It depends on the contract language and in-situ soil conditions. The geotechnical borings will only provide evidence of soil properties at the boring location and not necessarily be applicable between different borings (soil is chaos!)
The contract language should acknowledge this by including provisions for additional, or lesser, lengths of pile. The contractor may be asked to bid the piling based on some assumed (as accurate as possible) lengths and then also be asked to provide unit prices for additional/reduced lengths of piling.
Yes, there is always a risk that the piling goes deeper - but that has traditionally been a risk borne by the owner as it is his project and the designers and builders cannot be held liable for unknown conditions that only can be revealed during construction.
If a piling goes VERY deep, way beyond that predicted, I would then immediately get a geotech to consult with me on the situation.
Have you done any PDA analysis (or similar testing) to verify that your piles are intact, and have not been seriously damaged during driving?
I have seen a contractor driving piles up to 10 metres past forecast rock level, claim for extra payment, and then be very embarrassed indeed when a bore right beside a pile showed sound rock where it had been forecast.
The material above the rock base was general garbage in an abandoned municipal tip. It still remains a mystery to everyone why it was not obvious when the toe had reached rock, and how the piling crew could unknowingly destroy 10 metres of pile.
Well, AUSTIM, I'll offer an explanation that may fit.
Back in the old days of the interstate highway system here in the US, a generally ol' boy rule of thumb was to drive pile to refusal and then "beat it" 5 to 10 more times to really do the job! I about flipped over when I first heard this - its too ludicrous to even imagine. Naturally, one should always have the pile marked and the last foot of sound material and hard driving should be marked in smaller increments and watched for say a 1/4" in 10 or 12 blows. I don't think people realize how easy it is to damage a pile.
Now back to the previously scheduled program...
The language that we use in developing plans limits ours and the owners liability for sounding or coring information. Moreover, the contractor is encouraged thru additional language in the special provisions to do his own foundation testing if he beleives it is warranted. However, it is common practice to pay for the additional feet of pile and the labor to splice the pile by an additional 8 feet. No more than two splices are allowed and so the contractor is encouraged to be liberal with selecting a pile length.
Having noted that I have had several jobs where pile have exceeded the plan length considerably and not reached bearing or practical refusal. In one of those cases the pile cap was redesigned assuming a larger pile spacing - simply neglecting the pile which never seated. Another case pile was driven close to the original and luckily did seat. In all cases the contractor wanted to hit the pile again the next day or so and because of the great effort to overcome pile freeze - swore up and down that the pile were suddenly "seated". Don't buy that either.
If a pile is driven or bored to a depth greater or less than that predicted, it is essential that the technical reason(s) for this is/are understood. For example, the assumptions used in pile design may be wrong, or the construction process may be flawed. If only one pile is affected, then you MUST ask yourself why no other pile was affected, and you MUST be sure of your answer. If you do not already have a geotechnical engineer on board, you should consult one without delay.
If YOU have designed the pile length, then the client (or you?) pays for additional length. In the UK and Ireland, it is common for such work to be paid, at least in part, per unit length.
If the piling contractor has designed the piles, and the actual lengths prove to be (for example) beyond the reach of normal equipment, the question of the foreseeability of the conditions encountered really comes into play. More detailed advice would have to be based on a knowledge of the particular Conditions of Contract you are using.
The risk sharing may not be very clear if YOU have designed the piles. You must make it clear to your client from the outset what risks HE carries, and he needs to set aside a reasonable contingency sum to cover those risks. (But don't let the contractor know the money is there or how much it is, or he may make every effort to obtain this money, even if conditions are ideal.) If you haven't made it clear to your client what risk he carries, he may want to recover such extra costs from you.
Yes, a geotechnical engineer needs to know about contract as well as about geotechnics and many other areas.
I agree that the owner bears the greater risk, as soil conditions and response are certainly not exact. In my "neck of the woods" we deal with poorly defined limestone at depths below about 30 feet, often laden with solution cavities, so pile lengths sometimes don't necessarily follow the predicted. Even thorough geotechnical investigations don't always prevent this, though it can usually be mitigated by good investigation, experience with the area, and of course, standard geotechnical weasel words (those only help to mitigate the liability, not the problem!).
It happens everyday around here. There is now a provision in the contract for the contractor to get paid LF of pile installed. It is now a common practice to perform test piles, weaps, pda's prior to production pile installation. The soils engineer still bears responsibility in some instances where it is found that the borings were misrepresented.
