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Negative Skin Friction
Aaron S. Budge, Ph.D., P.E.Minnesota State University, Mankato
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A QUICK ITEM OF INTEREST
Mn/DOT-funded survey to State DOTs
41 out of 50 States eventually responded(with some prodding!)
One of several questions How much do
states use driven piles, vs. drilled shafts, vs.shallow foundations for transportation
applications?
Of the first 35 respondents, the following
results were obtained
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RESULTS OF THE STATE DOT SETUP
SURVEY2 For transportation applications in your state, which of the
following best describes the use of Drilled Shafts:
0 0% Our agency never uses Drilled Shafts for
transportation applications
2 6% Our agency rarely uses Drilled Shafts for
transportation applications
20 57% Our agency occasionally uses Drilled Shafts for
transportation applications
13 37% Our agency often uses Drilled Shafts fortransportation applications
0 0% Our agency almost exclusively uses Drilled
Shafts for transportation applications
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RESULTS OF THE STATE DOT SETUP
SURVEY3 For transportation applications in your state, which of the
following best describes the use of Shallow Foundations:
0 0% Our agency never uses Shallow Foundations for
transportation applications
8 23% Our agency rarely uses Shallow Foundations for
transportation applications
12 34% Our agency occasionally uses Shallow
Foundations for transportation applications
15 43% Our agency often uses Shallow Foundations fortransportation applications
0 0% Our agency almost exclusively uses Shallow
Foundations for transportation applications
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FUNDAMENTAL QUESTION #1:
WHAT IS NEGATIVE SKIN FRICTION(a.k.a. DOWNDRAG or DRAGLOAD)?
Downward load at pile topresisted by upward (pos.)
skin friction in soil layers and
end bearing at the pile toe- USUALLY --..
But, what if the soil moves
DOWNWARD with respect tothe pile???
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FUNDAMENTAL QUESTION #1:
WHAT IS NEGATIVE SKIN FRICTION(a.k.a. DOWNDRAG or DRAGLOAD)?
In what situations might this phenomenon occur?1 Embankment fill placed after pile driving?
(WHY?)
2 Site dewatering subsequent to pile driving?3 Soil liquefaction due to seismic activity or
other vibration?
4 - Other cases?
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FUNDAMENTAL QUESTION #1:
WHAT IS NEGATIVE SKIN FRICTION(a.k.a. DOWNDRAG or DRAGLOAD)?
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FUNDAMENTAL QUESTION #1:
WHAT IS NEGATIVE SKIN FRICTION(a.k.a. DOWNDRAG or DRAGLOAD)?
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As an example:Maple Grove, MN
New Interchange
Soil Profile at site =>
Predicted 3-5 inches
of settlement.Piles driven prior
to fill placement.
PROBLEMS?
(Stay tuned!)
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FUNDAMENTAL QUESTION #1:
WHAT IS NEGATIVE SKIN FRICTION(a.k.a. DOWNDRAG or DRAGLOAD)?
Should we estimate negative skinfriction using the same relationships
and calculations as we use for
positive skin friction (Nordlund,Tomlinson, etc.)?
Neutral plane and other approachesare available to identify where
negative friction stops and positive
friction begins.
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FUNDAMENTAL QUESTION #1:
WHAT IS NEGATIVE SKIN FRICTION(a.k.a. DOWNDRAG or DRAGLOAD)?
With respect to Load and ResistanceFactor Design (LRFD), how should
dragload be appropriately dealt with?
Reduced pile capacity!
Dead Load?
Live Load?Which load factor is appropriate?
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FUNDAMENTAL QUESTION #2:
HOW MUCH DOWNDRAG LOAD EXISTS?
Mn/DOT has funded several studies
to quantify the magnitude ofdowndrag experienced. Several
sites have been monitored where
settlement of the foundationmaterial with respect to the piling
was anticipated (i.e., fill placed near
piles AFTER pile driving.)
HOW CAN WE MEASURE Rs1?
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HOW TO MEASURE DOWNDRAG LOAD
VIBRATING WIRE GAGES
FIBER OPTIC or RUGGEDIZED RESISTANCE GAGES
Do you install gages before or after driving?
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INSTRUMENTATION OF A BRIDGE
(GAGES INSTALLED BEFORE DRIVING)
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INSTRUMENTATION OF A BRIDGE
(GAGES INSTALLED AFTER DRIVING)
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INSTRUMENTATION OF A BRIDGE
(GAGES INSTALLED AFTER DRIVING)
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INSTRUMENTATION OF A BRIDGE
WHAT ARE THESE?
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OPTIONS AVAILABLE TO REDUCE
NEGATIVE SKIN FRICTION?
BITUMEN COATING
PLASTIC COATING TEFLON COATING
PILE SLEEVES
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Instrumented
Sleeved PileInstrumented
Unsleeved Pile
INSTRUMENTATION OF A BRIDGE
MAPLE GROVE, MN
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INSTRUMENTATION OF A BRIDGE
MAPLE GROVE, MN
ShapeAccelArray (SAA)
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INSTRUMENTATION OF A BRIDGE
MAPLE GROVE, MNShapeAccelArray (SAA)
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INSTRUMENTATION OF A BRIDGE
MAPLE GROVE, MN
Gage A-48
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DATA COLLECTED FIRST 50 DAYS
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STRAIN DATA COLLECTED 11 MONTHS
0
50
100
150
200
250
300
0 50
100
150
200
250
300
350
Elapsed Time (days)
RawMicrostrain
.
