Retaining Wall Stability Check Against Sliding

[normm]

I was talking with someone who was coomenting on the sliding check for a soil retaining wall with water table part height of the retaind soil. His opinion is that since there is upward water pressure underneath the base, this will reduce the effective weight of the wall. And the frictional resistance W times(co-eff of friction) should be modified to reflect this reduced W. Although most engineers correctly include the horizontal water pressure for checking against overturning, not many check the resistance against sliding this way - they take the full W.

His comment seem logical to me. But in the past I have not considered this reduced weight of wall.

Also what is the appropriate Factor of safety against sliding? I come across cases where any value greater than 1.0 is taken as acceptable. And some ask for 2.0.

Your comments will be appreciated.

 

[msquared48]

If your water table is that high, should there not be a foundation drain to lower the table? Seems like there would be water on both sides of the wall with that condition without waterstops, or it being a tank, that would float.

What is the scenario of the function of the wall?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

I have never run into this situation, but as msquared48 already stated, if the water table was that high, I would think there would be water on both sides of the wall (if it was a simple cantilever retaining wall.

Also, thinking back to my soils engineering days, I would think you might have some soil stability problems with this type of wall (as water tried to flow under the wall).

 

[BAretired]

A water table behind a retaining wall should not arise because it is common practice to provide drain rock behind the wall and through-wall drains at ten foot centers or thereabouts.

BA

 

[a2mfk]

For there to be buoyant forces, would there not need to be water on both sides of the wall, not just one?

 

[BAretired]

You could have a water table at the lower grade. Then you would have a buoyant force for the footing and the portion of wall below grade.

BA

 

[msantana]

take a look as ASCE 7-05 Section 3.2.1 - "when a portion or the whole of the adjacent soil is below a free water surface, computations shall be based upon the weight of the soil diminished by buoyancy, plus full hydrostatic pressure"

regarding the factor of safety question, I think that the reduced dead load given by the load combination should account for that.

 

[BAretired]

Another point which may be relevant is that the presence of water at the bottom of the footing may reduce the coefficient of friction between soil and concrete, particularly in the case of clay soils.

The factor of safety against sliding should certainly be more than 1.0 and I would think 2.0 is a reasonable value.

BA