ens 110323 en jz rm lecture 2011 part 4

8/20/2019 Ens 110323 en Jz Rm Lecture 2011 Part 4

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Rock Mechanics

Mécanique des roches

Course Lectures

Part 4 – Rock Mass Properties and Classifications

Professor ZHAO JianEPFL−ENAC−LMR

Rock Mechanics and

Tunnel Engineering

Rock Mass Property and Classification


2/33

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 discontinuesrunning of highly fractured rock masses, depending

mainly on the existence of rock joints.


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


3/33

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 steelarches installed to

support a tunnel.


Rock Class DefinitionRock Load Factor Hp

(feet) (B and Ht in feet)Remark

I. Hard and intact

Hard and intact rock contains no joi nts 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 j oints 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 + H t) No si de pres sure.

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 + H t) L itt le o r no s ide p ressure.

VI. Completely

crushed but

chemically intact

Rock is not chemically weathered, and hi ghly 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 + H t)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 + H t)

IX. S welling 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 conditionsfor a meaningful application.


Active Span and

Stand-Up Time

Stand-up time is the

length of time whichan 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

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

RMR Parameters

4.

Condition of

joints

not

continuous,

very rough

surfaces,

unweathered,

no separation

slightly

rough

surfaces,

slightly

weathered,

separation 125

joint water pressure/major

in situ stress, or 0 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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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.


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).

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 T ig ht healed, hard , non-softening, impermeable filling, i.e., q uartz 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- o r 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

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, considerableoutwash 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.


11/33

Q-System Parameters

6. Stress Reduction Factor SRF

(a) Weakness zones intersecting excavation, which may cause loosening of roc k 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.

Q-System Parameters

(b) Competent rock, rock stress problems σc / σ1 σθ / σc 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 ≤ σ1 / σ3 ≤ 10,

reduce σc to 0.75 σc; when σ1 / σ3 > 10, reduce σc to 0.5 σc; where σc is unconfined

compressive strength, σ1 and σ3 are major and minor principal stresses, and σθ 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).


12/33

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 andintersections.

1.0

EUnderground nuclear power stations, railway stations,

sports and public facilities, underground factories.0.8

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 rockmass 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


15/33

Range of GSI for Grante Range of GSI for Mudstone and Shale

GSI for Heterogeneous Rock Masses such as Flysch


16/33

Example – Estimate RMR, Q and GSI

(a) 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 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 where noabnormal high in situ stress is expected.



Rock material strength 160 MPa Rating 12

RQD (%) 88% Rating 17

Joint spacing (m) 0.24 m Rating 10

Condition of joints very rough, unweathered, no separation Rating 30

Groundwater wet Rating 7

RMR 76

RQD 88% RQD 88

Joint set number 3 sets Jn

9

Joint roughness number rough stepped (⇒undulating) Jr 3

Joint alteration number unaltered, some stains Ja

1

Joint water factor wet only (dry excavation or minor inflow) Jw

1

Stress reduction factor σ

c /σ

1= 160/(150×0.027) = 39.5 SRF 1

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

Rock Mass Structure: Blocky Joint Surface Condition : Very good GSI = 75±5


17/33


(b) 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 beexcavated at 80 m below ground level and the

groundwater table is 10 m below the ground surface.



Rock Mass Structure: Blocky Joint Surface Condition : Very good GSI = 40±5

RQD 70% RQD 70

Joint set number 2 sets plus random Jn

6

Joint roughness number slightly rough (⇒rough planar) Jr 1.5

Joint alteration number highly weathered only stain, (altered non-

softening mineral coating)

Ja

2

Joint water factor 70 m water head = 7 kg/cm2 = 7 bars Jw

0.5

Stress reduction factor σc /σ1 = 85/(80×0.027) = 39.3 SRF 1

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

Rock material strength 85 MPa Rating 7

RQD (%) 70% Rating 13

Joint spacing (m) 0.11 m Rating 8

Condition of joints slightly rough, highly weathered, separation < 1mm Rating 20

Groundwater water pressure/stress = 0.32 Rating 4

RMR 52


18/33


19/33



RMR Quality Q Quality GSI Quality

(a) Granite 76 G 29 G 75 G

(b) Sandstone 52 F 4.4 F 40 F

(c) Siltstone 34 P 0.85 VP 20 VP

Other Rock Mass Classification Systems


Rock Mass Number, N

N is the rock mass

quality Q value when

SRF is set at 1, i.e.,

N = (RQD / Jn) (Jr / Ja) (Jw)

Rock Mass Index, RMi

RMi = σc Jp

σc is rock material strength.

Jp is jointing parameter for 4

joint characteristics: joint

density, size, roughness, and

alteration. Jp=1 for intact rock,

Jp=0 for crushed rock masses.


