Existing drilled shafts are too short
Existing drilled shafts are too short
(OP)
We are looking at constructing a bridge column foundation consisting of four-48 inch reinf. concrete drilled shafts. Two of the shafts are existing and two are proposed and all four have the same existing/plan lengths based on point bearing.
During excavation of the two proposed shafts, it was found that the plan lengths were too short and the shafts ran about 2.5 times the proposed length due to the bearing strata found.
If the existing shafts are not founded in good strata, are there ways to salvage them? Is it possible to extend them? Otherwise I'm envisioning extending the foundation cap and dropping two new shafts at the required length.
During excavation of the two proposed shafts, it was found that the plan lengths were too short and the shafts ran about 2.5 times the proposed length due to the bearing strata found.
If the existing shafts are not founded in good strata, are there ways to salvage them? Is it possible to extend them? Otherwise I'm envisioning extending the foundation cap and dropping two new shafts at the required length.





RE: Existing drilled shafts are too short
How do you know that the existing shafts do not have sufficient capacity? Why not load test them?
RE: Existing drilled shafts are too short
RE: Existing drilled shafts are too short
www.PeirceEngineering.com
RE: Existing drilled shafts are too short
If it is too short, any other methods for dealing with the shaft? Is removing it even an option? That sounds extremely tough. If the footing has not been set, what about expanding the footprint and dropping two new shafts (after a redesign to ensure this system works)?
RE: Existing drilled shafts are too short
Another test method is Statnamic testing. This method requires no reaction piles and is commonly used for your application. Here's one firm that does this...
http://www.testpile.com/
Good luck.
RE: Existing drilled shafts are too short
What is the purpose of the bridge? Is there just a single foundation of 4 x 1.2m diameter piles? Maybe not a major structure.
If you are only finding out information on the soils as you excavate for the piles perhaps the soil investigation has been minimal or doesn't exist. I suggest a couple of boreholes extending beyond the expected lengths of the piles. Then you can start the design with a good knowledge of the soils. BTW how long are the existing piles and what is the new assumed required length?
Where in the world are you? In the USA and Europe as Ron states you would usually use reaction piles for a static load test,but in Asia there are generally blocks of concrete available for hire which you can build up into a load to jack off.
Plunge piles are different, this is the technique of placing a steel column into the pile during construction.
RE: Existing drilled shafts are too short
There is a nationally known firm that has many ways to deal with odd situations like this. I'd look to them for ideas before doing much however.
Rather than giving their name here, all I can say is they send out a calendar each year with the views at their work "from the bottom up".
RE: Existing drilled shafts are too short
1. the new required length is wrong and a reasonable SI will show that the existing piles are capable of taking the load.
2. the existing piles are totally under designed and should be abandoned, 4 new piles to the required length should be installed.
I doubt there will be any advantage in trying to modify or improve the existing piles if they are so far from the required length. You have to mobilise a rig to site anyway so probably on cost and time any possible modification will not be advantageous.
RE: Existing drilled shafts are too short
RE: Existing drilled shafts are too short
RE: Existing drilled shafts are too short
If you don't know the length of the shaft, you can determine with ultrasonic testing.
RE: Existing drilled shafts are too short