Tunnelling and Underground Construction Technology

2015.04.10 1Tunnelling and Underground

Construction Technology

Tunnelling and Underground


Course Lectures

Part 3.1 – Tunnel support design based on

Rock mass Classification methods

Dr Ákos TÓTH

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Rock Mass Property and Classification

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Principal Geometrical Characteristics of Rock Joints

Number of joint sets

Joint persistence

Joint plane orientation

Joint spacing, joint frequency, block size, and RQD

Joint surface roughness and matching

Joint aperture and filling

Geometrical Properties of Rock Joints

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Number of Joint Sets

Joints are generally in sets, i.e.,

parallel joints. The number of joint

sets can be up to 5. Typically one

joint set cuts the rock mass into

plates, two perpendicular sets cut

rock into column and three into

blocks, and more sets cut rocks

into mixed shapes of blocks and

wedges.


1 set

3 sets

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I Massive, occasional random fractures

II One joint set

III One joint set plus random fractures

IV Two joint sets

V Two joint sets plus random fractures

VI Three joint sets

VII Three joint sets plus random fractures

VIII Four or more joint sets

IX Crushed rock, earth-like

ISRM suggested descriptionMechanical

properties of the

rock mass is

influenced by

joint sets. More

joint sets provide

more

possibilities of

potential slide

planes .

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Joint Persistence

Persistence is the areal extent or length of a

discontinuity, and can be crudely quantified by

observing the trace lengths of discontinuities on

exposed surfaces. The persistence of joint sets

controls large scale sliding or 'down-stepping'

failure of slope, dam foundation and tunnel

excavation.


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ISRM Suggested Description Surface Trace Length (m)

Very low persistence < 1

Low persistence 1 – 3

Medium persistence 3 – 10

High persistence 10 – 20

Very high persistence > 20

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Joint Plane Orientation

Orientation of joint sets controls

the possibility of unstable

conditions or excessive

deformations. The mutual

orientation of joints determines

the shape of the rock blocks.

Orientation is defined by dip angle

(inclination) and dip direction

(facing) or strike (running).


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Dip angle

Dip direction

StrikeN

N

Horizontal planeOrientation:

Dip direction / Dip

220/55Measured clockwise on horizontal plane: 220

Measured on vertical plane: 55

Vertical plane

Line of maximum dip

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A geological compass to

measure dip and direction of

joint plane

An electronic geological

compass

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Dip direction and strike direction are always

perpendicular. Dip direction/dip format is generally

used, e.g., 210/35, or 030/35. Sometimes, strike/dip

format is used, e.g., 120/35SW (=dip direction/dip

210/35), or 120/35NE (=dip direction/dip 030/35).

Normal (pole) to the plane is perpendicular to the

plane. Orientation of the normal is given by:

trend of normal = dip direction of the plane 180,

plunge of normal = 90 – dip.


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Orientation of a joint plane can be represented

graphically using hemispherical projection method.

The projection method is to represent a 3D plane by

a 2D presentation. Use the projection, joint

orientation data can be assessed in 2D form. It can

be used to analyse large number of joint data and

examine the rock slope stability and slide of rock

block in underground excavation.


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Spherical projections plot and analyze joint orientation data

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Plotting and analysis of joint orientation data performed by computer

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Joint Spacing, Frequency, Block Size, and RQD

Fracturing degree of a rock mass is controlled by

the number of joint in the rock mass. More joints

mean that less average spacing between joints.

Joint spacing controls the size of individual rock

blocks. It controls the mode of failure and flow. For

example, a close spacing gives low mass cohesion

and circular or even flow failure.


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Joint Spacing

Joint spacing is the perpendicular distance between

joints. For a joint set, is usually expressed as the

mean spacing of that joint set. Often the apparent

spacing is measured.

Measurements of joint spacing are different on

different measuring faces and directions. For

example, in a rock mass with mainly vertical joints,

measurements in vertical direction have far greater

spacing then that in horizontal direction.


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Apparent spacing in x, y and z directions

True spacingApparent spacingon the plane

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Description Joint Spacing (m)

Extremely close spacing < 0.02

Very close spacing 0.02 – 0.06

Close spacing 0.06 – 0.2

Moderate spacing 0.2 – 0.6

Wide spacing 0.6 – 2

Very wide spacing 2 – 6

Extremely wide spacing > 6

Classification of joint spacing

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Joint Frequency

Joint frequency (), is defined as number of joint per

metre length. It is therefore simply the inverse of

joint spacing (sj), i.e.,

= 1 / sj


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RQD

Rock Quality Designation (RQD) is defined as the

percentage of rock cores that have length equal or

greater than 10 cm over the total drill length.

RQD = Li / L x 100%, Li > 10 cm

RQD = (L1 + L2 + … + Ln) / L x 100%


<10 cm <10 cm <10 cm core loss

X X XX XL1 L2 L3 L4 L5 LnLi

L

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RQD = (L1 + L2 + … + Ln) / L x 100%

= number of joints / length = n / L


X X XX XL1 L2 L3 L4 L5 LnLi

<10 cm <10 cm <10 cm fault

L

Outcrop Face

Tape1 2 i n

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RQD can be correlated to joint frequency ():

RQD = 100 (0.1 + 1) e–0.1

For = 6 and 16/m, it can be approximated by:

RQD = 110.4 – 3.68

RQD was initially proposed as an attempt to

describe rock quality, in reality, it only describes

fracturing degree, but not other properties such as

joint alteration, groundwater and rock strength.


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Block Size and Volumetric Joint Count

Joint space also defines the size of rock blocks.

