Rock mass classification

ROCK MASS CLASSIFICATION
Prepared by: Binod Gurung (071BCE09) 1
ROCK MASS CLASSIFICATION

OBJECTIVES
WHY?
METHOD
HOW?

▪To divide a particular rock mass into groups of similar
behavior.
▪To provide a basis for understanding the
characteristics of each group.
▪To facilitate the planning and the design of excavations
in rock by yielding quantitative data required for the
solution of real engineering problems.
▪To provide a common basis for effective
communication among all persons concerned with a
tunneling project.
Prepared by: Binod Gurung (071BCE09)
3
WHY?

▪Developed by Bieniawski's in 1973.
▪Determined from 49 case histories
▪Parameters
➢a. Uniaxial compressive strength of intact rock material.
➢b. Rock quality designation (RQD).
➢c. Spacing of discontinuities.
➢d. Orientation of discontinuities.
➢e. Condition of discontinuities.
➢f. Groundwater conditions.
GEOMECHANICS CLASSIFICATION (RMR)

▪STRENGTH TEST
➢Point load strength index test
❖ 𝐼𝑆 =
𝑃
𝐷 𝐸
2
o P=Load at failure
o 𝐷 𝐸 = Distance between
cones.
❖ UCS=24* 𝐼𝑆 for 50 mm dia.
❖ UCS=(14+0.175*D)* 𝐼𝑆
a) Diametrical test
b) Axial test
c) Block test
d) Irregular lump test

➢Uni-axial compressive test
❖UCS=P/A
o P= Load at failure
o A= cross-sectional area
of specimen
❖ Specimen should not be
store more than 30 days.

▪RQD
❖If core sample not
available
o RQD=115-3.3Jv

▪SPACING OF DISCONTINUITIES
➢In a zone specified, spacing of at least three set of
discontinuities is measured in a direction perpendicular
to them.
➢ The measuring for two set of discontinuities will be
considered conservative.

▪CONDITION OF DISCONITINUITIES
➢Condition of discontinuities is measured by observing
the persistence, aperture, roughness, infilling materials
and degree of weathering.
➢Roughness or the nature of the asperities in the
discontinuity surfaces is an important parameter
characterizing the condition of discontinuities.

a.Very rough. Near vertical steps and ridges occur on the
discontinuity surface.
b. Rough. Some ridge and side-angle steps are evident; asperities
are clearly visible; and discontinuity surface feels very
abrasive.
c. Slightly rough. Asperities on the discontinuity surfaces are
distinguishable and can be felt.
d. Smooth. Surface appears smooth and feels so to the touch.
e. Slickensided.Visual evidence of polishing exists.

▪ORIENTATION OF DISCONTINUITIES
➢Orientation of discontinuities is determined by the
measurement of the dip and strike.
➢Tunnel axis orientation either parallel to strike or
perpendicular to strike affect the stability of the
tunnel.

▪GROUNDWATER CONDITIONS
➢In the case of tunnels, the rate of inflow of groundwater per 10 m length is
determined.
➢General condition can be described as completely dry, damp, wet, dripping,
and flowing.
➢If actual water pressure data are available
water pressure
major principal stress
➢Major principal stress is vertical stress which is determined from the depth
below surface and increases with depth at 1.1 psi per foot of the depth
below surface.

Consider a slightly weathered quartzite in which a 20-ft-span tunnel is to be driven.
The following classification parameters were determined:
Items Value Rating
Uniaxial strength 155 MPa 12
RQD 85 % 17
Spacing of discontinuity 2.1 m 20
Condition of discontinuity( gouge
absent)
12
Groundwater Moderate inflow (wet) 7
Orientation of discontinuity Fair -5

▪Final RMR value= 63.Then,
▪Grade of rock=Second Class; good rock
▪Stand up time= 1 year for 10 m span
Since our tunnel has 20 ft. span so stand up time = 1
month from graph.
Rock mass modulus= 0.37*10^6 psi.

