Additional Design Additional Design ConsiderationsConsiderations
2011 PDCA Professor Pile Institute
Patrick Hannigan
GRL Engineers, Inc.
Additional Additional Design ConsiderationsDesign Considerations
• Time EffectsTime Effects
• ScourScour
• DensificationDensification
• PluggingPlugging
• DrivabilityDrivability
Time Effects on Pile Time Effects on Pile CapacityCapacity
Time dependent changes in pile capacity occur with time.
Soil SetupSoil Setup RelaxationRelaxation
Vesic’ (1977) – NCHRP 42 Design of Pile Foundations
Time Time Effects Effects on Pile on Pile CapacityCapacity
Soil Setup
End of Initial Driving
Restrike 8 Days Later
Shaft = 3 x EOID
Relaxation
Restrike 24 Hours Later
End of Initial Driving
Toe = ½ EOID
Soil setupSoil setup is a time dependent increaseincrease in the static pile 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 SetupSoil Setup
In sands, setup is attributed primarily to aging effects and / or release of arching effects with time.
Soil setup frequently occurs for piles driven in saturated clayssaturated clays as well as loose to medium loose to medium dense silts and fine sandsdense 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 SetupSoil Setup
• Multiple Static Load Tests (time, $$)• Multiple Static Load Tests (time, $$)
How to Quantify Time Effects
How to Quantify Time Effects
• Dynamic Restrike tests- inexpensive, cost-effective- check for setup or relaxation- short-term restrikes establish setup trend- long-term restrikes provide confidence in
capacity estimates
• Dynamic Restrike tests- inexpensive, cost-effective- check for setup or relaxation- short-term restrikes establish setup trend- long-term restrikes provide confidence in
capacity estimates
• Non-instrumented Restrike Tests- uncertain if blow count change due to change in hammer performance or soil strength
• Non-instrumented Restrike Tests- uncertain if blow count change due to change in hammer performance or soil strength
1 d
ay1
day
10 d
ays
10 d
ays
100
day
s10
0 d
ays
1000
day
s10
00 d
ays
log timelog time
pile
cap
acit
yp
ile c
apac
ity
Technically desirable
Technically desirableEconomically desirable
Economically desirable
Restrike testing generally performedRestrike testing generally performed
1 to 10 days after installation1 to 10 days after installation
Quantification of Time Effects
Quantification of Time Effects
When to restrike?
When to restrike?
Semilog-linear process
Semilog-linear process
Efforts to Predict SetupEfforts to Predict Setup
Khan (2011) – Prediction of Pile Set-up for Ohio Soils
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 ])
Q = 0.9957 Qo t 0.087
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 SetupEfforts to Predict Setup
Bullock et al (2005) – Side Shear Setup: Results from Florida Test Piles
0.001 0.01 0.1 1 10 100 10000
500
1000
1500
2000
2500
Aucilla, Static TestAucilla, 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
Sh
ear
QS
(kN
)18” PSC with O-Cell at bottom18” PSC with O-Cell at bottom
Bullock et al (2005) – Side Shear Setup: Results from Florida Test Piles
For Closed-end Pipe Piles in Ohio For Closed-end Pipe Piles in Ohio SoilsSoils Q = 0.9957 Qot0.087
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)
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 DaysRs = 453, Rt = 1846, Ru = 2299 kN
BOR #2, t = 0.69 DaysRs = 1046, Rt = 2077, Ru = 3123 kN
BOR #3, t = 14.65 DaysRs = 1530, Rt = 2113, Ru = 3643 kN
BOR #4 t = 48.81 DaysRs = 2121, Rt = 2092, Ru = 4213 kN
BOR #1, t = 0.006 DaysRs = 818, Rt = 2047, Ru = 2865 kN
Time (log)Time (log)
CapacityCapacity(kN)(kN)
TotalShaft
Soil Setup FactorSoil Setup Factor
The soil setup factor is defined as failure load
determined from a static load test divided by the
ultimate capacity at the end of driving.
TABLE 9-20 SOIL SETUP FACTORS(after Rausche et al., 1996)
Predominant Soil Type Along Pile
Shaft
Range inSoil Set-up
Factor
RecommendedSoil Set-up
Factors*
Number of Sitesand (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
RelaxationRelaxation is a time dependent decreasedecrease in the static pile 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, dense, saturated non-cohesive silts, fine sands,
and some shalesand some shales.
RelaxationRelaxation
The relaxation factor is defined as failure load
determined from a static load test divided by the
ultimate 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 FactorRelaxation Factor
Time Effects on Pile Time Effects on Pile Drivability and Pile CapacityDrivability 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:
Piles Subject to ScourPiles Subject to Scour
Aggradation / Degradation Scour
Types of ScourTypes 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
Piles Subject to ScourPiles Subject to Scour
Pile Design Pile Design Recommendations in Soils Recommendations in Soils
Subject to ScourSubject to Scour1.1. Reevaluate foundation design relative to Reevaluate foundation design relative to
pile length, number, size and typepile length, number, size and type
2.2. Design piles for additional lateral restraint Design piles for additional lateral restraint and column action due to increase in and column action due to increase in unsupported lengthunsupported length
3.3. Local scour holes may overlap, in which Local scour holes may overlap, in which case scour depth is indeterminate and case scour depth is indeterminate and may be deeper. may be deeper.
Pile Design Pile Design Recommendations in Soils Recommendations in Soils
Subject to ScourSubject to Scour4. Perform design assuming all material above 4. Perform design assuming all material above scour line has been removed.scour line has been removed.
5. Place top of footing or cap below long-term 5. Place top of footing or cap below long-term scour depth to minimize flood flow scour depth to minimize flood flow obstruction.obstruction.