When a rock mass contains more joints numbers,

the joints have lower average spacing and smaller

block size.

RQD can be related to volumetric joint count Jv by:

RQD = 115 – 3.3 Jv, for Jv between 4.5 and 30.

Jv < 4.5, RQD = 100%, Jv > 30, RQD = 0%.


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Designation Volumetric Joint Count, joints/m3

Very large blocks < 1

Large blocks 1 – 3

Medium-sized blocks 3 – 10

Small blocks 10 – 30

Very small blocks > 30

Crushed rock > 60

ISRM suggested block size designations

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Joint Surface Roughness and Matching

A joint is an interface of two contacting surfaces.

The surfaces can be smooth or rough; they can be in

good contact and matched, or they can be poorly

contacted and mismatched.


The condition of contact

also governs the aperture

of the interface. The

interface can be filled with

intrusive or weathered

materials.

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Joint Roughness

Joint surface roughness is a measure of surface

unevenness and waviness relative to its mean plane.

The roughness is characterised by large scale

waviness (undulation) and small scale unevenness

(irregularity) of a joint surface. It is the principal

governing factor the direction of shear displacement

and shear strength, and in turn, the stability of

potentially sliding blocks.


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Joint Roughness

Roughness should first be described in metre scale

(step, undulating, and planar) and then in centimetre

scale (rough, smooth, and slickensided), as

suggested by ISRM. It is not a quantitative measure.

Joint Roughness Coefficient (JRC) is a quantitative

measure of roughness, varying from 0 for the

smooth flat surface to 20 for the very rough surface.

Joint roughness is affected by geometrical scale.


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JRC number is

obtained by directly

comparing the

actual joint surface

profile with the

typical profile in the

chart.

JRC20 is the profile

for 20 cm and

JRC100 for 100 cm.

The value of JRC

decreases with

increasing size.

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Joint Roughness in 3D

In realty, profiles of joint surfaces are 3D features.

ISRM and JRC descriptions are 2D based. It is

therefore suggested to take several linear profiles of

a surface for the description and JRC indexing.

Joint surface is a rough profile that can be described

by statistic method and fractal. Fractal method is

applicable not only in 2D (linear profile), but also in

3D (surface plane profile). It is a useful tool to

quantify the surface profile.


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A joint surfaces is 3D. Each

2D measurement may give

defferent linear profiles.

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A joint surfaces in 3D, also

noting change of linear

profiles in directions.

Calculating fractal for a 3D

surface profile.

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Joint Matching

A joint is an interface of two surfaces. Properties of

a joint are also controlled by the relative positioning

of the two surfaces, in addition to the profiles. For

example, joints in fully contacted and interlocked

positions has little possibility of movement and is

also difficult to shear, as compared to the same

rough joints in point contact where movement can

easily occur. Often, joints are differentiated as

matched and mismatched. A Joint Matching

Coefficient (JMC) has been suggested.


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JMC is 1 for completely matched

joint and two surfaces fully in

contact.

JMC is 0 for completely

mismatched joint and two

surfaces in contact at a few points

only.

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Joint Aperture and Filling

In a natural joint, it is very seldom that the two

surfaces are in complete contact. There usually

exists an opening or a gap between the two

surfaces. The perpendicular distance separating

the adjacent rock walls is termed as aperture. Joint

opening is either filled with air and water (open joint)

or with infill materials (filled joint). Open or filled

joints with large apertures have low shear strength.

Aperture also associates with flow and permeability.


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Aperture Description

< 0.1 mm Very tight

"Closed feature"0.1 ~ 0.25 mm Tight

0.25 ~ 0.5 mm Partly open

0.5 ~ 2.5 mm Open "Gapped

Feature"2.5 ~ 10 mm Widely open

1 ~ 10 cm Very widely open

"Open feature"10 ~ 100 cm Extremely widely open

> 1 m Cavernous

Classification of discontinuity aperture

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Joint Aperture and Filling

Aperture can be the real aperture and equivalent

hydraulic aperture. The later is particularly

important when permeability is concerned.

Filling is material in the rock discontinuities

separating the adjacent rock surfaces. In general,

properties of the filling material affect shear

strength, deformability and permeability of the

discontinuities.


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Rock Mass Properties

Rock mass is a matrix consisting of rock material

and rock discontinuities. Properties of rock mass

therefore are governed by the parameters of rock

joints and rock material, as well as boundary

conditions.

The behaviour of rock changes from continuous

elastic for intact rock materials to discontinues

running of highly fractured rock masses, depending

mainly on the existence of rock joints.


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Point Load Index

Point load test is a simple

index test for rock material.

It gives the standard point

load index, Is(50).

Strength and Deformation

Granite 5 – 15

Gabbro 6 – 15

Andesite 10 – 15

Basalt 9 – 15

Sandstone 1 – 8

Mudstone 0.1 – 6

Limestone 3 – 7

Gneiss 5 – 15

Schist 5 – 10

Slate 1 – 9

Marble 4 – 12

Quartzite 5 – 15

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Correlation between Point Load Index and Strengths

c 22 Is(50)

Correction factor can vary between 10 and 30.

t 1.25 Is(50)

Is(50) should be used as an independent strength

index.

Strength and Deformation

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Prime parameters governing rock mass property


Joint Parameters Material

Parameters

Boundary Conditions

Number of joint sets

Orientation

Spacing

Aperture

Surface roughness

Weathering and

alteration

Compressive

strength

Modulus of

elasticity

Groundwater

pressure and flow

In situ stress

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Rock Mass Clasification

Rock Load Factor

It classifies rock mass

into 9 classes. The

concept used in this

classification system is to

estimate the rock load to

be carried by the steel

arches installed to

support a tunnel.


