Environmental Engineering Reference
In-Depth Information
TABLE 5.26
(
Continued
)
(d) Joint Alteration Number
J
a
r
(approx.)
No rock wall contact when sheared
Zones or bands of disintegrated or
6, 8, or 8-12
(6-24)
crushed rock and clay (see above for
description of clay condition)
Zones or bands of silty or sandy clay,
5.0
small clay fraction (nonsoftening)
Thick, continuous zones or bands of
10, 13, or 13-20
(6-24)
clay (see above for description of clay
condition)
(e) Joint-Water Reduction Factor
e
J
w
Approx water
pressure (kg/cm
2
)
Dry excavations or minor inflow, i.e.,
1.0
1
5 min locally
Medium inflow or pressure, occasional
0.66
1-2.5
outwash of joint fillings
Large inflow or high pressure in
0.5
2.5-10
competent rock with unfilled joints
Large inflow or high pressure,
0.33
2.5-10
considerable outwash of joint fillings
Exceptionally high inflow or water
0.2-0.1
10
pressure at blasting, decaying with time
Exceptionally high inflow or water
0.1-0.05
10
pressure continuing without noticeable
decay
(f) Stress Reduction Factor
SRF
Weakness zones intersecting excavation,
which may cause loosening of rock mass
when tunnel is excavated
f
Multiple occurrences of weakness zones
10
containing clay or chemically
disintegrated rock, very loose
surrounding rock (any depth)
Single weakness zones containing clay
5
or chemically disintegrated rock (depth
of excavation
50 m)
Single weakness zones containing clay
2.5
or chemically disintegrated rock (depth
of excavation
50 m)
Multiple shear zones in competent rock
7.5
(clay-free), loose surrounding rock (any
depth)
Single shear zones in competent rock
5.0
(clay-free) (depth of excavation
50m)
Single shear zones in competent rock
2.5
(clay-free) (depth of excavation
50m)
Loose open joints, heavily jointed or
“sugar cube,” etc. (any depth)
5.0
σ
c
/
σ
1
σ
t
/
σ
1
SRF
Competent rock, rock stress problems
g
Low stress, near surface
200
13
2.5
Medium stress
200-10
13-0.66
1.0
High stress, very tight
10-5
0.66-0.33
0.5-2
structure (usually favorable to
stability, may be unfavorable for wall stability)
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