lecture6(rockmassclassification)

8/13/2019 Lecture6(RockMassClassification)

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Lecture 6

Rock Mass Classification


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Terzaghi's rock mass classification (1946)

Rock loads (gravity being driving force), carried by steel sets, are estimated on the

basis of a descriptive classification.

Intact rock contains neither joints nor hair cracks. Hence, if it breaks, it breaks across sound

rock. On account of the injury to the rock due to blasting, spalls may drop off the roof

several hours or days after blasting. This is known as aspallingcondition.

Hard, intact rock may also be encountered in thepopping condition involving the

spontaneous and violent detachment of rock slabs from the sides or roof.

Stratified rock consists of individual strata with little or no resistance against separationalong the boundaries between the strata. The strata may or may not be weakened by

transverse joints. In such rock the spalling condition is quite common.

Moderately jointed rock contains joints and hair cracks, but the blocks between joints are

locally grown together or so intimately interlocked that vertical walls do not require lateral

support. In rocks of this type, both spalling and popping conditions may be encountered.Blocky and seamy rock consists of chemically intact or almost intact rock fragments which

are entirely separated from each other and imperfectly interlocked. In such rock, vertical

walls may require lateral support.

Crushedbut chemically intact rock has the character of crusher run. If most or all of the

fragments are as small as fine sand grains and no recementation has taken place, crushedrock below the water table exhibits the properties of a water-bearing sand.

Squeezing rock slowly advances into the tunnel without perceptible volume increase. A

prerequisite for squeeze is a high percentage of microscopic and sub-microscopic particles

of micaceous minerals or clay minerals with a low swelling capacity.

Swelling rock advances into the tunnel chiefly on account of expansion. The capacity to

swell seems to be limited to those rocks that contain clay minerals such as montmorillonite,

with a high swelling capacity.


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Concept of Stand-up Time


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Rock quality designation index (RQD) (Deere et al 1967)

RQD is defined as the percentage of intact core pieces longer than 100 mm (4 inches) in the

total length of core. The core should be at least NW size (54.7 mm or 2.15 inches in diameter)

and should be drilled with a double-tube core barrel.


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RQD = 115 3.3Jv Palmstrm (1982), when no core is available

Jv is the sum of the number of joints per unit length for all

joint (discontinuity) sets known as the volumetric joint count.


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Rock Structure Rating (RSR) Wickham et al (1972)

RSR =A +B + C.

1.Parameter A, Geology: General appraisal of geological structure on the basisof:

a. Rock type origin (igneous, metamorphic, sedimentary).

b. Rock hardness (hard, medium, soft, decomposed).

c. Geologic structure (massive, slightly faulted/folded, moderatelyfaulted/folded, intensely faulted/folded).

2.Parameter B, Geometry: Effect of discontinuity pattern with respect to the

direction of the tunnel drive on the basis of:

a. Joint spacing.

b. Joint orientation (strike and dip).

c. Direction of tunnel drive.

3.Parameter C: Effect of groundwater inflow and joint condition on the basis of:

a. Overall rock mass quality on the basis of A and B combined.

b. Joint condition (good, fair, poor).

c. Amount of water inflow (in gallons per minute per 1000 feet of tunnel).

Favorable

Unfavorable


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Geomechanics Classification or the Rock Mass Rating (RMR) system. Bieniawski (1976)

Also known as the South African Council for Science and Industrial Research (CSIR) system

Six parameters are used to classify a rock mass using theRMR

system:

1. Uniaxial compressive strength of rock material.

2. Rock Quality Designation (RQD).

3. Spacing of discontinuities.

4. Condition of discontinuities.

5. Groundwater conditions.

6. Orientation of discontinuities.

Ratings are summed to give a value ofRMR.


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Bieniawski (1989) published a set of guidelines for the selection of support in tunnels

Guidelines have been published for a 10 m span horseshoe shaped tunnel, constructed using drill and blast methods, in a

rock mass subjected to a vertical stress < 25 MPa (equivalent to a depth below surface of


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Estimate of the support load (kN) = B100

RMR100P

= Unal (1983)

B = tunnel width (m)

= rock density (kg/m3

)

50)2(RMRE = Bieniawski (1978)

E = modulus of deformation of rock mass


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Rock Tunnelling Quality Index, Q, Barton et al (1974) of the NGI

Q varies on a logarithmic scale from 0.001 to a maximum of 1,000 and is defined by:

SRF

Jx

J

Jx

J

RQDQ w

a

r

n

=

where,

RQD Rock Quality Designation

Jn Joint Set Number

Jr Joint Roughness NumberJa Joint Alternation Number

Jw Joint Water Reduction Factor

SRF Stress Reduction Factor

R k T lli Q li I d Q B l (1974) f h NGI


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The first quotient (RQD/Jn) represents the structure of the rock mass or a

measure of the rock block size.

