Silly question regarding buoyancy and effective weight for Retaining Wall Stability 1

[gmoney731]

Hello,

I have the following problem (attached). My question is this:

When I am calculating the dead weights of the retaining wall, the foundation, and the soil, do I need to account for the groundwater level? So, for example, B.O.F. = 10', T.O.W. = 25', and G.W.L. = 19'.

Assuming a 2' thick wall and 3' FTG, does this mean the total weight that is resisting the overturning is as follows:

1) Submerged Wall: 2' thick x 6' submerged height x 1' (unit) x (150 pcf - 62.4 pcf)
2) Unsubmerged Wall: 2' thick x 6' unsubmerged height x 1' (unit) x (150 pcf)
3) FTG: 3' thick * 10' wide x 1' (unit) x (150 pcf - 62.4 pcf)
4) Soil: Calculate both submerged reduced weight and unsubmerged total weight

Is this approach correct?

Also, if I do this, do I stil l need to account for the uplift resultant force when calculating the overturning moment?

 

[Settingsun]

That looks right. You don't reduce the stabilising weight then also consider an additional overturning force due to buoyancy as they are the same effect. The buoyant weight is a shortcut/alternative for calculating the resultant external force due to hydrostatic pressure.

If you're doing an FOS design, whether you treat it as a destabilising effect or a reduction to the stabilising force will make a difference.

 

[gmoney731]

Thank you very much, Steve. I was overthinking it and confusing myself.

 

[HTURKAK]

This approach is OK as long as the water levels are high ( HWL) at both sides.. In case of rapid draw down , soil side WL could be still high ( depending on the soil type and drainage cond) . The uplift force developing under the ftg will be triangular and could trigger the rotational failure together with horizontal hydrostatic load.

 

[gmoney731]

HTURKAK - Thanks.

From what I was discussing with Steveh49 above, it seems that I am double dipping - by subtracting 62.4 pcf from submerged concrete and soil, AND also accounting for the trapezoidal uplift resultants when calculating my overturning moment. I am now with the understanding that if I subtract the water unit weight from my materials, I won't have to include the uplift as part of my OT calcs since I am already accounting for the uplift.

For your case, if the water level is high on one side, but nonexistent on the other side: I understand that the uplift will be a triangular distribution under the footing. However, what if I did the subtract 62.4 pcf way? Would I consider the concrete as "submerged" and only use the effective weight for my overturning calculations?

 

[HTURKAK]

- No... not really.. if the WL at both sides are the same, and the soil is permeable, the uplift will develop but will be uniform..In this case when you subtract the unit wt of water from submerged wt of soil and concrete ( literally working with effective stresses ) the uplift is accounted ..



If the water level is high on one side, but nonexistent on the other side, and if the soil is permeable so leaking.. a triangular uplift will develop under the footing. In this case, you SHALL work with total stress analysis.You can not consider the concrete as "submerged".
In this case, for OT check ;

- Stabilizing forces = wt of concrete T wall+ submerged wt of soil above heel cantilever.
- Destabilizing forces = active force of soil+ hor. hydrostatic load of water + triangular uplift forces under the ftg.

The following picture is useful to get the concept. Notice that , (Fv + W )are stabilizing forces while (Fh and U ) are destabilizing forces..