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Rock Class DefinitionRock Load Factor Hp

(feet) (B and Ht in feet)Remark

I. Hard and intact

Hard and intact rock contains no joints and fractures. After

excavation the rock may have popping and spalling at

excavated face.

0Light lining required only if

spalling or popping occurs.

II. Hard stratified

and schistose

Hard rock consists of thick strata and layers. Interface

between strata is cemented. Popping and spalling at

excavated face is common.

0 to 0.5 B

Light support for protection

against spalling. Load may

change between layers.

III. Massive,

moderately jointed

Massive rock contains widely spaced joints and fractures.

Block size is large. Joints are interlocked. Vertical walls do

not require support. Spalling may occur.

0 to 0.25 BLight support for protection

against spalling.

IV. Moderately

blocky and seamy

Rock contains moderately spaced joints. Rock is not

chemically weathered and altered. Joints are not well

interlocked and have small apertures. Vertical walls do not

require support. Spalling may occur.

0.25 B to 0.35 (B + Ht) No side pressure.

V. Very blocky

and seamy

Rock is not chemically weathered, and contains closely

spaced joints. Joints have large apertures and appear

separated. Vertical walls need support.

(0.35 to 1.1) (B + Ht) Little or no side pressure.

VI. Completely

crushed but

chemically intact

Rock is not chemically weathered, and highly fractured with

small fragments. The fragments are loose and not

interlocked. Excavation face in this material needs

considerable support.

1.1 (B + Ht)

Considerable side pressure.

Softening effects by water at

tunnel base. Use circular ribs or

support rib lower end.

VII. Squeezing

rock at moderate

depth

Rock slowly advances into the tunnel without perceptible

increase in volume. Moderate depth is considered as 150 ~

1000 m.

(1.1 to 2.1) (B + Ht)Heavy side pressure. Invert

struts required. Circular ribs

recommended.VIII. Squeezing

rock at great

depth

Rock slowly advances into the tunnel without perceptible

increase in volume. Great depth is considered as more than

1000 m.

(2.1 to 4.5) (B + Ht)

IX. Swelling rock

Rock volume expands (and advances into the tunnel) due to

swelling of clay minerals in the rock at the presence of

moisture.

up to 250 feet,

irrespective of B and Ht

Circular ribs required. In extreme

cases use yielding support.

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Comments on the Rock Load Factor Classification

(a) It provides reasonable support pressure

estimates for small tunnels with diameter up to 6

metres.

(b) It gives over-estimates for large tunnels with

diameter above 6 metres.

(c) The estimated support pressure has a wide

range for squeezing and swelling rock conditions

for a meaningful application.


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Active Span and

Stand-Up Time

Stand-up time is the

length of time which

an excavated

opening can stand

without any mean of

support . Rock

classes are assigned

according to the

stand-up time.


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Rock Quality

Designation (RQD)

RQD represents

fracturing degree

of the rock mass.

It partially

reflecting the rock

mass quality.


RQD Rock Mass Quality

< 25 Very poor

25 – 50 Poor

50 – 75 Fair

75 – 90 Good

90 – 100 Excellent

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Rock Mass Rating RMR

RMR system incorporates 5 basic parameters.

(a) Strength of intact rock material: uniaxial compressive

strength or point load index;

(b) RQD;

(c) Spacing of joints: average spacing of all rock

discontinuities;

(d) Condition of joints: joint aperture, roughness, joint surface

weathering and alteration, infilling;

(e) Groundwater conditions: inflow or water pressure.


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RMR Parameters

1.

Strength

of intact

rock

material

Point load

strength index

(MPa)

> 10 4 10 2 4 1 2

Uniaxial

compressive

strength (MPa)

> 250 100 250 50 100 25 50 5 25 1 5 < 1

Rating 15 12 7 4 2 1 0

2.RQD (%) 90 100 75 90 50 75 25 50 < 25

Rating 20 17 13 8 3

3.

Joint spacing

(m)> 2 0.6 2 0.2 0.6 0.06 0.2 < 0.06

Rating 20 15 10 8 5

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RMR Parameters

4.

Condition of

joints

not

continuous,

very rough

surfaces,

unweathered,

no separation

slightly

rough

surfaces,

slightly

weathered,

separation <1

mm

slightly rough

surfaces,

highly

weathered,

separation <1

mm

continuous,

slickensided

surfaces, or

gouge <5 mm

thick, or

separation 15

mm

continuous

joints, soft

gouge >5

mm thick,

or

separation

>5 mm

Rating 30 25 20 10 0

5.

Ground-

water

inflow per 10 m tunnel

length (l /min), ornone < 10 10 25 25 125 > 125

joint water pressure/major

in situ stress, or0 0 0.1 0.1 0.2 0.2 0.5 > 0.5

general conditions at

excavation surface

complete

ly drydamp wet dripping flowing

Rating 15 10 7 4 0

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RMR and rock mass quality


RMR Ratings 81 100 61 80 41 60 21 40 < 20

Rock mass class A B C D E

Descriptionvery good

rockgood rock fair rock poor rock

very poor

rock

Average stand-

up time

10 year for

15 m span

6 months for

8 m span

1 week for

5 m span

10 hours for

2.5 m span

30 minutes for

0.5 m span

Rock mass

cohesion (KPa)> 400 300 400 200 300 100 200 < 100

Rock mass

friction angle > 45 35 45 25 35 15 25 < 15

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Rock Tunnel Quality Q-System

Q = (RQD / Jn) (Jr / Ja) (Jw / SRF)

Block size Inter-block strength Active stress

RQD - Rock Quality Designation.

