Hydrostatic Pressure on Basement Retaining Walls 1

[Jeng79]

I have discussed this topic in my office, but I would like additional feedback/points of view:

We have a basement wall that we are modifying. The existing basement wall has no footing drainage. No footing drain can be provided without using a sub-pump.

Per our geotechnical investigation, the water table is 6'-0" below our wall.

What hydrostatic loading should be considered to act on the wall and how high up should the hydrostatic load be applied. Should hydrostatic pressure loading be considered to act all the way to the top of the wall, which is 19' above where the water table was measured at the time of the survey.

I will be calling the geotech to discuss this issue. I would appreciate learning how others would deal with this issue.

Thanks

 

[Ron]

The water table is 6' below your wall? Is the seasonal high water table any higher? If not, don't worry about it. If it is, then account for it to the influence on the wall. I can't imagine a 6' variation in seasonal water table....but then my practice is in an area where the water table is rarely below 6'!

 

[jgailla]

The design should be for the highest reasonable groundwater elevation.
Generally, the geotech will give you the seasonal high groundwater elevation, which would be the design high water.
You should look at the consequences if the groundwater rises above this level and the likelihood of it doing so.
If the bottom of your wall is 6 feet above the seasonal high groundwater, you would be conservative to design for water pressure on any part of the wall.
The design water level should be somewhat above the observed level, but only a geotechnical engineer can provide you with the correct design groundwater level based on the subsurface conditions.
In my area, observed groundwater can differ from high groundwater by 4 feet or more, but usually less. In some areas, South Florida for instance, high groundwater is unlikely to deviate from observed groundwater at any time by more than 2 feet.
If the observed groundwater is 6 feet below your wall level, you most likely would not need to take any hydrostatic pressure into account.

 

[VAD]

Some Cautions. Despite that the water table is below the wall level, one should look at a few other issues. Is there a likelihood for water from surface runoff to seep downwards in the subsoils. Remember the placement of soil against a wall is like putting a frictionless wound at the join between the wall and soil. This is a vulnerable area. Often we rely on positive slopes away rom the wall to ensure that water does not sit close to our wall. However, this often looks nice when constructed and shortly after. Unfortunately we rarely return to see how that slope is doing. Is it still the same, or does it have birdbaths etc. If it is a house rainfall beating against the walls can result in water getting below ground.

In your case with no weeping tile I would want to invoke the following. Placement of an impermeable liner geomembrane say at about a foot below the final grade of the ground. This memebrane can be sandwiched between non woven geotextile layers to prevent it from puncturing. The membrane should be sealed against the foundation wall using "Blueskin" tape or other so that it prevents water from going between the backfill and wall. Generally some slack should be in the sandwiched membrane to avoid it from pulling away if the backfill settles. This way one can minimize the infiltration of water in the subsoil.

The same procedure should be used even if one has a drainage board system installed with weeping tile. The more one can provide means to prevent or minimize the access of water the better one can assure himself/herself that water associated problems would be minimal.

Every situation has its own features and characteristics. The above is provided to stimulate thinking about an issue that despite how much we have done over the years re housing etc with basements and non basements, we still do not seem to be able to defeat this foe (sometimes a friend as well).