Depth of hole for sump pump

How deep do I really have to dig a sump hole in my basement floor if I want to install a sump pump?
Background:
Lotsa rain here in Boston has brought about 2 inches of water in my baseement. I own half of a two-family house, so my neighbor shares the foundation and basement structure. I've been in the house for a year; she's been here for 14 years and says that this happens ever few years - lots of rain in the spring brings in about 2 inches of water. We set up one of those "sits on the floor" pumps and pumped the water out into the street and away from the house; the 2 inches of water comes back in a couple of hours, even though the rain stopped yesterday. Sounds like a 'water table' issue to me.
Everywhere I read about installing a sump pump says that the hole ought to be around 2 feet deep or more. But I don't understand why a pit only a few inches into the floor wouldn't be enough. I mean, assuming that our basement floor (concrete slab) is relatively flat (and it is), as long as the intake of the pump was a few inches below the floor level, wouldn't that be enough to keep the floor dry? Do we really need a sump pit that's 2 feet deep?
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Matthew H. wrote:

You're correct; a "pit" only a few inches deep would suffice to get the water out. A deeper pit and bigger "bucket" is strongly recommended to provide a greater hysteresis for the poor pump motor between ON and OFF. The shallow pit will force the pump to cycle frequently, affecting its life. (May also be impossible to set the float for small level changes on some pumps.)
Jim
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Most sump pumps are introduced into basements with the idea that they're also evacuating water from footing drains (ie: french drains). That means they have to be recessed far enough into the floor to connect to the drains. Thus, you have a sump pump pumping away without the floor getting wet.
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Chris Lewis, Una confibula non set est
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You want to set the sump pump at least two feet deep to give it some leeway in case it can't keep up. Now is not the time to do thins. Wait a month or so till the water table drops again. Get your self a piece of plastic pipe at least 15 inch diameter and at least two feet deep. Drill as many 3/4 inch holes in it as you can fit on the bottom 18 inches. Break out a small portion of your concrete at least a 2 foot square. Dig to a depth of at least 2 1/2 feet to 3 ft deep. Put some 3/4 crushed stone in the bottom to make the top of you pipe lever with floor. then surround pipe with more 3/4 stone up to bottom of concrete floor. Mix up some concrete and repair you floor. Hopefull there is some crushed stone under you entire floor to help the water get to your pump. If not as mentioned above french drains may be needed. They require breaking the floor out along all four walls about a foot wide digging out the dirt to a depth of one foot or so and installing a 3 or 4 inch perforated pvc pipe pitching towards you sump from two differnt directions. Not an easy task but better then a flood everynow and then. Hope this helps, Rick
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15" plus plastic pipe isn't easy to find at a reasonable cost.
Pre-made sump pit liners are much easier to obtain. ;-)
For "cheap", take a plastic barrel, cut off one end to the right length, and drill holes as randd01 suggested - you may need to cobble together a cover of some kind.
55 gallon plastic barrels are a trifle on the large size but easy to find. Slightly smaller barrels (ie: used food grade ones) are also usually easy to find too.
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You'll be lucky if the 2 ft deep hole solves the problem. To not flood the floor, the water has to have an easier path to the sump pump than to come into the basement somewhere else, like on the other side of the basement. With a 2ft hole, this may work, but a hole only a few inchs deep definitely won't, because there is considerable resistance to water getting there from a distance away. Also, you're going to need something to put in the hole and the std sump pump liners are that deep anyway.
The most effective way to do this is to have drain pipe with holes laid in a gravel base under the perimeter of the basement floor, leading to the sump pump hole. Of course, that needs to be done before the floor is poured.
Even before the sump pump, have you checked what's going on outside? The fact that it happens only after a heavy rain suggests you may have a problem with not diverting water away from the house. Make sure all the gutters are clear and working. You have gutters right? Make sure the grading flows away from the foundation. All gutter leaders should divert away from the house, as far as possible. The minimum is a long splash block, for some locations an underground routing to an appropriate low point may also be appropriate. Go outside dooring a heavy rain and see what's going on, if water is pooling, etc. It's a lot easier to deal with it before it gets in the basement.
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A pit 2" deep is enough if you don't mind the entire floor flooding about 3" - 4" deep between each pump cycle. The object is to keep the water below the floor at all times and to have the pump run as long as possible as infrequently as possible (within reason), hence a 2' deep pit. An 18" pit should suffice though, but a 12" is probably too shallow as the pump wants to stop with perhaps 6" of water around it (and ideally the pump, if it isn't on short legs -- the bigger higher capacity ones may be -- should be elevated a few inches from the bottom of the pit to avoid sucking in sediment and debris that ends up settled in the pit, so a 12" pit would mean it could only drop the level about 5" at a cycle, leading to lots of short inefficient cycles.
In a bind (such as Boston in the last week or so where we received up to 7.75" of rain in about 40 hrs), a pump on the floor can help.
Zoeller pump has good info on construction of sump pump pits.

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