Drain tile design??? 3

[nutbutter]

I've been asked to determine a quantity of drain tiles needed for the construction of a large concrete pad. The project consists of the construction of a large concrete pad approximately 340' x 175' for storage.

Now, I'm not too familiar with drain tiles and I've never had to do any design involving drain tiles. Fortunately, I'm just coming up with a quantity for the proposal and I'm not doing any design.

Anyway, my understanding of drain tiles is that they are basically just perforated pipe, correct? I assume that they're basically laid out around the perimeter of a foundation at the same height as the bottom of the slab (or footings) and sloped to pick up water and move it away from the building. Is that the basic idea? Don't worry, someone with some experience will be doing the design, I'm just coming up with a rough estimate of quantities for the proposal.

Now, with a slab that large (340' x 175') how do I determine the size of drain tile that I will need. The research I've done shows standard is about 4" dia., but that's for residential applications I believe. Would I want to bump this up a few sizes, say 12" or so? Is there a quick, simple way to determine what size is needed without knowing any info on the groundwater at the site? We have very little info.

Lastly, and most importantly, will drain tiles around the perimeter be enough for a slab this large? Would I want to make a network of drain tiles under the slab to ensure better drainage? This slab is going to be elevated above the current grade at the project location by at least a few feet, so I don't foresee high groundwater being too much of an issue. However, if drain tiles are needed under the slab, is there a quick way to estimate an approximate quantity needed without any available information other than size of slab.

Any help on this would be much appreciated. I've been asked to help out someone in another discipline and the guy I know who has experience with drain tiles is gone through next week.

Peace,
Stoddardvilla

 

[cvg]

is this "pad" a building foundation slab? Are there footings for the pad below grade? You say high groundwater is not an issue, then is the drainage system picking up storm water? Drain tile as you describe are generally installed to pick up subsurface water and move it away from a footing or basement wall. Not generally necessary for a slab.

 

[nutbutter]

There are VERY strict requirements on the settlement allowed for this foundation. Although, in my opinion groundwater won't be an issue, I think they want to install some sort of subsurface drainage system to be (very) conservative.

For proposal purposes, I'm going to assume that we'll be running 8" perforated pipe at 25' intervals across the entire slab. They will daylight on the hill nearby. I assume they'll be about 2' below the slab, but that information isn't really necessary now because this is just for gathering quantities for a proposal.

If it were me, I don't think I'd even bother with drain tiles, but they want them, so they're going to get them.

Peace,
Stoddardvilla

 

[cvg]

then you will also want to install that pipe in a trench backfilled with a granular drainage material (sand or gravel). Material should have a filter fabric or be filter matched to the surrounding soil to keep soil from plugging the drainage material or entering the pipe. Search the forum for french drain / subsurface drainage and you should find considerable information / debate on the proper method for doing this

 

[TDAA]

In the absence of high GW, the 4" pipe should be plenty, especially if there is a granular material around the pipe. Upgrading to a 6" is not a bad ides for the larget commercial settings. Going larger I think is likely overkill.

Just figure the capacity of that pipe at the slope you will put it in at. Now, where will you get all the water to fill it? From your description, you will only have infiltration through the backfill around the building (possible pipe breaks, etc. excluded).

 

[dik]

Unless you have a serious groundwater problem, the 4" perf drain with geotextile 'sock' is likely adequate.Because of the long dimension, you might want to consider 6" for the perimeter. If you require underslab drainage, 4" at about a 40' spacing should be adequate. These should drain to a sump pit and should be pumped to a stormdrain sewage system or be pumped to free drainage away from the building to prevent a 'short circuit' of the fluid.

Dik

 

[nutbutter]

Thanks all you guys who replied. I'll probably bump it down a size because you're right. This pipe is going to only be picking up infiltration. I mean we're building the pad anywhere from 1'-8' above existing grade, so GW issues are non-existant.

Thanks,
Stoddardvilla

 

[dik]

If elevated 1'-8" above existing grade, 4" would likely be lots...

Dik

 

[nutbutter]

No, that's 1 ft through 8 ft, not 1'-8". MY BAD! Anyway, I would imagine that 4" will still by lots of extra capacity.

Peace,
Stoddardvilla