Jn - joint set number.

Jr - joint roughness number.

Ja - joint alteration number indicating the degree of

weathering, alteration and filling.

Jw = joint water reduction factor.

SRF = stress reduction factor.


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Q-System Parameters

1. Rock Quality Designation 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 reported or measured as 10 (including 0), a nominal value of 10

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

accurate.

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Q-System Parameters

2. Joint Set Number Jn

A Massive, no or few joints 0.5 – 1

B One joint set 2

C One joint set plus random joints 3

D Two joint set 4

E Two joint set plus random joints 6

F Three joint set 9

G Three joint set plus random joints 12

H Four or more joint sets, heavily jointed 15

J Crushed rock, earthlike 20

Note: (i) For intersections, use (3.0 Jn). (ii) For portals, use (2.0 Jn).

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Q-System Parameters

3. Joint Roughness Number Jr

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

A Discontinuous joints 4

B Rough or 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

Note: (i) Descriptions refer to small and intermediate scale features, in that order.

(c) No rock-wall contact when sheared

H Zone containing clay minerals thick enough to prevent rock-wall contact 1.0

J Sandy, gravelly or crushed zone thick enough to prevent rock-wall contact 1.0

Note: (ii) Add 1.0 if the mean spacing of the relevant joint set 3 m. (iii) Jr = 0.5 can be used for planar

slickensided joints having lineations, provided the lineations are oriented for minimum strength.

Note: Jr and Ja classification is applied to the joint set or discontinuity that is least

favourable for stability both from the point of view of orientation and shear

resistance.

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Q-System Parameters

4. Joint Alteration Number r approx. Ja

(a) Rock-wall contact (no mineral fillings, only coatings)

A Tight healed, hard, non-softening, impermeable filling, i.e., quartz or epidote – 0.75

B Unaltered joint walls, surface staining only 25 – 35 1.0

C Slightly altered joint walls. Non-softening mineral coating, sandy particles, clay-

free disintegrated rock, etc.

25 – 30 2.0

D Silty- or sandy-clay coatings, small clay fraction (non-softening) 20 – 25 3.0

E Softening or low friction mineral coatings, i.e., kaolinite or mica. Also chlorite,

talc, gypsum, graphite, etc., and small quantities of swelling clays

8 – 16 4.0

(b) Rock wall contact before 10 cm shear (thin mineral fillings)

F Sandy particles, clay-free disintegrated rock, etc. 25 – 30 4.0

G Strongly over-consolidated non-softening clay mineral fillings (continuous, but <

5 mm thickness)

16 – 24 6.0

H Medium or low over-consolidated softening clay mineral fillings (continuous, but

< 5 mm thickness)

12 – 16 8.0

J Swelling-clay fillings, i.e., montmorillonite (continuous, but < 5 mm thickness).

Value of Ja depends on percent of swelling clay size particles, and access to

water, etc.

6 – 12 8 – 12

(c) No rock-wall contact when sheared (thick mineral fillings)

K, L, M Zones or bands of disintegrated or crushed rock and clay (see G, H, J for

description of clay condition)

6 – 24 6, 8, 8 – 12

N Zones or bands of silty- or sandy-clay, small clay fraction (non-softening) - 5

O, P, R Thick, continuous zones or bands of clay (see G, H, J for clay condition

description)

6 – 24 10, 13, 13 – 20

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Q-System Prameters

5. Joint Water Reduction Factor Water pressure Jw

A Dry excavation or minor inflow, i.e., < 5 l/min

locally

< 1 (kg/cm2) 1.0

B Medium inflow or pressure, occasional outwash

of joint fillings

1 – 2.5 0.66

C Large inflow or high pressure in competent rock

with unfilled joints

2.5 – 10 0.5

D Large inflow or high pressure, considerable

outwash of joint fillings

2.5 – 10 0.33

E Exceptionally high inflow or water pressure at

blasting, decaying with time

> 10 0.2 – 0.1

F Exceptionally high inflow or water pressure

continuing without noticeable decay

> 10 (kg/cm2) 0.1 – 0.05

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

(ii) Special problems caused by ice formation are not considered.

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Q-System Parameters

6. Stress Reduction Factor 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

B Single weakness zone containing clay or chemically disintegrated rock

(depth of excavation 50 m)

5

C Single weakness zone containing clay or chemically disintegrated rock

(depth of excavation > 50 m)

2.5

D Multiple shear zones in competent rock (clay-free) (depth of excavation

50 m)

7.5

E Single shear zone in competent rock (clay-free) (depth of excavation

50 m)

5

F Single shear zone in competent rock (clay-free) (depth of excavation >

50 m)

2.5

G Loose, open joint, heavily jointed (any depth) 5

Note: (i) Reduce SRF value by 25-50% if the relevant shear zones only influence but not

intersect the excavation.

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Q-System Parameters

(b) Competent rock, rock stress problems c / 1 / c SRF

H Low stress, near surface, open joints > 200 < 0.01 2.5

J Medium stress, favourable stress condition 200 – 10 0.01 –

0.03

1

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

L Moderate slabbing after > 1 hour in massive rock 5 – 3 0.5 - 0.65 5 – 50

M Slabbing and rock burst after a few minutes in

massive rock

3 – 2 0.65 – 1 50 – 200

N Heavy rock burst (strain-burst) and immediate

dynamic deformation in massive rock

< 2 > 1 200 – 400

Note: (ii) For strongly anisotropic virgin stress field (if measured): when 5 1 / 3 10,

reduce c to 0.75 c; when 1 / 3 > 10, reduce c to 0.5 c; where c is unconfined

compressive strength, 1 and 3 are major and minor principal stresses, and is

maximum tangential stress (estimated from elastic theory).

