Joint Alteration Number |
φr |
Ja |
Rock-wall contact (no mineral fillings, only coatings)
|
A |
Tightly healed, hard, non-softening, impermeable filling, i.e., quartz or epidote. |
- |
0.75 |
|
B |
Unaltered joint walls, surface staining only. |
25-35°
|
1 |
|
C |
Slightly altered joint walls. Non-softening mineral coatings; sandy particles, clay-free disintegrated rock, etc.
|
25-30° |
2 |
|
D |
Silty or sandy clay coatings, small clay fraction (non-softening).
|
20-25°
|
3
|
|
E |
Softening or low friction clay mineral coatings, i.e.,kaolinite or mica. Also chlorite, talc gypsum, graphite, etc., and small quantities of swelling clays.
|
8-16°
|
4
|
|
Rock-wall contact before 10 cm shear (thin mineral fillings) |
F |
Sandy particles, clay-free disintegrated rock, etc. |
25-30°
|
4
|
|
G |
Strongly over-consolidated, non-softening, clay mineral fillings (continuous, but <5mm thickness). |
16-24°
|
6
|
|
H |
Medium or low over-consolidation, softening, clay mineral fillings (continuous, but <5mm thickness). |
12-16°
|
8
|
|
J
|
Swelling-clay fillings, i.e., montmorillonite (continuous, but <5mm thickness). Value of J depends on percent of swelling clay-size particles. |
6-12°
|
8-12 (10) |
|
No rock-wall contact when sheared (thick mineral fillings) |
K
|
Zones or bands of disintegrated or crushed rock. Strongly over-consolidated. |
16-24°
|
6
|
|
L
|
Zones or bands of clay, disintegrated or crushed rock. Medium or low over-consolidation or softening fillings.
|
12-16°
|
8
|
|
M
|
Zones or bands of clay, disintegrated or crushed rock. Swelling clay. J depends on percent of swelling clay-size particles.
|
6-12°
|
8-12
(10) |
|
N
|
Thick continuous zones or bands of clay. Strongly over-consolidated.
|
12-16°
|
10
|
|
O
|
Thick, continuous zones or bands of clay. Medium to low over-consolidation.
|
12-16°
|
13
|
|
P
|
Thick, continuous zones or bands with clay. Swelling clay. Ja depends on percent of swelling clay-size particles.
|
6-12°
|
13-20
(16) |
|
Stress Reduction Factor |
SRF |
|
Weak zones intersecting the underground opening, which may cause loosening of rock mass |
A |
Multiple occurrences of weak zones within a short section containing clay or chemically disintegrated, very loose surrounding rock (any depth), or long sections with incompetent (weak) rock (any depth). For squeezing, see 6L and 6M |
10 |
|
B |
Multiple shear zones within a short section in competent clay-free rock with loose surrounding rock (any depth) |
7.5 |
|
C |
Single weak zones with or without clay or chemical disintegrated rock (depth ≤ 50m) |
5 |
|
D |
Loose, open joints, heavily jointed or “sugar cube”, etc. (any depth) |
5 |
|
E |
Single weak zones with or without clay or chemical disintegrated rock (depth > 50m) |
2.5 |
|
Competent, mainly massive rock, stress problems |
σc / σ1 |
σΘ / σc |
SRF |
|
F |
Low stress, near surface, open joints |
>200 |
<0.01 |
2.5 |
|
G |
Medium stress, favourable stress condition |
200-10 |
0.01-0.3 |
1 |
|
H |
High stress, very tight structure. Usually favourableto stability.May also be unfavourable to stability dependent on the orientation of stresses compared to jointing/weakness planes |
10-5 |
0.3-0.4 |
0.5-2 (1.25) |
|
J |
Moderate spalling and/or slabbing after > 1 hour in massive rock |
5-3 |
0.5-0.65 |
5-50 (27.5) |
|
K |
Spalling or rock burst after a few minutes in massive rock |
3-2 |
0.65-1 |
50-200 (125) |
|
L |
Heavy rock burst and immediate dynamic deformation in massive rock |
<2 |
>1 |
200-400 (300) |
|
Squeezing rock: plastic deformation in incompetent rock under the influence of high pressure |
σΘ / σc |
SRF |
|
M |
Mild squeezing rock pressure |
1-5 |
5-10 (7.5) |
|
N |
Heavy squeezing rock pressure |
>5 |
10-20 (15) |
|
Swelling rock: chemical swelling activity depending on the presence of water |
SRF |
|
O |
Mild swelling rock pressure |
5-10 (7.5) |
|
P |
Heavy swelling rock pressure |
10-15 (12.5) |
|