questions on digging for sump pit in basement

In my ongoing basement reno project, I am going to install a sump pit and pump in the furnace room to deal with rare occurences of heavy rains and high water table. I have a few questions.
The pits I've seen at HD and Lowes are 18" diameter and 22" deep. How close to the foundation wall can I install it? Any concerns with hitting the footing? My basement is solid concrete walls. Due to clearance issues in the furnace room, the closer I am to the wall the better. For a 18" diameter, should the hole be 21" diameter?
Also as far as the discharge pipe which 1 1/2" PVC. I have to go up approx 8 feet, then 3 feet across horizontally .If I get a 1/3 HP pump, can it handle that ?
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Foundation footings can be anything from 12" to 24" depending on the code at the time and the size of the structure. I'm guessing you are going to jack hammer through the existing concrete floor? I'd do enough to accomidate a range. You are going to have to patch the floor when you are done anyway. Then when you dig the pit you will probably find the footing and can adjust from there. Just fill with gravel if you have to widen your hole to avoid the footing. You want to not dig below the bottom of the footing. If you are using one of those plastic pits you can always shorten it if you need to.
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why not go below level of footing, for a sump pit? its not like a regular sized pit will endanger the structure of the home...
lower has a better chance of getting the water table down
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Because you run the chance of undermining the foundation if you are try to get close to it. In many juristictions foundation depth will be fine. Unless the house is old and down south somewhere.
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if you are going to use a pump with a float switch that's on the end of a wire and floats up and down, make sure the diameter of the pit is big enough for the float to move in an arc up and down...
Mark
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The 18" diameter is too small. You need more room for the float to move freely and you need more volume of water in the sump. The small sumps are emptied in a few seconds by the pump and this causes the pump to cycle too much. More volume makes the pump operation more reliable. Get at least a 24" diameter, 30" is better yet. Don't go less than that 22" depth of sump. Locate the pump either right at the bottom or only an inch above bottom so it can suck the water table as low as possible. The reason you need holes at the bottom is twofold - lowest water table and to prevent sump from floating when it is empty.
You don't mention weeping tiles or any other way for ground water to reach the sump. If your soil is porous enough to transmit a fair amount of water pipes are not required. Most soils, however, are not porous enough and some pipes are needed. Since you are retro-fitting it is not too bad an idea to try going without pipes first to see if that is good enough. If you still get flooding it could mean a lot of things like too small a pump but most likely it will mean that water can't get to the sump so it floods into the basement. If that happens you will need some sort of weeping tile system. Maybe there are enough gaps under the basement floor that water will reach the sump without rising to floor level - need a bit of luck.
As to how close sump can be to wall - depends entirely on soil type and whether you do a good job with the geotextile. If the soil is sandy or very silty it will cave in when you dig and undermine the foundation. In that case move as far from wall as possible and use bracing to hold soil up. If the soil has enough clay that hole walls do not cave in at all then put it as close as you can but make sure that geo fabric is well done or soil will wash into sump and undermine foundation. A good fabric will hold fine soil particles back. You need to keep the soil from the sand layer as well as the sand from the sump.
Make sure the sump pump has a check valve either inside the pump or screwed onto the discharge line. Don't listen to anyone who tells you to remove the valve. Without a check valve all of the water in the pipes will drain back into the sump every time the pump stops. That refills a good portion of the sump volume and leads to more frequent pump cycling.
Make sure the sump is totally wrapped in a non-woven geotextile fabric - the garden grade is OK if it is non-woven and you use two layers. Non- woven means it is more like felt than sacking material. The fabric prevents small soil particles from being washed into the sump and pumped away. That way the foundation will not get undermined.
Most geotextile fabric has very good water flow capacity - test yours by running a hose into a bowl shape of fabric - water should flow right through very quickly. Try 2 layers to see that capacity is still Ok. Some of the cheaper fabrics are too tight. The construction grade stuff is great but comes in massive rolls. If you see some at a construction site stop in and ask if they will cut you off a few yards - it sells for a few bucks a square yard so it is not free but some project folks are glad for the public relations of being helpful to locals.
The dug hole can be 4" larger than the sump. Install non-woven geotextile around the walls and bottom of the hole as well. Make it tight and smooth, use large nails to hold the material in place while you install the sump. Center sump and backfill space between the sump and hole walls with coarse sand. Install the sand in layers of 4" to 6" and stop to pack it fairly tightly - don't pound hard, just enough to make sure all lumps are broken up and the sand moves into all gaps. So your layers will be; dirt wall of hole, geotextile, 2" of sand, geotextile, wall of sump. You can see that if hole is only 4" larger diameter than the sump there is only 2" to work sand into. You have to work very accurately which is fine if your soil is sticky enough to stand up by itself from when you first dig the hole untill you get the sand backfill done. A larger hole makes it easier to work but is not needed for the hydraulics.
If the sump does not have holes all the way to the bottom then drill more holes lower down - a lot of 1/2" holes in two rings around the bottom will give enough capacity. The rings should be 2 or 3 inches above each other so the sump wall is not weakened.
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.

