basement sump basin question

Joined
Jul 8, 2020
Messages
90
Location
Alaska
I have a 60 year old house with a basement that has always had a sump basin and pump. This year I have been working to finish the basement with a 3/4 bath and have the shower drain area nearly ready to concrete around the p-trap (there is a 2'x2' square cut out in the basement floor with exposed sand/fill. My house sits at the bottom of the hill and the water table is never very far below the slab floor, perhaps a 18 inches in the winter. The pump keeps the water in the basin at 12 inches below the basement floor.

This year has been an extremely wet summer, with almost all of the year's rainfall coming since mid July and the water table has come way up, in fact to above basement floor level. I have done quite a bit of vacuuming and mopping of water coming up through cracks in the floor and especially where the slab meets the foundation. Basically the basement is a hull that is taking on water. If the basement was divided into thirds the sump would be in one third of the floor area. Water intrusion got so extreme at one point in a portion of the basement that I dug another sump basin and plumbed another pump...what a treat that was. The new basin fills much faster than the original, that is, when pumped dry water literally shoots through the holes to fill it. Today it is pumping every 12 minutes. The old basin pump which is set at the same level pumps about every 20 minutes, maybe a little slower. Its basin's water holes only weep water. The holes are big enough that I can stick a long thin screwdriver into them and into the sand behind; there does not seem to be any screening or fabric on the outside of the original basin, only very wet sand. The house sits on nice sand, it is in the yard, under the slab where the new basin is installed and everywhere I trenched to install all the new under-slab bathroom drain pipes, presumably around the old basin as well as these new pipes run within a few feet of the old basin.

Ok, here is my question: Can the soil around a sump basin "wear out" over time, that is, could the sand immediately surrounding the old basin become impacted with finer sediments that migrated over the years from elsewhere and decrease the permeability enough to slow the flow of water into the basin? I mentioned my open slab area for the future shower: it is about 10 feet from the original sump and the water level where the concrete is removed is completely up to the top of the slab (draining down the open shower drain, thankfully) yet the nearby (old) basin water level is always more than 12 inches lower. It is like there isn't even a sump in the area at all. The new sump basin has a great effect: before I installed it water was spurting up through cracks 20 feet away, now they are bone dry. I don't want to replace the old basin but if it is possible that the several inches of sand surrounding it could be prone to losing permeability over the decades I would be motivated to break open the concrete, pull it out and replace the surrounding sand with pea gravel to match the work I did for the new basin. My mind is obviously working overtime to find a rational explanation here, maybe my theory is ridiculous. It has been a difficult summer, to say the least. If anyone has any experience or thoughts to share I would appreciate it.
 
Joined
Apr 8, 2016
Messages
228
Location
GA
I think that the difference between the two is that water has an easier time passing through the pea gravel than through the sand. When there is a danger of flooding, sandbags are used to hold back the water, not pea gravel bags.
 
Joined
Oct 30, 2009
Messages
290
Location
New York
Water tables throughout the years have been rising. From what you have been describing the most likely solution is to install an interior French drain. The French drain pipe goes around the interior of the foundation and flows to a sump basin with a pump ejecting it out. Once you install the French drain the water level will even out throughout the the drain tile and pump will rarely kick on.

Many years ago I was building a house on a crawlspace and put in a sump pump and basin. It was a failure as I kept having wet spots throughout the floor due to hi water table.

I think your new basin is filling fast because water will find the path of less resistance.

Any time hi water table might be an issue I put in French drain before basement slab is poured.
 

Attachments

  • tempImagew3ToSr.jpg
    tempImagew3ToSr.jpg
    72.6 KB · Views: 28
  • 0914D0C1-A10D-45BF-95C4-2A2D89246523.jpeg
    0914D0C1-A10D-45BF-95C4-2A2D89246523.jpeg
    86.6 KB · Views: 27
Last edited:
Joined
Mar 4, 2017
Messages
24,016
Location
...
You have had an exceptionally wet year. The pea gravel idea is good. The old sand is likely full of debris as well.

