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INITIAL RESULTS IN THE DETERMINATION OF GEOTECHNICAL SCOUR NUMBERS FOR
FOUNDATION ROCK AT FIFTEEN WEST VIRGINIA BRIDGE SITES
William L. Niemann, Ph.D., P.G., E.I.T.
Marshall University Geology Dept.
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM (NCHRP) PROJECT 24-29 / REPORT 717:
“SCOUR AT BRIDGE FOUNDATIONS ON ROCK” (http://www.trb.org/Main/Blurbs/167222.aspx)
KEATON, MISHRA & CLOPPER (2012)
Context of Report: 1) Current methodology for scour prediction is inexact--considers rock as either erodible or non-erodible.
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM (NCHRP) REPORT 717: “SCOUR AT
BRIDGE FOUNDATIONS ON ROCK” (2012)
Context (cont.): 2) If rock is deemed erodible, scour prediction is typically based on behavior of sand, resulting in
overestimates of scour and unnecessarily expensive bridge designs.
3) Criteria for bridge design should better reflect realistic response of erodible rock to hydraulic forces. 4) Methodology required for estimating scour along with corresponding design and construction guidelines.
WEST VIRGINIA DEPARTMENT OF HIGHWAYS (WVDOH) RESEARCH PROJECT 273:
“CRITERIA FOR PREDICTIONG ERODIBLE ROCK IN WEST VIRGINIA”
• MICHAEL PUMPHREY (PC),
WVDOH RESEARCH AND SPECIAL STUDIES
• JOE CARTE (PM) & DOUG KIRK (PM), WVDOH
• BILL NIEMANN (PI) & ISAAC WAIT (PI), MARSHALL UNIVERSITY
• WAEL ZATAR (PI)
RAHALL TRANSPORTATION INSTITUTE
• TERRY DOWNS, BRAIN GREENE, GANNETT FLEMING, INC..
• JEFF KEATON, AMEC, INC.
WEST VIRGINIA DEPARTMENT OF HIGHWAYS (WVDOH) RESEARCH PROJECT 273:
“CRITERIA FOR PREDICTIONG ERODIBLE ROCK IN WEST VIRGINIA”
1) Apply methodology from NCHRP 24-29 to bridge sites in West Virginia where scour has occurred and can be quantified. 2) Select sites from all 10 WVDOH districts and across a range in rock types and stream flow conditions. 3) Develop models that can more accurately predict rock scour under conditions typically found in West Virginia.
WEST VIRGINIA DEPARTMENT OF HIGHWAYS (WVDOH) RESEARCH PROJECT 273:
“CRITERIA FOR PREDICTIONG ERODIBLE ROCK IN WEST VIRGINIA”
Deliverables (2013): 1) Summary of research including investigation and data analysis. 2) Map of scour-able areas in state. 3) Best management practices (BMP) to account for rock scour in bridge design and evaluation.
MODES OF SCOUR
• Cavitation
• Solution
• Quarrying / plucking (“dislodgement”) *
• Erosion / flaking * *
* NCHRP 24-29: developed methodology for
estimating depth of scour.
** NCHRP 24-29: developed methodology for estimating depth and rate of scour.
SITE SELECTION CRITERIA
• Geographic distribution
• Exposed rock
• Variety of rock types / rock structure
• Measurable scour
• Initial assessment of scour mode
• Availability of hydraulic / hydrologic data
• Drill-rig accessible
USGS, Prof Pap. 1110A-1110-L
10 of 15 sites
11
4
1
0
2
4
6
8
10
12
Sandstone Mudrocks Carbonate
No.
of B
ridge
s
Axis Title
Dominant Rock Types
1 2
5
2 3
2
0123456
No.
of B
ridge
s Presumed Modes of Scour
DRILLING AND SAMPLING
Bridge Plan View
Deck
Left Abutment
Right Abutment Flow
Flow
Boring 1
Boring 2
DRILLING AND SAMPLING
Bridge Cross-sectional View
Deck
Abutment Fill / Soil
Rock
Borehole extended 10 feet below
abutment
Scoured interval
Water level (variable)
DRILLING AND SAMPLING
• 15 bridges
• 10 districts
• 15 counties
• 17 days
• 694 feet of drilling
• 30 rock cores
• 391 feet of rock coring
• 45 samples for Modified Slake Durability (MSD)
ROARING CREEK BRIDGE Pendleton Co., WV • Sandstone: massive, hard, jointed • Pocono Group (Mississippian age) • Mode of scour: dislodgement
ROARING CREEK BRIDGE Pendleton Co., WV
COON CREEK BRIDGE Summers Co., WV • Sandstone: weathered, vuggy, poorly cemented • Princeton Formation (Mississippian age) • Mode of scour: erosion / flaking
COON CREEK BRIDGE Summers Co., WV
Process for estimating depth and
time rate of scour for bridges
affected by erosion / flaking
of foundation rock*
*no gage or historical cross-section data
MODIFIED SLAKE DURABILITY (MSD) TESTING
MODIFIED SLAKE DURABILITY (MSD) TESTING
• Conventional slake durability tests (ASTM D4644) consist of two cycles of 10-minutes each and mass
is determined in a dry state.
• Modified slake durability tests consist of multiple cycles (5-9) of 60-minutes each
and mass is determined in a saturated-surface dry (SSD) state per ASTM.
