Rock mass classification systems - Geotechnical Investigation and Design Tables - page 233

Flow Joint flow Approx. water J w value

pressure (kPa)

Dry excavations or ie

<

5 L/min locally

<

100 1.0

minor inflow
Medium inflow or Occasional outwash of joint fillings 100-250 0.66

pressure
Large inflow or With unfilled joints 0.5

250-1000

high pressure in
competent rock
Large inflow or Considerable outwash of joint fillings 0.33

high pressure

Or water pressure at blasting, decaying with time 0.2-0.1

Exceptionally Or water pressure continuing without noticeable

>

1000 0.1-0.05

high inflow delay

Table 18.14 Stress reduction factor (Barton et al., 1974 with updates).

Zone characteristics

SRF
value

Rock type

Weakness zones Material in zone Depth

Multiple occurrences,
very loose surrounding rock
Single
Single

Any
50m
50m

10
5
2.5

Weakness zones
intersecting excavations
which may cause
loosening of rock mass
when tunnel is excavated

Chemically
disintegrated
rock

Clay

Multiple shear zones, loose
surrounding rock
Single Shear zones
Single Shear zones
Loose, open joints,
heavily jointed

Any
50m
50m
Any

7.5
5.0
2.5
5.0

No
clay

Stress σ / σ c

UCS/ σ 1

Near surface, open joints
favourable stress condition
very tight structure. Usually
favourable to stability, may be
unfavourable for wall stability
moderate slabbing after 1
hour in massive rock Slabbing
and rock bursts after a few
minutes in massive rock
heavy rock burst
(Strain burst) and immediate
dynamic deformations in
massive rock

Competent rock,
rock stress problems

Low
Medium
High

0.01
0.01- 0.3
0.3-0.4

2.5
1
0.5-2

200
200-10
10-5

5-3 0.5-0.65 5-50

50-
200
200-
400

3-2 0.65-1

2 1

Squeezing rock, plastic
flow of incompetent rock
under the influence of
high rock pressure

Mild squeezing rock pressure
Heavy squeezing rock pressure

1-5
5

5-10
10-20

Swelling rock, chemical
swelling activity depending
on pressure of water

Mild swelling rock pressure
Heavy swelling rock pressure

5-10
10-15