Rock Tunnelling Quality Index

Q =(RQD/Jn)(Jr/Ja)(Jw/SRF)
(click on a term in the equation above to review its definition and application)


1. ROCK QUALITY DESIGNATION RQD VALUE NOTES
A. Very poor 0 - 25 1. Where RQD is reported or measured as < = 10 (including 0), a nominal value of 10 is used to evaluate Q.

2. RQD intervals of 5, i.e. 100, 95, 90 etc. are sufficiently accurate.
B. Poor 25 - 50
C. Fair 50 - 75
D. Good 75 - 90
E. Excellent 90 - 100
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2. JOINT SET NUMBER Jn NOTES
A. Massive, no or few joints 0.5 - 1.0 1. For intersections use (3.0 x Jn )

2. For portals use (2.0 x Jn )
B. One joint set 2
C. One joint set plus random 3
D. Two joint sets 4
E. Two joint sets plus random 6
F. Three joint sets 9
G. Three joint sets plus random 12
H. Four or more joint sets, random, heavily jointed, 'sugar cube', etc. 15
J. Crushed rock, earthlike 20
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3. JOINT ROUGHNESS NUMBER Jr NOTES
  a. Rock wall contact   1. Add 1.0 if the mean spacing of the relevant joint set is greater than 3 m.

2. Jr = 0.5 can be used for planar, slickensided joints having lineations, provided that the lineations are oriented for minimum strength.
  b. Rock wall contact before 10 cm shear  
A. Discontinuous joints 4
B. Rough and irregular, undulating 3
C. Smooth undulating 2
D. Slickensided undulating 1.5
E. Rough or irregular, planar 1.5
F. Smooth, planar 1.0
G. Slickensided, planar 0.5
  c. No rock wall contact when sheared  
H. Zones containing clay minerals thick enough to prevent rock wall contact 1.0 (nominal)
J. Sandy, gravely or crushed zone thick enough to prevent rock wall contact 1.0 (nominal)
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4. JOINT ALTERATION NUMBER Ja ør degrees (approx.) & NOTES
  a. Rock wall contact  
A. Tightly healed, hard, non-softening, impermeable filling 0.75
B. Unaltered joint walls, surface staining only 1.0 25-35
C. Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc. 2.0 25-30
D. Silty-, or sandy-clay coatings, small clay-fraction (non-softening) 3.0 20-25
E. Softening or low-friction clay mineral coatings, i.e. kaolinite, mica. Also chlorite, talc, gypsum and graphite etc., and small quantities of swelling clays. (Discontinuous coatings, 1 - 2 mm or less) 4.0 8 - 16

1. Values of ør, the residual friction angle, are intended as an approximate guide to the mineralogical properties of the alteration products, if present.
  b. Rock wall contact before 10 cm shear  
F. Sandy particles, clay-free, disintegrating rock etc. 4.0 25-30
G. Strongly over-consolidated, non-softening clay mineral fillings (continuous < 5 mm thick) 6.0 16-24
H. Medium or low over-consolidation, softening clay mineral fillings (continuous < 5 mm thick) 8.0 12-16
J. Swelling clay fillings, i.e. montmorillonite, (continuous < 5 mm thick). Values of Ja depend on percent of swelling clay-size particles, and access to water. 8.0-12.0 6-12
  c. No rock wall contact when sheared  
K. Zones or bands of disintegrated or 6.0
L. crushed rock and clay (see G, H and J 8.0
M. for clay conditions) 8.0 - 12.0 6-24
N. Zones or bands of silty- or sandy-clay, small clay fraction, non-softening 5.0
O. Thick continuous zones or bands of clay 10.0 - 13.0
P. & R. (see G.H and J for clay conditions) 6.0 - 24.0
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5. JOINT WATER REDUCTION Jw Approx. Water Pressure (kgf/cm2) & NOTES
A. Dry excavation or minor inflow i.e. < 5 l/m locally 1.0 < 1.0
B. Medium inflow or pressure, occasional outwash of joint fillings 0.66 1.0 - 2.5
C. Large inflow or high pressure in competent rock 0.5 2.5 - 10.0

1. Factors C to F are crude estimates; increase Jw if drainage installed.
D. Large inflow or high pressure 0.33 2.5 - 10.0
E. Exceptionally high inflow or pressure at blasting, decaying with time 0.2 - 0.1 > 10

