Rock-mass properties

The material properties are given in Table 1. Except for permeability, the properties of the high-permeability zones (Lamprophyres) and the surrounding rock are the same. The mechanical rock-mass properties are obtained from the geological description of the GTS (Keussen et al., 1989). Significantly, the Young s modulus of the rock mass was reduced to 70% of its value for intact rock. 

Other than in the early stage of heating and regardless of the rock-mass properties or failure criteria used, the anchor displacements relative to the MPBX assembly head, located at the collar of the heated drift, were larger for the anchors located farther away from the heated drift. This displacement pattern indicated that the neighboring anchors of an MPBX continued to move away from each other as heating proceeded, suggesting an expansion of the rock mass between the two anchors. This behavior was not strictly observed in the... 

The bentonite properties have been calibrated from various tests results during DECOVALEX II (Rutqvist et al., 2001) and have been used, together with the rock mass properties, for verification in Task A of the BMTl exercise (Chijimatsu et al., 2003).The backfill material consists of a mixture of bentonite (15%), sand and gravel. 

Japan (Figure 1). The in-situ state of stress and the natural thermal gradient are also based on Japanese geological data however as we will illustrate in the next section, the rock mass properties in term of strength and permeability are based on typical Canadian Shield s data. 

The rock mass properties such as deformability and permeability are highly dependent on the existence of cracks (Eshelby (1920), Bieniawski (1979), Kinoshita (1992), Yoshida (1999)). Especially, open cracks give crucial effects to these factors. Moreover, the deformability and the permeability of rock mass are strongly related to each other, and the coupling analysis between these factors is inevitable for the rock mass evaluation. 

As TBM performance is the result of the interaction between rock and TBM machine, both of the rock mass properties and TBM design/operation parameters should be included for this issue. The factors influencing TBM performance in squeezing ground can be briefly summarized as shown in Table 1. 

In the lower portion of the shaft—the rock layers, structure and support should take the rock mass properties into account. Shotcrete, rockbolt, steel rib or their combinations can be used based on the rock mass classification and rock mass properties. 

Reese (1997) proposed a procedure to calculate p-y curves for rock using basic rock and rock mass properties such as compressive strength of intact rock q. Rock Quality Designation (RQD), and initial modulus of rock E-. A description of the procedure is presented in the following. 

While a full-scale field loading test, properly performed and interpreted, will reliably define the foundation soil interaction, such tests are generally not feasible. Thus, the way a soil or rock mass responds to being stressed is usually determined by previous experience in similar conditions, by extrapolating the results of small load tests or by using specific soil properties in various empirical formulae. These soil properties are sometimes inferred from previous experience, but more often reflect the results of laboratory and field tests on soil and rock samples. 

Samples of rock can only be obtained by coring , that is, by drilling into a rock mass with a hollow drill bit. Bits are available in many sizes, with the cutting edges made of tempered steel or of steel in which many tiny industrial diamonds are imbedded. The solid portions of the rock cores obtained are considered undisturbed , and representative of the properties of the in-situ rock. The makeup of a rock sample is a function of the relation between the sample size and the joint or fracture system, as shown in Figure 1.3. 

The results for flow on a single fracture surface are incorporated in the derivation of hydraulic properties of unsaturated fractured rock mass. Liquid retention and hydraulic conductivity in partially saturated fractured porous media are modeled in angular pores and slit-shaped spaces representing rock matrix and fractures, respectively. A bimodal distribution of pore sizes and apertures accounts for the two disparate pore scales and porosity. These considerations provide a framework for derivation of retention and hydraulic conductivity functions for fractured porous media (Or Tuller, 2001). 

It is obviously not possible to make experiments with duration of hundreds of thousands of years and over distance of hundreds of meters in the tight rock formations of interest. It is therefore essential to understand the key processes so well that credible predictions can be made using models based on well-established laws of nature. The models must be supported by experiments that can credibly be extrapolated. The models used are based on the laws of mass and energy conservation and on laws of thermodynamics. The difficulties in applying these laws arise mainly from the fact that the rock mass cannot be described in detail. The location, orientation and detailed hydraulic properties of the fractures cannot be measured in detail. The diffusion and sorption properties of the interior of the rock mass under natural stress cannot be readily measured. Mixing processes of different water packages in fractures and at intersections are not fully understood. All this makes it difficult to build models that account... 

