Basement in Ground Water 1

[kxa]

I have a client who wants to have his house constructed with a full basement in an area that after the excavation and boring it will be in at least in 2 feet of water. The area also has a few feet of dark grey peat and clay. Once it was excavated and water started getting in the standing water stabilized and in about two weeks the water almost turned green with a lot of algae. My questions are the following and would appreciate any advice or references:

1.How can we compact soil that is wet? Do I have to dewater the area first?

2.Is there any way to make a basement fully waterproof?

3.Should I suggest to replacing some of the clayey/silty material ( 2-6 blow counts) with RCA for good compaction?

4.Why would so much algae is growing in the water in such a short time? Does peat cause that?

 

[hokie66]

You need some good geotechnical assistance on this. Based on your description, I would suspect that the soil will shrink and swell as the moisture content changes. If that is the case, you may have to consider deep footings, with the basement floor spanning between these footings and isolated from the soil with void formers.

You definitely will have to dewater in order to construct the basement.

Theoretically, you can waterproof the basement. The membrane has to be on the outside, that is under the slab and on the earth side of the walls, and continuous. The effectiveness of waterproofing depends highly on the workmanship.

As to the algae, swimming pools do that if the chlorine level is not high enough, especially in warm weather.

 

[msquared48]

In addition to what Hokie says, I would seriously consider importing enough fill to raise the floor of the basement 3 feet to get it above the water table. I can just see mold problems here with time if it is allowed to have the water table where your client proposes.

Mike McCann
MMC Engineering

 

[phuduhudu]

It's quite normal to have basements going into water. Of course it is nice if at the time of construction your water table is lower but even then you need to waterproof the basement for the case where the water table will rise. Look in any good text book on basements for waterproofing details. There are many ways to do it. Tank it to keep the water out with an impermeable exterior layer or by designing water retaining concrete. Let the water in and have a dry wall inside with a cavity behind and then have a sump and pump out the water. It depends on what the basement will be used for. For the greatest defence often a belt and braces approach is used with a gravel drain outside the wall or a draining membrane and then a tanked basement and then a sump inside in case of any leaking.

The advantage of a basement is that you might get through the peat layers and into something decent that you can then found on otherwise you will need to found deeper. Remember to design the base slab and walls for the water pressure which can be large forces. That often gives you quite a thick base which then acts nicely as a stiffened raft foundation.

Carl Bauer

http://www.bauerconsultbotswana.com

 

[msquared48]

It's normal to have basements below the water table where you can drain the water out by gravity, or sump pumps, through the foundation drains.

That is not the scenario that I read here. If you cannot control the water by gravity, as power to run sump pumps is frequently lost during storms (auxiliary generator needed), I would still seriously consider raising the level of the basement floor and mporting fill to achieve the level of basement cover desired. Without doing that, you are looking at a long term maintenance problem.

Mike McCann
MMC Engineering

 

[dgillette]

Dewatering and eplacing the 2-6 blow clay and silt will probably both be necessary to compact a good foundation. The clay won't drain, so dewatering on its own is unlikely to dry things up enough to compact.

I'm with Mike on the undesirability of relying on sump pumps. Unless the client wants a large indoor swimming pool.

DRG

 

[lovethecold]

I agree with the above comments on building a basement below the water table in highle permeable soils. Although people do it, a good drainage system is a must. Multiple sump pumps that are interconnected are also highly recommended. The owner must also be very aware of the risks should the sumps stop working or become insufficient.

My big concern would be the layer of peat. The gray clay could be organic. Were soil borings done? Many jobs where I have seen peat near the surface, layers of peat and organic silt are also be found near the bottom of the basement elevation and have extended up to 50 feet below the surface. Is the house on or near a lake?

 

[hokie66]

lovethecold,

By the OP's description, I don't think this soil is permeable.

 

[oldestguy]

Hey you guys, didn't anyone advise that sealing the outside makes this a boat that can float? With 2 feet of head and maybe a slab weighing 70 pounds per sq. ft. is a dangerous situation.

Even if it is held down, what sort of structural slab is needed to keep the slab from blowing up?

 

[hokie66]

Well, in advising him to use piles and and a suspended slab, I thought it was a given that all the hydrostatic forces would have to be resisted. But you are correct. Either mass or a combination of mass and tension will be required.

 

[msquared48]

Oldestguy:

That's partof the reason why I want to raise the building!

Mike McCann
MMC Engineering

 

[MiketheEngineer]

Agree - raise the building

 

[oldestguy]

Mike you are right. Why struggle with the grief that the water table presents?

Can you imagine the Madison, WI engineers shock when Frank Lloyd Wright designed a large auditorium next to their lake in 1960 placing the main floor one foot below the elevation of the nearby lake at normal conditions. At flood times it would be a few feet higher.

If finally got built with some common sense.

 

[geomtl]

Having both worked with clients and remediated an old building that I bought with ground water problems I would highly recommend bringing the basement above the GW level. No matter what you build there will be problems down the road, and homeowners are notoriously more sensitive to water than larger building owners, and the home will lose a lot of value if there are water problems down the road.

 

[escrowe]

My first (default) recommendation would be to raise the foundation.
Most clients are persuaded to do so by the significant costs and risks
associated with construction below the water table.
The client should be prepared to spend at least 2 to 3 times more for the subaqueous foundation system,
with no guarantee of future performance.

I think the more difficult cost analysis
will be the use of a pile and structural slab system vs. soil replacement.

 

[darthsoilsguy2]

i would rather lose my basement space and have a frostwall below groudwater elevation than a problem basement.

 

[sundale]

Raising the basement slab is the easy solution. Obtaining this this added square footage by going sideways or upwards with the house would not be a bad call either.

Learn from New Orleans and think about the wisdom of constructing structures below copious amounts of water... You may perhaps win the battle against Mother Nature, but she will ultimately win the war.