Delivering Solutions Use of RMR to Improve Determination ... STGEC - Use of RM… · Use of RMR to...
Transcript of Delivering Solutions Use of RMR to Improve Determination ... STGEC - Use of RM… · Use of RMR to...
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Creating Value …
… Delivering Solutions
Scott Zang, P.E. Michael Baker Jr., Inc.
Use of RMR to Improve Determination of
the Bearing Resistance of Rock
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ASD Design
Q
v max < v allowable
• v allowable is a presumptive
allowable bearing capacity
• Obtained from AASHTO Specs
• Based on a limiting settlement
only (usually ½ to 1 “)
• Shear failure of foundation
assumed to be not controlling
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LRFD DesignService Limit State• Compute
displacements and
compare to tolerable
displacement
Strength Limit State• Check sliding failure
• Check overturning (e)
• Check bearing failure
z
x
Controlled for soft,
fractured rocks
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1. 10.6.3.5 allows flexibility in the method used
2. Many engineers use equation 10.8.3.5.4c-2
3. This is equivalent to the Nms method that was
presented in the old ASD specifications
Current LRFD Methodology
un qssmsq
Nms Coqult =ASD
V
qn > V 1 + 6e
B B
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Spread Footings
Drilled Shafts
Distribution
of Data
Comparison of Presumptive to Nms
Method based on RMR
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1
10
100
PresumptiveNms
RMR=50B
ea
rin
g R
es
ista
nc
e (
KS
F)
1000
0 20 40 60 80 100RMR
WV data usingm & s method(AASHTO 2006)
PresumptiveBearingResistance
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7
• Presumptive (AASHTO 2006 Table 10.6.2.6-1 from
NAVFAC DM-7)
Methods for Determining Bearing Resistance
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8
• Empirical correlation to RQD (AASHTO ASD
4.4.8.1.1)
Methods for Determining Bearing Resistance
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• Methods based on GSI
Methods for Determining Bearing Resistance
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• Modified bearing resistance equations and bearing
capacity factors (COE EM 1110-1-2908)
Methods for Determining Bearing Resistance
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• Empirical correlation of RMR to C and f (Serafim
and Pereira, 1983; Bieniawski, 1989) and General
bearing resistance equation
Methods for Determining Bearing Resistance
25
104
RMRFriction
RMRCCohesion
f
(in PSF)
qn = c Ncm + Df Nqm Cwq + 0.5 B N m Cw
WINNER for
RMR less than 50
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• Nms method (AASHTO ASD 4.4.8.1.2) or m & s
method (AASHTO 2006 10.8.3.5.4c-2)
Methods for Determining Bearing Resistance
un qssmsqWINNER for
RMR greater
than 50
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1
10
100
1000
0 20 40 60 80 100
RMR
Beari
ng
Resis
tan
ce (
ksf)
WV data usingm & s method(AASHTO 2006)
PresumptiveBearingResistance
RMR 83, C & f,
Gen. Eq.
B’ = 15’
RMR 83, m & s,
AASHTO 2006
Approx. Upper
Limit for Soil
24
Design
as Soil
Be
ari
ng
Re
sis
tan
ce
(K
SF
)Spread Footings
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1
10
100
1000
10000
0 20 40 60 80 100
RMR 1983
Be
ari
ng
Re
sis
tan
ce
(ksf)
WV data using m & smethod (AASHTO 2006)Presumptive BearingResistanceLoad Test Data
RMR 83, C & f,
Gen. Eq.
D = 5’
RMR 83, m & s,
AASHTO 2006
Approx. Upper
Limit for Soil
24
Design
as Soil
KR1
KR2 PM2
PM1
Be
ari
ng
Re
sis
tan
ce
(ksf)
Drilled Shafts
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Implementation of RMR
Inspector HandbookContract Documents
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RMR
rock mass strength
• qu
• RQD
• Spacing
• Condition
• Water
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Unconfined Compression StrengthHCSI – Hardness and Compressive Strength Index
HCSI Field Test RMR
0 Indented by Thumb Nail 0
1 Crumble under firm blows with point of geological pick.
Can be peeled by a pocket knife.
1
2 Can be peeled by a pocket knife with difficulty. Shallow
indentations made by firm blow of geological pick.
