Stability analysis of deep-buried hard rock underground laboratories based on stereophotogrammetry and discontinuity identification

Bulletin of Engineering Geology and the Environment - In a tunnel, instabilities in the surrounding rock mostly occur within the sidewalls and crown. After acquiring the rock mass structure, a...     

Multi-crack interaction in limestone subject to stress and flow of chemical solutions

This paper presented a number of experimental results on investigating mechanism of multi-crack interaction in limestone specimens with two or three flaws of different arrangements under the coupled uniaxial compressive stress and chemical solutions NaCl, CaCl₂ and NaHCO₃ with different ionic concentrations and pH values. The experiments were conducted on a testing system with a microscope and a CCD camera. The process of initiation, propagation and coalescence of cracks under the influence of the chemical corrosion were observed and digitally recorded. Cracking velocity, strength, cracking patterns, initial angle of new cracks and multi-crack interaction mechanism affected by chemical corrosion were also analyzed and compared with air and distilled water cases. The results indicated that the effect of chemical corrosion was quite complicated, depending on chemical ions and their concentrations and pH values, mineral components of rock, geometry and the number of flaws. The results are helpful guidance to establishing a coupling model for mechanical-hydro-chemical behavior of rocks.     

Stability assessment of the Three-Gorges Dam foundation, China, using physical and numerical modeling—Part I: physical model tests

Foundation stability is one of the most important factors influencing the safety of a concrete dam and has been one of the key technical problems in the design of the Three-Gorges Project. The major difficulties lie in two facts. The first one is that the dam foundation consists of rock blocks, with joints and so-called ‘rock bridges’ and the gently dipping joints play a critical role in the foundation stability against sliding. The second one is that, even in the regions where the gently dipping fractures are most developed, there are no through-going sliding paths in the rock mass due to the existence of the rock bridges; so the dam could slide only if some of the rock bridges fail, so as to create at least one through-going sliding path. To date, due to unavoidable shortcomings in physical and numerical modeling techniques, there is not a single satisfactory method to solve the problem completely. For this reason, the integration of multiple methods was adopted in this study and proved to be an effective and reliable approach.This Part I paper describes work based on the results of geological investigations and mechanical tests, relating to the geological and geomechanical models of the Three-Gorges Dam, and then a systematic study procedure was developed to carry out the stability assessment project. Then, 2D and 3D physical model tests for some critical dam sections were performed. In the physical tests, based on similarity theory, various testing materials were selected to simulate the rock, concrete, fracture and rock bridge. The loading and boundary conditions were also modeled to meet the similarity requirements. The failure mechanism was derived through a progressive overloading that simulated the upstream hydrostatic pressure applied to the dam, and the factor of safety was defined as the ratio between the maximum external load inducing the start of sliding instability of the dam foundation and the upstream hydrostatic load. The experimental results indicated that the stability of the Three-Gorges Dam foundation satisfies the safety requirements. Nevertheless, further discussions demonstrated that because of the incomplete definition of factor of safety adopted in the physical model tests, it is also essential to study the stability of the Three-Gorges Dam foundation using numerical modeling, which will be presented in the companion Part II paper.     

Estimating mechanical rock mass parameters relating to the Three Gorges Project permanent shiplock using an intelligent displacement back analysis method

Establishing the mechanical rock mass parameters is one of the important tasks for the highwall stability analysis of the permanent shiplock at the Three Gorges Project in China. Existing back analysis methods are not sufficient to provide the necessary accuracy and to recognize non-linear relations. The new displacement back analysis method proposed in this paper is a combination of a neural network, an evolutionary calculation, and numerical analysis techniques. The non-linear relation involving displacement and mechanical parameters is adequately recognized by the neural network techniques. The neural networks learn using an evolutionary technique, with samples created by orthogonal design and tested with new cases given by event design. With the neural network model established, the mechanical parameters are recognized using a genetic algorithm over a large search space in the global range. The predicted displacement occurring for each excavation step from January 1998 to the end of excavation and their cumulative values for 5 later excavation steps are closely characterized by the new analysis technique.     

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.     

