Weishen Zhu

Numerical Study on Crack Propagation in Brittle Jointed Rock Mass Influenced by Fracture Water Pressure

The initiation, propagation, coalescence and failure mode of brittle jointed rock mass influenced by fissure water pressure have always been studied as a hot issue in the society of rock mechanics and engineering. In order to analyze the damage evolution process of jointed rock mass under fracture water pressure, a novel numerical model on the basis of secondary development in fast Lagrangian analysis of continua (FLAC3D) is proposed to simulate the fracture development of jointed rock mass under fracture water pressure. To validate the feasibility of this numerical model, the failure process of a numerical specimen under uniaxial compression containing pre-existing fissures is simulated and compared with the results obtained from the lab experiments, and they are found to be in good agreement. Meanwhile, the propagation of cracks, variations of stress and strain, peak strength and crack initiation principles are further analyzed. It is concluded that the fissure water has a significant reducing effect on the strength and stability of the jointed rock mass.

Experimental and numerical investigations on the shear behavior of a jointed rock mass

The original forming process of the earth crust is companied with internal in situ stress, which gradually complicates while the earth crust evolves with geological conformation movements, leading to the generation of large amounts of faults, joints and fissures. These structural planes, to some extent, remarkably reduce the strengths of rock mass, including the shear behavior. In this paper, the authors report a physical model test on jointed rock mass under direct shear stress state and also adopt a numerical method, Discontinuous Deformation Analysis for Rock Failure (DDARF), to simulate the shear failure process, the variation of stresses and displacements of some key monitoring points. The comparative analysis demonstrates that the numerical results are favorable with those obtained in the physical model test. Therefore, it is concluded that the method of DDARF could effectively simulate the shear behavior of jointed rock mass. Furthermore, other than the original physical model test, the numerical models with echelon joints under different axial loadings are also simulated. The crack initiation, extension, coalescence, and the ultimate shear failure are totally investigated, after which the shear behavior of numerical models in different cases are comparatively analyzed.

Structural Stability Monitoring of a Physical Model Test on an Underground Cavern Group during Deep Excavations Using FBG Sensors

Fiber Bragg Grating (FBG) sensors are comprehensively recognized as a structural stability monitoring device for all kinds of geo-materials by either embedding into or bonding onto the structural entities. The physical model in geotechnical engineering, which could accurately simulate the construction processes and the effects on the stability of underground caverns on the basis of satisfying the similarity principles, is an actual physical entity. Using a physical model test of underground caverns in Shuangjiangkou Hydropower Station, FBG sensors were used to determine how to model the small displacements of some key monitoring points in the large-scale physical model during excavation. In the process of building the test specimen, it is most successful to embed FBG sensors in the physical model through making an opening and adding some quick-set silicon. The experimental results show that the FBG sensor has higher measuring accuracy than other conventional sensors like electrical resistance strain gages and extensometers. The experimental results are also in good agreement with the numerical simulation results. In conclusion, FBG sensors could effectively measure small displacements of monitoring points in the whole process of the physical model test. The experimental results reveal the deformation and failure characteristics of the surrounding rock mass and make some guidance for the in situ engineering construction.

Numerical investigations on slope stability using an elasto-brittle model considering fissure water pressure

The initiation, propagation, coalescence evolution behaviors of cracks in rock mass, and even fissure water pressure have significant impacts on the strength and stability of fractured rock mass in engineering projects. In this paper, we propose an elasto-brittle constitutive model considering fissure water pressure based on the computer code three-dimensional fast Lagrangian analysis of continua (FLAC3D). This constitutive model is initially validated through a pre-cracked rock-like-material specimen in the laboratory experiments and then proved to actually simulate the initiation, propagation, and coalescence characteristics of cracks considering fissure water pressure in brittle fractured rock mass. Afterwards, it is used to investigate the stability of the right bank slope in Dagangshan hydropower station which is located in Sichuan province, China. The whole stability of this rock slope during construction has been simulated using the above constitutive model. The development principles of cracks in a selected area and the displacement field of the slope under different fissure water pressures are obtained in the process of excavations. It is concluded that fissure water pressure has obvious and significant effect on the strength and stability of fractured rock mass.

An experimental investigation on mechanical property and anchorage effect of bolted jointed rock mass

Apparent discontinuities can be easily found in natural rock medium owing to the constant motion and change of the Earth’s crust, which contains large amount of discontinuous surfaces such as faults, joints, cracks and so forth. Rock bolt is one of the most effective and economical reinforcing tools in practical geotechnical engineering for a long time. This paper investigates the mechanical properties, anchorage effect, cracking, and coalescence process of intact rock-like specimens, rock-like specimens containing flaws, and bolted rock-like specimens containing flaws. A series of uniaxial compression tests, splitting tests, and biaxial compression tests are performed on these specimens. Some findings can be observed from this study. (1) The number of rock bolt(s) and the anchoring angle have a great influence on the anchorage effect of rock bolt(s). With the increasing number of rock bolt(s), the uniaxial compressive strength (UCS), the splitting peak strength (which can be converted to the tensile strength), and biaxial compressive strength (BCS) can all be improved, whose variation tendencies do not follow a linear relationship. (2) The contributions to the tensile strength for rock bolt is greater than that to the UCS for the same-type specimen. (3) In the biaxial compression tests, with the increasing of the lateral pressures, the anchorage effect of rock bolt gradually declines and the lateral pressure plays a dominant role in improving the strength of the specimen. The failure characteristics of three types of laboratory tests have also been systematically analyzed in this paper.