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A CLOSER LOOK AT TEMPERATURE
With the apparent creep during periods ofminimal construction activity, the thermal
strain effects were of interest Basic idea if the temperature changes in the
soil/pile such that the pile wants to change
length but cannot (due to friction forces of thesoil), thermal induced strains will bemeasured by the gages that should NOT be
included in the downdrag/pile load response.
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TEMP DATA COLLECTED 11 MONTHS
0
5
10
15
20
25
30
050
100
150
200
250
300
350
Elapsed Time (days)
Temperatu
reReading(Celsius).
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A CLOSER LOOK AT TEMPERATURE
-20
-15
-10
-5
0
5
10
-15.0 -10.0 -5.0 0.0 5.0
Delta Temp (deg F)
Delta
Strain(micros
train).
A-01 A-11 A-21 A-31 A-41 A-48
A-50A A-50B A-55A A-55B
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CORRECTED DATA 11 MONTHS
0
50
100
150
200
250
300
0 50
100
150
200
250
300
350
Elapsed Time (days)
Microstrain
.
Raw Microstrain Temperature Corrected Microstrain
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CORRECTED DATA 11 MONTHS
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ESTIMATED VS. MEASURED
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LOAD PROFILES
0
10
20
30
40
50
60
70
0 50 100 150 200 250 300
Depthfrom
PileCap
(ft)
.
Measured Total Load
Pile A Data -Unsleeved
8/1/2009 9/1/2009 10/1/2009 12/1/2009 1/1/2010 2/1/2010 3/1/2010 4/1/2010
5/1/2010 6/1/2010 7/1/2010 8/1/2010 9/1/2010 10/1/2010 11/1/2010 11/30/2010
"Original grade" at approx. 12 ft from
Pile Cap
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LOAD PROFILES
0
10
20
30
40
50
60
70
0 50 100 150 200 250 300
Depthfrom
PileCap(ft)
.
Measured Total Load
Pile B Data -Sleeved
8/1/2009 9/1/2009 10/1/2009 12/1/2009 1/1/2010 2/1/2010 3/1/2010 4/1/2010
5/1/2010 6/1/2010 7/1/2010 8/1/2010 9/1/2010 10/1/2010 11/1/2010 11/30/2010
"Original grade" at approx. 12 ft fromPile Cap
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LOAD PROFILE REGIONS
0 50 100 150 200 250 3000
10
20
30
40
50
60
70
0 50 100 150 200 250 300
UnsleevedSleeved
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Region 2
0 50 100 150 200 250 3000
10
20
30
40
50
60
70
0 50 100 150 200 250 300
UnsleevedSleeved
LOAD PROFILE REGIONS
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Region 3
0 50 100 150 200 250 3000
10
20
30
40
50
60
70
0 50 100 150 200 250 300
UnsleevedSleeved
LOAD PROFILE REGIONS
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INSTRUMENTATION OF A BRIDGE
CROSSTOWN COMMONS EXPANSION
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INSTRUMENTATION OF A BRIDGE
CROSSTOWN COMMONS EXPANSION
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INSTRUMENTATION OF A BRIDGE
CROSSTOWN COMMONS EXPANSION
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INSTRUMENTATION OF A BRIDGE
50
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-400
-350
-300
-250
-200
-150
-100
-50
0
50
4/2/2008
6/1/2008
7/31/2008
9/29/2008
11/28/2008
1/27/2009
3/28/2009
5/27/2009
7/26/2009
9/24/2009
11/23/2009
1/22/2010
Time Stamp
ChangeinStrain(microstr
ain).
Gage C12 Load Gage C8 Load Gage C6 Load Gage C1 Load
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CONCLUSIONS
At both sites, downdrag was measurable; at
Maple Grove observed pile strains began to
increase immediately when embankment fill wasplaced over compressible soils, prior to the
application of any structural loads.
At Crosstown, increases in strain continued forweeks after wall construction was completed,
corresponding to continued consolidation of
foundation soils.
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CONCLUSIONS
The strain and deflection data suggest that
additional strain, and therefore loading, is
intimately linked with the soil consolidationbehavior and construction and placement of the
bridge components.
Temperature effects were observed to addcomplexity to strain gage data evaluation,
although a period of steady-state load and
variable temperatures offered a method todevelop a correction factor.
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CONCLUSIONS
Pile strains were observed to decrease only with
the removal of the temporary soil surcharges;
added loads always appeared to result incorresponding increases in pile strain.
Vibrating wire strain gages and SAA deflection
monitoring systems were shown to be effectivesensors for the long term performance monitoring
of downdrag impacts.
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CONCLUSIONS
Mn/DOT continues to move forward with similar
monitoring projects (currently one underway with
Steele County and a second underway withWashington County) to improve understanding
with respect to the magnitude of negative skin
friction with the intent of developing a betterdesign strategy in the future.
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Questions???
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