(iii) Few cases 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).

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Q-value and rock mass quality


Q-value Class Rock mass quality

400 ~ 1000 A Exceptionally Good

100 ~ 400 A Extremely Good

40 ~ 100 A Very Good

10 ~ 40 B Good

4 ~ 10 C Fair

1 ~ 4 D Poor

0.1 ~ 1 E Very Poor

0.01 ~ 0.1 F Extremely Poor

0.001 ~ 0.01 G Exceptionally Poor

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Excavation Support Ratio (ESR)


Excavation Category ESR

A Temporary mine openings. 3 – 5

B

Permanent mine openings, water tunnels for hydro-

electric projects, pilot tunnels, drifts and headings for

large excavations.

1.6

C

Storage rooms, water treatment plants, minor road and

railway tunnels, surge chambers and access tunnels in

hydro-electric project.

1.3

D

Underground power station caverns, major road and

railway tunnels, civil defense chamber, tunnel portals and

intersections.

1.0

EUnderground nuclear power stations, railway stations,

sports and public facilities, underground factories.0.8

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Geological Strength Index GSI

GSI was aimed to estimate the reduction in rock

mass strength for different geological conditions.

The system gives a GSI value estimated from rock

mass structure and rock discontinuity surface

condition. The direct application of GSI value is to

estimate the parameters in the Hoek-Brown strength

criterion for rock masses.


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GSI and rock mass quality


GSI Value 76 95 56 75 41 55 21 40 < 20

Rock Mass

Quality

Very

goodGood Fair Poor

Very

poor



Rock Support using Q-System

• Rock Mass Classification Q-System

• General Support Design Methodology

• Excavation Support Ratio and Equivalent Span

• Roof Support Requirements from Q-Chart

• Design of Wall Support

• Design of Temporary Support

• Maximum Unsupported Span

• Bolt Length and Spacing

• Limitation of Q-System

• Examples



Rock Mass Classification Q-System

Rock Tunnel Quality Q-value is a measure of rock mass quality, given as:

Q = (RQD / Jn) (Jr / Ja) (Jw / SRF)

RQD - Rock Quality Designation.

Jn - joint set number.

Jr - joint roughness number.

Ja - joint alteration number (weathering, alteration, filling).

Jw = joint water reduction factor.

SRF = stress reduction factor.




Q-value Class Rock mass quality

400 ~ 1000 A Exceptionally Good

100 ~ 400 A Extremely Good

40 ~ 100 A Very Good

10 ~ 40 B Good

4 ~ 10 C Fair

1 ~ 4 D Poor

0.1 ~ 1 E Very Poor

0.01 ~ 0.1 F Extremely Poor

0.001 ~ 0.01 G Exceptionally Poor




General Support Design Methodology

Q is also a measure of rock stability in relation to

opening size.

It is empirical design based on thousands of cases, to

provide the design of permanent support using Q value.

Support design depends on effective span (diameter or

height) and safety requirement.




Excavation Support Ratio and Equivalent Span

Safety requirement is measured by a term called

Excavation Support Ratio (ESR), which in turn, gives the

Equivalent Dimension (De).

De =Actual excavation span or height

Excavation support ratio, ESR




Excavation Support Ratio (ESR)

Excavation Category ESR

A Temporary mine openings. 3 – 5

B

Permanent mine openings, water tunnels for hydro-electric

projects, pilot tunnels, drifts and headings for large

excavations.

1.6

C

Storage rooms, water treatment plants, minor road and

railway tunnels, surge chambers and access tunnels in

hydro-electric project.

1.3

D

Underground power station caverns, major road and railway

tunnels, civil defence chamber, tunnel portals and

intersections.

1.0

EUnderground nuclear power stations, railway stations, sports

and public facilities, underground factories.0.8





Roof Support Requirements from Q-Chart

The design chart using Q-value is the support design requirement for tunnel roof.

1. Horizontal axis is the Q-value of the surrounding rock masses.

2. Left vertical axis is the equivalent dimension of the designed tunnel.

3. Intersection point defines the support requirement zone, which gives type of support and thickness of sprayed concrete.

4. Vertical up from the intersection gives bolt spacing.

5. Horizontal to the right from the intersection gives bolt length.



Design of Rock Support

1

2

3

5

Q = 1.33, tunnel span = 16m

ESR = 0.8, De = 20m

4

3




Notes on Q-Chart

Bolt length is determined based on the actual span or height (i.e., ESR=1). It is in fact estimated by the following equation, L = 1.4 + 0.184 Span.

Bolt spacing is determined by Q-value, but be aware the thickness for plain and SFR shotcrete are different.

In support zone (2) to (7), ERS effectively is to change the support measure by changing only the thickness of shotcrete.




Design of Wall Support

Wall height should be used in equivalent dimension.

When Q values are used in design wall support, following adjustment should be used:

For Q > 10, Qwall = 5 Q

For 0.1 < Q < 10, Qwall = 2.5 Q

For Q < 0.1, Qwall = Q




Design of Temporary Support

Temporary support measures are primarily bolts and shotcrete (sometimes, steel sets).

For temporary support, the following adjustment can be used:

Increase ESR to 1.5 ESR

or

Increase Q to 5 Q (applicable to roof and wall)




Maximum Unsupported Span

For reasonably good quality rock mass, it is possible to have the tunnel unsupported for a long period.

Maximum unsupported span = 2 ESR Q0.4

Example:

Q = 10, ESR = 1 maximum unsupported span = 5 m




Bolt Length and Spacing

Spot/Local bolting is used to secure individual blocks of rock.