Table 1. Material properties of the rock mass used in modelling of TBM drilling of the FEBEX tunnel...

The material properties were obtained from various field and laboratory tests that were performed before the emplacement of the buffer and heaters. For example, the inflow into the open drift was utilized to calibrate the in situ hydraulic properties of the rock mass and Lamprophyre dykes. Moreover, several laboratory tests were utilized for numerical back-analyses of coupled THM properties of the bentonite. The properties of the bentonite and rock are listed in Tables 1 and 2. 

For the Yucca Mountain site, incorporation of stress effects into hydraulic properties is based on a conceptual model of a highly fractured rock mass that contains three orthogonal fracture sets, as shown in Figure 2b. Porosity correction factor (F,) and permeability correction factors (Fu, Ft, FtJ calculated from the initial and current apertures (bii, b i, bsi and b , b , bj, respectively) in fracture sets 1, 2, and 3, according to ... 

The conceptual model in Figure 2, combined with a continuum model approach, is shown to be appropriate for the analysis of THM processes at the DST because the rock mass is highly fractured, forming a dense, wellfracture network for fluid flow. This differs from many other fractured rock sites in Canada, Europe, and Asia, where underground tests have been conducted in sparsely fractured crystalline rocks (Rutqvist and Stephansson, 2003). In those formations, fluid flow is dominated by a few widely spaced fractures, which means that a continuum approach may not apply on the drift scale. In relation to other fractured rock sites, the rock mass at Yucca Mountain is relatively homogenous (ubiquitously fractured), with much less variability in rock-mass mechanical and hydrological properties. 

Table 3. Rock-mass mechanical properties for sensitivity analyses. ...

Abstract Geological disposal of nuclear fuel wastes relies on the concept of multiple barrier systems. In order to predict the performance of these barriers, mathematical models have been developed, verified and validated against analytical solutions, laboratory tests and field experiments within the international DECOVALEX project. These models in general consider the full coupling of thermal (T), hydrological (H) and mechanical (M) processes that would prevail in the geological media around the repository. This paper shows the process of building confidence in the mathematical models by calibration with a reference T-H-M experiment with realistic rock mass conditions and bentonite properties and measured outputs of thermal, hydraulic and mechanical variables. 

Table 3 Main different properties for rock mass...

In this paper, we provided the rationale and definition of a benchmark test called BMTl to look at the implications of THM couplings on safety parameters in the near field of a hypothetical repository. This hypothetical repository possesses composite features since it is based on a Japanese design, with a Japanese bentonite used as buffer material and the heat output characteristics of Japanese spent fuel. However, the permeability and strength characteristics of the rock mass are based on typical properties of granites of the Canadian Shield. 

To test the significance of the calculated initial mechanical apertures, a constant initial aperture of 77 pm was used and the simulations repeated. The results produced different results to those for the calculated hydraulic apertures but the trend was similar (Table 3). Thus, the initial mechanical apertures emerge to have an impact on the resulting hydraulic apertures but, for the results presented, the significance appears to be less than the impact of the variation of the mechanical properties. From these results the importance of the mechanical properties and their spatial distribution in the rock mass to the estimation of hydraulic aperture appears to be strong. 

An extensive examination of the fracture network and mechanical data has been undertaken to determine models of the fracture characteristics of the three formations, the uncertainties in the parameterisation of the models, and the sensitivity of the upscaled flow properties to the underlying parameter variations. The methodology used to calculate effective hydraulic conductivity values and their sensitivity to the small-scale model is described in Blum el al. (2003). The study undertaken to obtain the effective hydraulic conductivity under different stress conditions and presented in Blum et al. (2003) revealed that the important parameters in modelling HM processes in the fractured rock mass are the fracture density, the mechanical (M) properties and the M property variations through the rock mass. 

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