21- Tunnelling

Question 1

Discontinuities

Answer 1

Discontinuities(faults and jolts) can lead to “blocks” of rock forming that are not securely held in place, making them unstable. Since they are not firmly attached gravity can cause them to fall or slide

Question 2

Rock quality designation

Answer 2

Quality of the rock mass
RQD= (sum of length of core pieces >10cm/ total length of core run) x100%

Question 3

In situ stress

Answer 3

Natural stress present in rock before disruption (pre-existing state of stress)

Question 4

Water inflows

Answer 4

Water is under pressure, which reduces the effective stress and/or induces instabilities

Question 5

Tunnel hazard - Bursting

Answer 5

Involve sudden releases of stored strain energy through the brittle failure of strong rock (ex granite)
Manifests as sudden ejection of rock into excavation
Special concern in deep tunneling where high stress concentrations form due to overburden loads or tectonic activity

Question 6

Tunnel hazard - squeezing

Answer 6

Refers to weak, plastic rock material which displaces into the tunnel excavation under the action of gravity and induced stress
Results in damage/failure to ground support system, or require costly re-excavation of tunnel section
Common in fault zones, with weak or fractured rock

Question 7

Tunnel hazard- running ground

Answer 7

Where ground posses the ability to flow freely, for example with loose sand like material
Support and control difficulties can arise
Major inflow can flood tunnel work, or develop chimney collapse features which can propagate to the surface