Spacing is depending on the size of the block which can be estimated from observation of the joints.

The bolts should be long enough to obtain adequate anchorage in stable rock beyond the block (1~2 m into the solid rock).

For systematic bolting, the bolt spacing (function of joint spacing) and length is to be estimated from Q-chart.





Alternative to the chart, bolt length can be estimated using the equation below:

L = 1.4 + 0.184 Span (m)

Typical bolts are steel bars between 20 and 40 mm diameter, with bolt capacity up to 250 kN.



Support description

•The length L of rock bolts can be

also estimated from the excavation

width B and the Excavation Support

Ratio ESR

•The maximum unsupported span

can be estimated from Q and ESR

•The permanent roof support

pressure Proof is estimated from Q, Jn

and Jr

0.152

BL

ESR

0.4Max span (unsupported) =2ESR Q

Grimstad and Barton (1993)

1/ 32

3

n

roof

r

J QP

J



65 m2

101 holes

115 m2

151 holes

PilotSlash

Bench, 3.5-m high

Chamber section: 275 m2

Tunnel Sections in a Granite




Support Applied in Tunnels in a Granite

Q Support Span 10 m Span 15 m Span 30 m

>40Bolt Spot Spot Spot

SC 40 mm 40 mm 40 mm

10-40Bolt L3, S2.4 m L4, S2.4 m L5, S2.4 m


4-10Bolt L3, S2.2 m L4, S2.2m L5, S2.2 m


1-4Bolt L3, S1.9 m L4, S1.9 m L5, S1.9 m


< 1Bolt L3, S1.5 m L4, S1.5m L5, S1.5 m

SC 75 mm 75 mm 100 mm




Limitation of Q-System

The Q system can be used correctly for:

• Normal hard rock condition

• Very Poor to Good rock mass (0.1<Q<40)

• Jointed blocky rock mass where instability is caused by block falls

• Practical excavation span between 3 and 30 m




Limitation of Q-System

For support zones 2 and 3, with Q-value between 4 and 40, the chart suggests no shotcrete. However, a thin layer of shotcrete at roof is highly recommended.



Example (a): hydropower access tunnel of 20 m span, wall

height 10 m

Granite rock mass containing 3 joint sets, average RQD is 88%,

average joint spacing is 0.24 m, joint surfaces are generally

stepped and rough, tightly closed and un-weathered with

occasional stains observed, the excavation surface is wet but

not dripping, average rock material uniaxial compressive

strength is 160 MPa, the tunnel is excavated to 150 m below

the ground where no abnormal high in situ stress is expected.




RQD 88% RQD 88

Joint set number 3 sets Jn 9

Joint roughness

numberrough stepped (undulating) Jr 3

Joint alteration number unaltered, some stains Ja 1

Joint water factorwet only (dry excavation or minor

inflow)Jw 1

Stress reduction factor c/1 = 160/(1500.027) = 39.5 SRF 1

Q (88/9) (3/1) (1/1) 29





1

2

3

5

Q = 29, ESR = 1.3, Span = 20m, De = 15.4m

4

3

Qwall=5Q

Ht = 10m



Example (a): hydropower access tunnel of 20 m span, in granite, with Q=29

Roof support requirement from the Q-chart:

Systematic bolting

Bolt spacing at 2.5 m

Bolt length of 5 m

Thin shotcrete layer (approx. 2 cm) at roof

Side wall support requirement:

Generally no support needed




Example (b): highway tunnel of 20 m span, 10 m high

A sandstone rock mass, fractured by 2 joint sets plus random

fractures, average RQD is 70%, average joint spacing is 0.11

m, joint surfaces are slightly rough, highly weathered with

stains and weathered surface but no clay found on surface,

joints are generally in contact with apertures generally less

than 1 mm, average rock material uniaxial compressive

strength is 85 MPa, the tunnel is to be excavated at 80 m

below ground level and the groundwater table is 10 m below

the ground surface.




RQD 70% RQD 70

Joint set number 2 sets plus random Jn 6

Joint roughness

numberslightly rough (rough planar) Jr 1.5

Joint alteration number

highly weathered only stain,

(altered non-softening mineral

coating)

Ja 2

Joint water factor70 m water head = 7 kg/cm2 = 7

barsJw 0.5

Stress reduction factor c/1 = 85/(800.027) = 39.3 SRF 1

Q (70/6) (1.5/2) (0.5/1) 4.4





1

2

3

5

Q=4.4, ESR=1.0, Span=20m, De=20m 4

3

Qwall=2.5Q

Htl=10m



Example (b): highway tunnel of 20 m span, 10 m high, in sandstone, with Q=4.4



Bolt length of 5 m

SFR shotcrete of 7 cm


Bolting at 2.4 m spacing

Bolt length of 3 m

Thin shotcrete to cover or no shotcrete




Example (c): railway tunnel of 10 m diameter

A highly fractured siltstone rock mass, has 2 joint sets and

many random fractures, average RQD is 41%, joints appears

continuous observed in tunnel, joint surfaces are slicken-sided

and undulating, and are highly weathered, joint are separated

by about 3-5 mm, filled with clay, average rock material uniaxial

compressive strength is 65 MPa, inflow per 10 m tunnel length

is observed at approximately 50 litre/minute, with considerable

outwash of joint fillings. The tunnel is at 220 m below ground.




