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Rock mass quality, QRock mass quality, Q--systemsystem::

QQ--system (Tunnel Quality Index, Barton system (Tunnel Quality Index, Barton et. alet. al., 1974) is based on ., 1974) is based on 200 case studies of tunnels & cavern in, 6 parameters is defined200 case studies of tunnels & cavern in, 6 parameters is defined: :

Q = [RQD/Q = [RQD/JJnn] ] ×× [[JJrr/J/Jaa] ] ×× [[JJww/SRF]/SRF]

RQDRQD = Deere= Deere’’s Rock Quality Designation s Rock Quality Designation ≥≥ 1010JJnn = Joint set number (1, 2, 3= Joint set number (1, 2, 3……))JJrr = Joint roughness number for critical joint set= Joint roughness number for critical joint setJJaa = Joint alteration number (weathering) for critical joint= Joint alteration number (weathering) for critical joint

set set JJww = Joint reduction factor due to presence of water= Joint reduction factor due to presence of waterSRFSRF = Stress reduction factor= Stress reduction factor

For various rock conditions, the numerical ratings for the aboveFor various rock conditions, the numerical ratings for the above 6 6 parameters are defined as follows:parameters are defined as follows:

(1) (1) Rock quality designation RQDRock quality designation RQD::

RQD as previously defined. The RQD value in % is the rating of RRQD as previously defined. The RQD value in % is the rating of RQD QD for the Qfor the Q--system. system.

In the case of a poor rock mass where RQD < 10%, a minimum In the case of a poor rock mass where RQD < 10%, a minimum value of 10 should be used to evaluate Q (see Table 1) value of 10 should be used to evaluate Q (see Table 1)

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Table 1: Rock Quality Designation RQD (Barton Table 1: Rock Quality Designation RQD (Barton et al., 1974)et al., 1974)

Condition RQD (%) A. Very poor 0 – 25 B. Poor 25 – 50 C. Fair 50 - 75 D. Good 75 - 90 E. Excellent 90 - 100 Note: (i) Where RQD is measured as ≤ 10 (including 0), a nominal

value of 10 is used to evaluate Q (ii) RQD intervals of 5, i.e. 100, 95, 90 etc. are sufficiently

accurate

(2) (2) Joint set number (Joint set number (JJnn))::

The parameter The parameter JJnn, representing the number of joint sets, is often , representing the number of joint sets, is often affected by foliations, affected by foliations, schistocityschistocity or beddings, etc. If strongly or beddings, etc. If strongly developed, these parallel discontinuities should be counted as adeveloped, these parallel discontinuities should be counted as acomplete joint set. complete joint set.

If there are few joints visible or only occasional breaks in rocIf there are few joints visible or only occasional breaks in rock k core due to these features, then one should count them as core due to these features, then one should count them as ‘‘a a random joint setrandom joint set’’ while evaluating while evaluating JJnn from Table 2. Rating of from Table 2. Rating of JJnn is is approximately equal to square of the number of joint sets. approximately equal to square of the number of joint sets.

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Table 2: Joint set number Table 2: Joint set number JJnn (Barton et al., 1974)(Barton et al., 1974)

Condition Jn A. Masssive, none or few joints 0.5 – 1.0 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.

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I. Crushed rock earth like 20 Note: (i) For intersection use (3.0 × Jn)

(ii) For portals use (2.0 × Jn)

(3) (3) Joint Roughness & Joint Alteration Number (Joint Roughness & Joint Alteration Number (JJrr & & JJaa))::

The parameters The parameters JJrr & & JJaa, given in Table 3 & Table 4, respectively, , given in Table 3 & Table 4, respectively, represent roughness & degree of alteration of joint walls or filrepresent roughness & degree of alteration of joint walls or filling ling materials. materials.

The parameters The parameters JJrr & & JJaa, should be obtained for the weakest critical , should be obtained for the weakest critical jointjoint--set or clayset or clay--filled discontinuity in a given zone.filled discontinuity in a given zone.

