additional design considerations 2011 pdca professor pile institute patrick hannigan grl engineers,...

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


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


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 #4 t = 48.81 DaysRs = 2121, Rt = 2092, Ru = 4213 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 ?


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)


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


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


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.


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.


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 ? ? ?? ? ?

additional design considerations 2011 pdca professor pile institute patrick hannigan grl engineers,...

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