20/33

Correlation between Q, RMR and GSI

RMR = 9 lnQ + (44±18)

RMR = 13.5 logQ + 43

GSI = RMR – 5

(for GSI > 25)


Rock Mass Strength

Strength and deformation properties of a rock mass

are governed by the existence of joints. Those rock

mass properties are also related to the quality of therock mass. In general, a rock mass of good quality

(strong rock, few joints and good joint surface

quality) have higher strength and higher deformation

modulus than that of a poor rock mass.



21/33


β

σ1

σ3

σ1

σ3

Failure of rock

material

σ1

β900

Failure of rock mass


σ1

σ3

σc

σt

r o c k

m a t

e r i a l

g o o d

q u a l i

t y r o c k

m a s

s

p o o r

q u a l i t

y r o c k

m a s s


22/33

Hoek-Brown Rock Mass Strength Criterion

Generalised Hoek-Brown Criterion

or

σ1 = σ3 + (mb σ3 σci + s σci2)a

H-B criterion for rock material is a special form of

the generalised equation when s =1, a = 0.5, mb=mi.

σ1 = σ3 + (mi σ3 σci + σci2)0.5


σ1 σ3 σ3= + (mb + s)

a

σci σci σci


σci is consistently the uniaxial compressive strength

of intact rock material, used in the Hoek-Brown

criterion for rock material and for rock mass.

σ1 is the rock mass strength at a confining pressure

σ3. σci is the uniaxial strength of the intact rock in the

rock mass. Parameter a is generally equal to 0.5.

Constants mb and s are parameters that changes

with rock type and rock mass quality. Next table

shows mb and s values.



23/33

Hoek-Brown Failure

Criterion

σ1 /σc = σ3 /σc + (mbσ3 /σc + s)

0.5

Carbonate

rocks -

dolomite,

limestone,marble

Argillaceous

rocks -

mudstone,

siltstone,shale, slate

Arenaceous

rocks -

sandstone,

quartzite

Fine grained

igneous -

andesite,

dolerite,basalt,

rhyolite

Coarse

metamorphic

& igneous -

gabbro,gneiss,

granite

Intact rock material

RMR = 100 ,Q = 500

mi = 7.0

s = 1.0

mi = 10.0

s = 1.0

mi = 15.0

s = 1.0

mi = 17.0

s = 1.0

mi = 25.0

s = 1.0

Very good quality

rock mass

RMR = 85, Q = 100

mb = 3.5

s = 0.1

mb = 5.0

s = 0.1

mb = 7.5

s = 0.1

mb = 8.5

s = 0.1

mb = 12.5

s = 0.1

Good quality rock

mass

RMR = 65, Q = 10

mb = 0.7

s = 0.004

mb = 1.0

s = 0.004

mb = 1.5

s = 0.004

mb = 1.7

s = 0.004

mb = 2.5

s = 0.004

Fair quality rock

mass

RMR = 44, Q = 1.0

mb = 0.14

s = 0.0001

mb = 0.20

s = 0.0001

mb = 0.30

s = 0.0001

mb = 0.34

s = 0.0001

mb = 0.50

s = 0.0001

Poor quality rock

massRMR = 23, Q = 0.1

mb = 0.04

s = 0.00001

mb = 0.05

s = 0.00001

mb = 0.08

s = 0.00001

mb = 0.09

s = 0.00001

mb = 0.13

s = 0.00001

Very poor quality

rock mass

RMR = 3, Q = 0.01

mb = 0.007

s = 0

mb = 0.01

s = 0

mb = 0.015

s = 0

mb = 0.017

s = 0

mb = 0.025

s = 0


Development and application of the Hoek-Brown

criterion lead to better definition of the parameters

mb and s.

Determination of mi is improved, as in the next table.

With GSI estimated, mb can be calculated,

mb = mi exp [(GSI–100)/28]



24/33

Rock Type Rock Name and mi Values

I gn e o u s

IntrusiveGranite 32±3

Granodiorite 29±3

Diorite 25±5

Dolerite (16±5)

Gabbro 27±3

Norite 22±5

Peridotite (25±5)

Extrusive Rhyolite (16±5) Andesite 25±5Basalt (16±5)

Diabase (16±5)Porphyries (20±5)

VolcanicAgglomerate

(19±3)Tuff (13±5)

S e d i m en t ar y

Clastic

Conglomerate

(4±18)

Breccia (4±16)

Sandstone 17±4Siltstone 7±2

Marls (7±2)

Mudstone 4±2

Shale (6±2)

CarbonateCrystalline

limestone (12±3)

Sparitic limestone

(10±2)

Micritic limestone

(9±2)Dolomite (9±3)

Chemical Gypsum 8±2 Anhydrite 12±2

Organic Coal (8±12) Chalk 7±2M e t am

or ph i c

Foliated Gneiss 28±5 Schist 12±3 Phyllites (7±3) Slate 7±4

Slightly

FoliatedMigmatite (29±3) Amphibolite 26±6

Non

FoliatedQuartzite 20±3

Meta-sandstone

(19 ±3)Hornfels (19±4) Marble 9±3

Be careful with large uncertainty


For GSI > 25, i.e. rock masses of good to reasonable

quality,

s = exp [(GSI–100)/9]

a = 0.5

This is the original Hoek-Brown criterion.