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Table 3: Joint roughness number Table 3: Joint roughness number JJrr (Barton et al., 1974)(Barton et al., 1974)

Condition Jr (a) Rock wall contact, and (b) Rock wall contact before 10 cm shear

A. Discontinuous joint 4 B. Rough or irregular, undulating 3 C. Smooth, undulating 2.0 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 was sheared H. Zone containing clay minerals thick enough to prevent rock wall contact

1.0

I. Sandy, gravelly, or crushed zone thick enough to prevent rock wall contact

1.0

Note: (i) Add 1.0 if the mean spacing of the relevant joint set is

greater than 3.0 m. (ii) Jr = 0.5 can be used for planar, slickensided joint having

lineation, provided the lineations are favourably orientated. (iii) Description B to G above refer to small scale and

intermediate scale features, in that order.

Table 4: Joint alteration number Table 4: Joint alteration number JJaa (Barton et al., 1974)(Barton et al., 1974)

Condition φr (0) Ja (a) Rock wall contact A. Tightly healed, hard, non-softening, impermeable filling (e.g. quartz or epidote)

- 0.75

B. Unaltered joint walls, surface staining only 25 – 35 1.0 C. Slightly altered joint walls. Non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc.

25 – 30 2.0

D. Slickensided, undulating 20 – 25 3.0 E. Softening or low-friction clay mineral coatings (e.g. kaolinite, mica). Also chlorite, talc, gypsum & graphite etc., & small quantities of swelling clays (discontinuous coating 1 to 2 mm or less in thickness)

8 – 16 4.0

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Table 4: Joint alteration number Table 4: Joint alteration number JJaa ……..cont..cont

Condition φr (0) Ja (b) Rock wall contact before 10 cm shear F. Sandy particles, clay-free disintegrated rock, etc.

25 – 30 4.0

G. Strongly over-consolidated, non-softening clay mineral fillings (continuous, < 5 mm in thickness).

16 – 24 6.0

H. Medium or low over-consolidation, softening, clay mineral fillings (continuous, < 5 mm in thickness).

12 – 16 8.0

J. Swelling clay fillings, e.g. montmorillonite (continuous, < 5 mm in thickness). Value of Ja depends on the % of swelling clay-sized particles & access to water, etc.

6 – 12 8 – 12

Table 4: Joint alteration number Table 4: Joint alteration number JJaa ……cont.cont.

Condition φr (0) Ja (c) No rock wall contact when sheared K. Zones or bands of disintegrated or crushed rock & clay (see G, H, J for description of clay condition).

6 – 24 8 – 12

L. Zones or bands of silty or sandy clay, small clay fraction (non-softening).

- 5

M. Thick, continuous zones or bands of clay (see G, H, J) for description of clay condition).

6 – 24 13 – 20

Note: Values of φr are intended as an approximate guide to the mineralogical properties of the alteration products, if present.

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(3) (3) Joint Roughness & Joint Alteration Number (Joint Roughness & Joint Alteration Number (JJrr & & JJaa))::

If the joint set or discontinuity with the minimum value of (If the joint set or discontinuity with the minimum value of (JJrr / / JJaa) is ) is favourablyfavourably orientated for stability, then a second less orientated for stability, then a second less favourablyfavourablyorientated joint set or discontinuity may be of greater significorientated joint set or discontinuity may be of greater significance, ance, and its value of (and its value of (JJrr / / JJaa) should be used when evaluating Q value from ) should be used when evaluating Q value from equation: equation:

Q = [RQD/Q = [RQD/JJnn] ] ×× [[JJrr/J/Jaa] ] ×× [[JJww/SRF]/SRF]

For the effect of the joint sets, Table 7 (in RMR system) may beFor the effect of the joint sets, Table 7 (in RMR system) may bereferred.referred.

