Selecting support type for underground opening

The support type for an underground opening is selected from the quality of the rock mass, the size of the opening and the expected loads, using classification systems and site conditions.

Steps of selection

1. Investigate and classify: map the rock, estimate RQD, joint sets, strength and groundwater, and determine RMR and/or Q for each tunnel section.
2. Find the span and use: the larger the span (or equivalent dimension $D_e = \text{span}/ESR$), the heavier the support. A permanent hydropower tunnel needs more safety than a temporary adit.
3. Use the charts and tables: RMR class guidelines (bolts, shotcrete, ribs) or the Barton Q-chart.
4. Check special conditions: high stress, squeezing or swelling ground, running water, fault zones, joint orientation relative to the axis.

Typical selection (RMR)

Justification

• Good rock carries itself, so only local support is economical.
• Poor and very poor rock loosens rapidly and needs immediate and stiff support (shotcrete and ribs) to prevent collapse.
• Bolts reinforce the rock so it supports itself, shotcrete seals the surface, and ribs carry the loose rock load.
• The final choice is checked and adjusted by observation and monitoring during construction.