Additional Pile Design Considerations - · PDF fileAdditional Pile Design Considerations...
Transcript of Additional Pile Design Considerations - · PDF fileAdditional Pile Design Considerations...
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Additional Pile Design Considerations
Patrick Hannigan
GRL Engineers, Inc.
PDCA 2015 Professor Driven Pile Institute
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What Are Additional Pile Design Considerations ?
• Time Dependent Soil Strength Changes
• Scour
• Densification
• Plugging
• Drivability
• Predrilling / Jetting
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Time Effects on Soil Resistance
Time effects are changes in soil resistance
that occur with time.
Soil Setup Relaxation
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Soil setup is a time dependent increase in the
soil resistance or capacity.
In clay soils, setup is attributed to increases in
effective stress as large excess positive pore
pressures generated during driving dissipate as
well as due to thixotropic effects.
Soil Setup
In sands, setup is attributed primarily to aging
effects and / or release of arching effects with time.
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Soil setup frequently occurs for piles driven in
saturated clays as well as loose to medium
dense silts and fine sands as the excess pore
pressures dissipate.
The magnitude of soil setup depends on soil
characteristics as well as the pile material and type.
Soil Setup
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Relaxation is a time dependent decrease in the
soil resistance or capacity.
During pile driving, dense soils may dilate thereby
generating negative pore pressures and
temporarily higher soil resistance.
Relaxation has been observed for piles driven in
dense, saturated non-cohesive silts, fine sands,
and some shales.
Relaxation
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How to Quantify Time Effects
• Non-instrumented Restrike Tests - uncertain if blow count change due to change
in hammer performance or soil strength
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• Multiple Static Load Tests (time, $$)
How to Quantify Time Effects
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How to Quantify Time Effects
• Dynamic Tests - inexpensive, cost-effective
- check for setup or relaxation
- short-term restrikes establish setup trend
- long-term restrikes provide confidence in capacity estimates
L
0 2L/c
L = Pile Length (ft)
c = wave speed (ft/s)
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Soil Setup
End of Initial
Driving
Restrike 8
Days Later Shaft = 3 x EOID
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Relaxation
Restrike 24
Hours Later
End of Initial
Driving
Toe = ½ EOID
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Are Time Effects Included in the Analysis Methods?
Static Analysis Methods
Dynamic Formulas
Wave Equation Analysis
Dynamic Testing
Static Load Testing
Setup -Yes
Setup - Yes
Time of Test
Time of Test
Depends
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1 d
ay
10 d
ays
100 d
ays
1000 d
ays
log time
pil
e c
ap
acit
y
Restrike testing generally performed
1 to 10 days after installation
Quantification of Time Effects
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Efforts to Predict Setup
Bullock et al (2005) – Side Shear Setup: Results from Florida Test Piles
Skov and Denver (1988) – Time-Dependence of Bearing Capacity of Piles
1log1log00000
t
t
Q
m
t
tA
f
f
Q
Q
S
S
S
S
S
S
Q = Qo (1+A log [ t / to ]) Sand: t0 = 0.5, A=0.2
Clay: t0 = 1.0, A=0.6
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where: A = Dimensionless setup factor
QS = Side shear capacity at time t
QS0 = Side shear capacity at reference time t0
fS = Unit side shear capacity at time t
fS0 = Unit side shear capacity at reference time t0
t = Time elapsed since EOD, days
t0 = Reference time, recommended to use 1 day
mS = Semilog-linear slope of QS vs. log t
1log1log00000
t
t
Q
m
t
tA
f
f
Q
Q
S
S
S
S
S
S
