CIVL3811 Chap.6 Tunnel Rock Support
Tunnel Rock Support
Tunnel support begins with observed rock mass behaviour, not intact strength alone. This chapter interprets the six Q-system inputs as three physical ratios for block size, joint friction and water or stress environment. The worked example computes the quality index transparently and preserves every selected rating beside its description.
Classification is then combined with equivalent dimension, excavation purpose and local mechanism review rather than treated as a complete design. A dedicated support page separates reinforcement by bolts from retention by mesh and shotcrete and explains why bolt direction, anchorage beyond a wedge, overlap, drainage and installation timing matter.
Field prompts add clay-coated joints, increasing water and a roof wedge beyond the current bolt length. They require a safe pause, updated mapping, authorised support change, inspection and continued convergence monitoring through the affected chainage.
What this chapter covers
- 01
Rock-mass observations
- 02
Q-system ratios
- 03
Equivalent dimension
- 04
Rock-bolt reinforcement
- 05
Surface support
- 06
Observational control
Worked example · free
Compute a trial Q value
- +1Compute the block-size ratio.
- +1Compute the friction ratio.
- +1Compute the water and stress ratio.
- +1Multiply to obtain Q = 2.05.
- +1Combine with equivalent dimension and mechanism review.
Key terms
- Rock quality designation
- A core-based index contributing to the block-size term of Q.
- Joint roughness
- A rating describing the surface character affecting discontinuity shear resistance.
- Stress reduction factor
- A Q-system factor representing stress-related rock-mass difficulty.
- Rock bolt
- Reinforcement crossing discontinuities and tying rock into a stronger zone.
- Shotcrete
- Sprayed concrete providing continuous surface support and confinement.
- Joint alteration
- A rating of infill or surface change that can reduce shear resistance along the governing discontinuity.
- Equivalent dimension
- Excavation span or height divided by the excavation-support ratio for use with Q-system guidance.
- Rock wedge
- A potentially removable block defined by intersecting discontinuities and the free surface of an excavation.
- Convergence monitoring
- Repeated measurement of tunnel deformation used to compare observed movement with expected and trigger behaviour.
- Support timing
- The distance and elapsed time between excavation and installation that influence relaxation before support engages.
Tunnel Rock Support FAQ
Does the Q-system design tunnel support by itself?
The Q-system organises rock-mass observations and informs support guidance, but it still needs excavation span, purpose, wedge analysis, timing, mapping and observed behaviour. Local mechanisms can require reinforcement beyond the general chart category.
Why use both rock bolts and surface support?
Bolts reinforce larger blocks and mobilise a stronger arch around the opening, while mesh and shotcrete retain smaller material between discrete reinforcement points. The combined pattern must match the actual wedge, ravelling or stress mechanism.
What evidence can change a tunnel support class?
New joint orientation, alteration, water, stress or deformation evidence can change both classification and the mechanism requiring support. Record the observation at its chainage, update through the authorised process and verify the revised installation.
Why keep Q-system descriptions beside the numbers?
The ratings embed engineering judgement about observed rock, joints, water and stress. Recording descriptions makes the result auditable, exposes uncertain inputs and supports a range calculation when field conditions cross two categories.
How should rock bolts be oriented?
Place bolts to cross controlling release planes and develop anchorage beyond the potentially moving block. A convenient radial pattern may miss a wedge, so geometry should be drawn before length and direction are selected.
What does surface support contribute?
Mesh and shotcrete retain fragments between discrete bolts and provide more continuous confinement. Overlap, thickness, surface preparation, water management and early placement determine whether that support function is actually achieved.
How should a face change be communicated?
Record chainage, round, joint geometry, water, installed support and observed movement. Pass an updated ground model and authorised response to the next shift, then compare subsequent behaviour with the revised expectation.
What should happen when a wedge exceeds bolt length?
Restrict access, map and analyse the release geometry, then install reinforcement oriented across its planes and anchored beyond the apex. Add retention and drainage appropriate to smaller material and water conditions.
Why can one precise Q value be misleading?
Several ratings depend on field description and the least favourable joint set. When an observation lies between categories, calculate a plausible range and investigate the input that can change the support decision.
Exam move
Learn the Q expression through three physical questions: what is the block size, how resistant is the governing joint surface, and how difficult are water and stress conditions? Recompute the example with each ratio on a separate line and write the selected field description beside the number. Test sensitivity by changing one uncertain rating and identifying whether the support region might change.
Sketch intersecting joints forming a roof wedge, draw the movement direction, and place bolts so they cross the release planes and anchor beyond the apex. Add mesh, shotcrete and drainage for material between bolts. Finish with a face-response drill that includes access restriction, chainage record, revised ground class, installation checks and convergence monitoring.
Avoid choosing a chart category until span and excavation-support ratio are stated.
