Pile driving is often a cost-effective and time-efficient method of driving support posts (piles) into the earth. But because you're working with soil and other hidden features in the earth, there is an element of the unknown, and things don't always go as planned. Whether you are driving concrete, steel, or wood piles, any number of problems are common.
Not surprisingly, problems with pile driving usually are related to adverse or unexpected soil conditions, which can lead to pile damage, hammer-pile alignment problems, and other issues. In most cases, stopping the work and testing the soil, reviewing or revisiting borings or calculations, and inspecting the equipment will help highlight the most likely cause of the problem.
Blow Count Is Above Pile Penetration Requirements
Soil analysis should accurately predict the number of blows required to drive a pile to its planned depth, and if the number of blows is substantially higher than expected, it indicates a problem with either the driving system, the soil, or both.
Confirm that the pile has sufficient drivability and that the driving system is matched to the type of pile. If the pile and driving system are suitably matched, check the driving system operation to make sure it is conforming with the manufacturer’s guidelines.
If no obvious problems are found, take dynamic measurements to determine if the problem is related to the driving system or to soil behavior. Driving system problems could include preignition, preadmission, low hammer efficiency, or a cushion that is too soft. Possible soil issues can include greater soil strength than anticipated, temporarily increased soil resistance with later relaxation (this requires a restrike to check), large soil quakes, or high soil damping.
Piles Are Driving Significantly Deeper Than Estimated
When piles are driving considerably deeper than expected, it may be because soil resistance at the time of driving is lower than anticipated, or because the driving system performance is better than anticipated. Perform restrike tests after an appropriate waiting period to evaluate soil strength changes.
- If the ultimate capacity based on restrike blow count is still low, check the drive system performance and restrike capacity, using dynamic measurements.
- If the drive system performance is as assumed and restrike capacity is low, the soil conditions are probably weaker than anticipated. Foundation piles will probably need to be driven deeper than originally estimated, or additional piles will be required to support the load. Contact the structural engineer/designer to identify the proper design changes.
Blow Count for Bearing Piles Abruptly Changes or Decreases
If you find that the blow count for a group of pilings suddenly changes or decreases, first check the soil conditions using borings. If the borings do not indicate a weathered profile above the bedrock/bearing layer, damage to the pile toe is likely. For piles that allow internal inspection, reflect light into the pile toe and inspect for indications of damage. For piles that cannot be internally inspected, take dynamic measurements to evaluate the problem, or consider pile extraction.
Blow Count Is Slightly Lower Than Expected
When blow counts are somewhat lower than you expect, it is usually because soil resistance is lower than anticipated. Perform restrike testing. Establish the setup factor, and drive to lower capacity. Hammer performance may also be better than anticipated, so this should also be checked.
Blow Count Is Significantly Lower Than Expected
When the blow count to drive the pile is much lower than expected, first review the soil borings. If they do not indicate soft layers, the problem may be that the pile is damaged below grade.
If the pile was spliced, re-evaluate the splice detail and field procedures for possible splice failure.
Investigate both tensile stresses along the pile as well as compressive stresses at the toe. If the calculated stresses are acceptable, investigate the possibility of obstructions or uneven toe contact on hard layer or other causes of pile toe damage.
Previously Installed Piles Show Lateral Movement When New Piles Are Driven
When existing piles move, it is often because of soil displacement as new piles are driven. Possible solutions include redriving the installed piles, changing the sequence of pile installation, or predrilling the pile locations to reduce ground movements. Lateral pile movements can also be caused by soil failure in the adjacent slope.
Piles Are Driving Out of Alignment
When piles move out of alignment tolerance, it is often due to control issues with hammer-pile alignment or to soil conditions. If due to poor hammer-pile alignment control, a pile gate, template, or fixed lead system may improve the ability to maintain alignment tolerance. Soil conditions, such as near-surface obstructions or steeply sloping bedrock having minimal overburden material, can also disrupt alignment.
Piles Are Driving Out of Location
Piles that shift locations out of accepted tolerance can also be caused by problems with hammer-pile alignment control or due to soil obstructions. If due to poor hammer-pile alignment control, a pile gate, template, or fixed lead system may improve the ability to maintain location tolerance.
For piles encountering shallow obstructions, excavation/removal of the obstruction is probably feasible if the obstructions are within 3 feet of working grade. If the obstructions are at deeper depth or below the water table, or if the soil is contaminated, excavation may not be feasible. In these cases, spudding or predrilling of pile locations may provide a solution.
Deep Pile Obstructions Are Encountered
If deep obstructions are encountered during driving, contact the engineer for remedial design. Ultimate bearing capacity of piles hitting obstructions will need to be reduced based on pile damage potential and soil matrix support characteristics. Additional piles may be necessary to compensate.
Concrete Piles Develop Partial Horizontal Cracks in Easy Driving
Cracks in concrete piles are more common in hard-driving situations, but when they occur during easy driving, check hammer-pile alignment because bending may be causing the problem. If the alignment appears to be normal, the combined tension and bending may be too high.
Concrete Pile Is Spalling or Slabbing Near the Head
When damage to a concrete pile occurs near the head of the pile, determine pile head stress for observed blow count and compare with allowable stresses. If the calculated stress is high, add pile cushioning. If the calculated stress is low, possible causes are inferior pile quality, hammer performance, and hammer-pile alignment issues.
Concrete Piles Develop Complete Horizontal Cracks in Easy Driving
If complete cracks develop in a pile during easy driving, determine the tension stresses along the pile for the observed blow count. If the calculated tension stresses are high, add cushioning or reduce the length of the hammer stroke. If the calculated tension stresses are low, the cause may be hammer performance.
Concrete Piles Develop Complete Horizontal Cracks in Hard Driving
When hard-driving produces complete horizontal cracks in a concrete pile, calculate the tension stresses along the pile. If the calculated tension stresses are high, consider using a heavier ram. If the calculated tension stresses are low, take measurements and determine the quakes, which are probably higher than anticipated.
Concrete Piles Develop Partial Horizontal Cracks in Easy Driving
When partial horizontal cracks appear in a concrete pile during easy driving, check the hammer-pile alignment, because bending may be the problem. If the alignment appears to be acceptable, then combined tension and bending may be too high.
Head of a Steel or Timber Pile Is Deformed
A number of problems can cause the head of steel pile to deform or a timber pile to mushroom. Check the following:
- Helmet size and/or shape
- Steel strength
- Evenness of the pile head
- Banding of the timber pile head
If all of these check out, calculate the stress on the pile head. If the calculated stress is high, reduce the hammer energy (stroke) for low blow counts; for high blow counts, a different hammer or pile type may be required.
Diesel Hammer Stroke Is Higher Than Calculated
When using a diesel hammer, if the field stroke is less than 90 percent of the calculated stroke, confirm that the ram friction is not a problem.
Compare the calculated and observed blow counts. If the observed count is lower, it's likely that the soil resistance is less than anticipated. If the blow counts are comparable, reanalyze with lower combustion pressure to match the observed hammer stroke.
Cannot Find Hammer in Data File
When you cannot find your specific hammer when entering data for the wave analysis, look for a hammer of the same type and with a similar ram weight and energy rating, then modify its data to fit the application.
No Hammer That Meets Driving Stress and Resistance Limits
When there is no hammer available that matches the driving stress and resistance limits required by the wave analysis, both the calculated stresses and blow counts on the piles may be too high. When this is the case, you can Increase the pile impedance or material strength, or redesign for lower capacities. If the soil is fine-grained or known to exhibit setup gains after driving, the end-of-driving capacity may be selected to be lower than required. Confirm the capacity by restrike testing or static load testing.