Rock mass class Excavation
Rock bolts (20 mm
diameter, fully
grouted)
Shotcrete Steel sets
I - Very good
rock RMR: 81-
100
Full face, 3 m advance. Generally no support required except spot bolting.
II - Good rock
RMR: 61-80
Full face , 1-1.5 m advance.
Complete support 20 m from
face.
Locally, bolts in
crown 3 m long,
spaced 2.5 m with
occasional wire mesh.
50 mm in
crown where
required.
None.
III - Fair rock
RMR: 41-60
Top heading and bench
1.5-3 m advance in top heading.
Commence support after each
blast. Complete support 10 m
from face.
Systematic bolts 4 m
long, spaced 1.5 - 2 m
in crown and walls
with wire mesh in
crown.
50-100 mm
in crown and
30 mm in
sides.
None.
IV - Poor rock
RMR: 21-40
Top heading and bench
1.0-1.5 m advance in top
heading. Install support
concurrently with excavation,
10 m from face.
Systematic bolts 4-5
m long, spaced 1-1.5
m in crown and walls
with wire mesh.
100-150 mm
in crown and
100 mm in
sides.
Light to medium ribs
spaced 1.5 m where
required.
V – Very poor
rock RMR: < 20
Multiple drifts 0.5-1.5 m
advance in top heading. Install
support concurrently with
excavation. Shotcrete as soon
as possible after blasting.
Systematic bolts 5-6
m long, spaced 1-1.5
m in crown and walls
with wire mesh. Bolt
invert.
150-200 mm
in crown, 150
mm in sides,
and 50 mm
on face.
Medium to heavy ribs
spaced 0.75 m with
steel lagging and fore
poling if required.
Close invert.
Guidelines for excavation and support of 10 m span rock tunnels in accordance
with the RMR system

23

NORWEGIAN Q-SYSTEM
▪ Developed by Norwegian scientist in 1974 A.D.
▪ Proposed after the study of 200 tunnel case histories.
▪ It is a quantitative classification system
▪ Engineering system enabling the design of tunnel supports
▪ Six parameters:
➢ RQD
➢number of joint sets
➢roughness of the most unfavorable joint or discontinuity
➢degree of alteration or filling along the weakest joint
➢water inflow
➢stress condition

▪ 𝑄 =
𝑅𝑄𝐷
𝐽 𝑛
×
𝐽 𝑟
𝐽 𝑎
×
𝐽 𝑤
𝑆𝑅𝐹
Where,
➢
𝑹𝑸𝑫
𝑱 𝒏
= measure of relative block size
➢
𝑱 𝒓
𝑱 𝒂
= interlock block strength of joints
➢
𝑱 𝒘
𝑺𝑹𝑭
= empirical factor describing the 'active stress'.

▪The following steps are involved in applying the Q-
System:
➢Classify the relevant rock mass quality.
➢Choose the optimum dimensions of excavation.
➢Estimate the appropriate permanent support.

▪ 𝐃 𝐞 =
𝐄𝐱𝐜𝐚𝐯𝐚𝐭𝐢𝐨𝐧 𝐬𝐩𝐚𝐧,𝐝𝐢𝐚𝐦𝐞𝐭𝐞𝐫 𝐨𝐫 𝐡𝐞𝐢𝐠𝐡𝐭 (𝐦)
𝐄𝐒𝐑
Excavation Support Ratio (ESR) is related to the use for which the
excavation is intended and the extent to which some degree of
instability is acceptable.

▪The length of bolt is not specified in the support table but
must be calculated from the equation
𝐿 =
2 + 0.15𝐵
𝐸𝑆𝑅
where B is the excavation width
▪The maximum unsupported span can be obtained by
Maximum unsupported span=2(ESR) Q0.4

▪Permanent pressure at roof;
𝑃𝑟𝑜𝑜𝑓 =
2.0
𝐽𝑟
∗ 𝑄−1/3
If no of joint set is less than three, the equation is
expressed as,
𝑃𝑟𝑜𝑜𝑓 =
1.0
𝐽𝑟
∗ 𝑄−
1
3 ∗
2
3
∗ 𝐽 𝑛
1/2

▪The following is known:
Joint set 1: Smooth, planar Jr=1.0
Chlorite coatings Ja=4.0
15 joints per meter
Joint set 2: Smooth, undulating Jr = 2
Slightly altered walls Ja = 2
5 joints per meter
Consider a water tunnel of 9-in (29.5 ft) span in a phyllite rock mass.