Efforts to Predict Setup
Bullock et al (2005) – Side Shear Setup: Results from Florida Test Piles
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0.001 0.01 0.1 1 10 100 10000
500
1000
1500
2000
2500
Aucilla, Static Test
Aucilla, Dynamic Test
1 min
15 min
60 min
QS0 =1021 kN (at t0 = 1day )
mS = 293.4 kN
Elapsed Time, t (days)
Pil
e S
ide S
hear
QS
(kN
)18” PSC with O-Cell at bottom
Bullock et al (2005) – Side Shear Setup: Results from Florida Test Piles
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For Closed-end Pipe Piles in Ohio Soils
Q = pile capacity (kips) at time t (hours) since EOID
Qo = pile capacity (kips) at EOID
Khan (2011) – Prediction of Pile Set-up for Ohio Soils
( Q includes shaft + toe)
Q = 0.9957 Qo t0.087
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Ng, Suleiman, and Sritharan (2013) – Pile Setup in Cohesive Soil. II:
Analytical Quantifications and Design Recommendations
Rt = Resistance at time t (kN)
REOD = Resistance at EOD from WEAP-SA or CAPWAP (kN)
Cha = Weighted average horizontal coefficient of consolidation (cm2/min)
Na = Weighted average uncorrected SPT N value
rp2
= Equivalent pile radius (cm)
fc = Consolidation factor
fr = Remolding recovery factor
t = Elapsed time since EOD, min
tEOD = Reference time of 1 min
Lt / LEOD = Normalized pile embedded length
EOD
t
EOD
r
a
hac
L
L
t
tf
rN
CfRR
p
1log 102
EODt
For H-piles in Cohesive Iowa Soils
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For Puget Sound Lowlands
Reddy and Stuedein (2014) – Time Dependent Capacity Increases of Piles
Driven in Puget Sound Lowlands
Rs = shaft resistance at time “t” of driving (kips)
Rso = initial shaft resistance at “to” of driving (kips)
A = constant based on pile type (1.72 for PSC, 0.70 for CEP, 0.77 for OEP)
t = time after driving (hours)
to = time of driving (hours) (to = 1 hour for all tests)
k1 = regression factor 1 (0.17 for PSC, 0.12 for CEP, 0.15 for OEP)
k2 = regression factor 2 (0.00044 for PSC, 0.00078 for CEP, 0.00060 for OEP)
Rs
=R
so 𝐴 log
tt
o
𝑘1 + 𝑘2 Rso
𝐴 logtt
o
+ Rso
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Soil Setup Factor
The soil setup factor is defined as failure load
determined from a static load test divided by the
capacity at the end of driving.
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TABLE 9-20 SOIL SETUP FACTORS
(after Rausche et al., 1996)
Predominant Soil
Type Along Pile
Shaft
Range in
Soil Set-up
Factor
Recommended
Soil Set-up
Factors*
Number of Sites
and (Percentage
of Data Base)
Clay 1.2 - 5.5 2.0 7 (15%)
Silt - Clay 1.0 - 2.0 1.0 10 (22%)
Silt 1.5 - 5.0 1.5 2 (4%)
Sand - Clay 1.0 - 6.0 1.5 13 (28%)
Sand - Silt 1.2 - 2.0 1.2 8 (18%)
Fine Sand 1.2 - 2.0 1.2 2 (4%)
Sand 0.8 - 2.0 1.0 3 (7%)
Sand - Gravel 1.2 - 2.0 1.0 1 (2%)
* - Confirmation with Local Experience Recommended
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5 80
-4000
0
4000
8000
ms
kN
9 L/c
Force Msd
Velocity Msd
5 80
-4000
0
4000
8000
ms
kN
9 L/c
Force Msd
Velocity Msd
5 80
-4000
0
4000
8000
ms
kN
9 L/c
Force Msd
Velocity Msd
5 102
-4000
0
4000
8000
ms
kN
9 L/c
Force Msd
Velocity Msd
5 102
-4000
0
4000
8000
ms
kN
9 L/c
Force Msd
Velocity Msd
EOID, t = 0.0007 Days Rs = 453, Rt = 1846, Ru = 2299 kN
BOR #2, t = 0.69 Days Rs = 1046, Rt = 2077, Ru = 3123 kN
BOR #3, t = 14.65 Days Rs = 1530, Rt = 2113, Ru = 3643 kN
BOR #4 t = 48.81 Days Rs = 2121, Rt = 2092, Ru = 4213 kN
BOR #1, t = 0.06 Days Rs = 818, Rt = 2047, Ru = 2865 kN
RSEOD x 4.7
RUEOD x 1.8
RSEOD x 3.4
RUEOD x 1.6
RSEOD x 2.3
RUEOD x 1.4
RSEOD x 1.8
RUEOD x 1.2
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Time (log)
Capacity
(kN)
Total Shaft
SLT
DLT
For Closed-end Pipe Piles in Wisconsin
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Relaxation is a time dependent decrease in the
soil resistance or capacity.
During pile driving, dense soils may dilate thereby
generating negative pore pressures and
temporarily higher soil resistance.
Relaxation has been observed for piles driven in
dense, saturated non-cohesive silts, fine sands,
and some shales.