Real-time computerized tomography (CT) experiments on sandstone damage evolution during triaxial compression with chemical corrosion

The computerized tomography (CT) images and CT values for the process of compression, micro-cracking, and dilation up to the failure of sandstone specimens under different loading levels have been obtained using the real-time CT technique for triaxial loading of sandstone subjected to chemical corrosion. Clear CT images and CT value of the stages from compression of micro-cavities→emergence of micro-cracks→bifurcation→development→fracture→collapse→unloading can be observed. The CT value, equivalent to rock density at the CT scan layer, is the most important parameter describing the damage evolution process of rock. The paper also presents results of the corrosive influence of chemical solutions with different pH values and ionic concentrations on the sandstone strength. Stronger acidity (pH<7) or causticity (pH>7) has a stronger effect on the rock micro-fracturing evolution. The mechanism of damage evolution of sandstone is analyzed and a damage model based on the chemical corrosive influence and CT values is proposed.     

Strength and failure characteristics of jointed marble under true triaxial compression

The rock structure and three-dimensional stress state play a vital role in the mechanical behaviour of rock masses. Here, a series of true triaxial compression tests (σ₁ > σ₂ > σ₃) are conducted on jointed marble (50 × 50 × 100 mm³) containing a natural stiff joint, taken from the China Jinping Underground Laboratory (CJPL-II) project. The purposes of this study are to investigate the joint effect and estimate the stress dependency of jointed marble. The test results show that jointed marble can fail in four distinct forms, namely, splitting or shearing of intact marble, opening of the joint or sliding along the joint, and these failure modes are influenced by the joint configuration and the minimum and intermediate principal stresses. Generally, jointed marble has more brittle post-peak behaviour than intact marble. The linear Mogi-Coulomb failure criterion can be modified to describe the strength of the jointed marble under true triaxial compression. The jointed marble strength is more sensitive to the minimum principal stress than to the intermediate principal stress. A maximum decline of 25% in strength is observed, which corresponds to a joint dip angle of 60° at σ₂ = 60 MPa and σ₃ = 30 MPa. The link between the experimental results and in situ fracturing at CJPL-II is also demonstrated.

     

Technical auditing of rock mechanics modelling and rock engineering design

We present a procedure for technically auditing rock mechanics modelling and rock engineering design. As rock mechanics modelling becomes more sophisticated, with the ability to include more parameters and to be coupled with other disciplines such as hydrogeology, and as the requirements on rock engineering projects become ever more challenging, it is prudent, if not essential, to have some form of procedure for checking that the modelling and design are suitable within the limits of current knowledge. Accordingly, we present here an appropriate auditing procedure, and demonstrate its use with the examples of measurement of rock stress (as a soft audit) and modelling for the design of hydropower caverns in China (as a semi-hard audit), the auditing questions being tailored to the nature of the work being audited. The types of procedures demonstrated can be used from the initial project concept, through site investigation and modelling, to construction, monitoring and back analysis, allowing the whole process to be concurrently checked providing a transparent and traceable audit trail of results and associated decisions made.     

Microwave response characteristics and influencing factors of ores based on dielectric properties of synthetic samples

In order to understand the influence of different factors on the microwave response characteristics of ores, the effects of electrical conductivity, metal mineral content, compactness, metal mineral distribution, microwave frequency and temperature on the dielectric properties of synthetic ores (metal mineral and quartz) were studied. Microwave heating tests were carried out on three types of natural ores (Hongtoushan copper ore, Sishanling iron ore and Dandong gold ore) with significant differences in metal mineral contents. The test results showed that under microwave irradiation, the stronger the electrical conductivity of the metal minerals, the smaller the penetration depth in synthetic ore. For those metal minerals with lower electrical conductivity, the microwave absorption coefficient of the synthetic samples increases with increasing metal mineral content. For those metal minerals with higher electrical conductivity, the microwave absorption coefficient of the samples first increases and then decreases as the metal mineral content increases. When the metal minerals are distributed in layers, the penetration depth is much less than that given a uniform distribution. The penetration depth in the sample at microwave frequency of 915 MHz is greater than that at 2.45 GHz. The higher the electrical conductivity of metal minerals used in synthetic ores, the higher the high-temperature sensitivity of electromagnetic shielding coefficient (0 °C–500 °C). The Hongtoushan copper ore with high metal mineral content exhibits obvious size effect. The effects of ore structure and crystal particle size on the distribution characteristics of microcracks were discussed. Based on the test results, a quantitative prediction model of microwave sensitivity of ore was proposed, which provides guidance for the prediction of ore heating effect and the selection of microwave heating sequence of ore.