MODIFIED SLAKE DURABILITY (MSD) TESTING
R² = 0.9595
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
0 50 100 150 200 250 300 350 400 450 500
Wei
ght L
oss
(%)
Time (min)
Example test data
GEOTECHNICAL SCOUR NUMBERS • Stream Power: rate at which hydraulic energy is
expended
ω = Unit stream power [L*force/T/L2] = [force/T/L]
γ = specific weight of water [force/L3]
d = depth [L]
S = slope of the channel [L/L]
V = velocity of water flow [L/T]
GEOTECHNICAL SCOUR NUMBERS • Equivalent Stream Power from MSD:
ω = L * t [ W(X) + W(X+1) ] / [2 * A(1/8) ]
ω = equivalent stream power (force/L*T),
L = equivalent length (L),
t = incremental time of test interval (T),
[ W(X) + W(X+1) ] / 2 = average weight of sample during
test interval (force),
A(1/8) = fractional surface area of drum occupied by sample (L2).
GEOTECHNICAL SCOUR NUMBERS • Equivalent Depth of Scour (from MSD)
Hourly measurements result in a series of sample volume
losses, Vi , which are then normalized by a unit area to derive a depth of scour for each interval:
D(X) = Vi (X) / unit area
D(X) = depth of scour corresponding to test interval
x (= 60, 120, 180…) minutes (L),
Vi (X) = incremental volume lost during test interval x minutes (L3),
unit area = area in appropriate units (L2).
GEOTECHNICAL SCOUR NUMBERS
NCHRP 24-29
y = 2.42E-04x - 1.75E-03 R² = 7.95E-01
y = 1.38E-04x - 1.15E-03 R² = 5.43E-01
y = 1.00E-04x - 7.83E-04 R² = 8.87E-01
0.0E+00
1.0E-04
2.0E-04
3.0E-04
4.0E-04
5.0E-04
6.0E-04
7.0E-04
6.0 6.5 7.0 7.5 8.0 8.5 9.0
Eq. S
cour
Dep
th (f
t)
Eq. Stream Power (ft-lb/s//ft2)
Roaring Creek
ROA-SEXP-01-01
ROA-01-11.2'-11.9'
ROA-02-11.2'-11.6'
Linear (ROA-SEXP-01-01)Linear (ROA-01-11.2'-11.9')Linear (ROA-02-11.2'-11.6')
presumed mode of scour:
dislodgement
Sandstone: massive, hard, jointed
y = 2.34E-04x - 1.69E-03 R² = 7.95E-01
y = 5.32E-04x - 4.66E-03 R² = 8.82E-01
y = 5.68E-04x - 4.84E-03 R² = 9.65E-01
0.0E+00
1.0E-04
2.0E-04
3.0E-04
4.0E-04
5.0E-04
6.0E-04
7.0E-04
6.0 6.5 7.0 7.5 8.0 8.5 9.0
Eq. S
cour
Dep
th (f
t)
Eq.Stream Power (ft-lb/s//ft2)
Coon Creek
COO-SEXP-01-01
COO-01,9.7'-10.2'
COO-02,7.4'-8.0'
Linear (COO-SEXP-Linear)
Linear (COO-01,9.7'-10.2')
Linear (COO-02,7.4'-8.0')
presumed mode of scour:
erosion / flaking
Sandstone: weathered, poorly cemented, vuggy
NCHRP 24-29
y = 1.0E-03x
y = 1.0E-04x
y = 1E-05x
y = 3.0E-04x
y = 2.00E-04x
0.00E+00
5.00E-04
1.00E-03
1.50E-03
2.00E-03
0 1 2 3 4 5 6 7 8 9 10
Eq. S
cour
Dep
th (f
t)
Eq. Stream Power ( ft-lbs / s / ft2 )
WVDOT -- GEOTECHNICAL SCOUR NUMBERS
Paden Fork SsCoon Creek SsRoaring Creek SsLaurel SsBridge Fork SsCucumber SsGrassy Run SsBeverly Siltst/Sh/ClaystLaurel X Sh (carb.)Caldwell Run Sh / CoalMish Ls
NCHRP 24-29
NCHRP 24-29
Non
-dur
able
rock
s –
sc
our b
y er
osio
n / f
laki
ng
THE NEXT PHASE: INTEGRATING LAB AND FIELD DATA
NCHRP 24-29
THE NEXT PHASE: INTEGRATING LAB AND FIELD DATA D
urab
le ro
cks
-- sc
our b
y
Qua
rryi
ng /
Pluc
king
Block thickness
y = 1.42E-04x - 1.07E-03 R² = 7.90E-01
y = 1.32E-04x - 1.02E-03 R² = 4.85E-01
y = 1.19E-04x - 6.72E-04 R² = 9.97E-01
0.0E+00
1.0E-04
2.0E-04
3.0E-04
4.0E-04
5.0E-04
6.0E-04
7.0E-04
6.0 6.5 7.0 7.5 8.0 8.5 9.0
Eq. S
cour
Dep
th (f
t)
Stream Power (ft-lb/s//ft2)
Beverly
BEV-SEXP-001
BEV-01-18.7'-19.5'
BEV-02A-17.4'-18.0'
Linear (BEV-SEXP-001)
Linear (BEV-01-18.7'-19.5')
presumed mode of scour:
primary--dislodgement secondary--erosion/flaking
Claystone RQD?
0
1
2
3
4
5
6
7
8
9
10
0.0E+00 1.0E-04 2.0E-04 3.0E-04 4.0E-04 5.0E-04 6.0E-04
Thre
shol
d (E
quiv
alen
t Str
eam
Pow
er)
GSN
GSN vs. Threshold
Paden Fork Ss
Coon Creek Ss
Roaring Creek Ss
Laurel Ss
Bridge Fork Ss
Cucumber Ss
Grassy Run Ss
Mish Ls
Beverly Siltst/Sh/Clayst
Caldwell Run Sh / Coal
Laurel X Sh (carb.)
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