Build Pile Cap In High water area, under Active Airport terminal 3

[dragondipaula]

Does anyone have a way of stopping water intrusion into a pile cap while I excavate, chip piles down, set the rebar mats, and then place concrete in a pile cap. Other than well point. Multiple pile caps but, the largest is about 15'x13'. Here are the tricky parts.
1. Under an active airport terminal, with the closest daylight area about 500' away.
2. Soil is contaminated and thus the water smells like old jet fuel.
3. Encounter ground water at approximatly 2' into the excavation, going 4' deep.

 

[DRC1]

Why not well points?
What type and how permiable is soil?
How much headroom?

 

[civilperson]

Hire good contractor and do not interfer with means and methods!

 

[dragondipaula]

DRC1, this job is in south florida and the watertable is directly connected to the tides. Which would require constant pumping for days on end of water that smells of jet fuel. Expensive to get rid of. The soil is very permiable. The head height is about 12 feet.

Civilperson, good advice but, not an option.

 

[civilperson]

Pile caps and other formed structures can have the concrete easily placed under water using correct tremie practices. You are assuming that the pile cap must be built in the dry and experience shows otherwise. A evaporation pond for the fuel contaminated water is probably the most cost effective method of dealing with the water.

 

[dragondipaula]

An evaporation pond is a good idea but, I don't have the room.

 

[escrowe]

Who forgot to consider dewatering in south Florida?
Well, not the first time, but always fun.

I hope you're using a local geotech, but it doesn't sound like it.
Tremie will save you dough by allowing you to work wet,
but the inspection requirements and capping details may not allow it.

If dewatering is required, consider constructing a sheetpile coffer dam, overexcavating,
and pouring a tremie seal following pile installation (auger cast?).
That will reduce the dewater volume significantly.
Get a package water treatment system to handle the effluent, and pump it right back into the ground.
You will likely need a SFWMD/DEP permit for that (2 to 6 weeks). Get it, you don't want to be shut down in flagrante.

I suspect the budget will experience a bit of strain.

 

[concretemasonry]

Do not collect and discharge the "smelly" ground water. That is asking for bigger problems. Live with what is there, unless there is a federal grant for hazardous site, which is unlikely.

Live with the water and look toward tremie placement of concrete even if you add several feet of "filler or sealer" concrete under the real pile cap. Rough excavation is much cheaper than total dewatering.

 

[BigH]

Can I enquire about what type of structure the pile foundation is supporting?

 

[hokie66]

So the piles are there already, and you want now to build a pile cap. Why are the piles there? If they are just supporting the ground floor, you may want to localize your penetration of the floor by installing a new pile of large enough capacity to support whatever new load you have. In a working terminal building, access could be a problem for equipment, but I imagine a large area will have to be blocked off anyway.

 

[DRC1]

I would not recomend a trmmie pour. Trmmie pours are often used for non structural concrete, but rarely with srtuctural concrete. Contaminated groundwater makes the pour that much risker.

12 feet of headroom is not bad. I see two possible options or a combination of both.

1.) Drive sheets to effect a cut off dam. For a four foot cut heavy sheets will not be needed and the top 2 feet could be pushed back to further reduce the height. Light wt sheeting can be driven by handheld breakers or pressed by jacking against the overhead if it is strut. adequate. The sheeting will significantly reduce pumping requirements.
2.) use precastpile caps that fit over the piles and attach mechanicallyor are grouted in place. This has been done on limted access projects.

 

[hokie66]

Funny how we read things differently. When the OP said the water level was tidal and he would have to pump for days on end, I thought the 12' was water head, not head height in the building.

 

[dragondipaula]

thank you everyone
Escrowe, I was already thinking along the same lines as you but, that's quite an expense and what if it doesn't keep the water out? FYI, no one forgot dewatering, yes a local geotech, and we apply today for the dewatering permit. The work is down the road a few months.
concretemasonry, thanks for the info
Big H, augercast already installed
hokie66, pile cap to support a new elevator pit. we are to excavate about 12 feet down to include a sump pit.
DRC1, your idea 1 will be helpful, idea 2, I forgot to tell you it's an elevator pit/pile cap

 

[escrowe]

Yes, supported excavations drastically increase cost,
but they come with the territory.

If the excavation is properly sealed, and the sheet pile
properly installed, you should be able to work 'in the dry'
with minimal pumping.
That should satisfy the inspection requirements.

Unfortunately, construction of a tremie concrete seal
will require overexcavation to a bottom depth
at least twice the depth below groundwater of the top of the seal...

...unless some other means than dead weight is employed
to resist hydrostatic pressure/bouyancy.
And of course the top of the seal will have to be set
below the planned bottom of the pile cap.
That's a lot of digging.

Have you considered perm-grouting the perimeter of the excavation area?
That could significantly decrease permeability
and lateral loads on the excavation support system.
It could also be possible to injection-grout an excavation
floor, and pour a relatively thin mat
instead of a full-depth seal.

I would suggest checking in with Hayward Baker or similar.
Of course the expense of grouting may approach
that of a tremie seal.

 

[dragondipaula]

escrowe, perm grouting and injection grouting? sounds expensive

 

[escrowe]

I agree. But then again, these expenses are relative,
and it sounds like something needs to be done...

http://www.haywardbaker.com/services/chemical_grouting.htm

 

[hokie66]

I think escrowe has hit on the solution. You need to grout, grout, and grout some more to make the underlying sand impermeable before you start digging. Rather than expensive, I would expect this would be by far the cheapest option. Find a specialist grouting company.

 

[DRC1]

Can you reinject the groundwater? Some states allow it some don't. If so, with sufficent toe on th sheets you will be able to reduce pumping considerably w/o grout.

 

[dragondipaula]

DRC1, The port won't let us reinject the water. Once it's out it's our problem to get rid of.

Thanks everyone, I'm going to over excavate and concrete the bottom about two feet and the slope the sides about 45 degrees and concrete. Basically build a swimming pool with 45 degree sides and try to keep the water to a managable level so they can cut and chip the piles, set the mats, dowels and forms. I can get the inspections and pour the pile cap/bottom of elevator pit. Then I should be out enough to form the walls of the pit.
Thanks again for everyones advice. This was the first time I used this forum. Extreamly easy and helpful

 

[escrowe]

Good luck-- if you are where I think you are,
watch out for the pitchfork waving, torch bearing townfolk...