The second quotient (Jr/Ja) represents the roughness and frictional

characteristics of the joint walls and filling materials.

The third quotient (Jw/SRF) represents the total stress state of the rock mass

which is affected by the presence of weaknesses and water inflow in the joints.



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Rock Tunnelling Quality Index Q Barton et al (1974) of the NGI


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Barton et al (1974) defined an additional parameter called the

Equivalent Dimension,De, of the excavation. This dimension isobtained by dividing the span, diameter or wall height of the

excavation by a quantity called theExcavation Support Ratio,ESR.



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g Q y , Q, ( )


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Barton (1989)


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Barton et al (1980) provide additional information on rockbolt length, maximum unsupported spans and roof

support pressures to supplement the support recommendations

The length L of rockbolts can be estimated from the excavation widthB and the Excavation Support

RatioESR:

The maximum unsupported span can be estimated from:

Grimstad and Barton (1993) suggest that the relationship between the value ofQ and the permanent roof

support pressureProof is estimated from:

Correlation between Q and RMR systems:

44Q9lRMRn +=


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The use of at least two rock massclassification schemes is advisable.

Estimation of TBM Advance Rate and Penetration Rate based on modified Q


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Q

index (QTBM) by Barton (1999)

Penetration Rate (PR) : Rate of cutting and breaking the rock into fragment bythe cutter head of TBM (m/hr)

Advance Rate (AR) : Rate of advancing the TBM that include changing cutter

head, gripper advance, support installed and much removal etc. (m/hr)

Extremely weak rock (squeezing rock, clay, fault zone etc. where Q=0.001)

require extensive support and slow down TBM advance

Extremely sound rock (unjointed hard massive rock where Q=1000) require

frequent cutter head changes, also unfavorable to TBM advance

Barton (1999), based on case records, back calculate and modify the Q

index (call QTBM) for predicting the PR and AR taking into account of:

Average cutter force

Rock mass strength

Orientation of fabric or joint structure in the direction of tunnel

Compressive and point load strength index of the rock

Cutter life index

Rock stress level


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x

qx

20x

SIGMAx

Jx

Jx

RQDQ wr0=


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5x

20x

CLIx

/20Fx

SRFx

Jx

JQ

910

an

TBM=

RQD0 = RQD in the tunnelling direction

Jn, Jr, Ja, Jw and SRF same as those used in the Q index

F = average cutter load (ton force)

SIGMA = rock mass strength (MPa)take the lesser of SIGMACM (compression) or SIGMATM(tension) where,

c = uniaxial compressive strength = density (gm/cm3)

I50 = point load strength index

CLI = cutter life index (e.g. 4 for quartzite, 90 for limestone)

q = quartz content in percentage terms

= induced biaxial stress on tunnel face (MPa)

3

1

503

1

tTM

3

1

c3

1

cCM

4

I5Q5SIGMA

100

Q5Q5SIGMA

==

==

Based on 145 Tunnels (total length more than 1000 km) in hard

rock soft rock faulted rock etc the following relationship is


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rock, soft rock, faulted rock etc., the following relationship is

empirically established:

0.25me,performancexcep.poor

0.21me,performancpoor

0.19me,performancfair0.17me,performancgood

0.17to0.13me,performancbest

hourintimeTTxPRAR

m

=

=

==

=

==

m is further correlated to cutter wear (cutter life index CLI, abrasiveness of rock),

quartz content (q), porosity (n) of rock, tunnel diameter (D), support needed

0.050.100.150.20

12n

20q

CLI20

5Dmm

-0.21-0.19-0.17-0.22-0.5-0.7-0.9m1

10001001010.10.010.001Q

where, m1 is given below

Approximate values of m in relation to Q

Finally use with caution the following prediction:


