Eiffel 101: Pile Driver Troubleshooting Guide
Some pile driving problems are more common that others. Here, we take a look at the most prevalent pile driving issues and their solutions.
The Bearing Pile Blow Count Changes Quickly or Decreases
If the blow count for a group of pilings quickly changes or decreases, check the soil conditions using borings. Borings can be used to indicate a weathered profile above the bedrock or bearing layer. If the borings do not indicate a weathered profile, damage to the pile toe is possible. Internally inspect the pile toes for damage, if possible. If piles cannot be internally inspected, dynamic measurements can be taken to evaluate the problem. As a last resort, piles can be extracted to determine the issue.
The Blow Count Is Lower than Expected
If blow counts are slightly lower than expected, it's usually due to soil resistance that is also lower than expected. Restrike testing can be used to determine if this is the case. If blow counts are significantly lower than expected, review the soil borings. If they don't show soft layers, the problem could be that the pile is damaged below grade.
Piles Drive Much Deeper Than Expected
If piles are driving much deeper than expected, it could be because the soil resistance is lower than originally thought, or because the driving system is performing better than estimated. Restrike tests can be performed to evaluate soil strength changes. If the ultimate capacity based on restrike blow count remains low, check the drive system performance and restrike capacity. It's possible that soil conditions are weaker than estimated, and foundation piles will need to be driven deeper than estimated.
The Blow Count Exceeds Penetration Requirements
Before driving a pile, a soil analysis can be used to predict the number of blows required to drive the pile to its planned depth. If this analysis shows that the number of blows exceeds the expectation, there could be a problem with the driving system or the soil. Make sure that the pile has good drivability and that the driving system matches the type of pile. You can also check the driving system operation to ensure it conforms with the manufacturers guidelines.
If neither options result in a solution, take dynamic measurements to see if the problem is coming from the driving system or the soil. Some common problems with driving systems include preignition, preadmission, low hammer efficiency, or a too soft cushion. Issues with the soil include greater soil strength than expected, high soil resistance, large soil quakes, or high soil damping.
Installed Piles Move Laterally When New Piles are Driven
If existing piles move during the installation of new piles, it's usually because of soil displacement. Possible solutions include redriving the installed piles, switching the sequence of pile installation, or predrilling the pile locations to reduce ground movement.
Driven Piles Are Moving Out of Alignment
If piles are driving out of alignment, it's usually due to control issues with the hammer-pile alignment, or because of soil conditions. If the problem has to do with hammer-pile alignment, a pile gate, template, or fixed lead system can fix the problem. If the problem stems from soil conditions, check to see if there are near-surface obstructions or bedrock that slopes steeply, which can disrupt alignment.
The Head of the Pile Is Deformed
The head of a pile might deform because of an incorrect helmet size, too week of steel strength, or unevenness of the pile head. To determine the problem, check for each of these issues, as well as for the calculated stress on the pile head. If the calculated stress is high and the blow count is low, reduce the number of hammer strokes. For high blow counts, a different type of pile may be required.
There are No Hammers Available Meeting the Resistance Limits
If there are any aren't hammers available that meet the driving stress and resistant limits set by the wave analysis, it's possible that both the calculated stresses and blow counts on the piles are too high. To solve this problem, you can increase the pile impedance or redesign for lower capacities.
Piles Hit Underground Obstructions
If piles encounter underground obstructions during driving, the project engineer will need to create a remedial design. The ultimate bearing capacity of piles that have hit obstructions will be reduced based on the damage to the pile and the soil composition. It's possible that the new plan will include the use of additional piles to compensate.
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