You might need more capacity.
 
Joined
Mar 8, 2012
Messages
1,782
Location
MO
Is there anything at all you can do to divert water away from the house outside? I’d be very reluctant to do any finish work in a basement with that much water. Water will flow through pea gravel easier than through sand but I’d consider a third basin if possible as you are one pump failure from a wet basement as it sits.

Ideally you’d get water away from the foundation with grading, downspouts, etc but if the water table really is that high there’s not much you can do. Are you sure the groundwater is that high or could it just be filling the hole that your foundation happens to be in?

For the best flow I’d break up the perimeter of the basement floor and run drain tile to the sump basins if you don’t have that already. Possibly a third sump or dig an existing one even deeper to give you a bit more margin before your basement floor gets wet.
 
Joined
Jun 22, 2022
Messages
284
Do you have proper drainage outside the house - clean gutters, extended downspouts, rainwater catchment and ground sloping away from the house? I know you live in a low ground but the immediate surroundings can have a big impact.

When we bought our house, the sump pump would work frequently during storms. When I investigated the cause, it turned out that on one side of the house the ground was sloping towards the house. I raised and compacted the ground on that side and extended the downspouts away from the foundation. Now the sump pump hardly gets engaged.

There is a youtube channel that is about these issues, the guy is passionate about proper drainage. You might get some nuggets of good advice there

 

snowshew

Thread starter
Joined
Jul 8, 2020
Messages
90
Location
Alaska
Thank you all for your replies, here is a quick rundown of additional information based on your suggestions:

I am not certain what footer tiles are, and I cannot assess what the footer looks like, however, the slab floor is at least one course of blocks above the footer (I was able to view this course of cmu at one point). There are no drain tiles in the floor, just slap pour up to the foundation, with about a 1/4 gap between the foundation cmu and the slab. Sand underneath everywhere. Wet sand was squirted up through this gap during the 2018 earthquake. The foundation is not perfect to be sure, but I have never observed liquid moisture entering through the blocks, only up through the floor. There is some staining on the wall from past owners but they had no gutter extensions, poor grading etc.

Understand the comment on pea gravel vs sand and it got me thinking, and leads to a refinement of my original question. The new sump is located near some crack in the slab floor and much of the flow that I noticed entering the hole when I dug it came from the top of the sand right under the slab. I may have located it near voids between the floor and slab, and easy pathway for water to move. So, I don't think the pea gravel is doing much by itself to speed the water movement as it is only the several surrounding inches, rather, I got lucky with location.

I thought a lot about the French drain, however, this year water would enter through cracks in the interior of the floor, several feet away from the walls. The house is in a low spot and ground water seems to flow to me first before subsiding. On Mon we had about a 1/4 inch of rain and I observed more than an inch of rise in the water in my shower cut out, it may have been more but the water level reached the drain. It should drop and inch or so every 48 hours we don't have rain. I don't know what construction science looked like in 1959, and I don't know when the sump was put in nor why they located it where they did. I wonder how much of an effect it actually has on relieving water pressure under the house; perhaps in a normal year it is more than adequate.

I will likely give into the urge to improve the existing sump basin but may wait until winter so I don't have to deal with as much water management. It is pretty sporting digging in a lake under a houseboat.

Gutters and water diversion is excellent. Grade is good, soil is compacted. Gutters are 100%. The shortest outflow point is more than 12 feet from the house and that is a small section of roof; the next shortest is 25 feet from the basement portion of the house (garage downspout). Some are as more than 40 feet as I have underground piping for the sumps that the gutters share. The neighbors shed very little water near my property. The storm drains have been inspected (I called the city right away) and they are old but good. The camera showed water intrusion as the ground water level was so high. I have no reason to suspect the underground home water supply line either, pressure is good in the house.

I don't think water is getting under my house especially fast as the rise in water level I observe in the basement is more than the rainfall amount and I am pushing roof water far away. I think the ground water got high due to a surge of rain in July and doesn't go where ever it goes at a rate that keeps up with the rainfall.