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( )

( )

m

PR

LT

+

=

1

1

m0.2

TBM

0.2

TBM

LlengthwithtunnelpenetratetoTTime

TQ5AR

Q5PR

Engineering Rock Mass Classification in China


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(China Planning Publication No. GB 50218-94)Basic Quality (BQ) of a Rock Mass

BQ = 90 +3Rc+250Kv

where,

Rc = Uniaxial compressive strength of intact rock (in MPa)

2

pr

pm

V

V

Kv = Intactness index of a rock mass =

Vpm = Velocity of longitudinal elastic wave in rock mass (km/s)

Vpr = Velocity of longitudinal elastic wave in intact rock (km/s)



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Table 1 Strength Description of a Rock Mass based on Uniaxial Compresssive Strength Rc

Rc (MPa) > 60 60 30 30 15 15 5 < 5

Strength

Description

Hard

Rock

Relatively

Hard Rock

Relatively

Weak Rock

Weak

Rock

Extremely

Weak Rock


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Basic Quality (BQ) of a Rock Mass (Amended so as to consider stress conditions,

groundwater and joint orientation

[BQ] = BQ - 100 (K1+ K2+ K3)

BQ = Basic Quality from Equation above

K1 = Correction Factor for Groundwater Conditions

K2 = Correction Factor for Joint Plane Orientation

K3 = Correction Factor for In-situ Stress Conditions

[BQ] = Amended Basic Quality Value



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Host Rock Rating (HRR) for Underground Excavation Projects in China


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A national standard that has been specifically designed for underground excavation related to

water resources and hydropower projects in China (Ministry of Water Resources and Ministry ofElectric Power)

HRR = A + B + C + D + E

where,

A = Rating Factor related to Rock Strength (provided in Table 5)

B = Rating Factor related to Rock Intactness (provided in Table 6)

C = Rating Factor related to Joint Conditions (provided in Table 7)D = Rating Factor related to Groundwater Conditions (provided in Table 8)

E = Rating Factor related to Joint Plane Orientation (provided in Table 9)


Table 5 Rating Factor A related to Rock Strength


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Table 5 Rating Factor A related to Rock Strength

Description of Rock Strength

Hard Rock Moderately HardRock

Relatively WeakRock

Weak Rock

Uniaxial

Compressive

Strength of

Saturated Rock

(MPa)

100 60 60 30 30 15 15 5

Rating Factor A 30 20 20 10 10 5 5 0

For Uniaxial Compressive Strength Rc > 100 MPa, Rating Factor A is 30


T bl 6 R i F B l d R k I


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Table 6 Rating Factor B related to Rock Intactness

Description of Intactness

Intact Relatively

Intact

Poor

Intactness

Relatively

Fractured

Fractured

Intactness

Factor Kv

1.0 0.75 0.75 0.55 0.55

0.35

0.35 0.15 < 0.15

Rating

Factor

B

Hard

Rock

40 30 30 22 22 14 14 6 < 6

Weak

Rock

25 19 19 14 14 9 9 4 < 4



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Table 8 Rating Factor D related to Groundwater Conditions


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Table 8 Rating Factor D related to Groundwater Conditions

Sum of Rating

Factors

(A + B + C)

State Wet, Dripping Small inflow Large Inflow

Flow Rate

(l/min/m)

or

Water Head (m)

< 25

or

< 10

25 125

or

10 - 100

> 125

or

> 100

100 - 85 Rating Factor (D) 0 0 -2 to -6

85 - 65 0 to -2 0 to -2 -6 to -10

65 - 45 -2 to -6 -2 to -6 -10 to -14

45 - 25 -6 to -10 -10 to -14 -14 to -18

< 25 -10 to -14 -14 to -18 -18 to -20



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Design Applications of HRR

E(x 104MPa)C (MPa)f

Support

Requirements

Rock PropertiesStrength vs

Stress Ratio

(S)

Host

Rock

Rating

Rock Mass StabilityRock

Type


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E(x 10 MPa)C (MPa)f

0.2 - 0.020.3 - 0.050.55 - 0.40< 25Very unstable, could not stand

up; severe deformation/failure

V

Bolts and shotcrete,

with re-bar mesh

and concrete lining

0.5 - 0.20.7 - 0.30.8 - 0.55Downgrade

to V for S

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