21: Tunneling & Underground Construction Flashcards
What is special about urban tunneling?
When in an urban environment, we’re dealing with infrastructure from the last hundred of years
- recent things are well documented
- older activities can be unforeseen hazards
Costs are increased when dealing with uncertainty an the unseen factors you have to deal with
What are some reasons you’d want to tunnel in urban settings?
- alleviate serious traffic congestion
- eliminate a troublesome eyesore
- reconnect old neighborhoods
- create open space in the middle of a historic city
- big savings related to transport time and less congestion!
Excavating through glacial till on its way under false Creek, which geological hazard do you think impacted the Canada Line Tunnel boring machine the most?
a) High stresses
b) Water leaking through the till
c) weak material strength
d) intersecting active volcanism
e) boulders mixed in the glacial till
a) not a deep tunnel (might suffer if drilling through mountains)
b) till has low permeability, so water isn’t an issue
c) these machines are designed for this and act as a shield preventing the ground from caving in
d) no active volcanism here
e) BIG ISSUE
If its all weak, we can design for that. If its all strong, vice versa. If we have strong mixed with weak, that’s when we have problems!
What are the 5 key factors that control the ground conditions encountered in underground engineering?
We have the
- INTACT ROCK,
which is itself divided by
- DISCONTINUITIES
to form the rock mass structure
We find then the rock is already subjected to a
- IN SITU STRESS.
superimposed on this are the influence of
- PORE FLUID / WATER FLOW
as well as
- TIME!
What are the 2 key factors affecting rock-mass deformation and stress response?
- how fractured the rock is
- how much in-situ stress there is
How does
- unfractured,
- fractured, and
- highly fractured rock
behave in
- low,
- medium, and
- high stress environments?
Unfractured rock:
low stress: linear elastic response, aka normal rock
med stress: brittle failure on edges of tunnel
high stress: brittle failure of intact rock around the entire tunnel (and deeper too)
med fractured rock:
low stress: falling or sliding of blocks and wedges
med stress: brittle failure starts to happen on intact rock. Pieces still moving, but less
high stress: Brittle failure starts to happen everywhere, rocks still moving
highly fractured rock:
low stress: tiny blocks, so whole tunnel starts to unravel
med stress: localized brittle failure of intact rock and unravelling along discontinuities
high stress: rock starts getting deformed, almost like toothpaste squeezing out!
What can discontinuities such as faults and joints lead to in rock?
Discontinuities such as faults and joints may lead to structurally-controlled instabilities whereby BLOCKS form through the intersection of several discontinuities.
They are kinematically free to FALL or SLIDE from the excavation periphery as a result from gravity
Summed up, what happens to rock mass failure in low-stress environments?
Blocky rock (rock that isn’t perfectly intact and kind of looks like a wall of protruding blocks):
- wedge failure (rock kind of come together to form wedge-shaped gaps
Very block/disturbed rock/disintegrated rock:
- we usually get gravity collapse of tunnel, as the small rock pieces don’t have anything to hold them from simply falling
What is the rock mass characterization?
Rock Mass characterization is an important feature for engineering that estimates the overall quality of the rock mass
Equation:
RQD = (sum of length of core pieces >10cm) /
(total length of core run)
Higher percentage = more pieces = higher quality
What is in-situ stress? How does this relate to total stresses?
IN situ stresses are stress that already exists in the rock
total stress = in-situ stress + excavation-induced stress
Summed up, what happens to rock mass failure in High-stress environments?
intact/blocky rocks:
- we get rock burst failure, where if the rock is clamped with high horizontal forces, it might just explode. Fractures are created instantly and it explodes!
block/disturbed/disintegrated rock:
- squeezing failure, where rock gets deformed (like toothpastes squeezing out)
How does water underground tunneling?
Water and the pressure it can bring acts to reduce the effective stress and/or induce instability
Groundwater can also chemically alter the rock and fracture surfaces by fluid movement
Aside from flooding, which in itself is a massive concern!
What are some hazards present in tunneling environments?
- temperatures exceeding 40 C
- Silicosis caused by quartz dust (ventilation is a big issue)
- environmental issues from large amounts of waste rock
- fault zone squeezing
- bursting
- running ground
What is fault zone and squeezing?
squeezing: weak, plastic rock material displaced into tunnel excavation under the action of gravity and induced stresses.
- Could result in damage/failure to the ground support system
fault zone:
- sometimes you can get super unlucky and have a horizontal fault along the path where you want to drill. This causes major issues!
What is running ground?
Running ground is when the ground has the ability to flow freely
Ex/ loose sands or other cohesion less materials
Special supports and control difficulties can arise
