Hydrovac excavated pile capacity

[CED]

Hello.

Could anyone direct me to papers or documents with info
on how to determine friction/bearing capacities for piles
advanced using hydrovac excavating?

Any links or direction would be greatly appreciated.

Thanks,
Corey Dale
cdale_shelbyeng@hotmail.com

 

[lacykh]

Have you had any luck finding any references?

 

[Focht3]

Corey -

I haven't seen any - I assume that you have checked the usual references (GT Journal, Geotechnique, etc.)

Personally, I don't have any experience with this technique (at least not with a construction method by that name.) Can you provide some additional details about the site and hydrovac construction in general?

 

[CED]

I haven't found any references on this yet. After talking with one of the senior engineers in my firm, we decided to treat the pile as a skin friction pile.

A hydrovac'd pile is simply a pile hole excavated using a high pressure water stream to blast the soil, and a vacuum truck to subsequently remove the material blasted away by the water. This type of excavation is used when the pile is to be located near existing buried ulities so as to not damage the utilities.

A hole with very rough sides is created. Our feeling was that the skin friction would actually be greater than that of a bored friction pile due to the much rougher sides, but we treated it as a regular skin friction pile to be conservative.

Regards,
Corey Dale

 

[Focht3]

I'd be concerned about soil disturbance. Do you plan on running load tests? I'd suggest at least three - all to plunging failure if possible. Loading them to twice design doesn't help you evaluate the effect of installation on soil properties.

My gut says that piers constructed this way should be adequate, but my gut doesn't have an engineer's license...

 

[BantrelStructural]

On projects that I have worked on in the last few years hydrovac has been used many times for excavation for installing piles, and I don't recall ever seeing different friction values being presented by our geotechnical consultants. Having said that, in most cases hydrovac was only used for starting piles (i.e. to get past potential uncharted underground utilities in refineries, petrochemical plants, etc.), and then once past this danger zone (which in most cases is close to the same length as the dead zone on the pile - where we don't count on any friction capacity anyway), the contractor continued drilling the pile with a conventional drilling rig.

There are strong caveats for drilling deeper with hydrovac, not the least of which is that depending on deeper soil layers/materials your pile might be a whole lot larger in diameter than you expect, even to the point where you could fail adjacent structures. The deeper you go, the harder it is to see what is actually happening. And in some circumstances even shallow excavations have resulted in undesirable effects (such as cave-ins) due to a sandy-gravelly layer washing away, rather than cutting a relatively nice clean hole in some clay/clay till.

Discussion with a colleague reveals that while he has never seen a different value for friction capacity, he has seen higher values suggested by the geotech for frost jacking effects.