Existing drilled shafts are too short

[mehr27]

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.

 

[Ron]

If they were driven piles, you could splice and continue driving; however, since they are drilled shafts, extending is not really an option.

How do you know that the existing shafts do not have sufficient capacity? Why not load test them?

 

[mehr27]

You're right, we don't yet know if the existings are sufficient and that's a better place to start. Load testing sounds like a good first step. Are there any other tests to determine the capacity?

 

[PEinc]

If the existing shafts are too short, possibly you could drill a micropile down the center of each.

http://www.PeirceEngineering.com

 

[mehr27]

Thanks, that's another good option to consider. Is that called a plunge pile?

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)?

 

[Ron]

You can do a static load test on the shaft. You'll have to have reaction shafts to do that.

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.

 

[Zambo]

Load testing is expensive, but unless there is good knowledge of the soils perhaps unavoidable.

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.

 

[oldestguy]

I'd look to a good grouting contractor to install compaction grout under and around the tips. The ease of pumping grout will tell a lot about how good the "bearing " is there. You might even get the grouting contractor to guarantee the capacity.

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".

 

[Zambo]

We are informed that the required pile length is 2.5 times the installed length of the existing piles. I think there are 2 possibilities:

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.

 

[hokie66]

There is a third possibility unless mehr27 actually knows the length of the existing piles rather than just how they were shown on drawings: 3. During the existing pile installation, the poor bearing strata was found just as now, and they were extended. You could do a drill probe to find out how long they are.

 

[BigH]

Was there any soil investigation done before the new work? You might try to put down some boreholes to confirm the strata- and suitable founding levels. The other thing - did you not review the soil at the base of the drilled shafts before you concreted? Sometimes boreholes are put down when reaching the base to confirm. I know of one GE in NJ who, as a result of my Canadian experience, used dynamic cones (Canadian pentests - 140 lb hammer dropping 30 inches driving a 2 inch 60deg apex cone) to prove that there were no "soft layers" within the next 4 or 6 pile diameters below the drilled shaft.

 

[Ron]

hokie makes a good point; although hopefully there was logging and observation during the initial installation that would be available.

If you don't know the length of the shaft, you can determine with ultrasonic testing.

 

[BigHarvey]

totally agree with hokie66. We are talking of mobilizing a rig for two piles anyway. The cost of mob/demob will certainly be the biggest part of the bill, so just do your 4 new piles and disregard the existing 2 ones : you will save time and money !