Program Assumptions

The main assumptions in the RocSupport analysis method are as follows:

• tunnel is circular
• in-situ stress field is hydrostatic (i.e. equal stress in all directions)
• rock mass is isotropic and homogeneous. Failure is not controlled by major structural discontinuities.
• support response is elastic-perfectly plastic
• support is modeled as an equivalent uniform internal pressure around the entire circumference of the circular tunnel

This last assumption in particular, should be carefully considered, when comparing actual tunnel behavior, and calculated results using RocSupport. The assumption of uniform support pressure implies that:

• shotcrete and concrete linings are closed rings
• steel sets are complete circles
• mechanically anchored rockbolts are installed in a regular pattern which completely surrounds the tunnel.

Because this will not usually be the case, actual support capacities will be lower, and deformations larger, than those assumed in RocSupport.

The idealized model used for a RocSupport analysis is not intended to replace detailed final design and analysis requirements for tunnel support. In general, this will require numerical analysis (e.g. finite element), particularly for tunnels with large strain.

However, a great deal can be learned about the interaction of tunnels in weak rock, with various support systems, by carrying out parametric studies using RocSupport, in which different combinations of in-situ stress levels, rock mass strengths and support characteristics are evaluated.

In spite of the limitations discussed above, rock-support interaction analysis has many attractions and, when used in conjunction with numerical analysis, it can provide valuable insights into the mechanics of rock support and reasonable guidelines for the design of this support.