Basement slab correct water pressure

[Enhineyero]

I received an SI for a building project with a basement. Basement would be 3 meters below ground. There are two sources of ground water level reading one is from a borehole log another is from the water standpipe monitor, both are located in exactly the same place. Results are as follows:

a. Borehole log - varies widely from 0.3m below ground to 9.0m below ground level (taken once a day for 9 straight days)

b. water standpipe monitor - constant around 5.0m to 5.5m below ground level (taken once a day for 5 straight days)

I asked several colleagues on their opinion on how should i interpret these result for the purpose of designing the basement slab. I heard different recommendations from different people. Here are the opinions i heard:

a.) take the result from water standpipe, adopt ground water level of 5.0m, since basement is above the ground water level we can ignore water pressure

b.) ignore result of water stand pipe, adopt full height water pressure level (3m x 9.81 = 29.43 kpA uplift)

c.) disregard results from borehole log and water standpipe, ground water level might change due to heavy rains or due to construction activities of adjacent properties, adopt a ground water level of 1 meter below ground (2m x 9.81 = 19.62 kPa uplift). I think he is referencing BS 8210 on this.

Any comments or recommendations? I am very confused on where i will take this.

 

[Canuck67]

In my area, we are required to design for the 100 year flood elevation, which will be above typical standpipe results. The flood elevation may also be at a level above finished grade. This flood elevation level information is readily available in my area so it does not require additional study to determine. If you are not in a flood prone area, then the ground water level will determine the design parameter. Ground water levels can vary significantly and you do not appear to have any long term data. Therefore, to be on the conservative side, I would design for the worst case where ground water is at the surface, it is only 0.3m different than what has been found in a borehole log. To limit my liability, I would also want to receive an written opinion on a recommendation from the geotech or hydrogeologist.

 

[Enhineyero]

Canuck67 - thanks, unfortunately we dont have data for 100 yr flood elevation. Though I get why some people would advise on the full height water pressure on the basement. Since there is no historical data on how much the ground water level would rise during extreme rains or events of the same sort.

 

[JStephen]

Are there other buildings with basements nearby?

I grew up in a coastal area with shallow groundwater- and mostly, there just weren't any basements built around there.

 

[dhengr]

Enhineyero:
You do not want to deal with water pressure under the floor slab. This invariable means water standing on the floor slab. Put a proper drainage system outside the foundation, at or slightly below the bottom of the footing elevation. Drain this to daylight or into a sump with a pump. Drain the entire foundation wall down into this drain pipe. There are details and products which truly waterproof all the various joints and surfaces of the foundation, so as to make it literally a floating tub, but these are difficult to do and very expensive.

 

[hokie66]

There is no tried and sure way of knowing how high the water will come. You just have to use the best local information available, and then use your engineering judgment. The consequences of a higher water table than assumed then need to be evaluated, with full disclosure to the owner.

Unlike my friend dhengr, I tend to assume that drainage systems will stop working at some stage. When building below ground, you have to either make the structure watertight and able to resist buoyant uplift, or live with the consequences.

 

[manstrom]

Provide proper drainage and design for zero water pressure. Don't fight an uphill battle.

If you do design for water surcharge, you will also need to design for buoyancy effects. It doesn't take long to realize that even if you waterproof the basement properly, you need up needing to design a houseboat if you are under the waterline.

 

[Enhineyero]

Thanks for all your valuable input. Putting a drainage and designing for uplift pressure is the route I chose. It may seem an over kill, however, the cost of add'l mat'l is less compared to the damage a leaking basement would impose.

 

[DST148]

@Enhineyero>>> We have a situation something similar to yours.
The project is located in a Flood Zone.
The lower level parking is about 8 feet below finished grade. The Civil Engineer has given us the
Design Flood Elevation which is about 7 feet above lower parking level.
The natural water table is 1 foot below lower parking level.
Given poor soil conditions, we are proposing 2 feet thick foundation mat.
The Geotechnical Engineer is of the opinion that the ground water level will not rise to the level of
Design Flood Elevation during the event of flash floods. His opinion..... the water table may be assumed to rise
to a level some where midway between natural water table and Design Flood Elevation for uplift purposes.
We have designed the flood walls for Design Flood Elevation. However, we have designed the foundation mat
per Geotechnical Engineer's suggestion, for about 4 feet rise in water table.
We have submitted calculations to DEP and awaiting their comments / approval.

 

[Ron]

If you do a drainage design, it has to perform for the life of the structure. If it fails, potentially so does your slab if not designed for the excess hydrostatic uplift. Consider the skill level of those who might install the drainage system.

Unless YOU have full time inspection with authority to re-direct errant work along with a foolproof drainage design, then design for full uplift and waterproof everything for the life of the structure....which will likely include a mud slab, waterproofing, slab, and exterior waterproofing of the basement walls contiguous with the underslab waterproofing...all with a robust system. My recommendation would be coal tar; however, that's pretty nasty stuff and few roofers/waterproofers will still do it.

 

[oldestguy]

You need more than some variable ground water reading wells. They may be reading infiltrating surface water and should not be depended on for long term conditions or temporary rise of water table. There should be some government agency that can give at least a general opinion of what to expect. Geology departments of large universities may also help with this question. Lacking that, see the above recommendations for design (Ron in particular). For a "proper drainage system", do not just use a cook-book design that does not consider filtering the fines out which can plug the system and cause failure. And of course provide provision for emergency power that is dependable.