25/33


For GSI < 25, i.e. very poor rock masses, s = 0,

a = 0.65 – GSI/200

When σ3 = 0, it gives the uniaxial compressivestrength as,

σ

cm = σ1 = sa σci

For very poor rock masses, s=0, σcm=0.


Example on Hoek-Brown Criterion and GSI


(a) Granite rock mass, σci= 150 MPa, GSI=75, a = 0.5.

mi for granite is 32,

mb = mi exp[(GSI – 100)/28] = 13.1

s = exp[(GSI – 100)/9] = 0.062

σ1 = σ3 + (1956 σ3 + 1395)0.5

When σ3 = 0, σcm = 13950.5 = 37.3 MPa



26/33

Example on Hoek-Brown Criterion and GSI


(c) Siltstone rock mass, σci=65 MPa, GSI=20.

mi for siltstone = 7

mb = mi exp[(GSI – 100)/28] = 0.40

s = exp[(GSI – 100)/9] = 0.00014

GSI < 25, a = 0.65 – (GSI/200) = 0.55

σ1 = σ3 + (26 σ3 + 0.59)0.55

σcm = 0.590.55 = 0.75 MPa


Applicability of Hoek-Brown Criterion


27/33

Hoek-Brown and Mohr-Coulomb Criteria

There is no direct correlation between linear M-C

criterion and the non-linear H-B criterion.

When Mohr-Coulomb parameters c and are needed

for design and modelling,

(i) Use direct test results on rock mass if available.

(ii) Use H-B to generate a series σ1 –σ3 data, plotthem by Mohr circles, and fit them with the ‘best’

linear tangent envelope, to find c and .


Getting c and using Hoek-Brown Equation


σ3 σ1

0 6

2 12

4 17

6 21

8 25

10 28

12 32

15 37

20 45

30 61

40 75σ

τ

σci=100 MPa, mb=0.3, s=0.004, a=0.5

H-B

M-C

M-C low stress

c


28/33

Correlation of Rock Mass Quality and Properties

Correlations between rock mass strength and

quality are by mb and s in the Hoek-Brown criterion.

Better rock mass quality gives higher mb and s,

hence higher rock mass strength. When rock mass

is solid and massive with few joints, rock mass

strength is close to rock material strength. When

rock mass is very poor (GSI < 25), rock mass hasvery low uniaxial compressive strength close to zero.



Rock mass modulus (Em, GPa) can be estimated

from RMR and Q, for fair and better rock mass,

Em = 25 log10Q, for Q > 1

Em = 10 (Q σci /100)1/3

Em = 2 RMR – 100, for RMR > 50

Em = 10(RMR–10)/40 for 20 < RMR < 85

Em = 10(15 logQ+40)/40



29/33


For poor rocks with σci


30/33

Squeezing Behaviour of Rock Mass

Squeezing of rock is the time dependent large

deformation, which occurs around an openings, and

is essentially associated with creep caused by

exceeding shear strength.

Classification of squeezing degree,

(i) Mild squeezing: closure 1-3% of opening D;

(ii) Moderate squeezing: closure 3-5% of D;

(iii) High squeezing: closure > 5% of D.


Squeezing Behaviour of Rock Mass

Behaviour of rock squeezing is typically represented

by rock mass deforms plastically into the opening.

Rate of squeezing is time and stress dependent.Usually the rate is high at initial stage, say, several

cm/day closure at beginning, reduces with time.

Squeezing may continue for a long period.

Squeezing may occur at shallow depths in weak and

poor rock masses. Poor rock masses with moderate

strength at great depth may also suffer from

squeezing.



31/33

Squeezing Estimation by Rock Mass Classification Q


Squeezing:

Overburden

H > 350 Q1/3

Non-squeezing:

H < 350 Q1/3

Squeezing Estimation by Rock Mass Classification N


Non-squeezing:

H < 275 N1/3) B –0.1

Mild squeezing:> (275 N1/3) B –0.1

H< (450 N1/3) B –0.1

Moderate squeezing:

> (450 N1/3) B –0.1H

< (630 N1/3) B –0.1

High squeezing:

H > (630 N1/3) B –0.1


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Yacambu-Quibor, Venezuela

Tunnel squeezing case histories

compared with prediction for

squeezing (Hoek 2000)

Prediction curve for squeezing for

different rock mass strength to in situ

stress ratios (Hoek 2000)

predictions from analysis

Squeezing in the Yacambu-

Quibor tunnel, Venezuela

ens 110323 en jz rm lecture 2011 part 4

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