Basement slab correct water pressure
Basement slab correct water pressure
(OP)
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.
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.






RE: Basement slab correct water pressure
RE: Basement slab correct water pressure
RE: Basement slab correct water pressure
I grew up in a coastal area with shallow groundwater- and mostly, there just weren't any basements built around there.
RE: Basement slab correct water pressure
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.
RE: Basement slab correct water pressure
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.
RE: Basement slab correct water pressure
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.
RE: Basement slab correct water pressure
RE: Basement slab correct water pressure
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.
RE: Basement slab correct water pressure
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.
RE: Basement slab correct water pressure