Rock Mass Strength and Scale Effects
Transcript of Rock Mass Strength and Scale Effects
![Page 1: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/1.jpg)
Rock Mass Strengthand Scale Effects
School of Civil & Environmental EngineeringThe University of New South Wales
Sydney, Australia
![Page 2: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/2.jpg)
Rock Mass Strength
DO NOT USE IT!Unless you have
investigated all possible structurally controlled
mechanisms first
![Page 3: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/3.jpg)
From an international journal:
“Different methods can be used to assess the stability of rock and/or soil slopes – the selection of a suitable method being primarily a function of the availability of geotechnical data.”
![Page 4: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/4.jpg)
![Page 5: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/5.jpg)
Nattai escarpment failure
Failure is through numerous defects and/or weak intact rockImpossible to determine precise failure path
![Page 6: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/6.jpg)
![Page 7: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/7.jpg)
structure
mass
![Page 8: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/8.jpg)
![Page 9: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/9.jpg)
Consideration of GSI for slopes
![Page 10: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/10.jpg)
Strength of Intact Material
• Intact rock exhibits a scale effect for block sizes up to at least one metre.
Specimen Diameter (mm)2500
1.3
0.7
UCSSAMPLE / UCS50mm
Hoek & Brown (1980)
![Page 11: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/11.jpg)
RQD
• Based on a fixed length of 100mm– relevance for large rock masses??
• Includes all joints in borehole– may not be significant to large scale
behaviour• Is in addition to spacing
![Page 12: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/12.jpg)
Defect Spacing
• Similar problems to RQD• Developed for underground tunnels of the
order of 10 - 20m in span.• Maximum rating for spacing intervals of:
> 3m - RMR76
> 2m - RMR89
![Page 13: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/13.jpg)
Slopes with Equivalent GSI
10m 100m
Spacing = 2m
![Page 14: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/14.jpg)
Joint Condition
• The scale of a problem affects:– persistence (maximum length >20m)– aperture– roughness– infilling– weathering
![Page 15: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/15.jpg)
Very rough
Smooth&
infilled
Very roughSmooth
![Page 16: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/16.jpg)
Q
• Depends on:– RQD– Number of joint sets– Joint roughness– joint alteration
• Similar problems to RMR factors.
a
r
n JJ
JRQDQ ×=
![Page 17: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/17.jpg)
GSI80
50
10
30
Dec
reas
ing
of in
terlo
ckin
g ro
ck p
iece
s
Decreasing surface quality• GSI Table includes structure & surface conditions.
• Scale independent, providing ‘scale of problem’ is used.
• Intact or massive with very good surfaces GSI > 80 (Hoek, 1999)
• Foliated/sheared not included
![Page 18: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/18.jpg)
Hoek & Browna
cc sm
+
′+′=′
σσσσσ 3
31
• Intact Rock– si = 1– ai = 0.5– mi = triaxial tests or function of rock type
• Rock Mass– sb, ab, mb are functions of the Geological Strength
Index, GSI
![Page 19: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/19.jpg)
20 40 60 80 1000
0.5
1.0
0
abmb/mi
sb
GSI
−−
=D
GSImm
i
b
1428100exp
−−
=D
GSIs39100exp ( )32015
61
21 −− −+= eea GSI
Hoek et al (2002)
![Page 20: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/20.jpg)
Issues with Hoek-Brown
• Intact rock:– Assumes a constant ai = 0.5 (c.f. 0.4-0.9)– mi often based on rock type– Discussed in detail GEOENG 2000
• Rockmass:– Maximum ab = 0.62 (GSI = 5 from Table)– Could expect that a weak rock mass should
have an ab approaching unity– Rockfill has an ab of approximately 0.9
![Page 21: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/21.jpg)
Laboratory test database & analysis
• Data from many sources• 3817 test results forming 485 sets• Most commonly adopted criterion is the
Hoek-Brown• Fitted the Hoek-Brown criterion to these
data• Method of fitting extremely important• For many data sets, mi and σc are not
independent, σc → 0 as mi → ∞
![Page 22: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/22.jpg)
Regression of Intact Data
Sigma 3 (MPa)
Sig
ma
1 (M
Pa)
0
50
100
150
200
250
-10 10 30 50 70
UCS miArtificial data 10.012.0Normal eqn & LS14.97.75Extended eqn & LS 8.4615.5Ext eqn & mod LS 10.712.0Fix UCS & LS 10.05.21Excl Sc or St & LS 6.1921.4DS^2 & LS 3.9735.2Log & LS 8.094.12
