consequences on the mechanical properties: the critical pressure, strength and elastic moduli were significantly increased and the brittle-to-ductile transition was shifted towards higher pressures. We compared our results to the models of Zhang et al. [Zhang, J., Wong, T. -F. and Davis, D. M., Micromechanics of pressure-induced grain crushing in porous rocks. J. Geophys. Res., 1990, 95, 341–352] and Wong et al. [Wong, T. -F., David, C. and Zhu, W., The transition from brittle faulting to cataclastic flow in porous sandstones: Mechanical deformation. J. Geophys. Res., 1997, 102, 3009–3025]. We conclude that Zhang et al.'s microstructural parameter φD (i.e. the product of porosity φ by grain size D) appeared to be a scaling parameter for both the failure envelopes and the critical pressure as defined in these models. Intuitively, the contact length L is expected to play a crucial role in the mechanical properties of granular materials. Accordingly, we made a statistical analysis of this microstructural parameter in our synthetic materials and in a suite of natural sandstones. A positive correlation with Young's modulus and a negative correlation with porosity were found. This last result gives a physical background for the use of parameter (φD) in theoretical models. We want to emphasize that working on synthetic sandstones allows for a better control of the structural parameters (grain size, sorting, cement content, etc.) which appear to be so important for the mechanical properties of granular rocks.     

Enlarging a large-diameter shield tunnel using the Pile–Beam–Arch method to create a metro station

It is an effective approach to enlarge a large-diameter shield tunnel using the Pile–Beam–Arch (PBA) method to construct a metro station, especially when traditional excavation methods cannot be used because of the narrow roads and dense buildings. Based on the structural characteristics of a metro station, FLAC^3D is used to build a three-dimensional model and to analyse the mechanical performance of structurally critical joints and their effect on the settlement of the ground surface. The critical joints include segment joints, connections between blocks of key segments, connections between longitudinal beams and segments, and connections between primary linings and segments. The results show that the calculated stress in the structure does not exceed the strength during the enlarging process. A control standard for the project is also presented based on a comparison between the results of the numerical simulation and the control reference of a Beijing metro station. The numerical analysis result is used to guide the construction. Based on the in-situ monitoring data, the feasibility of the expansion method is verified, as is the accuracy of the numerical simulation.     

Interactions of overburden failure zones due to multiple-seam mining using longwall caving

This paper presents an investigation on the interactions of overburden failure zones induced by the mining of adjacent coal seams using the longwall caving method. Overburden failure is an important factor in safety assessments in the fields of mining engineering geology and safety geology, especially when mining under water bodies. In this study, the influence of the thickness and properties of the interburden between seams on the development and interactions of caving and fractured water flow zones are investigated by using in situ measurements, scale model testing and numerical simulations. The height of the fractured water flow zones in the scale model tests and numerical simulations are basically in good agreement with measurements after mining of the upper and lower seams of Seam No. 3 in the Cuizhuang Coalmine. Therefore, the scale tests and numerical simulations in the study are verified. The results show that interaction and superposition between two close distance seams cannot be ignored when the ratio (h/M) of the interburden thickness (h) to the cutting height of the lower seam (M) is less than a defined critical value. A dividing line, Line D, has been proposed to judge whether the interactions exist. When the (M, h/M) points are located above Line D, the caving zone induced by excavation of the lower seam will not propagate to the caving zone induced by the upper seam. Otherwise, for the (M, h/M) points below Line D, the interactions and superposition of the overburden failure must be considered when predicting the heights of the caving and fractured water flow zones.

     

阿坝铝厂地基与边坡变形稳定性预测

阿坝铝厂座落于岷江右岸Ⅲ级阶地上。由于紫坪铺水库的兴建蓄水 ,可能引起铝厂新的地基沉降变形及边坡的变形和稳定问题。本文在对铝厂地基、边坡工程地质条件的分析基础上 ,利用快速拉格朗日 (FLAC3D2 .0 0 )、赤平投影、刚体极限平衡等方法对水库蓄水后边坡与地基的稳定性进行了预测。     

Shaft resistance of a pile embedded in rock

A rational calculation procedure is proposed for establishing the shaft resistance of a pile embedded in rock, based upon the Hoek and Brown failure model. The state of the art of the calculation of the pile shaft resistance is analysed. Nearly all the recommendations that have appeared in the technical literature, for calculating the ultimate shear strength of a shaft embedded in rock (τ_ult) propose that τ_ult=ασ_c^k(σ_c;τ_ulten MN/m²) where the coefficient α, considered as a constant dimensional value, ranges from 0.1 to 0.8, if the unconfined compressive strength (σ_c) is expressed in MN/m². In most cases, the exponent k is 0.5.A comparison is made between the results yielded and the different empirical theories that have been put forward with respect to this shaft resistance. It can generally be stated that the results obtained with this theory are reasonable for long and deeply socketed piles (high confining pressures) but the results are on the safe side in some cases where short piles (low confining pressures) are involved.This paper is a continuation of the works developed by the same authors with piles working at the tip, socketed in rock.