Table 7 (RMR): Assessment of joint orientation effect Table 7 (RMR): Assessment of joint orientation effect on tunnels (dips are apparent dips along tunnel axis) on tunnels (dips are apparent dips along tunnel axis)

((BieniawskiBieniawski, 1989), 1989)

Strike perpendicular to tunnel Drive with dip Drive against dip

Strike parallel to tunnel axis

Irrespective of strike

Dip 450 - 900 Dip 200 - 450 Dip 450 - 900 Dip 200 - 450 Dip 200 - 450 Dip 450 - 900 Dip 00 - 200 Very favourable

Favourable Fair Unfavoura-ble

Fair Very un- favourable

Fair

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(4) (4) Joint Water Reduction Factor (Joint Water Reduction Factor (JJww ))::

The parameter The parameter JJww (Table 5) is a measure of water pressure, which (Table 5) is a measure of water pressure, which has an adverse effect on the shear strength of joints. This is dhas an adverse effect on the shear strength of joints. This is due to ue to reduction in the effective normal stress acting across joint surreduction in the effective normal stress acting across joint surface. face.

Water in addition may cause softening & possible washWater in addition may cause softening & possible wash--out in the out in the case of claycase of clay--filled joints.filled joints.

Water may also acts as lubricant (reducing shear strength) and Water may also acts as lubricant (reducing shear strength) and causes swelling of causes swelling of montmorillonitemontmorillonite..

Table 5: Joint water reduction factor Table 5: Joint water reduction factor JJww (Barton et (Barton et al., 1974)al., 1974)

Condition Approx. water press., MPa

Jw

A. Dry excavations or minor inflow, i.e. 5 lit/min locally.

< 0.1 1

B. Medium inflow or pressure occasional out-wash of joint filings.

0.1 – 0.25 0.66

C. Large inflow or high pressure in competent rock with unfilled joints.

0.25 – 1.0 0.5

D. Large inflow or high pressure, considerable out-wash of joint filings.

0.25 – 1.0 0.33

E. Exceptionally high inflow or water pressure at blasting, decaying with time.

> 1.0 0.2 – 0.1

F. Exceptionally high inflow or water pressure continuing without noticeable decay.

> 1.0 0.1 – 0.05

Note: (i) Factors C to F are crude estimates. Increase Jw if drainage

measures are installed.

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(5) (5) Stress Reduction Factor (SRF)Stress Reduction Factor (SRF)::

The parameter SRF (Table 6) is a measure of the followings:The parameter SRF (Table 6) is a measure of the followings:

i) Loosening pressure in the case of an excavation through sheai) Loosening pressure in the case of an excavation through shear r zones & clay bearing rock masses.zones & clay bearing rock masses.ii) Rock stress qii) Rock stress qcc//σσ11 in a competent rock mass where in a competent rock mass where qqcc is is uniaxialuniaxialcompressive strength of rock material & compressive strength of rock material & σσ11 is the major principal is the major principal stress before excavation.stress before excavation.iii) Squeezing or swelling pressures in incompetent rock massesiii) Squeezing or swelling pressures in incompetent rock masses, & , & SRF can also be regarded as a total stress parameterSRF can also be regarded as a total stress parameter. .

Table 6: Stress Reduction Factor SRF (Barton et al., 1974; Table 6: Stress Reduction Factor SRF (Barton et al., 1974; GrimstadGrimstad & Barton, 1993)& Barton, 1993)

Condition SRF (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

B. Single-weakness zones containing clay or chemically decomposed rock (depth of excvation ≤ 50 m).

5.0

C. Single-weakness zones containing clay or chemically decomposed rock (depth of excvation > 50 m).

2.5

D. Multiple-shear zones in competent rock (clay-free), loose surrounding rock (any depth).

7.5

E. Single-shear zones in competent rock (clay-free) (depth of excvation ≤ 50 m).

5.0

F. Single-shear zones in competent rock (clay-free) (depth of excvation > 50 m).

2.5

G. Loose open joints, heavily jointed or ‘sugar cube’, etc. (any depth)

5.0

Note: Reduce these SRF values by 25-50 % if the relevant shear zones only influence but do not intersect the excavation.

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Table 6: Stress Reduction Factor SRF Table 6: Stress Reduction Factor SRF …….. cont.. cont

Condition qc/σ1 qt/σ1 SRF (old)

SRF (new)

(b) Competent rock, rock stress problems H. Low stress, near surface open joints.