▪ Jv = 15 + 5 = 20
▪ RQD = 115 - 3.3 Jv =50 percent
▪ Jn =4
▪ Most unfavorable Jr/Ja = 1/4
▪ Minor water inflows: Jw = 1.0
▪ Uniaxial compressive strength of phyllite: 40 MPa
▪ Major principal stress: 3 MP
▪ Minor principal stress: 1 Mpa
▪ Major and minor principal stress are virgin stress.
▪ Thus: Ma /mi= 3 and UCS/ Ma =13.3 (medium stress), SRF = 1.0
▪ Q=3.1 (poor)
▪ Support estimate: B - 9 m, ESR - 1.6 for water tunnel
▪ B/ESR = 4.6

▪ Thus from graph,
➢ Reinforcement categories= Systematic bolting with unreinforced
concrete 4-10cm.
▪ 𝐿 =
2+0.15𝐵
𝐸𝑆𝑅
=
2+0.15∗9
1.6
=2.09 m
▪ Maximum unsupported span=2(ESR) Q0.4
▪ = 5.03 m
▪ 𝑃𝑟𝑜𝑜𝑓 =
2.0
𝐽 𝑟
∗ 𝑄−1/3
▪ =1.37 psi
▪ For temporary support, determination, either Q is increased to 5Q
or ESR is increased to 1.5 ESR.

RMI METHOD
▪ Developed by Palmstorm in 1995.
▪ It is composed of mainly four jointing characteristics
➢ Block volume or density of joints
➢ Joint roughness
➢ Joint alteration and
➢ Joint size

▪ This main principle of Rock Mass index is expressed as
𝑹𝑴𝒊 = 𝝈 𝒄 × JP
where, 𝜎𝑐= uniaxial compressive strength of intact rock (Mpa)
JP= the jointing parameter
𝑱𝑷 = 𝟎. 𝟐 𝒋 𝑪 × (𝑽 𝒃) 𝑫
(𝐷 = 0.37𝑗 𝐶
−0.2
)
𝑗 𝐶 = 𝑗 𝑅 ×
𝑗 𝐿
𝑗 𝐴
Where
Vb=block volume
Jc=joint condition
jR = the joint roughness depend on smoothness factor(js) and waviness factor (jw)
jA = the joint alteration
jL = the joint length

50

▪RMI in massive rock
➢Rmi=compressive strength of intact rock* massivity factor
➢ Massivity factor> Joint parameter
➢ Massivity factor is generally taken as 0.5

▪Ground condition factor( similar to Q value) can be determined
from RMi value.
➢Gc=Rmi*GW*SL*C
where GW=Groundwater condition i.e. inflow through opening
SL=stress level
C=an adjustment factor for wall and inclined roof

▪SUPPORT ESTIMATE BY RMI METHOD
➢ For discontinuous (blocky) ground
➢ For continuous ground
❖ For discontinuous (blocky) ground
✓ Determine Ground condition factor (Gc=Rmi*K1)
where K1=GW*SL*C
✓ Determine size ratio (Sr)
𝑆𝑟 =
𝐷𝑡
𝐷𝑏
∗
𝐶0
𝑁𝑗
=
𝐷𝑡
𝐷𝑏
∗ 𝐾2
where Dt= Diameter or span of tunnel(m)
Db= Equivalent block diameter ( Vb^(1/3))

▪C0= adjustment factor for orientation of joint
▪Nj=adjustment factor for number of joint
▪These K1 and K2 are determined from table shown below:

❖ For continuous ground
✓Rock mass with few joints generally stable and generally does not
need any support, except for some scaling work in drill and blast.
✓But, it may squeeze or burst due to time dependent deformation
due to over stressed ground.
✓Thus, though continuous ground is of few rock joints, it should also
be provided with support.
✓The chart of support estimation is shown below:

Rock mass classification

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