When driving piles, the resistance from the soil is constantly measured. What happens when the expected resistance values from the soil are not obtained? For a single pile or for multiple piles? What do we do in that case?
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3 Answers
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Since you said that the piles are not reaching the desired capacity, I am assuming that these are friction piles instead of bearing piles.
There are two ways in which the capacity of friction piles can be increased:
- Increase the length
- Wait
The first option is fairly obvious. To some degree, every additional unit length of pile will increase the total capacity of the pile just through additional contact with the soil.
The second option is less intuitive. In certain soil conditions, the act of driving the pile disrupts the soil structure as it is driven. In these cases, after a certain amount of time, the soil will press more firmly against the pile and the capacity will be increased. (I am not a geotechnical engineer, but they tell me that it has something to do with pore water pressure.) On projects that I have been involved with, the capacity of a pile has almost doubled when tested immediately after pile driving and after a month.
To answer your general question, if a pile is not reaching its desired capacity while driving, the first option is to drive more length of pile. The soil conditions of a site may not be completely known, so there will be some local variability. This is why on my plans we always give an estimated pile tip elevation.
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$\begingroup$ Increasing the pile length or increase the pile diameter too? or just pile length? I wouldnt have guessed / had heard about waiting though. Is this common? So you wait and after a while it is ok ? What about the construction schedule? $\endgroup$– upstreamCommented Oct 10, 2020 at 3:38
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$\begingroup$ Increasing pile diameter would also work, but it is not something that is easy to change in the field. The geotechnical engineer would have to know if the soil is suitable for increase in capacity over time. $\endgroup$– hazzey ♦Commented Oct 10, 2020 at 18:27
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It is assumed we are still in fill layer. Usually we keep going down till we hit the competent strata. Then we log the depth and keep going. In some jobs variation of the fill depth is normal.
Unless soils engineer orders to stop the job and possibly take a sample for the lab.
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I worked for a main contractor as project engineer. Taking my experience from one of my project.
To be more specific, when we are driving piles into the soil, say we are using hydraulic hammer pile rig to do the driving, we would measure and record the number of blows that required to drive each 300mm depth and consequently the final 10 blows settlement and the temporary compression.
We would limit the final 10 blows of the driven pile to be capped not more than certain settlement. Say final 10 blows, the maximum settlement allowed is 15mm. Depending on the soil profile and estimated depth, generally there are several empirical formula that we use to calculate the final settlement requirement, Hiley formula is one of them.
$$Q_u = \dfrac{\eta_h Wh}{s+c} \times \dfrac{1 + C_r^2 R}{1+R}$$
where
- $\eta_h$ is the efficiency of the hammer
- $s$, the set (final penetration of the pile per blow)
- $c$ is half of the total elastic compression (pile + pile cap + soil)
- $C_r$ is the coefficient of restitution
- $R$ is the pile-to-ram weight ratio
However, some engineers may argue that using this formula is not reliable, thus, for safety concerns we would do PDA test on selected pile. If the results from the PDA is satisfied, then the remaining piles driven according to the requirement derived from the Hiley Formula is justified.
We will do these before commencing the work for all other piles, but say if the piles are driven before the PDA Test and it indicates that the result was not satisfied, then all the piles shall be driven again based on the variation.