> 200 < 0.01 2.5 2.5

J. Medium stress, favourable stress condition.

200-10 0.01-0.3 1.0 1.0

K. High stress, very tight structure (usually favourable to stability, may be unfavourable to wall stability.

10-5 0.3-0.4 0.5-2 0.5-2

L. Moderate slabbing after > 1 hour in massive rock

5-3 0.5-0.65 5-9 5-50

M. Slabbing & rock burst after a few minutes in masssive rock.

3-2 0.65-1.0 9-15 50-200

N. Heavily rock burst (strain-burst) & immediate deformations in massive rock.

< 2 > 1 15-20 200-400

Note: (i) For strongly anisotropic stress field (if measured): when

5≤(σ1/σ3)≤10, reduce qc & qt to 0.8qc and 0.8qt; when (σ1/σ3)>10, reduce qc & qt to 0.6qc and 0.6qt; (where qc is unconfined compressive stress & qt is tensile strength (point-load), σ1 and σ3 are major & minor principal stress).

(ii) 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).

Table 6: Stress Reduction Factor SRF Table 6: Stress Reduction Factor SRF …….. cont.. cont

Condition SRF (c) Squeezing rock; plastic flow of imcompetent rock under the influence of high pressures. O. Mild squeezing rock pressure. 5 – 10 P. Heavy squeezing rock pressure. 10 – 20 (d) Swelling rock; chemically swelling activity depending on presence of water. Q. Mild swelling rock pressure. 5 – 10 R. Heavy swelling rock pressure. 10 – 15 Note: (i) Reduce these SRF values by 25-50% if the relevant shear

zones only influence but do not intersect the excavation. (ii) For getting the rating of SRF in case of squeezing ground

condition, the degree of squeezing can be obtained from Table 7.5

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QQ--systemsystem::

Ratings of all the 6 parameters (as given in Table 1 to Table 6)Ratings of all the 6 parameters (as given in Table 1 to Table 6) for for given rock mass are substituted in the equation to get the rock given rock mass are substituted in the equation to get the rock mass quality: mass quality:

Q =Q = [RQD/[RQD/JJnn]] ×× [[JJrr/J/Jaa]] ×× [[JJww/SRF]/SRF]

QQ--systemsystem::

The QThe Q--system may be considered a function of only 3 parameters system may be considered a function of only 3 parameters which are approximate measures of:which are approximate measures of:

Q = Q = [RQD/[RQD/JJnn]] ×× [[JJrr/J/Jaa]] ×× [[JJww/SRF]/SRF]

Block size [RQD/Block size [RQD/JJnn]] : It represents overall structure of rock mass. : It represents overall structure of rock mass.

InterblockInterblock shear strength [shear strength [JJrr/J/Jaa]] : It has been found that tan: It has been found that tan--1 1 [J[Jrr/J/Jaa] is ] is a fair approximation to the actual peak sliding angle of frictioa fair approximation to the actual peak sliding angle of friction n along the clay coated joint (along the clay coated joint (see Table 7see Table 7). ).

Active stress [Active stress [JJww/SRF]/SRF] : : It is an factor describing the active stress.It is an factor describing the active stress.

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Table 7: Estimation of angle of internal friction from the Table 7: Estimation of angle of internal friction from the parameters parameters JJrr and and JJaa (Barton et al., 1974)(Barton et al., 1974)

Description tan-1(Jr/Ja) (a) Rock wall contact Jr Ja = 0.75 Ja = 1.0 Ja = 2.0 Ja = 3.0 Ja = 4.0 A. Discontinuous joint. 4.0 790 760 630 530 450 B. Rough, undulating. 3.0 700 720 560 450 370 C. Smooth, undulating. 2.0 690 630 450 340 270 D. Slickensided, undulating. 1.5 630 560 370 270 210 E. Rough, planar. 1.5 630 560 370 270 210 F. Slickensided, planar. 0.5 340 270 140 9.50 7.10 (b) Rock wall contact when sheared Jr Ja = 4.0 Ja = 6 Ja = 8 Ja = 12 A. Discontinuous joint. 4.0 450 340 270 180 B. Rough, undulating. 3.0 370 270 210 140 C. Smooth, undulating. 2.0 270 180 140 9.50 D. Slickensided, undulating. 1.5 210 140 110 7.10 E. Rough, planar. 1.5 2.10 140 110 7.10 F. Slickensided, planar. 0.5 70 4.70 3.60 2.40 (c) No rock wall contact when sheared Jr Ja = 6 Ja = 8 Ja = 12 Disintegrated or crushed rock or clay. 1.0 9.50 7.10 4.70 Jr Ja = 5 Bands of silty or sandy clay. 1.0 110 Jr Ja = 10 Ja = 13 Ja = 20 Thick continuous bands of clay. 1.0 5.70 4.40 2.90

