This paper describes the validity of the Multiple Yield Model (MYM) based on the comparison between the prediction by MYM analysis and the measurement results of two cases history about the large vertical excavation about 30m in depth and 100m in width for nuclear power plants. MYM is a kind of finite element method constituted the mechanical properties of intact rock and discontinuity systems in rock mass, and can be analyzed the non-linearity of deformation under loading and unloading stress paths. For analyzing, the geometrical model of rock mass were determined from test adit and borehole observations about the discontinuity conditions such as orientation, spacing, persistence, and the physical parameters were determined by laboratory test using core specimens and also considering scale effect. As the results of MYM analysis, both of the deformation mode and displacement were well corresponded to the measurement and we have been confirmed that the actual behavior of discontinuous rocks can estimate by MYM in practical accuracy"


A number of infrastructure such as dam, nuclear power plant, long-span bridge, rock slope and underground excavation have been constructed on and /in rock masses that have a lot of discontinuities like as joints, bedding planes, fractures. Generally we call this kind of rock mass with term of discontinuous rock.

As well known, the mechanical properties of discontinuous rocks are strongly influenced by the geometrical distribution and its mechanical properties of discontinuities which those strength and deformation behavior are non-linear. However, the practical parameters for design are generally setting by performing in-situ tests such as plate loading test, rock shear test, and assuming that rock mass is elastic and/or homogeneous. But the actual behavior of deformation is non-linear and/or greatly influenced by discontinuity conditions such as orientation (dip direction and dip), spacing, roughness, number of joint set and others. Therefore, it is extremely important to establish the appropriate analytical method of considering the discontinuity. Sasaki et al. (1994, 2004) have proposed Multiple Yield Model (MYM) for analyzing discontinuous rock, that is, the equivalent model of FEM by using compliance matrix method. This paper describes the tow practical examples of the results of the prediction by the numerical analysis using MYM and the measurements of deformation behavior due to large scale vertical excavations 20~30m in depth.


Figure 1 shows the concept of multiple yield models. The theory assumed that the total strain of rock mass is expressed summations of joint sets and intact rock's strains shown by equation (1);

  • (Equation in full paper)

Thus it is assumed that the joints are distributed periodically and the volume of each joint set is ignored in comparison with the volume of intact rock. And it is assumed that the stresses of the intact rock and joints coincide. The stiffness matrix of joint set I in the local coordinate system is transformed for the global coordinate system using the coordinate transformation matrix by equation (2).

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