Installing sump basin in Very High Water Table

[BeamAce99]

I just bought a house next to a brackish tidal marsh. The water table is only a few inches below the 3" think basement/crawl slab. Someone cut a 3ft square hole in the slab and filled out with rock and a poor attempt at a sump pump with a 5 gal bucket that looked like swish cheese set in the stone. Again, the water is at 3" below the slab during most days. It will over flow the slab during full moon high tides and other such situations.
Clearly pumping out the entirety of the marsh didn't work because there were 3 dead pumps piled in the basement.
Using 2 utility pumps, I dug out all the stone, and emptied the hole. But I can only dig down about 18" until another slab. So, I am going to make a pit only that deep. 'Its not an ideal situation but it's what I got.

The question I have is: do the holes into the basin go lower than the water line? .... I am afraid if I do that, the basin will always be filled and filling more and will run non stop because even at low tide the hope gets filled.
I am considering only putting holes into the new basin between the top of the regular water line and the bottom of the slab. Thoughts?

 

[Reach4]

I like the holes to be high. I agree that you don't want to be the one to try to lower the water table for the whole neighborhood.

How do you plan to make your pit? Pour walls with a form and pour in hydraulic cement? Push some kind of plastic liner into the square hole?

Make sure your new liner does not float up when you pump the water out.

If you will use a plastic liner, and hold it down with mortar at the rim, fill the liner with water and rocks/weights to keep it from floating until the mortar is hardened.

 

[Sponsor]

 

[BeamAce99]

Thanks! I drilled the holes in the space below the slab, just above the water table. We'll see how they work....
As for the install, when I dug it out three of the sidewalls were mostly sand/dirt combo and the fourth wall was the foundation wall. Then on the bottom, there was some sort of hard slab about 18" below my slab. The hole was much bigger than the cut in the slab shown in the picture, it was almost 2.5ft x 3ft. So, because it seemed that water was seeping in from all directions, I used about 80lbs of hydraulic cement to coat the interior. It wasn't perfect, but it made for a decent barrier for me to work with. I was tempted to create a concrete block pit in the middle of all of this, but I ended up using a thick-walled plastic planter I found at home depot and placed it in the deepest part of the hole. Then I lined it with landscape fabric, a thin layer of drainage stone, then more landscape fabric and more drainage stone.
For now I have paver stones in it, and water to hold it down, but I put a few blocks on top of it for the short-term. I am going to give it a test for a while, and if all goes well, I may cement over the remainder of the hole. Also, in this pic I am using a temp flexible drainpipe until I finalize it all.
Its a 1HP Drummond pump with a mechanical switch. Again, when I have an understanding of how well the installation works, I can upgrade the pump and switch. The drummond was quickly, cheaply available and @ 1HP, it should hold up to the tides.