QQ--system, Q system, Q == [RQD/[RQD/JJnn]] ×× [[JJrr/J/Jaa]] ×× [[JJww/SRF] /SRF]

The first quotient The first quotient [RQD/[RQD/JJnn] ] represents the rock structure & is a represents the rock structure & is a measure of block size of the wedge measure of block size of the wedge fromedfromed by the presence of by the presence of different joint sets. In a given rock mass, the rating of the different joint sets. In a given rock mass, the rating of the parameter parameter JJnn could increase with the tunnel size in certain could increase with the tunnel size in certain situations where additional joint sets are encountered. situations where additional joint sets are encountered.

Hence it is not advisable to use QHence it is not advisable to use Q--value obtained from a small drift value obtained from a small drift to estimate the support pressure for a large tunnel or a cavern.to estimate the support pressure for a large tunnel or a cavern.

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QQ--system, Q system, Q == [RQD/[RQD/JJnn]] ×× [[JJrr/J/Jaa]] ×× [[JJww/SRF] /SRF]

The second quotient The second quotient [[JJrr/J/Jaa] ] represents the roughness & frictional represents the roughness & frictional characteristics of joint walls or filling materials. characteristics of joint walls or filling materials.

It should be noted that value of It should be noted that value of JJrr/J/Jaa is collected for the critical joint is collected for the critical joint set, i.e. the joint set which is most set, i.e. the joint set which is most unfavourableunfavourable for stability of a key for stability of a key rock block.rock block.

QQ--system, Q system, Q == [RQD/[RQD/JJnn]] ×× [[JJrr/J/Jaa]] ×× [[JJww/SRF] /SRF]

The third quotient The third quotient [[JJww/SRF] /SRF] is an empirical factor describing is an empirical factor describing ‘‘active active stress conditionstress condition’’. The stress reduction factor SRF, is a measure of . The stress reduction factor SRF, is a measure of the the 3 items mentioned previously3 items mentioned previously..

The water reduction factor The water reduction factor JJww is measure of water pressure, which is measure of water pressure, which has an adverse effect on the strength of joints due to reductionhas an adverse effect on the strength of joints due to reduction in in effective normal stress. In the hydroelectric projects where roceffective normal stress. In the hydroelectric projects where rock k masses get charged with water after masses get charged with water after commisioningcommisioning of projects, of projects, JJww

should be reduced accordingly on the basis of should be reduced accordingly on the basis of judgementjudgement, while , while using Q for estimating the final support requirements. using Q for estimating the final support requirements.

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Q = [RQD/Q = [RQD/JJnn] x [] x [JJrr/J/Jaa] x [] x [JJww/SRF]/SRF]JJnn= Joint set number= Joint set numberJJrr= Joint surface roughness= Joint surface roughnessJJaa= Joint alteration number due to weathering = Joint alteration number due to weathering JJww= Joint reduction factor due to water= Joint reduction factor due to waterSRF= Stress reduction factorSRF= Stress reduction factor

Which rock mass will yield a higher QWhich rock mass will yield a higher Q--value ?value ?-- rock with 2 or 4 joint sets (rock with 2 or 4 joint sets (JJnn))-- rock with fresh or weathered joints (rock with fresh or weathered joints (JJaa))-- rock with higher RQD or lower RQDrock with higher RQD or lower RQD-- rock with smooth or rough joints (rock with smooth or rough joints (JJrr))-- rock with dry or wet joints (SRF)rock with dry or wet joints (SRF)

HOW QHOW Q--SYSTEM EVALUATES A ROCK MASS ?SYSTEM EVALUATES A ROCK MASS ?