Foundation Drain to Pump

[civilman72]

I'm working on a design to install a foundation drain around an existing building. The site is flat and footers are 4' deep, so I have no place to discharge by gravity - it will have to feed back to a sump pump in the crawl space and I have some questions.

Is it important to have a separate sump pump for the foundation drain, or can I connect to the existing sump pump, which is used to discharge moisture in the crawl space floor (gravel surface)?

What is the best way to go from the outside footer into the crawl space? I was going to propose that the pipe go under the footer. The trench would only be 1' wide and should not affect the integrity of the foundation. If I try to go over the footer and through the foundation wall it will cause a negative grade in the pipe (since the pipe will be at the bottom of the footer). I could consider installing the drain at the top of the footer, which would not create a negative grade to get into the crawl space.

The foundation drain will be 4" perforated pipe, or could be a grouping of 4" perf pipe. The pipe that goes under (or over) the footer and to the sump pump in the crawl space will more than likely be solid pipe. What kind of transition design should I consider to ensure that the majority of the water in perf-pipe foundation drain (and surrounding select material) gets into solid discharge pipe?

I've read a lot of other posts concerning the type of materials to surround the perf pipe (i.e. concrete sand, clean rock, well-graded aggregate, etc.), and it seems that in all discussions there is material placed beneath the perf pipe. I recently spoke with a local geotech, and they are changing their typical foundation drain detail to not show material beneath the perf pipe - they want the pipe placed at the bottom of the trench and backfilled with select material. Their theory is that gravity pulls most of the water to the bottom of the trench, and that the select material below the pipe will not convey this water as quickly as the pipe will, so by placing the pipe at the bottom of the trench they argue they are moving the water in the trench more efficiently. Thoughts?

Your input and thoughts are appreciated.

 

[Ron]

First of all, I'm a bit of a purist, so it is a "footing" not a "footer".

If all you have below the building is a crawl space, why do you feel the need to drain to the foundation level? (assuming the crawl space is adequately drained and the sump pump is working)

If your groundwater level is above the level of your perforated pipe, it makes no difference whether you put material below the pipe or not...same effect. If you have low permeability material below the pipe and the groundwater is below the top of the pipe, then you might get an increase in conveyance efficiency, but a decrease in drainage efficiency...in short, it doesn't matter.

If you are trying to lower the water table by these underdrains, then you need to be careful about lowering the water table below the footing bearing level. This will likely cause settlement.

Is there more to this than what you have stated?

 

[civilman72]

Ron - sorry for misnomer - duly noted "footing."

My client is dealing with high groundwater and standing water in the crawl space during spring runoff, so the foundation drain is necessary.

I see your point about location of perf pipe not making a difference with high groundwater.

Your comment about settlement is a concern - wouldn't the bearing pressure be maintained below the footing if the foundation drain is located at the top of the footing?

 

[MiketheEngineer]

Pretty much agree w/Ron

I usually show the bottom of the drain at the bottom of the footing - only because in these here parts the basement floor (if there is one) is usually just a couple of inches above the footing and I want a few inches of "safety". And you haven't "dried" out the area under the footing.

I would trench the outside of the footing which is what I think you are planning. What is wrong w/ another pump? They are only about $100 and may give you some redundancy if the flows could seep either direction.

Of course you could trench a small pipe under the footing and shove a 4'' piece in there and just let the hyrdualics take care of it self.

 

[Ron]

cm72...the buoyancy of the footing/soil interface will change. Probably not a big deal if you don't go below the top of the footing.

As for combining the crawl space drain and the foundation drain, you can do that, but I would put duplex pumps in the sump and cycle them. That way you always have redundancy and the pumps will last longer if you cycle them rather than letting one do all the work.

Good luck.

 

[hokie66]

Don't forget the geotextile sock around your perforated pipe, and probably around the trench.

 

[civilman72]

Great points - thanks for the quick responses.

Ron - like your idea of duplex pumps and will likely propose this.

Hokie - I am planning on wrapping pipe and trench with geotextile. I may also require the trench geotextile to be placed along the face of the foundation, attempting to tie-in the proposed foundation drain tile to the foundation drain. I'm also probably going to propose 3"-4" of well-graded gravel below the pipe topped with 8"-12" of concrete sand.

 

[Ron]

cm72...don't mix materials within the trench..if you put gravel in the bottom and sand on top, the sand will ravel into the gravel voids...resulting in settlement of the trench from the top. Use one or the other, or separate them with filter fabric.

 

[civilman72]

Ron - appreciate your input. Here's my initial thoughts - let me know where I went wrong:

I understand the concept of ravelling resulting in settlement, but there has to be some voids to fill for this to take place, and also something pushing it from above, a situation most commonly found under roads. But this is in an open space area and, unlike washed rock or p-gravel, the well-graded gravel is very dense and does not have voids (at least big enough for sand to easily penetrate). If the bottom of the gravel layer is wrapped in geotextile, there should be no significant future voids created within this gravel layer for the sand to migrate.

I expect groundwater movement to cause some of the very fine material to get pushed through the fabric with which will create some voids. So I'm anticipating some migration, but not enough to cause noticeable settling on the surface.

 

[Ron]

The only thing required to move the sand into the gravel voids is water movement...which you will have.

 

[cvg]

what is the point of placing the ABC below the pipe? You might consider a thin bedding layer of sand, but ABC is not particularly good for pipe bedding. Either way, I agree the sand will not mix with aggregate base course material.

 

[civilman72]

Ron - Could you expand a bit on your comment? Why does pure gravel or pure sand pipe bedding not create voids (and settling) when compared to layered gravel/sand pipe bedding? And migration through this layered bedding creates enough voids to cause settling in an open area 4' above?

cvg - I wanted to place a tighter material beneath the pipe, to try to keep the water in and above the pipe, and not underneath it. In high groundwater this will not have a noticeable effect, but in low groundwater situations it should prove effective.

Also, the material beneath the trench will surely be a fluffed backfill, so I wanted to get as much compaction beneath the pipe to discourage uneven settling of trench.

Thanks for the comments.