Light weight fill and retaining wall loads? 1

[connect2]

Hi All,

So when you are using a lightweight fill such as blocks of extruded polystyrene on the fill side of a retaining wall do you treat the lateral earth pressures on the wall as you normally would? Or do you negate or say there is no lateral earth pressure on the wall due to the use of the light weight fill? Thanks.

 

[dcarr82775]

I have seen it done both ways. In my opinion the lateral loads from the retained earth still needs to be resisted by something so the wall still provides the resistance.

 

[briancpotter]

Unless you can remove the retaining wall and have the fill remain, the retaining wall will necessarily take lateral load.

One possible change is that if the lightweight fill is sufficiently compressible, it might allow the use of the active lateral coefficient Ka, instead of the at-rest coefficient Ko.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[BigH]

Maybe I am missing something here . . . but 70 m of geofoam is approximately 1 m of earth as far as mass/m3 is concerned. So, even if you consider the wall to undergo "active pressure", what is the pressure? Even if it is at-rest pressure? Of course you might have to consider some surcharge loading and the loading, say, of a metre or so of top covering earth - but . . . so enlighten me if I don't see this as a "big" design issue . . .

 

[hokie66]

Depends on the width of the polystyrene.

 

[structSU10]

Yea, I believe if the earth behind the polystyrene has a chance to mobilize, you must design for some portion of it. Generally the geotechs I work with make the normal weight fill run at a slope equal to or less than its friction angle to ensure the load is not transfered. For a tall wall, this is prohibitive, so likely some pressure gets the wall.

 

[hokie66]

Never having used these polystyrene blocks for fill in this manner, I am surprised that it is considered. While it may reduce the lateral loading on the stem, it also reduces the mass holding the heel down.

The other issue is economy. When polystyrene is used as a void former in concrete structures, it is to save mass rather than concrete per se. As I understand it, polystyrene costs about the same as concrete on a volume basis. If that is correct, it would cost a lot more than earth fill. So why do it?

 

[connect2]

Well I think you would want to design for full 'normal' lateral pressures, at least I would. So far nobodies convinced me otherwise.
For those that believe you don't need to design for full 'normal' lateral pressures, but some portion of it, quess my question is 'What portion and how do you know it's that portion, percemtage or fraction?.
briancpotters reply seems the most straight forward, 'If's there's no lateral pressure, why do you need a retaining wall?', to paraphrase.
Hokie66 of course I agree, stability becomes the problem when you have no mass on/over the heel. Unless of course one believes there is no lateral force.

 

[BigH]

@Hokie - check out retaining walls on I-15 in Utah . . . They built many walls using styrofoam (geofoam). It has also been used in bridge abutments for railways in Scandanavia

 

[RFreund]

Are you sloping the retained soil or the retained cut back as StructSU10 refers to?

EIT

http://www.HowToEngineer.com

 

[connect2]

Hi RFreund,
No the styrofoam fill is not being sloped as mentioned by sructSU10. The proposal is to install 4'x4'x8' blocks vertically up the back face of the retaining wall, the foam will come 8' out from the back of the wall, the 8' block direction is parrallel to the length of the wall and then backfill against the foam. The Contractors proposal makes little sense to me. The lateral load can not disappear in to the foam. The wall is 14' high. The backfill will be 2" minus crushed stone. The natural deposit is a compact silty-sand with the natural WT below the walls footing. Excavation into the slope is 3:1.

 

[hokie66]

BigH,
Thanks for the leader to the I15 in Utah. In that case, it looks like the focus was on roadway embankments, and limiting consolidation in the underlying clays...a noble objective. The "walls" were actually facing panels, similar to Reinforced Earth facades. Not cantilevered retaining walls, but the soil part of the embankments were sloped, so the styrofoam is wider as you go up.

http://www.civil.utah.edu/~bartlett/UDOT/UDOT EPS Final Report.pdf

 

[RFreund]

Connect2 -
In that case, I guess I too really don't see the benefit.

EIT

http://www.HowToEngineer.com

 

[connect2]

Thanks all for your considerations.