Relaxation
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The relaxation factor is defined as failure load
determined from a static load test divided by the
capacity at the end of driving.
Relaxation factors of 0.5 to 0.9 have been reported
in case histories of piles in shales.
Relaxation factors of 0.5 and 0.8 have been
observed in dense sands and extremely dense
silts, respectively.
Relaxation Factor
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Time Effects on Pile Drivability and Pile Capacity
Time dependent soil strength changes should
be considered during the design stage.
• SPT – torque and vane shear tests
• CPTu with dissipation tests
• Model piles
• Soil setup / relaxation factors
Tools that have been used include:
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Piles Subject to Scour
Aggradation / Degradation Scour
Types of Scour
Local Scour
Contraction and General Scour
- Long-term stream bed elevation changes
- Removal of material from immediate vicinity of foundation
- Erosion across all or most of channel width
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Piles Subject to Scour
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Pile Design Recommendations in Soils Subject to Scour
1. Reevaluate foundation design relative to
pile length, number, size and type
2. Design piles for additional lateral restraint
and column action due to increase in
unsupported length
3. Local scour holes may overlap, in which
case scour depth is indeterminate and
may be deeper.
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Pile Design Recommendations in Soils Subject to Scour
4. Perform design assuming all material above
scour line has been removed.
5. Place top of footing or cap below long-term
scour depth to minimize flood flow
obstruction.
6. Piles supporting stub abutments in
embankments should be driven below the
thalweg elevation.
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Plugging of Open Pile Sections
Open end sections include open end pipe
piles and H-piles.
It is generally desired that the open sections
remain unplugged during driving and behave
plugged under static loading conditions.
Why ?
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Plugging of Open Pile Sections
Factors influencing plug development include
hammer size and penetration to pile
diameter ratio (D/b)
During driving:
Large hammer plug slippage
Small hammer plug formation
Therefore, size pile for larger hammer.
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Plugging of Open Pile Sections
Under static conditions, plugging likely in
Dense sand & clay D/b ≥ 20
Medium dense sand D/b ≥ 20 to 30
Under dynamic conditions, plugging may occur
(b 30 inch diameter) in very dense sands unless
the penetration into the dense layer is very
shallow (<3b). For large diameter piles (b ≥ 60
inch diameter), plugging will rarely occur during
driving.
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Plugging of Open Pile Sections
H-piles are usually assumed to be plugged
under static loading conditions due to the
smaller section size.
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Plugging of
Open Pile
Sections
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Use of Constrictor Plates in Open End Pipe Piles to Force Plugging
Bottom View Top View
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Densification Effects on Soil Resistance
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Densification Effects on Soil Resistance
Densification can result in the soil resistance
as well as the pile penetration resistance
(blow count) being significantly greater than
that calculated for a single pile.
Added confinement from cofferdams or the
sequence of pile installation can further
aggravate a densification problem.
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Densification Effects on Pile Capacity
Potential densification effects should be
considered in the design stage. Studies
indicate an increase in soil friction angle of up
to 4˚ would not be uncommon for piles in loose
to medium dense sands.
A lesser increase in friction angle would be
expected in dense sands or cohesionless soils
with a significant fine content.
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Effects of Predrilling and Jetting on Soil Resistance
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Effects of Predrilling and Jetting on Soil Resistance
Predrilling in cohesive soils and jetting in
cohesionless soils are sometimes used to
satisfy minimum penetration requirements.
Both predrilling and jetting will effect the axial,
lateral, and uplift capacity that can be attained.
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Effects of Predrilling and Jetting on Pile Capacity
Depending upon the predrilled hole diameter,
the shaft resistance in the predrilled zone
may be reduced to between 50 and 85% of
the shaft resistance without predrilling.
In jetted zones, shaft resistance reductions of
up to 50% have been reported.
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Effects of Predrilling and Jetting on Pile Capacity
It is important that the effect of predrilling or
jetting be evaluated by design personnel
whenever it is proposed.
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Pile Drivability
The ability of a pile to be
driven to the desired
depth and / or capacity at
a reasonable blow count
without exceeding the
material driving stress
limits.
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Factors Affecting Pile Drivability
• Driving system characteristics
• Pile material strength
• Pile impedance, EA/C
• Dynamic soil response
Primary factor
controlling drivability
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Pile Drivability
Pile drivability should be checked by wave
equation analysis during the design stage for
all driven piles.
Pile drivability is particularly critical for
closed end pipe piles.
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Questions ? ? ?