Numerical methods such as FEM based on continuum mechanics do not seem to be able to analyze the behavior of discontinuous rock masses adequately nor effectively. Weak regions such as joints and faults dominate their behavioral features.
Recently some attempts have been made to solve this problem. TIWY have been successfully applied to various kinds of practical problems. However, they have some weak points at the same time.
This paper proposes a new effective modeling technique based on the assumption that a discontinuous rock mass consists of rigid blocks and joints.
Four current methods for analyzing the behavior of discontinua are summarized up in Table I. Fig. 1 shows examples of current methods in modeling a voussoir arch abutment.
A new modeling technique has been developed to avoid the weak points and to incorporate the strong points of the current methods described in Table I (Asai 1981). Fig. 2 shows an example in this technique named rigid-body jointelement method (RIM). The method based on the combination of Goodman's joint element and Cundall's distinct rigid element. This hybridization brings us more strong points than either of the two methods. Their weak points have vanished. The features of this method are summarized in Table 2. As Shown in Fig. 2, the three kinds of elements (URJS, STRE, RBE) represent the whole system with arbitrarily shaped rocks by superposing the element stiffness matrices in just the same way as FEM.
Fig. 3 shows the concept of URJS. This unit has two block Points. Each block point is located at the vertex opposite to each other. A block point has 3 degrees of freedom in a two-dimensional field. According to the notation shown in Fig. 3. the equilibrium equation for the two triangular rigid elements is:
(Equation in full paper)
A computer simulation model is illustrated in Fig. 8. The model has almost the same geometry as Voegele's (Voegele 1978).
The model in Fig. 8 has the properties:
density 2.6 t/m3,
joint thickness: 0.02 m,
joint stiffness: ks= 100 MN/m3, kn = 200 MN/m3.
We applied RIM to this model in two cases. The simulations were carried out within the limitations:
the material is linear and elastic,
the displacements are small enough,
the gravity IS turned on after excavation.
Case 1(gravity and excavation).- The joint stress distributions are illustrated in Fig. 9. An arrow means a resultant stress acting on the lower or the left-hand side rock.
Using DEM, Voegele has pointed out that
the ground arch forms along the line 6–7-8–9-10–11–1213–14, 2) the roof arch forms along the line 2–3-4–5, 3) rock I moves into the excavation and then the joint opens at A.
Fig. 9 leads to the following: I) compressive joint stresses are produced along the line 6–7-8–9-10- 11–12–13–14 and the line 2-C-4–5 instead of 2–3-4–5,
but the two lines are not clearly distingushed from each other,