2. Special problems caused by ice formation are not considered.
F. Exceptionally high inflow or pressure 0.1 - 0.05 > 10
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6. STRESS REDUCTION FACTOR σc/σ1 σtσ1 SRF NOTES
   a. Weakness zones intersecting excavation, which may cause loosening of rock mass when tunnel is excavated      
A. Multiple occurrences of weakness zones containing clay or chemically disintegrated rock, very loose surrounding rock (any depth) 10.0 1. Reduce these values of SRF by 25 - 50% but only if the relevant shear zones influence do not intersect the excavation.
B. Single weakness zones containing clay, or chemically disintegrated rock (excavation depth < 50 m) 5.0
C. Single weakness zones containing clay, or chemically disintegrated rock (excavation depth > 50 m) 2.5
D. Multiple shear zones in competent rock (clay free), loose surrounding rock (any depth) 7.5
E. Single shear zone in competent rock (clay free), (depth of excavation < 50 m) 5.0
F. Single shear zone in competent rock (clay free), (depth of excavation > 50 m) 2.5
G. Loose open joints, heavily jointed or 'sugar cube,' (any depth) 5.0
  b. Competent rock, rock stress problems  
H. Low stress, near surface > 200 > 13 2.5 2. For strongly anisotropic virgin stress field (if measured): when 5 < =σ1/σ3 < = 10, reduce σc to 0.8σc and σt to 0.8σt. When σ1/σ3 > 10, reduce σc and σt to 0.6σc and 0.6σt, where σc = unconfined compressive strength, and σt = tensile strength (point load) and σ1 and σ3 are the major and minor principal stresses.

3. Few case records available where depth of crown below surface is less than span width. Suggest SRF increase from 2.5 to 5 for such cases (see H).
J. Medium stress 200-10 13-0.66 1.0
K. High stress, very tight structure (usually favourable to stability, may be unfavourable to wall stability) 10-5 0.66-0.33 0.5 - 2
L. Mild rockburst (massive rock) 5-2.5 0.33-0.16 5 - 10
M. Heavy rockburst (massive rock) < 2.5 < 0.16 10 - 20
  c. Squeezing rock, plastic flow of incompetent rock under influence of high rock pressure  
N. Mild squeezing rock pressure     5 - 10
O. Heavy squeezing rock pressure 10 - 20
  d. Swelling rock, chemical swelling activity depending on presence of water
P. Mild swelling rock pressure 5 - 10
R. Heavy swelling rock pressure 10 - 15
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ADDITIONAL NOTES ON THE USE OF THESE TABLES
When making estimates of the rock mass Quality (Q), the following guidelines should be followed in addition to the notes listed in the tables:
1. When borehole core is unavailable, RQD can be estimated from the number of joints per unit volume, in which the number of joints per metre for each joint set are added. A simple relationship can be used to convert this number to RQD for the case of clay free rock masses: RQD = 115 - 3.3 Jv (approx.), where Jv = total number of joints per m 3 (0 < RQD < 100 for 35 > Jv > 4.5).
2. The parameter Jn representing the number of joint sets will often be affected by foliation, schistosity, slaty cleavage or bedding etc. If strongly developed, these parallel 'joints' should obviously be counted as a complete joint set. However, if there are few 'joints' visible, or if only occasional breaks in the core are due to these features, then it will be more appropriate to count them as 'random' joints when evaluating Jn.
3. The parameters Jr and Ja (representing shear strength) should be relevant to the weakest significant joint set or clay filled discontinuity in the given zone. However, if the joint set or discontinuity with the minimum value of Jr/Ja is favourably oriented for stability, then a second, less favourably oriented joint set or discontinuity may sometimes be more significant, and its higher value of Jr/Ja should be used when evaluating Q. The value of Jr/Ja should in fact relate to the surface most likely to allow failure to initiate.
4. When a rock mass contains clay, the factor SRF appropriate to loosening loads should be evaluated. In such cases the strength of the intact rock is of little interest. However, when jointing is minimal and clay is completely absent, the strength of the intact rock may become the weakest link, and the stability will then depend on the ratio rock-stress/rock-strength. A strongly anisotropic stress field is unfavourable for stability and is roughly accounted for as in note 2 in the table for stress reduction factor evaluation.
5. The compressive and tensile strengths (σc and σt) of the intact rock should be evaluated in the saturated condition if this is appropriate to the present and future in situ conditions. A very conservative estimate of the strength should be made for those rocks that deteriorate when exposed to moist or saturated conditions.