Cantilever retaining wall with wet backfill material

[diqmayer]

Hello,

We are looking to design a cantilever retaining wall as efficiently as possible out of reinforced concrete. Pah (Active) is the active horizontal component of the soil pressure behind the wall. In our situation, the backfill soil contains water and has a density of 138 pcf.

My question is this: if we use porous backfill material up against the wall (first 18" against the wall) with perforated drain tile to remove hydrostatic pressure, must we assume a high Pah such as 45 pcf (from the wet soil) or can we assume that the porous material will allow the water to drain off so we can use a lower Pah such as 30 pcf? 30 pcf is a value we see for dry backfill material.

I hope I stated this clearly. Thanks for your help.

 

[fattdad]

wet material may give you poor compaction (and thus low friction), but if there's no hydrostatic pressure, you'd just run the numbers and get your horizontal earth pressure. Whatever soils are in the 45-degree envelope from the toe of the wall will affect horizontal earth pressures. That angle will be steeper depending on the actual friction angle.

f-d

¡papá gordo ain’t no madre flaca!

 

[Doctormo]

It really depends on the source of the water that makes the soil wet and the type of soil behind the drainage zone. If it is ground water related, there could be all sorts of other design issues. True hydrostatic water pressure in soil is a lot more than 45 PCF and uplift can come into play which reduces effective friction.

 

[BigH]

Why don't you design to proper active pressures and the like rather than equivalent fluid pressure?

 

[moe333]

Putting drainage material against the wall will reduce the potential for developing hydrostatic pressure but probably would not significantly reduce the moisture content of the retained soil. I would still design using 138 pcf.

 

[diqmayer]

Thanks for the replies. @ moe333, is the Pah value more a function of the hydrostatic pressure (which would be reduced by the porous material and drain tile) or purely a function of the soil density? I see an argument here (

http://web.mst.edu/~rogersda/umrcou...on of Earth Pressures on Retaining Walls.pdf)

suggesting that:

"The placement of a foot or more of gravel behind the wall creates the need for averaging the Φ values of the gravel and the
backfill with respect to the wall."

Applying this suggestion to our design, we would use an average of 30 and 45 pcf for the Pah value.

 

[moe333]

The Pah is a function of the retained soil friction and density. I would use the friction value for the retained soil, not the 1-foot of gravel.