6. Piles supporting stub abutments in 6. Piles supporting stub abutments in embankments should be driven below the embankments should be driven below the thalweg elevation. thalweg elevation.
Densification Effects on Pile Densification Effects on Pile CapacityCapacity
Densification Effects on Densification Effects on Pile CapacityPile Capacity
Densification can result in the pile capacity as Densification can result in the pile capacity as
well as the pile penetration resistance to well as the pile penetration resistance to
driving being significantly greater than that driving being significantly greater than that
calculated for a single pile.calculated for a single pile.
Added confinement from cofferdams or the Added confinement from cofferdams or the
sequence of pile installation can further sequence of pile installation can further
aggravate a densification problem.aggravate a densification problem.
Densification Densification Effects on Pile Effects on Pile
CapacityCapacityPotential densification effects should be Potential densification effects should be
considered in the design stage. Studies considered in the design stage. Studies
indicate an increase in soil friction angle of indicate an increase in soil friction angle of up up
to 4to 4˚̊ would not be uncommon for piles in loose would not be uncommon for piles in loose
to medium dense sands. to medium dense sands.
A A lesser increaselesser increase in friction angle would be in friction angle would be
expected in dense sands or cohesionless soils expected in dense sands or cohesionless soils
with a significant fine contentwith a significant fine content..
Plugging ofPlugging of
Open PileOpen Pile
SectionsSections
Plugging of Open Pile Plugging of Open Pile SectionsSections
Open end sections include open end pipe Open end sections include open end pipe
piles and H-piles.piles and H-piles.
It is generally desired that the open sections It is generally desired that the open sections
remain unplugged during driving and behave remain unplugged during driving and behave
plugged under static loading conditions.plugged under static loading conditions.
Why ?Why ?
Plugging of Open Pile Plugging of Open Pile SectionsSections
Factors influencing plug development include Factors influencing plug development include
hammer size and penetration to pile hammer size and penetration to pile
diameter ratio (D/b)diameter ratio (D/b)
Plugging of Open Pile Plugging of Open Pile SectionsSections
During driving:During driving:
Large hammer Large hammer plug slippage plug slippage
Small hammer Small hammer plug formation plug formation
Therefore size pile for larger hammer.Therefore size pile for larger hammer.
See Reference Manual Page 9-185
Plugging of Open Pile Plugging of Open Pile SectionsSections
Under Under staticstatic conditions: conditions:
Dense sand & clay Dense sand & clay D/b D/b ≥ 20 = ≥ 20 = plug plug
Medium dense sand Medium dense sand D/b D/b ≥ 20-30 = plug≥ 20-30 = plug
H-piles are usually assumed to be plugged H-piles are usually assumed to be plugged
under static loading conditions due to the under static loading conditions due to the
smaller section size.smaller section size.
PILE PILE DRIVABILITYDRIVABILITY
PILE DRIVABILITYPILE DRIVABILITY
Pile drivability refers to the ability of a pile to be
driven to the desired depth and / or capacity at
a reasonable driving resistance without
exceeding the material driving stress limits.
FACTORS AFFECTING FACTORS AFFECTING PILE DRIVABILITYPILE DRIVABILITY
• Driving system characteristicsDriving system characteristics
• Pile material strengthPile material strength
• Pile impedance, EA/CPile impedance, EA/C
• Dynamic soil responseDynamic soil response
Primary factor controlling drivability
PILE DRIVABILITYPILE DRIVABILITY
Pile drivability should be checked during the
design stagedesign stage for all driven piles.
Pile drivability is particularly critical for closed end
pipe piles.
PILE DRIVABILITY EVALUATION PILE DRIVABILITY EVALUATION DURING DESIGN STAGEDURING DESIGN STAGE
1. Wave Equation Analysis1. Wave Equation Analysis
Computer analysis that does not require a pile to be driven.
2. Dynamic Testing and Analysis2. Dynamic Testing and Analysis
Requires a pile to be driven and dynamically tested.
3. Static Load Tests3. Static Load Tests
Requires a pile to be driven and statically load tested.
Effects of Predrilling Effects of Predrilling and Jetting on Pile and Jetting on Pile
CapacityCapacity
Effects of Predrilling Effects of Predrilling and Jetting on Pile and Jetting on Pile
CapacityCapacityPredrilling in cohesive soils and jetting in Predrilling in cohesive soils and jetting in
cohesionless soils are sometimes used to cohesionless soils are sometimes used to
achieve minimum penetration requirements.achieve minimum penetration requirements.
Both Both predrilling and jetting will effect the axial, predrilling and jetting will effect the axial,
lateral, and uplift capacitylateral, and uplift capacity that can be that can be
developed.developed.
Effects of Predrilling Effects of Predrilling and Jetting on Pile and Jetting on Pile
CapacityCapacityDepending upon the predrilled hole diameter, Depending upon the predrilled hole diameter,
the shaft resistance in the predrilled zone the shaft resistance in the predrilled zone
may be reduced to between 50 and 85% of may be reduced to between 50 and 85% of
the shaft resistance without predrilling.the shaft resistance without predrilling.
In jetted zones, shaft resistance reductions of In jetted zones, shaft resistance reductions of
up to 50% have been reported.up to 50% have been reported.
Effects of Predrilling Effects of Predrilling and Jetting on Pile and Jetting on Pile
CapacityCapacityIt is important that the effect of predrilling or It is important that the effect of predrilling or
jetting be evaluated by design personnel jetting be evaluated by design personnel
whenever it is proposed.whenever it is proposed.
Questions Questions ? ? ?? ? ?