Rock mass factor

An estimate of the numerical value of the deformation modulus of a jointed rock mass can be obtained from various in situ tests (see Chapter 7). The values derived from such tests are always smaller than those determined in the laboratory from intact core specimens. The more heavily the rock mass is jointed, the larger the discrepancy between the two values. Thus, if the ratio between these two values of deformation modulus is obtained from a number of locations on a site, the engineer can evaluate the rock mass quality. Accordingly, the concept of the rock mass factor, J, was introduced by Hobbs (1975), who defined it as the ratio of deformability of a rock mass to that of the intact rock (Table 2.5). 

A parameter variation showed that the HM-induced pressure responses depend on many parameters, including rock-mass deformation modulus, Biot s coupling constants, hydraulic permeability, and the magnitude and orientation of the in situ stress field. The three material parameters affect only the magnitude of the HM-induced pressure response. On the other hand, the magnitude and direction of the in situ stress field are important factors that determine where and when the fluid pressure will increase or decrease. 

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 ... 

Failure is conditioned to the effective stress state, according to equation (3). Figure 12, which compares the safety factor (defined by the ratio F/o i) after excavation, shows the necessity to consider full THM couplings when studying the rock mass stability ( Failure corresponds to a safety factor lower than 1, in dark blue on Figure 12). 

The main conclusions drawn from the small-and large-scale analyses of the studied rock mass are that for this data set, the uncertainty in the mechanical properties and their spatial variations are the most important factors in performance assessment of deep waste disposal, followed by the uncertainty of the fracture density and the spatial distribution of the fracture density. 

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 which, the above formula includes three items. The first item yb jllju) illuminates the classic cubic law for the ideal flow channel situation. While, the second item (JRC/JMC) illuminates the influence of the rock surface, which is the fundamental influence factor for the seepage. And the third item (1 - ef shows the normal and shear stress on the rock mass, especially for the structural plane, which should be considered seriously. 

Mathews diagrammatize method is used to analyze roof stability of working face, it need to calculate two major variables, one is the shape factor, which is represented with structure size and shape of working face, and another is stability coefficient, which is the self-stability of rock mass under stress condition (Milne et al. 1998). Shape factor is hydraulic radius of structure size of working face. Mathews diagrammatic stability coefficient is been expressed as ... 

A value is considered effect factor of rock mass stability under high stress condition in Mathews diagrammatize method. A value is a ratio of uniaxial compressive strength and maximum principal stress of parallel working face with complete block of coal and rock. The relationship of A value and shows a linear, and its variation range is from 0.1 to 1.0. 

B value is considered effect factor to cut direction of rock mass discontinuity surface, which decided by control joint and relative direction of working face roof When the angle of joint surface and working face roof is 90°, B coefficient is 1. When the angle of discontinuity joint surface and working face roof is 20°, B coefficient is 0.3. 

The roof of working face is influenced by dead weight in deep mining, roof stability of working face is smaller than slide. The gravity adjustment coefficient (Q is considered effect factor of selfgravity of coal and rock mass to slide roof collapse... 

It is attempted to evaluate uniaxial compressive strength (UCS) and elastic modulus of rock mass as a function of UCS or elastic modulus of intact rock and j oint factor. Figure 14 shows the variation of (the ratio of UCS of jointed rock to the intact rock) with joint factor (Jj.) for tested material. The term joint factor (Jj.), introduced by Ramamurthy (1993), which reflects the combined effect of j oint frequency, joint inclination and joint roughness (or wall strength) and is expressed as ... 

Exponential correlations were established for the prediction of uniaxial compressive strength ratio/ratio of static and dynamic elastic modulus of rock mass from the intact rock uniaxial compressive strength/elastic modulus and joint factor (Ramamurthy, 1993), which includes joint frequency, joint inclination andjoint strength. These relations are useful in characterisation of jointed rock mass by knowing the intact rock properties and the joint factor. 

In this paper the compressive strength/elastic modulus of the jointed rock mass was estimated as a function of intact rock strength/modulus and joint factor. The joint factor reflects the combined effect of joint frequency, joint inclination and joint strength. Therefore, having known the intact rock properties and the joint factor, jointed rock properties can be estimated. The test results indicated that the rock mass strength decreases with an increase in the joint frequency and a sharp transition was observed from brittle to ductile behaviour with an increase in the number of joints. It was also found that the rocks with planar anisotropy exhibit the highest strength in the direction perpendicular to the anisotropy and the lowest at an inclination of 30o-45o in jointed samples. The anisotropy of the specimen influences the dynamic elastic modulus more than the static elastic modulus. The results were also compared well with the published works of different authors for different type of rocks. 

ABSTRACT In this study the disturbance factor in the general Hoek-Brown (HB) criterion is considered to be a gradually-attenuated variable from the excavation surface to the deep surrounding rocks. The elasto-plastic analytical solution is formulated for an axisymmetrical cavern model in which there exist a supported pressure at the wall of tunnel and a far-field pressure at infinity. The presented analytical model can well reflect the disturbance of the HB rock mass triggered by drilling and blasting excavation. 

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