Sump Pump Installation 2

[HomerLee]

We have a home constructed in 1978. The crawl space is not ventilated to the outdoors but serves as the return air sysyem for the forced air heating system. The floor is concrete, apparently poured over black plastic. Occasionaly during periods of high rainfall we have water in this space. It does no damage; there is nothing that gets wet other than the concrete floor and concrete foundation walls. The water is usually no more than an inch or so deep in the lowest place, although one time it got about 4 inches deep during a severe period of rain (groundwater was at the surface of the soil in spots on our lawn).

The builder did not level the crawlspace floor carefully which left one area near a corner as a low spot, ideal for a sump pump. I have a submersible 1/3 hp float type pump with a differential of 5 inches between the on-off points. The pump mfg. specified a minimum sump of 18 in. wide x 24 in. deep. I talked to their tech department and was told these dimensions were determined by the desire of people to not have a wider sump in a basement. I said I thought a wider, shallower sump would be better for the pump as it would increase cycle time and the tech agreed.

My plan is to install a 22 in. wide X 16 in. deep sump in a 30 in. wide pit. I will have 3 in. of gravel around the sump with a membrane of fabric to prevent wash out of the soil.

My questions:

1. If the sump is a minimum of 25" in. from the foundation foot, and 16 in. deep, with the membrane liner, is there any probability of undermining the foundation nearby? (we have clay soil). I have heard of a general rule that if you draw an imaginary line at a 45 degree angle down from the base of the foundation foot and stay above this line then a sump or french drain is located safely.

2. Due to the construction of the house, there are obstructions such that I can get a sump liner to the location only if the liner is no more than 16 in. deep so I must fabricate my own. I plan to make one out of a heavy 55 gallon poly drum by cutting off the bottom 16 in. I will drill about 280 3/16" holes around the perimeter of the sump which, according to my calculations, will roughly equal one 3 in. diameter pipe. Is this adequate to allow water to flow in quickly enough to prevent the sump from floating out of the hole?

3. Should the membrane be next to the sump or outside the gravel? I assume outside the gravel as in a french drain. Is sturdy landscape fabric adequate for a membrane or must the woven type used for road construction be used?

4. Any cautions to my plan?

Thanks in advance for your assistance. I have tried other forums and gotton almost nothing.

 

[cvg]

I'm not a geotech, but 25" should be ok.

If you are worried about the liner floating, put some of the gravel in to weight it down. However, I don't think that should be a problem.

The 280 holes seems adequate, however remember that most of the holes will not flow until the water level in the gravel reaches the top of the bucket. You may want to go with larger holes.

the filter fabric should always be next to the fabric that you are filtering - in this case, next to the soil. However, with clay soil - I would not expect much groundwater flow from the clay into your sump.

 

[HomerLee]

cvg,

Thanks for your response! I will put a concrete pad about 2" thick in the bottom of the sump. I believe that will be adequate with the weight of the pump to prevent the sump from floating. I experimented with water flow through the 3/16" holes by trying to fill the sump as much as I could with a 5/8" water hose, faucet wide open, and when I reached about 4" in depth the water ran out as fast as I put it in.

 

[oldestguy]

You have the right idea about a filter. However, since gravel is not a filter, you are right that some protection against loss of ground into the bucket is needed.

There may be difficulty getting that fabric in there. Instead, why not make your "backfill" a filter? Instead of gravel, use a mix of sand and gravel, as one would use for a concrete mix. Concrete sand is an excellent filter of all soils by itself.

The 3/16 inch holes will ideally work also, since coarser particles keep the finer partcles from entering.

A belt and suspender approach would be to use this sand and gravel mix, plus the fabric.

I imagine this situation also adds humidity that you don't need in the air. Have you thought about catching that water before it gets there?