Drilled Shaft (or Pier or Caisson) Construction Clearances

[dison]

I have placed a few exploratory calls to contractors who install drilled shaft foundations, but I don't have any personal contacts with them and haven't received any info back yet, so I thought someone in the forums might be able to help.

I'm investigating overhead clearances required to install drilled shafts beneath an existing bridge. Some are on land and some are in a river. So, the contractor may use several different methods to construct the shafts. For example, over water, the contractor might use a crane on a barge and permanent casing combined with the slurry method while on land, the contractor will probably use a short upper temporary casing and slurry method with a crane or vehicle-mounted drilling equipment. These are my questions:

1)What is the absolute minimum overhead clearance required to construct a drilled shaft? What is the minimum lateral clearance? What are typical lateral clearances?

2)If only a portion of the new drilled shaft overlaps with the existing superstructure (in plan view), can the shaft still be constructed? How much overlap is OK?

3)What is the associated cost difference having clearance restrictions during construction vs. no clearance restrictions?

Normally, you would just design the drilled shafts to be clear of the existing superstructure and then you'd have no problems, however, the client would like to use a particular "standard" substructure and doesn't know if clearances will be a problem. Oh, and absolutely no removing the existing superstructure before construction.

Thanks in advance! ~dison

 

[Qshake]

~Dison

I can't recall what exactly the clearances are, but I can assure you that your shafts can be constructed by today's leaner, meaner drill rig.

A few years ago and continuing today, I have been involved in a large seismic retrofit of a long double deck bridge with all kinds of height restrictions and similar lateral restrictions. My fondest memory is the footing retrofit that was done in a Movie theater which was built around the bridge column. The contractor tracked this small compact rig into the theater and promptly began drilling away.

This same machine was used on the drilled shafts for much of the remaining job as there is only one contractor in this area doing that kind of work.

I will search through my correspondance file for some specifications. It might take a couple of days. I might even find some pictures!

 

[JAE]

Qshake is right...we had a project in San Antonio some years ago where an existing stiffened slab-on-grade house (mansion, actually) had to be re-supported by drilled piers due to swelling of expansive clays.

The contractor began excavating sequentially under the slab (which was a 5" thick slab with a grid of integral concrete turn-down beams spaced at about 16' o.c. As the excavation proceeded, he would start drilling shafts for the piers under the slab.

Very low head clearance - but with special equipment - and that is the key. You really should check with local drillers to verify that they have the proper equipment in your area.

 

[thaidavid]

dison,

We have used contractors with low-headroom drill rigs on some of our projects which need only 16 feet of headroom. There are some that go even lower - as low as 12 feet. As for lateral clearances, you need the width of the drill rig (about 8 feet), plus the area to dump the spoil (about the same width, located to one side). I hope this helps some.

David

 

[Focht3]

All the posts are good and on target. I am concerned by the casing, though. Generally speaking, your overhead clearance requirements are controlled by the casing length, which is controlled in turn by the pier design. You can install permanent casing in sections, but the welds are normally considered structural (since the casing is permanent and the casing must transfer skin friction to the soil.) Installation of segmented casing is both slow and expensive. Corrosion is also an issue.

Slurry processed piers with short temporary casing are probably your best answer. If you have extensive open-work gravels and cobbles, then you might consider grouting these zones before you drill through them. The grout doesn't need to be high strength; it only needs to be strong enough to hold the gravel/cobbles together during pier construction. Done right, this will be a lot cheaper and faster than fighting drilling fluid losses or stuck temporary casing. It will also result in safer construction with respect to the existing foundation elements.