Underslab drainage question

[jloehlein]

I'm starting to build a 54x81 metal building in Virginia. The ground is red clay. I am about to install the main sewer line, which has to run about 4' deep through the center of the building. I'll be backfilling the trench under the slab with clear stone (57s). I'm wondering if the trench of 57s surrounded by clay could create some odd moisture issue under the slab and if I should put a drainage pipe in there and run it out to daylight or if I'm overthinking it and it'll be fine without.

 

[Firebrick43]

I would trench to daylight. I would also trench around the perimeter or at least the eave sides and join it together before going to daylight.

Had to look up Virginia stone classifications but I think 57’s or 68’s is an excellent choice.

 

[Rusted Nut]

Yes, ditch lines are a conduit for water drainage/moisture. Can you go around the building? If not, install a high quality vapor barrier with taped seams, etc.., under the entire slab. If it were me, I would pour CDF mix in the ditch rather than fill with stone. I don’t know what 57 stone is, as I’m on the other side of the country, but in general rock/gravel lined ditches collect water.

 

[TomC750]

I'm starting to build a 54x81 metal building in Virginia. The ground is red clay. I am about to install the main sewer line, which has to run about 4' deep through the center of the building. I'll be backfilling the trench under the slab with clear stone (57s). I'm wondering if the trench of 57s surrounded by clay could create some odd moisture issue under the slab and if I should put a drainage pipe in there and run it out to daylight or if I'm overthinking it and it'll be fine without.

I had the identical situation in my building, red clay as well. surrounded pipe with 57's all the way to septic tank. Been 8 years, no problem.

 

[jloehlein]

I would trench to daylight. I would also trench around the perimeter or at least the eave sides and join it together before going to daylight.

Had to look up Virginia stone classifications but I think 57’s or 68’s is an excellent choice.

I guess it would be pretty easy for me to backfill with clear stone all the way to daylight versus just under the building. It'll be easy for me to do the same on the eave ends when I add the gutter drains, too. Thanks.

Yes, ditch lines are a conduit for water drainage/moisture. Can you go around the building? If not, install a high quality vapor barrier with taped seams, etc.., under the entire slab. If it were me, I would pour CDF mix in the ditch rather than fill with stone. I don’t know what 57 stone is, as I’m on the other side of the country, but in general rock/gravel lined ditches collect water.

No matter what, the sewer line will have to come through part of the slab, so we're taking the straightest shot to the septic tank. Will do a vapor barrier and look into CDF. 57s are just ~3/4 crushed and washed stone. It's the most common backfill around here.

 

[Firebrick43]

I would pour CDF mix in the ditch rather than fill with stone.

Why would you want to stem the movement away from the slab? You want the water taken away in this situation

but in general rock/gravel lined ditches collect water.

All ditches collect water and move it away from where it presents a problem. It it’s not constructed properly and holds water it’s a moat.

Rock filled ditches, aka French drains, have been important part of keeping our structures foundation clear of damaging water for centuries

 

[LopezBart]

When I had the area cleared for our 30x48 shop, I discovered that it was a local low spot and it looked more like a swimming pool than a future shop site. We had a French drain installed around the building footing and daylighted it some distance away; the slab has a moisture barrier and is 18" or so above the drain. The water drains nicely....

 

[TnClimber]

I put 57 fill on the clay before I poured the new floor in my garage and new shop build. There is a slight slope towards the buildings. You know that the water is going to run on top of the clay right through the #57. It will keep the moisture down and a barrier is a must. I put a corrugated pipe under the new garage floor at the low end to daylight. I also put a corrugated pipe on the high side of the shop to catch the water before the monolithic pour. During a heavy rain, the water ran out the end of both pipes. Fast toward...I put a French drain and catch basins on the high side of the garage and shop. The gutter and downspout run to the drain. I rarely get water on low side with the new French drain. I did overkill and it sounds like #57 under your pad maybe enough. Craig

 

[theoldwizard1]

If it really is red clay, you are over thinking it.

 

[Rusted Nut]

Why would you want to stem the movement away from the slab? You want the water taken away in this situation

A rock filled ditch would give water a place to sit, filling it with CDF would not allow to fill and sit there.

All ditches collect water and move it away from where it presents a problem. It it’s not constructed properly and holds water it’s a moat.

Rock filled ditches, aka French drains, have been important part of keeping our structures foundation clear of damaging water for centuries

Yes, French drains work well; but I wouldn’t put one under a slab on grade. The less water sitting under a slab, the better.

 

[ConCretin]

Where is all the water you are worried about coming from? Install a quality vapor barrier and carry on.

 

[LopezBart]

Unless one has an artesian well under a shop, or very high ground water levels, water will come from rain and perhaps water draining towards the building. To prevent problems, have a French drain around your building at or below the footing level, and daylight that (or use a pump if you must). This will drain away any water that accumulates around your building, and will keep the moisture away from your slab.
And install that moisture barrier!

 

[Firebrick43]

A rock filled ditch would give water a place to sit, filling it with CDF would not allow to fill and sit there.

Yes, French drains work well; but I wouldn’t put one under a slab on grade. The less water sitting under a slab, the better.

It does not sit it, it drains away.

Just because you apparently have seen rock filled ditches/drains that were graded incompetently doesn’t mean every one is placed by an incompetent person.

