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Sump crock holes

Sump crock holes

Sump crock holes

Sorry for the lengthy description... The soil in our project was identified by our engineer to be Marlboro Clay and as such the engineer recommended that we over excavate the basement by 3 feet and install 2 feet of #3 stone and then 1 foot of #57 stone under the entire foundation. (1.5+ million pounds of stone) The concrete company dug away roughly 4" of the #57 stone and build the forms for the 1' high footers. Once done with the footers, more #57 stone (guessing here at 8") was used to raise the inside slab area to the height of the footer. Then the slab was poured (approximately 4" thick). So from top of the slab to the filter fabric under the #3 stone is about 4'. I purchased a couple of sump crocks and extensions to get them a bit deeper. Each crock basin is about 4' high.

The lowest opening in each of the crock basins are about 18" from the bottom of the rock. This means that there is constantly 18" of water sitting inside the 18" of #3 rock around the sides of the bottom of the crocks. I'm wondering if anyone has opinions on if I should drill additional holes in the crock basins below the hole at 18" from the bottom to try to get as much water out of the stone or if I should just leave it alone.

There is constant water collecting as we are in a low spot in the community. Just rough guess here... Slab is 1900 sq/ft, if the #3 stone displaces 70%, then there is roughly 6,400 gallons of water sitting in the 18" below the lowest hole. (Math - 1900 * 1.5 * 30% = 855 sq/ft = 6,400 gallons)

Zoeller 31-1099, 18 x 30 in. Simplex Poly-Foam Deluxe Basin
Zoeller 31-1473, 12 Riser for 18 Diameter Basin

RE: Sump crock holes

You want to be draining the free water from the rock layer, so the lowest opening in the sump crock should be at that level or slightly lower. Make sure your discharge is sufficiently far away and downslope from the structure.

It sounds like you have shallow groundwater, so make sure you have reliable pumps.

RE: Sump crock holes

The ground water will eventually reach an equilibrium level. If you locate the sump pump pumping level below that equilibrium level, your sump pump may pump constantly.

The switches on the zoeller and other sump pumps are garbage and will only last 5-7 years before failing. The pumps last much longer. When my zoeller switch failed, I hard-wired around the switch. These sump pumps also have a limited adjustment of the pumping level.

If possible, buy a sump pump without the switch. I would strongly recommend that you use the Levelguard electronic switch to activate the sump pump. The Levelguard switch lasts forever and you can set the pumping level with the Levelguard.


Before the concrete slab is placed, be sure to install a plastic vapor barrier under the concrete to prevent water vapor from passing through the concrete.

RE: Sump crock holes


I'm sorry, I may not have been clear in what I was asking. I've included a crude sketchup of the current implementation. The foundation is in place already, so no easy changes. I also have a hand full of corrections to my original description so I'll try to summarize a fresh.

We have a 4" slab with approximately 3' 8" of #3 and #57 stone below it (this stone is mixed near the crock because it had to be excavated to put crock in). So from top of slab to bottom of rock is 4'. Our crock is only 3' 6" deep, so there is 6" of rock that we can never drain from the current sump crock. The rock is wrapped, bottom side and exposed top, with a filter fabric to attempt to delay the clay/soil penetration. Currently the only hole in the side of the crock is a 4" hole with drain tile tube, the bottom of, is approximately 2' below the slab. So currently below that hole there is 1.5' of water sitting inside the rock around the side of the crock, with another 6" below the crock. My question is, should I drill additional holes in the side of the crock below the existing 4" hole to let that 1.5' of water drain? Also should I be worried about the 6" of water that seemingly I will never be able to drain?

Thank you for your replies ahead of time and thank BIMR and TIGERGUY for your already great responses.

RE: Sump crock holes

Do you have a foundation drain around the outside perimeter of the basement? Is there still clay under the rock?

Many of the sump pumps can operate with the low level about 6” below the top of the pump motor.

You want the water level to be roughly 6" below the concrete surface, to avoid moisture wicking upward. The more the better, but beyond 10-12 inches it probably doesn't make much difference. Otherwise, you are trying to lower the water table on your property and wasting money.

RE: Sump crock holes

The next three items are informational, but love the fourth item, and gets to the heart of my question.

> Do you have a foundation drain around the outside perimeter of the basement?
Around the outside of the footers, there is 4" drain tile pipe, that drain tile pipe connects to a similar drain tile pipe inside the footer. Those connections happen every 10-15 foot of footer. All of that drain tile/footer, is 3-4' above the bottom of the "rock bridge".

> Is there still clay under the rock?
Yes, the engineer called the 3-4' of rock, "a bridge", but the same problematic clay that caused the issue is still under the 1.6 million lbs of rock that is surrounded by the filter fabric.

>Many of the sump pumps can operate with the low level about 6” below the top of the pump motor.
I'm familiar with how pumps work and am not terribly concerned how the current future pumps will work, if we need to change/replace them we will.

>You want the water level to be roughly 6" below the concrete surface, to avoid moisture wicking upward. The more the better, but beyond 10-12 inches it probably doesn't make much difference. Otherwise, you are trying to lower the water table on your property and wasting money.
This is great info, and something I have not understood. I assumed it was critical to drain all of the water from the rock. It sounds like what your are saying is that it doesn't matter if there is 2-2.5' of rock with water pooled in it. That will have no impact on the clay under it, and the only concern is that it doesn't rise up high enough to impact the slab.

RE: Sump crock holes

The answer to your question is no, don't cut holes. The inlet to the sump will help to keep the water level constant in the rock layer. You want to keep the water level constant so that the moisture level in the clay soils is stable. The reason for the moisture control is that some clay soils are expansive.

Expansive soils

If you have holes in the bottom of the sump, the minimum punp operating water level may not be maintained. If you don't maintain the minimum level specified by the pump manufacturer, the pump may burn out permaturely.

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