I've always heard to put 3/4 stone around the sump. What is the advantage with sand?
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The objective is too prevent fine soil particles from moving at all, not into the sand/gravel layer and not into the sump itself. Gravel has larger pore spaces and thus the geotextile could get pushed into the voids - the cloth may tear in the future. Sand is easier to pack tightly.
Gravel will interlock and create large pore spaces. You never put gravel into tight spaces, it is for laying on surface layers. In addition; clean gravel will interlock worse and what is called well-graded gravel will have a lot of small particles which fill the void spaces and greatly reduce hydraulic conductivity.
If you are retricted for space it may be better to use an 18" diameter sump than to place washer/dryer on top of the sump. You need to inspect the sump often to ensure that the float is not obstructed from moving up and down. The appliances are heavy and have water or vent connections that must be removed and replaced every time the appliance is moved. In my experience, maintanance that is difficult is not going to happen.
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Re: questions on digging for sump pit in basement:

That was a very thorough discussion. I'm not doing a sump, but found it interesting. Thanks for posting it.
--
Work is the curse of the drinking class.

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1/3 hp should be plenty, I also go up 8', and then across 22 feet and pump out in no time. Any bigger will work, of course, but if you need backup power, the bigger motor sizes will require a bigger power source and that is $$$$.
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On 1/18/2011 9:56 PM, hr(bob) snipped-for-privacy@att.net wrote:

18" is pretty puny, IMHO. If it ever gets full of mud and needs to be dug out, it will be hard. The pit here (bone dry with cobwebs the 5.5 years I have been here, in spite of wall leaks) is an old 20gallon lube oil drum- you can see the lip at the top and the bump halfway down. (If I ever do see water in there, I have a submersible I can throw in there, hooked to a garden hose run out the window and across the back yard.) Previous owner abandoned a 24?" diameter plastic pit and pedestal pump he never got around to installing. I really need to put that in next garage sale, or donate it to Habitat or something. When I was a kid, my father favored 24" glazed clay drain tile sections. The bell end was flushed with floor, and made a perfect fit for the kid-proofing lid. Of course, in southern IN, there were no slab drains feeding into the side, these had a foot of gravel on the bottom, and were solely for the occasional ground water spike. In Columbus IN, dig down 10-12 feet pretty much anywhere, and you are into gravel. Here in SW MI, dig down 5 feet, and you are into sand. Beyond that, it is turtles all the way down.
--
aem sends...

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I have space limitations in my furnace and laundry room, thats why 18" diameter would work for me.
If I go bigger, am I allowed to store things on top of the pit with the cover on? For example, can part of a washer or dryer cover the pit? Not the whole unit, just part of it.
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On Wed, 19 Jan 2011 03:46:01 -0800 (PST), Mikepier

The things you store on top should be readily movable. Before every wet season, (October to March here on the wet coast), the pump should be tested by lifting the float to ensure the pump runs.
Having said that, I never check mine :)
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