RQD 41% RQD 41

Joint set number 2 sets plus random Jn 6

Joint roughness

numberSlicken-sided and undulating Jr 1.5

Joint alteration numberhighly weathered filled with 3-5 mm

clayJa 4

Joint water factorlarge inflow with considerable

outwashJw 0.33

Stress reduction factor c/1 = 65/(2200.027) = 11 SRF 1

Q (41/6) (1.5/4) (0.33/1) 0.85





1

2

3

5

Q = 0.85, ESR = 1.0, Span = 10m, De = 10m

4

3

Qwall=2.5Q



Example (c): railway tunnel of 10 m diameter, in highly fractured siltstone, with Q=0.84



Bolt length of 3 m



Bolting at 1.9 m spacing





Rock Support using RMR

• Rock Mass Rating RMR

• General Support Design Methodology

• RMR Adjustment for Tunnelling

• Support Guide using RMR

• Maximum Unsupported Span

• Bolt Length and Spacing

• Guide on Shotcrete

• Limitation of RMR Design System

• Examples



Rock Mass Rating RMR

RMR system incorporates 5 basic parameters:

1. Strength of intact rock material: uniaxial compressive strength or point

load index;

2. RQD;

3. Spacing of joints: average spacing of all rock discontinuities;

4. Condition of joints: joint aperture, roughness, joint surface weathering

and alteration, infilling;

5. Groundwater conditions: inflow or water pressure.




RMR and Rock Mass Quality

RMR Ratings 81 100 61 80 41 60 21 40 < 20

Rock mass

classA B C D E

Descriptionvery good

rockgood rock fair rock poor rock

very poor

rock

Average stand-

up time

10 year

for 15 m

span

6 months

for 8 m

span

1 week

for 5 m

span

10 hours

for 2.5 m

span

30 minutes

for 0.5 m

span

Rock mass

cohesion (KPa)> 400 300 400 200 300 100 200 < 100

Rock mass

friction angle > 45 35 45 25 35 15 25 < 15





RMR Adjustment for Tunnelling

To use RMR for tunnel support design, RMR rating needs to be adjusted for tunnel alignment with respect to joint orientations.

Adjusted RMR = Original RMR + Adjustment

Adjustment is between 0 and -12.




Rating adjustment for joint orientations

Conditionvery

favourable

favourabl

efair

unfavoura

ble

very

unfavourable

Rating

adjustment0 2 5 10 12

Effects of joint orientation in tunnelling

Strike to tunnel axis, drive with dipStrike to tunnel axis, drive against

dip

Dip 45 90

very favourable

Dip 20 45

favourable

Dip 45 90

fair

Dip 20 45

unfavourable

Strike // to tunnel axis Sub-horizontal joint (Dip 0 20)

Dip 45 90

very unfavourable

Dip 20 45

fair

irrespective of strike

fair



General Support Design Methodology

RMR is a measure of rock mass quality, as well as a measure

of rock stability in relation to opening size.

It was initially developed

to estimate the stand-up

time for mines of various

opening size in rocks

of various quality




Support Guide using RMR

Support using RMR is empirical based on the RMR value.

The design guide is primarily for tunnels operation.

Tunnel sizes are generally between 2 and 15 metres.





Adjusted

ratings

Original RMR ratings

>80 70-80 60-70 50-60 40-50 30-40 20-30 10-20 0-10

>50 a a a a

40-50 b b b b

30-40 c, d c, d c, d, e d, e

20-30 g f, g f, g, j f, h, j

10-20 i i h, i, j h, j

0-10 k k l l



a) Generally no support, but joint intersections may require local bolting.

b) Patterned, grouted bolts at 1.0 m spacing.

c) Patterned, grouted bolts at 0.75 m spacing.

d) Patterned, grouted bolts at 1.0 m spacing, and shotcrete 100 mm thick.

e) Patterned, grouted bolts at 1.0 m spacing, and massive concrete 300 mm thick; only used if stress changes are not excessive.

f) Patterned, grouted bolts at 0.75 m spacing, and shotcrete 100 mm thick.

g) Patterned, grouted bolts at 0.75 m spacing, and mesh-reinforced shotcrete 100 mm thick.




h) Patterned, grouted bolts at 1.0 m spacing, and massive concrete 450 mm thick; if stress changes are not excessive.

i) Patterned, grouted bolts at 0.75 m spacing, and mesh-reinforced shotcrete 100 mm thick, plus yielding steel arches as repair technique if stress changes are excessive.

j) Stabilize with wire-mesh cover support and massive concrete 450 mm thick; if stress changes are not excessive.

k) Stabilize with wire-mesh cover support followed by 100-150 mm shotcrete (including face if necessary), plus yielding steel arches where stress changes excessive.

l) Avoid failure development in this ground if possible; otherwise, use support systems j or k.




Supplementary notes

1. The original RMR rating, as well as the adjusted ratings, must be considered in assessing ground-support requirements.

2. Rock bolts are generally ineffective in highly jointed rock masses and should not be used as the sole support when the joint spacing rating is less than 6.

3. Support recommendations in the table are applicable to mine openings with stress levels less than 30 MPa.

4. Large chambers should only be excavated in rock with adjusted total RMR of 50 or better.




Maximum Unsupported Span

Maximum unsupported span and stand-up time can be estimated from the chart.





The minimum bolt length is recommended to be the greatest of the following:

1. two times the bolt spacing;

2. three times the average discontinuity spacing for critical rock blocks;

3. 0.5B for spans of B < 6m, or 0.25B for spans of B = 18 to 30m.

For excavations higher than 18m, the lengths of sidewall bolts should be at least 1/5 of the wall height.





The maximum bolt spacing is recommended to be ½ bolt length or 1.5 times the average spacing of rock joints.

