Environmental Engineering Reference
In-Depth Information
to provide adequate support to the opening itself; consideration must also be given to the
possibility of surface subsidence and detrimental deflections of adjacent structures.
Design Criteria
Support requirements may be selected on the basis of experience and empirical relation-
ships with rock quality or on the basis of finite element methods applied to assess stress
conditions and deformations. Analytical methods require information on rock-mass stress
conditions and deformation moduli, which are measured in situ (see Section 3.5.3). In situ
test results may not be representative of real conditions for various reasons depending on
the type of test, and are generally most applicable to rock of good to excellent quality in
large openings.
A tunnel may become unnecessarily costly if all contingencies are provided for in
design, especially with regard to the problem of identifying all significant aspects of
underground excavation by exploration methods. The nature of tunnel construction
allows design on a contingency basis in which the support systems are modified to suit
the conditions encountered. This approach requires monitoring in situ conditions as
excavation proceeds because of the relative nonpredictability of rock-mass response to an
opening.
Construction Monitoring
Elements to be monitored during construction may include:
1.
Closure of the opening and strain gradients in the rock mass
2.
Support system strains and loads
3.
Ground surface subsidence in urban areas over mines, caverns, and tunnels (sur-
face subsidence may result from closure of the opening, from running ground
into the opening, or from the lowering of groundwater causing compression in
overlying soil formations)
4.
Vibrations from blasting affecting surface structures or adjacent underground
openings
Instrumentation (Figure 4.39)
Borehole extensometers (o) are installed to measure strain gradients and closure, and serve
in a number of purposes. Rock-mass loads can be deduced for support design, and the
modulus can be determined in good-quality rock when rock stresses are known from bore-
hole stress relief or flat-jack tests.
Rock moduli computed from radial deflections may be lower than those computed
from flat-jack tests. During tunnel driving, radial stresses diminish to zero at the edge of
the opening while tangential stresses become concentrated (Kruse, 1970). Joints that
have nearly tangential orientations have the greatest tendency to open. This condition
influences borehole extensometer measurement as well as the compression generated by
plate bearing loads. Borehole extensometer data are also used to estimate rock-bolt
lengths.
Convergence meters (g) monitor closure. Deflectometers (m) installed in advance of the
tunnel from a small pilot bore provide warning of mass deformations occurring in major
shear zones; groundwater conditions are revealed during the drilling process and pilot
holes serve to provide predrainage.
In situ stress meters (x,z) provide data for estimating roof and wall pressures that have
to be retained by the support system. Pressure cells (s) installed between the tunnel lining
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