Support Behaviour
In RocSupport, the characteristics of a support system are represented by two parameters:
- Maximum (radial) support pressure
- Maximum strain
There are two additional simplifying assumptions:
- The load-deformation curve of the support system is elastic-perfectly plastic.
- The support is applied uniformly around the tunnel opening.
Elasto-Plastic Behavior
The support system acts like a system of springs – support pressure increases with increasing deformation until the capacity of the system is exceeded. It is assumed that after yielding, support starts to show perfectly plastic behavior. Support pressure remains constant with increasing deformation, as illustrated in Figure 1. In reality, support systems might not demonstrate this behavior. Figure 2 shows actual load-deformation tests of several rock bolt types.
Figure 1. Elasto-plastic behavior of support
Figure 2. Results of rock bolt load-deformation tests. From Hoek et al. (1995).
Uniform Support Pressure
For all predefined support types, RocSupport makes the assumption that support is installed uniformly around the tunnel opening. This means that rock bolts must be installed in the roof, sidewalls and floor of the tunnel, and that steel sets and shotcrete linings are completely circular. Typical support installations are not uniform, which leads to drastic reduction in the overall capacity and stiffness of support. In this case, it is recommended that some reserve capacity is left, and the factor of safety is increased.
Combining Supports
RocSupport allows the user to combine all or selected types of support. When combining supports, it is assumed that:
- the maximum support pressure of the resultant system is the sum of maximum pressures of all individual systems
- the maximum strain of the resultant system is equal to the maximum value of maximum strain of all individual support systems (e.g. if you combine shotcrete with max. strain = 0.1% and steel sets with max. strain = 0.3%, it is assumed that the maximum strain of the composite system = 0.3%)