Date: Oct 11, 2022 Views:
Piling is generally a cost-effective and time-saving method of driving support columns (piles) into the ground. But because you're dealing with soil and other hidden features in the Earth, there are unknown elements and things don't always go according to plan. Whether you're hitting concrete, steel or wood piles, any number of problems are common.
Pile driving problems are often related to unfavorable or unexpected soil conditions, which can lead to pile damage, hammer pile alignment problems, and other issues. In most cases, stopping work and testing the soil, reviewing or revisiting the borehole or calculations, and checking the equipment will help highlight the most likely cause of the problem.
01 The blow count is higher than the pile penetration requirement
The number of hits required to drive the pile to the planned depth should be accurately predicted, and if the number of hits is significantly higher than expected, it could indicate a problem with the drive system, the soil, or both.
Verify that the pile has sufficient drive capacity and that the drive system matches the type of pile. If the piling system is properly matched to the piling system, check the operation of the piling system to make sure it meets the manufacturer's guidelines.
If no apparent problem is found, dynamic measurements are made to determine if the problem is related to the drive system or soil behavior. Drive system problems can include pre-ignition, preload, low hammer efficiency, or a cushion that is too soft. Possible soil problems may include greater than expected soil strength, temporarily increased soil resistance and subsequent relaxation (which requires re-examination), large soil earthquakes, or high soil damping.
02 The driving depth of the pile is much deeper than estimated
When the piling depth is much deeper than expected, it may be because the soil resistance was lower than expected when the pile was driven, or because the piling system was performing better than expected. A thump test was performed after an appropriate waiting period to assess soil strength changes.
· If the final capacity based on thump count is still low, use dynamic measurements to check drive system performance and thump capacity.
· Soil conditions may be weaker than expected if drive system performance is consistent with assumptions and refire capability is low. Foundation piles may need to be deeper than originally estimated, or additional piles may be required to support the load. Contact the structural engineer/designer to determine appropriate design changes.
03 Sudden change in bearing pile or reduced number of blows
If the number of hits on a set of piles suddenly changes or decreases, first check the soil condition using the borehole. If the borehole does not show a weathered profile above the bedrock/bearing layer, the pile toe may be damaged. For piles that allow internal inspection, reflect light to the pile toe and inspect for signs of damage. For piles that cannot be inspected internally, perform dynamic measurements to assess the problem, or consider pile extraction.
04 Blow count is slightly lower than expected
When the number of hits is slightly lower than expected, it is usually because the soil resistance is lower than expected. Perform a thump test. Build the set factor, and reduce the capacity. The hammer may also perform better than expected, so that should be checked as well.
05 The number of strikes was significantly lower than expected
When piling hits are much lower than expected, check the soil bore first. If they do not indicate a soft layer, the problem may be that the pile is damaged below the grade.
If piles are spliced, re-evaluate splicing details and field procedures to prevent possible splicing failures.
Investigate the tensile stress at the pile edge and the compressive stress at the toe. If the calculated stress is acceptable, investigate for obstructions or other causes of uneven toe contact on the hard layer or pile toe damage.
06 Previously installed piles experience lateral movement when driving new piles
When existing piles move, it is usually because of soil displacement as new piles are driven in. Possible solutions include re-driving the installed piles, changing the order in which the piles are installed, or pre-drilling the pile positions to reduce ground movement. Lateral pile movement can also be caused by soil damage on adjacent slopes.
07 Pile is being driven out of alignment
When the pile moves out of alignment tolerance, it is usually due to a control problem with the hammer pile alignment or soil conditions. Pile gates, formwork or fixed lead systems may improve the ability to maintain alignment tolerances due to poor hammer pile alignment control. Soil conditions, such as near-surface obstructions or steeply sloping bedrock with minimal covering material, can also disrupt alignment. 08 piles are leaving the position
Pile positions that are out of acceptable tolerances can also be due to hammer-pile alignment control problems or due to soil obstructions. Pile gates, formwork or fixed guidance systems may improve the ability to maintain positional tolerances due to poor control of hammer pile alignment.
For piles that encounter shallow obstacles, excavation/removal of the obstacle may be feasible if the obstacle is within 3 feet of the working slope. Excavation may not be feasible if the obstruction is at a greater depth or below the water table, or if the soil is contaminated. In these cases, excavation or pre-drilling at the pile location can provide a solution
08 Encountering a deep pile obstacle
If you encounter a deep obstacle during driving, please contact the engineer for remedial design. Depending on the pile damage potential and soil matrix support characteristics, the ultimate bearing capacity of the pile hitting the obstacle needs to be reduced. Additional stakes may be required to compensate. 10 Concrete piles produce localized horizontal cracks in easy-to-follow
Cracks in concrete piles are more common in hard driving situations, but when they occur on easy driving, check the hammer pile alignment as bending can cause problems. If the alignment looks normal, the combined tension and flex may be too high. 11 Concrete piles peeling or peeling near the head
When the damage of the concrete pile occurred near the pile head, the pile head stress was determined from the number of observed blows and compared with the allowable stress. If the calculated stress is high, add a pile buffer. If the calculated stress is low, the likely causes are poor pile quality, hammer performance and hammer-pile alignment issues. 12 Concrete piles form complete horizontal cracks in easy
If a complete crack occurs in the pile during an easy drive, determine the tensile stress along the pile in the number of hits observed. If the calculated tensile stress is high, add a cushion or reduce the length of the hammer. If the calculated tensile stress is low, the cause may be hammer performance. 13 Concrete piles form full horizontal cracks in hard drive
When the hard drive creates a complete horizontal crack in the concrete pile, the tensile stress along the pile is calculated. If the calculated tensile stress is high, consider using a heavier plunger. If calculated tensile stress is low, take measurements and determine earthquake, which may be higher than expected 14 Concrete piles create localized horizontal cracks in ease
When concrete piles show localized horizontal cracks during easy driving, check hammer pile alignment, as bending may be the problem. If the alignment seems acceptable, the combined tension and flex may be too high. 15 Head deformation of steel or wood stakes
A number of problems can cause steel pile heads to deform or wood pile mushrooms. Check the following:
· Helmet size and/or shape
·Steel strength
·Pile head flatness
· Binding of timber pile heads
If all of these are checked out, calculate the stress on the pile head. If the calculated stress is high, lower the hammer energy (stroke) to reduce the number of blows; for high number of blows, a different hammer or stake type may be required.