However, if wire mesh is to be anchored by the bolts, then a bolt spacing greater than 2 m makes attachment of the mesh practically impossible.

Rock bolt design for major zones of instability created by seams or persistent smooth joints should be the subject of stability analysis.





Guide on Shotcrete

1. Shotcrete (particularly fibre reinforced) thickness should not exceed 20 cm;

2. Thick layers of shotcrete may be applied occasionally to small areas of particularly poor rock.

In general, systematic bolting with fibre reinforced shotcrete should be used for permanent support of roofs of tunnels that will be occupied by people or will contain important processes or machinery.



Limitation of RMR System

RMR is primarily developed for mining, i.e., for tunnels of limited size. Design guide does not cover size effects.

Design does not consider the usage and safety requirements.




Example (a): hydropower access tunnel of 18 m span

Granite rock mass containing 3 joint sets (1 sub-horizontal, 1

sub-vertical and sub-vertical // to tunnel axis), average RQD

is 88%, average joint spacing is 0.24 m, joint surfaces are

generally stepped and rough, tightly closed and unweathered

with occasional stains observed, the excavation surface is wet

but not dripping, average rock material uniaxial compressive

strength is 160 MPa, the tunnel is excavated to 150 m below

the ground.




Rock material strength 160 MPa Rating 12

RQD (%) 88% Rating 17

Joint spacing (m) 0.24 m Rating 10

Condition of jointsvery rough, unweathered, no

separationRating 30

Groundwater wet Rating 7

RMR 76


Joint orientation condition: fair, v fav, v unfav v unfav

Adjustment = -12; Adjusted RMR = 64




a. Generally no support, but joint intersections may require local bolting.

Adjusted

ratings


>80 70-80 60-70 50-60 40-50 30-40 20-30 10-20 0-10

>50 a a a a

40-50 b b b b

30-40 c, d c, d c, d, e d, e

20-30 g f, g f, g, j f, h, j

10-20 i i h, i, j h, j

0-10 k k l l



Example (b): highway tunnel of 20 m span

A sandstone rock mass, fractured by 2 joint sets (1 // to tunnel axis

dipping at 30 and 1 to tunnel axis dipping at 70), plus random

fractures, average RQD is 70%, average joint spacing is 0.11 m, joint

surfaces are slightly rough, highly weathered with stains and

weathered surface but no clay found on surface, joints are generally

in contact with apertures generally less than 1 mm, average rock

material uniaxial compressive strength is 85 MPa, the tunnel is to be

excavated at 80 m below ground level and the groundwater table is

10 m below the ground surface.




Rock material

strength85 MPa Rating 7



Condition of jointsslightly rough, highly weathered,

separation < 1mmRating 20

Groundwater water pressure/stress = 0.32 Rating 4

RMR 52


Joint orientation condition: fair, fair to very favourable fair





b. Patterned, grouted bolts at 1.0 m spacing

Adjusted

ratings


>80 70-80 60-70 50-60 40-50 30-40 20-30 10-20 0-10

>50 a a a a

40-50 b b b b

30-40 c, d c, d c, d, e d, e

20-30 g f, g f, g, j f, h, j

10-20 i i h, i, j h, j

0-10 k k l l



Example (c): railway tunnel of 10 m diameter

A highly fractured siltstone rock mass, has 2 joint sets (1 sub-

horizontal and 1 sub-vertical // to tunnel axis), and many random

fractures, average RQD is 41%, joints appears continuous

observed in tunnel, joint surfaces are slicken-sided and undulating,

and are highly weathered, joint are separated by about 3-5 mm,

filled with clay, average rock material uniaxial compressive strength

is 65 MPa, inflow per 10 m tunnel length is observed at

approximately 50 litre/minute, with considerable outwash of joint

fillings. The tunnel is at 220 m below ground.




Rock material strength 65 MPa Rating 7



Condition of jointscontinuous, slicken-sided,

separation 1-5mmRating 10

Groundwater inflow = 50 l/min Rating 4

RMR 34


Joint orientation condition: fair, very unfavourable very unfavourable





g, j. Closely spaced (0.75 m) bolts, wire-mesh, and shotcrete 100 mm thick, followed by cast-in concrete 450 mm thick.

Adjusted

ratings


>80 70-80 60-70 50-60 40-50 30-40 20-30 10-20 0-10

>50 a a a a

40-50 b b b b

30-40 c, d c, d c, d, e d, e

20-30 g f, g f, g, j f, h, j

10-20 i i h, i, j h, j

0-10 k k l l



Comparison of Q and RMR Support Design Systems


System Rock Mass 1

(18 m span)

Rock Mass 2

(20 m span)

Rock Mass 3

(10 m span)

RMR Generally no

support, with

possible spot bolts

Systematic bolts at

1.0 m spacing.

Systematic bolts at 0.75 m

spacing, wire-mesh, 10 cm

shotcrete, 45 cm cast-in

concrete

Q Systematic bolts at

2.5 m spacing, no

or thin shotcrete

Systematic bolts at

2.1 m spacing, 7

cm SFR shotcrete

Systematic bolts at 1.6 m

spacing, SFR 10 cm

shotcrete

Major differences



Comparison and Comments

RMR does not consider tunnel size, it is generally suitable for tunnel between 3-10 m.

RMR does not differentiate roof and wall support.

For rock mass of fair and above quality, RMR and Q give similar support, though Q uses more shotcrete, while RMR uses more bolts (practical in mines).

For very poor rock mass, the difference in support is very large:

• It was recognised Q-system is not initially designed for very poor rock.

• So RMR system is recommended for support design in rock mass of poor and below quality.


Documents

Tunnelling and Underground Construction Technology