Large Overburden Behind a Retaining Wall

[Teguci]

I have read through previous posts on this and would like additional references.

Have a 12 ft tall retaining wall with 4,000 psf overburden starting at some, to be determined, distance behind it (probably 7 ft back). Using USS sheet pile equation, pressure equals = 2q/PI x [B - (SinB x Cos(2A))]. This gives me 4,000 psf horizontal soil pressure from the overburden if I have the overburden start at the wall.

However, using other equations for overburden the back pressure from overburden should be = Ka x 4,000 or conservatively half of what I am getting.

What am I missing? What have other engineers done?

 

[msquared48]

At the 7' lateral distance, the effect would only be seen by the lower third of the wall. More than likely, the overburden would primarily affect the sliding of the wall, but that would also be relative tho the normal soil pressure applied too.

Is there no geotech reoport here to help you with the lateral load application here?

Mike McCann
MMC Engineering

 

[muuddfun]

Look for the Caltrans Shoring Manual on the Internet, it has some stuff on surcharge loads and the equivalent horizontal pressures. I don't have a link at the moment.

 

[DRC1]

You do not give the extent of the surchage, so B is not determinable, (note that angular measure is radians) but the presure for the wall should be zero and increase rapidly to a maximum of less than the vertical overburden pressure, the decrease slightly with depth. If you move it out 7 feet, the wal pressure would still start at 0, reach a much smaller maximum value 5 -7 feet down, and then decrease much more rapidly with depth.
To find the location & magnitude of the resultant, the diagram can be approximated as 2 triangles, one from the top of the wall to the point of maximum pressure and one from the point of maximum pressure to zero at a point where the wall pressure is suffiently small.
using Ka times the overburden pressure is accurate for a uniform load spread over a large area starting at the wall. Since this is not your case, I think you will find the results too consevative.
4,000 psf is a significant load and should be carefully acoounted in the design

 

[BigH]

DCR1 - significant load? It is something like 30 to 35 ft of material (and he did say overburden)!! - you hit it on the head. At that height, if the toe is at 7 ft from the top of the wall, the overburden would be sloping up some 1.5H:1V I would say. Might be an idea to us the Culmann graphical method to determine if the "wedge" goes as far back as the surcharge or does the wedge daylight on the flat 7ft portion.

 

[msucog]

let's not forget: depending on the size of the area loaded, fill induced settlement could be excessive (perhaps very excessive depending on the geology and foundation conditions) even if it's backed off the wall a little ways. (i'm presuming the wall is cast-in-place concrete??)

 

[Teguci]

Yes - Cast in place wall with couterforts. The wall will be very stiff.

I have changed our approach from the above noted equation to one out of Bowles - "Foundation Analysis and Design." He corrects the mirror load mistake that puts an additional factor of 2 onto the surcharge load in the above equation.

We are now getting reasonable results that match with other accepted surcharge equations.

Thanks for your help.

As for settlement, it doesn't much matter. The surcharge loading is from steel slabs. The slabs will not have a problem adapting to any settlement.

 

[fattdad]

We need a drawing. From the thread, I have to wonder what the 4,000 psf suracharge looks like. I'd also agree for a unique case like this a Culman's construction is what's needed.

There is no guarantee that a line that extends up from the toe of the wall at an angle of 45+phi/2 (measured from the horizontal) will be accurate as the surcharge may affect this typical convention. This is what the Culman's construction will ascertain.

Is there anything critical about the surcharge load (i.e., is it the county courthouse)?

f-d

¡papá gordo ain’t no madre flaca!

 

[hokie66]

I think Teguci's use of the word "overburden" has confused the issue. His last post says the surcharge load is stacked steel slabs.