Transcript Special Design Considerations
Special Design and Construction Considerations
2009 PDCA Professor Pile Institute
Patrick Hannigan GRL Engineers, Inc.
SPECIAL DESIGN CONSIDERATIONS
• Time effects on pile capacity. • Pile driveability • Scour • Densification effects on pile capacity • Additional design and construction topics in FHWA Pile Manual Chapters 9.9 and 9.10
590 560 530 500 470 430 14” CEP
TIME EFFECTS ON PILE CAPACITY
Time dependent changes in pile capacity occur with time.
Soil Setup Relaxation
SOIL SETUP Soil setup
is a time dependent
increase
static pile capacity. in the Large excess positive pore pressures are often generated during pile driving.
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
Restrike testing generally performed 1 to 10 days after installation log time
SOIL 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 in Soil Set-up Factor Recommended Soil Set-up Factors* Number of Sites and (Percentage of Data Base) Clay Silt - Clay Silt Sand - Clay 1.2 - 5.5
1.0 - 2.0
1.5 - 5.0
1.0 - 6.0
2.0
1.0
1.5
1.5
Sand - Silt Fine Sand Sand 1.2 - 2.0
1.2 - 2.0
0.8 - 2.0
1.2
1.2
1.0
Sand - Gravel 1.2 - 2.0
1.0
* - Confirmation with Local Experience Recommended 7 (15%) 10 (22%) 2 (4%) 13 (28%) 8 (18%) 2 (4%) 3 (7%) 1 (2%)
RELAXATION Relaxation
is a time dependent
decrease
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, and some shales
.
RELAXATION FACTOR
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.
TIME EFFECTS ON PILE DRIVEABILITY AND CAPACITY
Time dependent soil strength changes that affect the soil resistance at the time of driving should be considered during the design stage.
• Remolded shear strength in clays • Estimate of pore pressures during driving • Soil setup / relaxation factors
PILE DRIVEABILITY
PILE DRIVEABILITY
Pile driveability 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.
Soil Profile Illustrating Driveability Considerations
Estimated Tip EL 14” Pipe Estimated Tip EL 12” H-pile
FACTORS AFFECTING PILE DRIVEABILITY
• Driving system characteristics • Pile material strength • Pile impedance, EA/C Primary factor controlling driveability • Dynamic soil response
PILE DRIVEABILITY
Pile driveability should be checked during the
design stage
for all driven piles.
Pile driveability is particularly critical for closed end pipe piles.
PILE DRIVEABILITY EVALUATION DURING DESIGN STAGE 1. Wave Equation Analysis
Computer analysis that does not require a pile to be driven.
2. Dynamic Testing and Analysis
Requires a pile to be driven and dynamically tested.
3. Static Load Tests
Requires a pile to be driven and statically load tested.
Soil Profile – 12.75 In CEP
3 ft Silty Clay = 127 lbs / ft 3 q u = 5.5 ksf 46 ft 20 ft Dense, Silty F-M Sand = 120 lbs / ft 3 = 35 ˚
Student Exercise #2 Revisited
Static analysis indicates a 12.75 in O.D. closed-end pipe pile driven to 63 ft below grade can develop an ultimate capacity of 420 kips. A static load test will be used for construction control.
No special design conditions exist (scour, downdrag, etc.). Therefore, a used.
Pick Pile Section – (Appendix C2-4)
Given: 12.75 in O.D. closed-end pipe pile Select: wall thickness – try 0.109 in wall (lowest cost) ASTM A-252 Grade 3 steel 28 day concrete strength – F Y = 45 ksi – f’c = 5 ksi
Check Allowable Design Load (10-5)
Design Load = 0.25 (F Y )(steel area) + 0.40
(f’c)(concrete area) = 0.25(45 ksi)(4.33 in 2 ) + 0.40(5 ksi)(123.0 in 2 ) = 48.7 kips + 246.0 kips = 294 kips
Check Pile Driveability
Driveability Requirements: (see pg 11-15) Driving stress limit of 0.9(F Y ) = _____ ksi (see pg 10-5)
Check Pile Driveability
Determine Hammer Size: (see pg 21-36) (see Appendix D-1) Perform Wave Equation Analysis
GRLWEAP Driveability Results For D-16-32
Piles Subject to Scour
Piles Subject to Scour
Types of Scour Aggradation / Degradation Scour - Long-term stream bed elevation changes Local Scour - Removal of material from immediate vicinity of foundation Contraction and General Scour - Erosion across all or most of channel width
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.
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.
Densification Effects on Pile Capacity
Densification Effects on Pile Capacity
Densification can result in the pile capacity as well as the pile penetration resistance to driving 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.
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.
Pile Driving Induced Vibrations
Vibrations from pile driving sometimes perceived as a problem.
Vibrations can cause damage.
Vibrations can cause soil densification and settlement.
Pile driving vibrations often a perceived problem than an actual problem and can often be controlled by construction procedures PDCA database – www.piledrivers.org
Charleston SC Project
• Hydraulic hammer (30 ft-kip) • HP 12 x53 H-piles • Predrill 12” diameter hole to 40 ft.
3 1 2 • 220 piles monitored • 2 failed vibration criteria • No significant movement (> 1 mm) recorded by crack monitoring devices • No damage to masonry facade 2 4 6 8 10 20 40 60 80 100 USBM (1985) Hajduk et al. (2004) - New Hajduk et al. (2004) - Older/Historic HP 12x53 Piles 12in OD CEP Pile s 60 40 1 20 124 123 8 10 6 4 0.1
2 1 2 4 6 8 20 10
Vibration Frequency (Hz)
40 60 80 100