A modified initial in-situ Stress Inversion Method based onFLAC3D with an engineering application

Abstract To improve the accuracy of an initial in-situ stress field determined by inversion, we describe a modi fied initial in-situ stress inversion method that uses partial least-squares regression based on FLAC3D. First, each stress component is regressed to improve the fitting accuracy of locally abnormal stress regions, and then the relationship between element stress and unbalanced node force is analyzed according to the computational principles of FLAC3D. The initial in-situ stresses obtained from these regression calculations are added to a numerical model, and the unbalanced node forces are recalculated. An external force equal to the recalculated unbalanced node force is then exerted on the node in the direction opposing the original unbalanced node force to satisfy the equilibrium condition. For the in-situ stresses of elements that do not satisfy the strength conditions, they are modi fied by assuming the average stress is constant and reducing the partial stress to satisfy the equilibrium and strength conditions, which also resolves the unreasonable distribution of the boundary nodal forces and results in good regression estimates. A three-dimensional hypersurface spline interpolation method is developed to calculate the in-situ stress tensor at arbitrary coordinates. Finally, we apply this method to an underground engineering project, and the results are shown to agree well with those obtained from field monitoring. Therefore, it is concluded that this modified in-situ stress inversion method could effectively improve the fitting accuracy of locally abnormal stress regions.

Splitting failure in side walls of a large-scale underground cavern group: a numerical modelling and a field study

Vertical splitting cracks often appear in side walls of large-scale underground caverns during excavations owing to the brittle characteristics of surrounding rock mass, especially under the conditions of high in situ stress and great overburden depth. This phenomenon greatly affects the integral safety and stability of the underground caverns. In this paper, a transverse isotropic constitutive model and a splitting failure criterion are simultaneously proposed and secondly programmed in FLAC3D to numerically simulate the integral stability of the underground caverns during excavations in Dagangshan hydropower station in Sichuan province, China. Meanwhile, an in situ monitoring study on the displacement of the key points of the underground caverns has also been carried out, and the monitoring results are compared with the numerical results. From the comparative analysis, it can be concluded that the depths of splitting relaxation area obtained by numerical simulation are almost consistent with the actual in situ monitoring values, as well as the trend of the displacement curves, which shows that the transverse isotropic constitutive model combining with the splitting failure criterion is appropriate for investigating the splitting failure in side walls of large-scale underground caverns and it will be a helpful guidance of predicting the depths of splitting relaxation area in surrounding rock mass.

Impact of In Situ Stress Distribution Characteristics on Jointed Surrounding Rock Mass Stability of an Underground Cavern near a Hillslope Surface

In this paper, a series of numerical simulations are performed to analyze the in situ stress distribution characteristics of the rock mass near different slope angles hillslope surfaces, which are subjected to the vertical gravity stress and different horizontal lateral stresses and the influence which the in situ stress distribution characteristics of 45° hillslope to the integral stability of surrounding rock mass when an underground cavern is excavated considering three different horizontal distances from the underground cavern to the slope surface. It can be concluded from the numerical results that different slope angles and horizontal lateral stresses have a strong impact on the in situ stress distribution and the integral surrounding rock mass stability of the underground cavern when the horizontal distance from the underground cavern to the slope surface is approximately 100 m to 200 m. The relevant results would provide some important constructive suggestions to the engineering site selection and optimization of large-scale underground caverns in hydropower stations.

Numerical Analyses and Elasto-Plastic Behavior Study on Surrounding Rock Mass of the Underground Caverns in a Hydropower Station during Deep Excavations

For the time being in China, aggressive development of the western hydroelectric resources is being performed. Taking the Shuangjiangkou (SJK) Hydropower Station as an engineering background, the elasto-plastic behavior is studied on the surrounding rock mass of the underground caverns, including the main powerhouse, the transformer house, the surge chamber, the busbar chamber, etc. in the whole process of deep excavations. A 3D numerical simulation is performed using FLAC3D. Four typical yield criteria which are Mohr-Coulomb (MC) criterion, Drucker-Prager model for the outer adjustment (DPO), Drucker-Prager model for the inner adjustment (DPI) and Zienkiewicz-Pande criterion (ZP) are all utilized in the elasto-plastic behavior study on the surrounding rock mass of the underground caverns. Among of the criteria, ZP criterion is written in the VC++7.1 compiling environment and the other three criteria are included in FLAC3D. The displacement field of the surrounding rock mass are obtained and compared under the four criteria in the process of excavations. The plastic zones are also obtained and compared in this paper. It is concluded that the results obtained through MC and DPI criteria are much greater than those obtained by the other two criteria, and the results obtained though ZP criterion are close to the practical engineering results.

Design and Manufacture of a New Large-Scale Three-Dimensional Geomechanics Model Test System

Geomechanics modeling has played important role in geotechnical engineering. In order to investigate on the stability of underground caverns at great depth, a large-scale geomechanics model test system was designed and manufactured. The system mainly consisted of a steel structural frame and a hydraulic loading control system, which can apply active loading on six sides with a true three-dimensional stress state. Newly developed combinational ball sliding walls were installed on each of the major loading surfaces, which were significantly reduced the friction due to model deformation. The system has apparent technical advantages such as high stiffness, great stability, and flexibility of assembly, and easy adjustment of its dimensions.