f-rock mass classification

8/3/2019 F-Rock Mass Classification

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ROCK MASS CLASSIFICATIONS

Numerous classifications proposed butTWO main ones in use :- Rock Mass Rating System RMR Rock Mass Rating System RMR

Q System (N.G.I.)Q System (N.G.I.)

Attempt to combine INDEX properties of INTACT rocks with the characteristics of field scale DISCONTINUITIES

Development of RockMass ClassificationsNeed for initial empirical design tool to assess

ground conditions and support requirements

Terzaghis Rock Load Classification (1946)

Rock Quality Designation RQD Index

Active Span and Stand-up time

RMR

Q System

ACTIVE SPAN

b. Support placed close to face

a. Support lagging behind face position

Support

S

S

RQD EXAMPLE (1)


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Core Logging

%100 x L x

RQD i

=

xi = Length of individual piecesof core => 100mmL = Total Length of core run

ENGINEERING LOGGINGOF ROCK CORE

R. Q. D

Used in support design Does not allow for discontinuity orientation

infill, weathering etc..

Very Good> 90Good75 - 90

Poor 50 - 75

Poor 25 - 50

Very Poor < 25Drill Core QualityRQD %

I. The Rock Mass Rating System(RMR)

Provides RMR of 0-100Based on 5 parameters INTACT ROCK STRENGTH RQD GROUNDWATER FRACTURE SPACING FRACTURE CONDITION

RATING ADJUSTMENT

BASED ON FRACTURE ORIENTATION

Other Adjustments

Blasting, In-situ stress, Major faults


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ROCK MASS RATING, RMRROCK MASS RATING SYSTEM

INFLUENCE

OF DIP

Drive with Dip

Drive against Dip

RMR and STAND-UP TIME

Immediatecollapse

No support required

10 4 10 5 10 610 310 210 -1 10 0 10 1Stand-up time in hours

20

30

40

50 6070 80

20

30

40

50

6070

80 901d 1wk 1mo 1yr 10yr

1

2

34

10

20

30

R o o f

S p a n

( m ) 8

65

R o c k M

a s s R a t i n

g

R o c k M

a s s R a t

i n g


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RMR SUPPORT GUIDELINES

THE NGI Q SYSTEM

RQD = Rock Quality DesignationJn = Joint Set No.Jr = Joint Roughness No.Ja = Joint alteration No.Jw = Joint water reduction No.SRF = Stress Reduction Factor

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

Q varies from 0.001 to 1000

THE Q SYSTEMRQD/ J n represents structure of rock mass (block size)

Range 100/0.5 to 10/20

J r /J a represents roughness and frictional properties of joint wall and infill (interblock shear strength)

Rock wall contact

Rock wall contact before 10cm shearNo rock wall contact when sheared

J w/SRF represents the active stresses

loosening load where excavation through shear zonesand clay bearing rock

rock stress in competent rock squeezing loads in plastic rocks


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THE Q SYSTEM

Q SYSTEM

EXCAVATION SUPPORT RATIO

related to use of excavation and to extent of to which some instability is acceptable

roughly analogous to inverse of factor of

safety

Used in Support Design via

THE EQUIVALENT DIMENSION, De.

De = Excavation span, diameter or heightExcavation Support Ratio

EXCAVATION SUPPORT RATIO


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Geomechanics Rock Mass Rating, RMR

Tunneling Quality Index, Q

I n - s

i t u

M o d u l u s o f

D e f o r m a t

i o n .

G P a

E m = 2RMR-100

E m = 25log 10Q

E m = 10 (RMR-10)/40

0

90

60

30

100500 20 803010 9060 7040

0.040.01 1 4 10 40 100 400

Case HistoriesSerafim and PereiraBieniawski

ROCK MASS MODULUS OF DEFORMATION

SUPPORT

Use of UNWEDGE and stereographic techniques todesign openings in jointed rock Use of SWEDGE for design of rock slopes

TYPES OF SUPPORT TYPES OF SUPPORT


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TYPES OFSUPPORT


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TYPES OF SUPPORT

HIGHWALL AUGERING

HIGHWALL AUGERING

f-rock mass classification

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