Back to my question, I guess, does the old sump seeming to have no effect on the water table 10 feet away strongly suggest an issue with the soil near that basin? The only thing I can really do now as a Hail Mary is replace the basin and the sand right next to it. It is pumping every 20 minutes, that is its fastest historical rate (for 5 years). The entire lot is sand, I have been told it is quite deep. I remain surprised that water could be 12 inches higher than the sump level only 10 feet away. I realize sand is not as good as gravel, but it is much better than many materials. I had thought that the sump would have depressed the level of the ground water over a much greater radial distance.

So, that's where this ends, I guess, maybe pull that old basin this winter and see if the pump cycles faster than 20 minutes next rainy season or do it right away and see if it relieves the water level in the shower area. That would be great, I could start the shower with piece of mind.
 
Joined
Apr 28, 2020
Messages
682
Location
North Dakota
Where I live a cycle every 20 minutes is nothing...my house is a raised ranch so my pump rarely kicks but my next door neighbors will kick every 30 seconds when it has been really wet.
 
Joined
Jun 3, 2002
Messages
8,971
Location
MI
I am not certain what footer tiles are, and I cannot assess what the footer looks like, however, the slab floor is at least one course of blocks above the footer (I was able to view this course of cmu at one point).
It's better defined as an (exterior) footing drain pipe: https://basc.pnnl.gov/resource-guides/footing-drain-pipe
As described in a post above, some people install an interior footing drain to solve basement water problems.
1663890662955.png


One interesting concept about french drain principles is that in some instances they can draw MORE water towards the area you are trying to dry up. For example, a porous french drain at the soil line where grade meets house is essentially a magnet that draws water towards your house. Water in the heavier soil around the house moves to the more open porous area next to the house. Sometimes you create a problem. I wonder if you might have this scenario underneath your basement floor.

You also might want to study the concept of "perched water table" and determine if this might be a situation you have.
 
Last edited:

snowshew

Thread starter
Joined
Jul 8, 2020
Messages
90
Location
Alaska
Thanks gents!
I should better have described the topography; the house sits in a bit of a depression in a road in an area that is otherwise flat, bottom to top is probably 40 feet. There was a creek next to my house before it was filled in and this house and the one next to it were built. I am probably 100 feet above sea level and 1/2 mile from salt water. This is a 1959 house that was no doubt built in a hurry with ever short cut imaginable taken, I highly doubt any specialized fill nor drainage piping was installed exterior of the house. I have dug in the yard to plant trees and place deck footings and found the same sand as under the house. The old sump basin may have been added after the house was built as well.

I read up on perched water table, thank you, but no, I don't believe so. I am sitting on sand left here by interaction between the ocean and glacial silt deposits. The sand goes quite deep I am told.

I suspect sitting on what was a (gentle) watershed path means I am essentially dealing with a creek flooding its banks, only this creek is 7 feet below ground. I had always believed that having a pit/pump lowering the water table by 18 inches in on place would create a lower water table for dozens of feet in each direction, even if only by a little bit after perhaps 20 feet. Seeing no discernable effect on the open concrete 10 feet away is puzzling. We might have a break in the rain for a week and if the water level goes down a bit I'll probably give this a shot. Will report back!
 
Joined
Oct 30, 2009
Messages
290
Location
New York
Sand is very porous and excellent for drainage. If you told me your soil was hard clay that's another story.

"Back to my question, I guess, does the old sump seeming to have no effect on the water table 10 feet away strongly suggest an issue with the soil near that basin"

I don't think so I went thru same thing as I described to you and French drain solved issue.


"There was a creek next to my house before it was filled in and this house and the one next to it were built. "

I think this answers a lot of questions.
 
Joined
Mar 21, 2004
Messages
27,778
Location
Near the beach in Delaware
I would dig a trench between the house and the hill. In a "C" shape around the house. Maybe 4'to 6' deep with pipe towards the bottom and the rest filled with gravel. This will catch water moving towards your house. Hopefully you have a place to drain this to.

Is it possible there are springs under the slab?
 
Top