Seepage from pits

[AUSue82]

I am working on a project which is an old manufacturing facility that is being dismantled, with everything being brought down to grade. The facility contains many u-drains and pits, which will remain in place, and will be backfilled. The contractor is planning to put holes in the bottom of the pits so that they don't fill with water when it rains.

They are looking for me to tell them how big to make the holes, and how far apart, and I have no idea.

Any suggestions?

Thanks in advance!

 

[CarlB]

Depends on how much rain, does an area drain into them or direct rainfall, soils type below them, how long to drain, etc... Could vary from 1 sq inch per 1000 sf, or remove the entire bottom & they could still fill up. So my SWAG is 1% to 5% of the bottom area.

 

[TDAA]

Get some perc rates. I suggest coring a hole, and digging a small hole with a hand auger. Fill it, and measure over time. That will let you know how fast the water will leave. Clay soils may hold water in the pits for some time.

 

[cvg]

What is the future plan for the site? If there is ever any plan to re-use the site, the concrete should be removed and then backfilled.

how big are these "pits" ?
if they are small - how will you get compaction inside of the small pit? they may need to use lean concrete to backfill the smaller ones which will eliminate the water problem. By U-drain, do you mean trench drain? they should be filled with lean concrete, otherwise removed.

Also, if they are well below the finish surface, will it matter if they do fill with water? Provide a minimum of two holes per pit, one at each end. Any convenient size obtainable with a jack hammer.

 

[fattdad]

How did this become your responsibility? I mean, just because a contractor wants you to give advice, doesn't necessarily make it your job to assume this responsibility. There should be a design and the contractor should follow it. If there is a design omission, then an RFI (request for information) should be issued and the appropriate designer should respond.

f-d

¡papá gordo ain’t no madre flaca!

 

[AUSue82]

We're just trying to come up with a preliminary cost estimate. The design will come later (hopefully soon!).

 

[dgillette]

Hate to bring a skunk to the picnic, but is this former manufacturing site known to be clean enough to be allowed to drain into the underlying groundwater? If that hasn't been verified, the investigation may cost 100 times as much as the holes.

 

[AUSue82]

Yes, everything that remains will be thoroughly cleaned before backfilling.

We went with filling the smaller pits and trench drains with lean concrete, and for the larger pits, putting in an approx. 6" diameter hole about every 20 sf.

That should give them a pretty reasonable cost estimate.

Thanks to all of you for your help!