Drilled Pier Foundation Vs Spread Footing Constructability in Tight Access Areas.

[EngDYIGuy]

Hello everyone,
I would like to know what types of small equipment are available to drill 3 ft diameter drilled pier foundations to a depth of 10 ft within an existing structure? Is there anything on the market that can fit through a standard door width? Drilled piers will extend through a concrete slab, after saw cutting and demolition of the slab. Which of the two details do you think is easier to construct in an existing building, and why? Please provide photos of similar jobs and equipment if you have any.

Thank you

 

[EngDYIGuy]

Here are the two details.

http://files.engineering.com/getfil...-f065-4a84-a7fd-d4e102ddcf39&file=Details.jpg

 

[jike]

You need to talk with those foundation contractors in the project area who most likely will bid on your project. Find out the limitations of their equipment. They will need a copy of the soil borings to evaluate the feasibility of the installation.

 

[bridgebuster]

Without knowing all the details, I'd go with the spread footing. It looks, from your sketch, to be fairly shallow. My concern is whether the influence of the existing footing would affect the new drilled shaft and conversely whether the stability of the existing footing would be affected by the construction of the shaft.

If you have to drill can you take out the wall? Can you go with micropiles on a pier cap? 3' diameter holes requires a machine with some B@!!$. There are very small rigs but they don't fit through doors. How much head room is available, at least 8'?

 

[thaidavid40]

The drilled pier eliminates the overlapping pressure bulbs below the two adjacent spread footings. There are any number of mini-excavators with sufficient hydraulic power to drill 36"-diameter holes, and these can in tight, restricted areas. Using the piers could be a real efficient, and safe way to accomplish your goals.
Dave

Thaidavid

 

[hokie66]

"Standard door width"? Will a 3' auger even fit through your door?

 

[JedClampett]

I hate to be "that guy" (hijack the thread with an alternative and not answer the OP's question), but have you considered micropiles? They're a lot less intrusive, and you probably could install a couple or three pretty economically.

 

[delagina]

if water table is low, 10' drilled pier is easier in construction. you would use mini excavator with auger attachment.

for spread footing you would excavate a wide area (footing area and some). so you have all those dirt around waiting specially if footing is not poured monolithic with pedestal.

 

[delagina]

i meant water table is deep.

 

[Arp32]

I worked on a project once with 15-18ft deep caissons (ranging from 36"-52" diameter) within 6 ft of an existing slab on grade structure where access was tight. We ended up going to the maximum depth with a vacuum truck - basically a diesel motor powering a huge vacuum with 12" diameter hoses that sucked up dirt, rock and cobble.

The truck was maybe 12-30ft from the caisson holes, they just added hose sections where needed. Was pretty easy to thread the hose through obstructions.

Word of caution, the truck was very loud and disorienting.

 

[EngDYIGuy]

Hey everyone,
The project location actually is in a shop with 12 ft tall roll up doors, and vehicle bays. So a small drill rig should be able to back into the locations. Sorry for confusion. I intentionally left out some details to see if anyone had some ideas on innovative excavation/construction techniques for tight access areas without a drill rig, backhoe, or equipment access (such as basements). Arp32's vacuum truck excavation is an interesting solution I've never seen or heard about.

The soils are stiff clays, with a very deep water table. Geotechnical engineer says either foundation system will work with the existing foundation system, and has designed for induced stresses, and consolidation. This thread is purely a constructability/cost comparison between the two options. Either designed system should work.

Undermining the slab for the verification of existing spread footing depth for the spread footing option was of concern. The spread footing requires someone to climb into the excavation and tie-wire a cage together (shallower then OSHA requirements for shored excavation), also requires compaction of fill under an existing undermined slab, and has (risk) assumptions made on existing foundation system, without as built verifications.

The temporary stability of the existing strip footing for a drilled pier excavation depth below the bottom of the wall strip footing, was of concern for this option. In lieu of the new information I provided with access for a small drill rig, it is my opinion that the drilled pier is a better/faster construction technique, maybe shore the wall first (damages during construction is contractors responsibility). Casing the hole doesn't make sense for such a shallow pier, in clay, in my opinion. This option just drops a reinforcement cage in place.

 

[PEinc]

It is also possible to hand-dig a foundation like this. The excavated pit may need to be a bit larger than 3' diameter and the pit will need to be shored for safety and to prevent collapse. If the foundation carries only vertical loads, the pit could be shored with either treated or untreated wood, like an underpinning pier. If you have lateral loads and/or an applied moment, you may want to shore the pit with steel line plates, that can also be grouted around their perimeter for full contact with the soil.

http://www.PeirceEngineering.com