Presentation on theme: "Special Design and Construction Considerations"— Presentation transcript:
1 Special Design and Construction Considerations 2009 PDCA Professor Pile InstitutePatrick HanniganGRL Engineers, Inc.
2 SPECIAL DESIGN CONSIDERATIONS Time effects on pile capacity.Pile driveabilityScourDensification effects on pile capacityAdditional design and construction topics in FHWA Pile Manual Chapters 9.9 and 9.10
4 TIME EFFECTS ON PILE CAPACITY Time dependent changes in pile capacity occur with time.Soil SetupRelaxation
5 SOIL SETUPSoil setup is a time dependent increase in the static pile capacity.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.
6 Economically desirable Technically desirable Soil Setup1 day10 days100 days1000 daysSemilog-linear processWhen to test?capacityEconomically desirableTechnically desirableRestrike testing generally performed1 to 10 days after installationlog time
7 SOIL SETUP FACTORThe soil setup factor is defined as failure load determined from a static load test divided by the ultimate capacity at the end of driving.
8 TABLE 9-20 SOIL SETUP FACTORS (after Rausche et al., 1996) Predominant Soil Type Along Pile ShaftRange inSoil Set-upFactorRecommendedFactors*Number of Sitesand (Percentageof Data Base)Clay2.07 (15%)Silt - Clay1.010 (22%)Silt1.52 (4%)Sand - Clay13 (28%)Sand - Silt1.28 (18%)Fine SandSand3 (7%)Sand - Gravel1 (2%)* - Confirmation with Local Experience Recommended
9 RELAXATIONRelaxation 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.
10 RELAXATION FACTORThe 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.
11 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 claysEstimate of pore pressures during drivingSoil setup / relaxation factors
13 PILE DRIVEABILITYPile 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.
14 Soil Profile Illustrating Driveability Considerations Estimated Tip EL 14” PipeEstimated Tip EL 12” H-pile
15 FACTORS AFFECTING PILE DRIVEABILITY Driving system characteristicsPile material strengthPile impedance, EA/CDynamic soil responsePrimary factor controlling driveability
16 PILE DRIVEABILITYPile driveability should be checked during the design stage for all driven piles.Pile driveability is particularly critical for closed end pipe piles.
17 PILE DRIVEABILITY EVALUATION DURING DESIGN STAGE 1. Wave Equation AnalysisComputer analysis that does not require a pile to be driven.2. Dynamic Testing and AnalysisRequires a pile to be driven and dynamically tested.3. Static Load TestsRequires a pile to be driven and statically load tested.
18 Soil Profile – 12.75 In CEP 3 ft Silty Clay = 127 lbs / ft3 qu = 5.5 ksfDense, Silty F-M Sand= 120 lbs / ft3 = 35˚3 ft
19 Student Exercise #2 Revisited Static analysis indicates a 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 maximum axial design load of _______ kips can be used.210
26 Piles Subject to Scour Types of Scour Aggradation / Degradation Scour - Long-term stream bed elevation changesLocal Scour- Removal of material from immediate vicinity of foundationContraction and General Scour- Erosion across all or most of channel width
27 Pile Design Recommendations in Soils Subject to Scour Reevaluate foundation design relative to pile length, number, size and typeDesign piles for additional lateral restraint and column action due to increase in unsupported lengthLocal scour holes may overlap, in which case scour depth is indeterminate and may be deeper.
28 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.
30 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.
31 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.
32 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 proceduresPDCA database –
33 Charleston SC Project 220 piles monitored 2 failed vibration criteria No significant movement (> 1 mm) recorded by crack monitoring devicesNo damage to masonry facadeHydraulic hammer (30 ft-kip)HP 12 x53 H-pilesPredrill 12” diameter hole to 40 ft.