Environmental Engineering Reference
In-Depth Information
(e)
(f)
(j)
(r)
(o)
(q)
(r)
(a)
(p)
(o)
(o)
Instrumentation notch
(z)
(x,z)
(x,z)
(s)
Pilot bore
(m)
(o)
(g)
(v or w)
(o)
(t)
(j)
Section to be excavated
FIGURE 4.39
Instrumentation for tunnels, caverns and mines in rock. Legend: (a) precise leveling; (e) tiltmeter; (f) pendulum;
(g) convergence meter; (j) vibration monitor; (m) deflectometer; (o) borehole extensometer; (p) multiple electric
strain meter; (q) acoustical emission device; (r) piezometers; (s) pressure or stress cells; (t) load cells; (v) strain
gages; (w) strain meter; (x) stress meter; (z) borehole residual stress measurements.
and the roof monitor stress changes occurring with time and forewarn of the necessity for
additional supports. Strain gages (v or w) are installed on metal supports. Load cells (t)
installed in the tunnel lining or on rock anchors monitor load changes. Load reductions in
anchors indicate movements of the rock and a relaxation of the anchor load and the
anchor's retention capacity. In this case, additional anchors, probably of greater length, are
required.
Vertical extensometers (o), surface strain meters (p), and precise leveling (a) are used to
monitor ground subsidence, and tiltmeters (e) and pendulums (f) monitor deflections in
overlying structures.
Acoustical emission devices (q) provide warning of the impending collapse of mines or
large caverns. Vibration monitoring (j) is required when blasting is conducted near an
adjacent existing tunnel or overlying structures.
Case 2: Tunnels in Soil in Urban Areas
Objectives
The most important objective in instrumenting tunnels in urban areas is the monitoring of
surface deflections and the prevention of damage to overlying structures.
Instrumentation
As illustrated in
Figure 4.40,
surface movements are monitored by precise leveling (a), but
important data are obtained from tiltmeters (e) or pendulums (f) installed in structures.
Borehole extensometers provide data on subsurface deformations in the vertical mode and
inclinometers (L) in the lateral mode, serving as early warnings of excessive deformations.
Pore pressures are monitored with piezometers (r), and load cells (t) installed in tunnel lin-
ings provide data on earth pressures.