2
3 Cannot be scraped or peeled with a pocket knife.
Specimen can be fractured with single firm blow of
hammer end of geological pick.
4
4 Specimen requires more than one blow with hammer end
of geological pick to fracture it.
7
5 Specimen requires many blows of hammer end of
geological pick to fracture it.
12
6 Specimen can only be chipped with geological pick. 15
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• Uniaxial compression tests of
laboratory specimens
• Point load tests conducted in the
field or laboratory
Unconfined Compression Strength
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19
RMR
rock mass strength
• qu
• RQD
• Spacing
• Condition
• Water
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RQD
Record for each core run
For stratum thinner than core
run length - record the RQD
separately for stratum
Assign points for RQD
according to AASHTO LRFD,
Table 10.4.6.4-1
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RMR
rock mass strength
• qu
• RQD
• Spacing
• Condition
• Water
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Fracture Spacing
Estimate average fracture spacing for core run or
identified stratum (which ever is smaller)
length of identified interval
Average spacing =
number of discontinuities in
interval
Assign point value in accordance with AASHTO
LRFD Table 10.4.6.4-1
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RMR
rock mass strength
• qu
• RQD
• Spacing
• Condition
• Water
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Fracture Condition
Different sub-categories
• Discontinuity length
• Separation
• Surface roughness
• Infilling joint material
• Joint weathering
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Parameter Ratings
Discontinuity
length (feet)
< 3
5
3-10
4
10-30
2
30-65
1
>65
0
Separation
(inches)
None
5
<0.005
4
0.005-0.05
3
0.05-0.2
1
>0.2
0
Roughness Very
rough
5
Rough
4
Slightly
rough
2
Smooth
1
Slickensided
0
Infilling
(inches)
Hard filling Soft filling
None
5
<0.2
4
>0.2
3
<0.2
2
>0.2
0
Weathering None
5
Slightly
4
Moderate
2
Highly
1
Decomposed
0
Fracture Condition
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Sub: Discontinuity Length
• Estimate based on exposed outcrops
and site geology
• Compare with adjacent boreholes
• Use default value of 2 where this
parameter is hard to estimate
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Sub: Discontinuity Separation
• Observe core in split core barrel prior to
removal
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Sub: Roughness• Very rough : discontinuity
surface angular, amplitude > 0.2”
• Rough : amplitude < 0.2”
• Slightly rough : undulating surface, amplitude < 0.2
• Smooth discontinuities : planer surface
• Slickensided : discontinuity shows visible polishing
0.2”
0.2”
0.2”
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Sub: Infilling
Hard and soft infilling as previously described
• None
• Hard , thickness < 0.2”
• Hard , thickness > 0.2”
• Soft , thickness < 0.2”
• Soft , thickness > 0.2”
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Sub: WeatheringTerm Description Points
Decomposed Original minerals decomposed to secondary
minerals
Original rock fabric not apparent
Material can be easily broken by hand
0
Highly
Weathered
Original minerals almost entirely decomposed to
secondary minerals
Although original fabric maybe intact
Material can be granulated by hand
1
Moderately
Weathered
More than half of the rock is decomposed 2
Slightly
Weathered
Rock is discolored and noticeably weakened, but
less than half is decomposed.
4
Unweathered Rock shows no discoloration, loss of strength, or
other effect of weathering/alteration
5
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RMR
rock mass strength
• qu
• RQD
• Spacing
• Condition
• Water
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• For bearing resistance determination: base on anticipated service conditions.
• Parameter may change from that observed during the field investigation.
• Record all 5 components of the RMR
• Allows correction of the RMR values based on the final design configuration and use.
Ground Water
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RMR
rock mass strength
• qu
• RQD
• Spacing
• Condition
• Water
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Method for Recording RMR
RMR =
10+20+10+20+10=70
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Conclusions
• The Nms method of bearing resistance
determination greatly under estimates
the bearing resistance of rocks with
RMR<50
• An alternate procedure for estimating
bearing resistance of rocks with
RMR<50 shows better correlation to past
successful practice
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Conclusions
• Use of RMR methods requires consistent
implementation of the RMR in the field
• Additional guidance On RMR
determination is helping provide more
consistent and less conservative
estimates of bearing resistance
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Questions