Not shownExcl St & LS 9.5213.7DS^2 with UCS fixed 10.013.8DS^2 & Least abs sum 9.1815.4Log with excl St 9.6712.2
![Page 23: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/23.jpg)
−≤=
−>
++=
ic
icc
ic
m
mm
σσσσ
σσσσσσσ
331
3
5.0
331
for
for 1
Sigma 3 (MPa)
Sig
ma
1 (M
Pa)
0
10
20
30
40
-2 0 2 4 6
UCS miArtificial data 10.012.0Normal eqn & LS14.97.75Extended eqn & LS 8.4615.5Ext eqn & mod LS 10.712.0Fix UCS & LS 10.05.21Excl Sc or St & LS 6.1921.4DS^2 & LS 3.9735.2Log & LS 8.094.12
Not shownExcl St & LS 9.5213.7DS^2 with UCS fixed 10.013.8DS^2 & Least abs sum 9.1815.4Log with excl St 9.6712.2
![Page 24: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/24.jpg)
Rock Type vs mi
mite
st
0
10
20
30
40
Cla
ysto
nFi
recl
ayG
reen
sto
Mud
ston
eS
erpe
nti
Schi
stS
hale
Cha
lkC
hlor
itiLi
mes
ton
Mar
ble
Silt
ston
Sla
teB
ioca
lca
Dol
omite
Anh
ydrit
Sal
tC
oal
Tuff
Pyr
ocla
sR
hyol
iteA
plite
Bas
alt
Lam
prop
hTr
achi
teA
gg tu
ffG
reyw
ack
Whi
nsto
nA
ndes
iteD
iaba
seD
oler
iteQ
uartz
doS
ands
ton
Gra
nite
1N
orite
Qua
rtzit
Dun
iteE
clog
iteG
abbr
oP
erid
oti
Am
phib
olD
iorit
eQ
uartz
diG
rano
dio
Gne
iss
Gra
nite
![Page 25: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/25.jpg)
Intact Rock Modificationsia
cic m
+
′+′=′ 13
31 σσσσσ
ti
ciim σ
σ≈
+
+≈
7exp1
2.14.0i
i ma
Note that triaxial testing is preferred
![Page 26: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/26.jpg)
Exponent vs mi
mi
Alp
ha
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40 50
φ0 15 25
35 45 55 65φ0
![Page 27: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/27.jpg)
Rock mass modifications
• New equations are required to account for the transition between a variable ai and miat GSI = 100 and ab min = 0.9 at GSI = “0”
GSI 100 “0”ab ai 0.9mb mi 2.5sb 1 0
Intact rock testing
Rockfill testing
![Page 28: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/28.jpg)
Rock mass modifications
• Parameter ‘m’– Predominantly affects friction angle at low stress– Should reduce with GSI (less interlocking)
• Parameter ‘s’– Predominantly contributes to cohesion– Expect rapid decrease in s with GSI– H-B use exponential drop
• Exponent ‘a’– Needs to drop from ai to amin limit
![Page 29: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/29.jpg)
Data
• Habimana et al (2002) data used– 35 triaxial tests on rock mass for a hydroelectric plant
& tunnel in Swiss Alps– Tests grouped into GSI = 15, 25, 50, 80– Best quality published data known to authors
• Process used by authors:– Data for each GSI was statistically analysed using the
H-B equation to get ab, mb and sb
– Statistical analysis of results to get new equations for ab, mb and sb
– Global analysis of data to check results
![Page 30: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/30.jpg)
Rock Mass Equations( )
−
=1
1585exp
min
GSI
sb
=5.2
100max
GSImm
i
b
( )
−−+=
i
biib m
maaa 3075exp9.0
![Page 31: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/31.jpg)
Min GSIand mbfrom rockfill
Note linear relationship compared to exponential H-B
0
5
10
15
20
25
0 20 40 60 80 100GSI
mb smb
Equation 6
![Page 32: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/32.jpg)
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100GSI
s b authors' eqn for sauthors' eqn for atest data - atest data - s
αbsb
αbsb
sbαbsb
sb
abab
ab
Equation 7
Equation 8
Max GSI for a well interlocked rockmass
Max abfrom rockfill
0
![Page 33: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/33.jpg)
Transition curve from GSI = 100 to GSI = 0 for mi = 40
mb
( )
−−+=
i
biib m
maaa 3075exp9.0
10.9
0.4
0
a
400 10
Intact rock relationship
+
+≈
7exp1
2.14.0i
i ma
mi
Rock mass limit GSI = 0
Intact sample GSI = 100GSI ≈ 25
=5.2
100max
GSImm
i
b
( )
−−+=
i
biib m
maaa 3075exp9.0
![Page 34: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/34.jpg)
Comparison for mi = 405
0 1
GSI = 100
GSI = 10
Hoek, 2002Author
σ′1/σc
σ′3/σc
![Page 35: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/35.jpg)
Comparison for mi = 45
0 1
GSI = 100
GSI = 10
Hoek, 2002Author
σ′1/σc
σ′3/σc
![Page 36: Rock Mass Strength and Scale Effects](https://reader030.fdocuments.net/reader030/viewer/2022012804/61bd270761276e740b0fe5aa/html5/thumbnails/36.jpg)
Conclusions
• Rock mass strength should only be used where the geological model shows that it is valid
• Parameters should be considered on the scale of the slope
• New equations have been developed for the Hoek-Brown parameters to address issues
• Further published data on rock mass would help to give more confidence in the equations