With lasers it’s even easier.

 

[larry4406]

A rock filled ditch would give water a place to sit, filling it with CDF would not allow to fill and sit there.

Yes, French drains work well; but I wouldn’t put one under a slab on grade. The less water sitting under a slab, the better.

What do you think is under your house slab? Your groundwork’s are bedded in stone as is the outfall. Then there’s the typical 4” of stone under the slab?

I did 18 front load townhouses where the sewer exited the house straight thru and under the garage slab to the main at the street. We backfilled with #57 stone to bed the piping and then backfilled fully with #57 stone up to slab sub grade. When the slabs were prepped we added rebar to span the trench and normal WWF.

Why 4’ deep under the slab? Run the groundwork’s at 1/4” per foot from bottom of slab (no horizontal pipe in slab).

If you have a deeper invert to tie into, make that drop external to the structure.

 

[Rusted Nut]

What do you think is under your house slab? Your groundwork’s are bedded in stone as is the outfall. Then there’s the typical 4” of stone under the slab?

I did 18 front load townhouses where the sewer exited the house straight thru and under the garage slab to the main at the street. We backfilled with #57 stone to bed the piping and then backfilled fully with #57 stone up to slab sub grade. When the slabs were prepped we added rebar to span the trench and normal WWF.

Why 4’ deep under the slab? Run the groundwork’s at 1/4” per foot from bottom of slab (no horizontal pipe in slab).

If you have a deeper invert to tie into, make that drop external to the structure.

You have an unattached garage with possibly a high side and low side grading, where water could collect and flow through a four deep rock filled ditch. under a slab. To me, I would not want the potential moisture issues that could cause, and I would fill it. Just my two cents from dealing with water infiltration issues for 40 years the wet PNW.

 

[Firebrick43]

You have an unattached garage with possibly a high side and low side grading, where water could collect and flow through a four deep rock filled ditch. under a slab. To me, I would not want the potential moisture issues that could cause, and I would fill it. Just my two cents from dealing with water infiltration issues for 40 years the wet PNW.

wow. I watch the news and see a lot of messed up stuff in the PNW but who would of figured they have been suffering for 40 years of people who can’t use a transit and grade stick or more recently laser levels to ensure proper drainage of ditches .

Maybe we need a late night night infomercial to raise money to train them how to dig a proper slope?

 

[jloehlein]

What do you think is under your house slab? Your groundwork’s are bedded in stone as is the outfall. Then there’s the typical 4” of stone under the slab?

I did 18 front load townhouses where the sewer exited the house straight thru and under the garage slab to the main at the street. We backfilled with #57 stone to bed the piping and then backfilled fully with #57 stone up to slab sub grade. When the slabs were prepped we added rebar to span the trench and normal WWF.

Why 4’ deep under the slab? Run the groundwork’s at 1/4” per foot from bottom of slab (no horizontal pipe in slab).

If you have a deeper invert to tie into, make that drop external to the structure.

The site necessitates the tank location is pretty close to the outside of the shop, where there is a 3-4' drop off just after crossing the driveway (which is right next to the shop). The pipe will be sleeved across the driveway, so we are making up most of the depth required at the vertical stack.

 

[kj_mustang]

In western Virginia with red clay pad under my building and septic line dug into the middle of the building. Septic line slopes away from building to septic tank. Surrounding earth around building sloped away from building and gutters installed. Septic line was backfilled totally with dirt. Zero settling or water issues in 10 years.

 

[Rusted Nut]

wow. I watch the news and see a lot of messed up stuff in the PNW but who would of figured they have been suffering for 40 years of people who can’t use a transit and grade stick or more recently laser levels to ensure proper drainage of ditches .

Maybe we need a late night night infomercial to raise money to train them how to dig a proper slope?

Even with slope, there will be water running through, which will cause moisture issues.

 

[larry4406]

The site necessitates the tank location is pretty close to the outside of the shop, where there is a 3-4' drop off just after crossing the driveway (which is right next to the shop). The pipe will be sleeved across the driveway, so we are making up most of the depth required at the vertical stack.

If you can, add drain tile in your 4’ deep trench, run your sewer lateral as needed to your tank, but then take the drain tile to a location where it can be daylight.

I’ve built several homes where perched water tables were encountered during excavation. The gravel bed under the slab includes drain tile which we ran to daylight. Normal vapor barrier then slab. Guess what? Water drains by gravity, sump pump doesn’t run, and house has no issues.

 

[rayra]

I'm starting to build a 54x81 metal building in Virginia. The ground is red clay. I am about to install the main sewer line, which has to run about 4' deep through the center of the building. I'll be backfilling the trench under the slab with clear stone (57s). I'm wondering if the trench of 57s surrounded by clay could create some odd moisture issue under the slab and if I should put a drainage pipe in there and run it out to daylight or if I'm overthinking it and it'll be fine without.

Aren't you already essentially creating a french drain by making the gravel-filled trench? No reason to put a pipe in it too.

 

[larry4406]

Aren't you already essentially creating a french drain by making the gravel-filled trench? No reason to put a pipe in it too.

His sewer later trench dead ends at the buried tank so no place for the water in the gravel to go.

By adding draintile to the trench, he can then take the draintile in a different direction and run it to day light.