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Retaining water and soil in excavation beneath water table

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BMRBrad

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Discussion starter · #1 ·
We are excavating for the foundation for a new home build. We have a footing depth of 9' and the water table is approx. 6' underground. The soil condition is sand with no rocks. Once we excavate beneath the water table, the sandy soil silts into the excavation and undermines the excavation walls causing collapses. Due to site conditions, we are very limited in where we can over dig.

Our current plan is to bring in concrete blocks and build a retaining wall inside the excavation and place gravel and weeping system behind it in hopes to catch the silt and control the water to a point where it can be gathered and pumped.

Anyone have any suggestions on retaining the silting sand when over digging is not an option?
 
#2 ·
I dealt with this issue on a retaining wall footer 2' away from a lake. Our soil where I'm at is a sandy loam with broken limestone mixed in until about 10~12', so I feel you about the water intrusion issue. Dewatering sucks.

A couple of suggestions, based on no room for overdig:

- If the intrusion is very fast, perhaps a couple of dewatering pumps in the bottom of your excavations. I've done this a few times with good results, as the pumps could easily beat the rate of water intrusion.

- Forgive me because I can't remember the name for this type of dewatering, so perhaps another poster can help me out here. I have seen an array of dewatering wells run around a site a couple times down here. Wells all over the site pumping simultaneously; It sucks all the water out of an entire area.

- Man-portable trench shoring in conjunction with the above two techniques. Use man-portable so you won't have to move a stack of 4000lb trench boxes for each leg of your pour, and they're cheaper to rent. Since you're having cave-ins right now anyway, I would consider using them. Besides, the great god of OSHA mandates the shoring for your soil type without overdig possibility.

- An extreme alternative might be to use a hydraulic mix for the footers that can cure immersed in water. Pumpdown for the footer pour, then pumpdown to construct the stem walls.

Unless I'm misunderstanding your post, you're just trying to pour the footers and not trying to do some exfiltration/drainage system as a permanent part of the foundation plan, correct?
 
Discussion starter · #3 ·
That's right. I need to keep the excavated area dry enough to form my footings and foundation wall. The other issue is the city inspector is most likely going to make us get a compaction test before we set out footings.
I will be running pumps in pits around the outside to get the water out. I've been having problems with the pumps burying themselves so I am going to put them in perforated 5 gal pails to keep the silt away. An issue is finding a reliable pump setup that we can trust to run overnight without clogging or failing.

We are starting the shoring and pumping tomorrow wish us luck! All input is appreciated!
 
#4 ·
With the fine sand you have to use gravel around your buckets to help keep out the sand. Gravel still may not stop it.

It sounds like well points are what you really need. If the sand is fine WellPoint are about your only solution. And with the sand they can be jetted in by hand and the ground dried up. You will need a good place to discharge the water. You can install some wells periodically but they need to be well below your footings in order to have enough draw down to get the water table below your footings in all areas.

Dewatering is a fun challenge to dry the ground up.
 
#5 ·
As for your pimps what type of pumps do you have? If you have some submersible pumps a piece of pipe with slots cut into it works better than buckets. You will need some stone though. We like to use a #8 washed gravel. It will hold back the sand yet let the water through. So what you will do is dig your hole as deep as possible and insert your pipe that your pump will be installed in. One you have your hole excavated you will put as much stone as possible around the pipe. This will keep the sand from getting to your pump so it will be able to get lots of water to the pump. Also by getting the screen deeper it will allow you to spread your well or pipes further apart
 
#6 ·
When you have a situation like this, the point wells should be set up to keep the water out and you definitely must use a Box or a Coffin (everybody calls it different) when doing excavation to prevent sides from caving in.
 
#8 ·
How big is this a foundation and is the house near a body of water? If it was mine I would seriously consider a 5'-6' crawl space instead of full foundation dig that includes well points, shoring, importing stone & compaction with tests. That is going to get real expensive.
Hope you covered yourself in your pricing.
 
Discussion starter · #9 ·
The house is about a mile away from Lake Ontario . If I could convince my client into a 7' basement I would. The basement is to have 9' clearance inside. Lots of houses in the area have 9' basements but the foundation sticks 2' to 3' out of the grade. unfortunately, if i raise the foundation, the house will be taller than the city will allow and my client will not lower the ceiling heights on the first and second floor to compensate. The most he is willing to go down to is 8'. At this rate taking a foot off the basement would help but won't get us above the water table.

We are running the additional work on a time and material basis. It is still not going to be cheap.

Does anyone have some experience on how long it took to lower the water table using wells?
 
#10 ·
What is the square footage? The draw down for wells depends on the depth of the wells and the ground material. If the sand is very tight the water will have a hard time traveling to get to the wells. If it is course the water will travel well but it will take more pumps to because more water will get to the wells.
 
#11 ·
How many lineal feet of footings that are on outside cuts?
You may have to shore each leg as much as possible, excavate, pour footings and move on to the next.

During the complete process you should run multiple well points, pumps, etc.

I would explain to your client the cost involved before starting so he understands what he is getting into. This could make it easier when you give him that big fat invoice.
 
#12 ·
raise the house 4'...haul in fill. even if you get site dewatered, once pumps are shut off, it's going to be a perpetual problem. what if the power goes off? i would not encourage any of my customers to have an indoor pool in their lower level.
 
Discussion starter · #13 ·
We managed to get to footing level today and discovered shale. So we know we have something stable to set the house on and now know why the water is trapped in the sand. Unfortunately our attempt to install a retaining system had to be scrapped. The size of the excavation was expanding to the point of threatening adjacent properties so we had to fill in that section of the excavation to prevent the earth from eroding into the neighbours.

Tomorrow I am meeting a geotechnical engineer on site to discuss shoring options and have a layout engineered so we can determine the cost. My client will not settle for a crawl space and is well aware that we are now into a $40,000 problem. Depending on what the engineer comes back with, my client will have to decide how bad he wants his 9' basement.
 
#14 ·
How in the hell are you going to keep the basement dry after it is constructed ??

The homeowner better have one hell of a pump set up with a back up pump in case the first one fails and a back up generator too.

You better get the homeowner to sign off on a liability due to flood damage, water seepage, mold and foundation cracking.
 
Discussion starter · #15 ·
Dewatering is still going to be an issue due to the shale trapping the water at our foundation. The wells are more effective when they draw water from several feet beneath the footings. The shale is holding the lowest point if the water at out footings, we would have to drain the local water table almost dry to get the footings clear.
 
Discussion starter · #16 ·
Houses in the area have engineered weeping systems. Some places have 2 or more pumps. They have floor drains placed beside the sump pits with an overflow tube connecting the pit to the drain. If the pump does not run the water is drained off into the sewer system. I visited several homes being constructed in a mile radius. Water is an issue everywhere but the basements are dry with the weeping systems.

This house will need sump pumps and sewage ejector with battery back ups and high level warnings. Did I mention the sewer line comes in higher than our floor slab? Hence the sewage ejector... One day I will look back on this and shake my head.
 
#20 ·
BMRBrad said:
...If the pump does not run the water is drained off into the sewer system. Did I mention the sewer line comes in higher than our floor slab?...One day I will look back on this and shake my head.
Click to expand...
So when the pump doesn't run the basement fills with water to the height above the floor slab at which the sewer line comes in? (or hopefully goes out rather).

Shake your head? Perhaps a more likely outcome will be your insurance company shaking its head within 30 days of the first extended power outage. I don't understand how a builder could, in good faith, site a home like that. It seems to me a disaster waiting to happen.
 
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Redneckpete
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