Neglect Surcharge

[slickdeals]

Folks,
How far away does a load have to be from the face of a retaining wall/sheet pile to not be considered as a surcharge load.

I was told to draw a 45 degree angle from the base of the ret. wall and then asked to place the load beyond where this line meets the surface. What is the basis for this? Is it valid for any soil type?

Where can I find additional information regarding the principles behind this?

 

[msquared48]

It can vary from that line based on a hemispherical shaped failure plane, but that is a good general rule to follow in the absence of good geotechnical guidance.

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

 

[msquared48]

And the top of that 45 degree line would start at the outside of the existing building footing too.

If you were placing a new building and did not want to impace an existing structure, you could placed it closer to the property line by simply lowering the footing closest to the property line.

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

 

[BigH]

That's usually a safe assumption. I you need something more exact, you could do the Culmann's graphical method to determine where the surcharge loading doesn't effect the active force.

 

[FixedEarth]

It depends on whether you have a point load, line load or strip load. If you use Boussinesq formulas, in my experience it ranges from 1.0H to 2.3H for all surcharges.

For example, a 1,000 psf intensity that is 1.0 ft wide & 20 ft distance from the ret. wall., will only have an average of 20 psf lateral pressure against the the entire Height, H (I assumed 20 ft wall Ht.) However, if you increase the strip width to 2 ft, but keep the same intensity of 1,000 psf and same setback of 20 ft, then the surcharge is felt in the upper 30 feet. So it is felt even below the base of the wall. By felt, I mean lateral pressure magnitude of at least 20 psf.

The only author, I know, that covers strip wall surcharge thoroughly is Andersen in his "Substructural Analysis and Design" book. 1956?. You can use Tschebotarioff or Muni Budhu book for the line and point loads. Also the Canadian Foundation Engineering Manual has the line and point loads but not the strip load.

It is a pain to setup the spreadsheet, because you have to watch for m/h being less than or greter than 0.4 That is why I use lateral stress software.

 

[Lion06]

If in doubt check out the Boussinesq equations - there is a good treatment of this in Bowles' "Foundation Analysis and Design", 5th edition in Chapter 11.

 

[slickdeals]

Thanks, I will dig up the book at home and look in it.

 

[dcarr82775]

It depends on the soil type and magnitude of the surcharge. If you do a 45degree line it is almost always conservative. Boussinesq equation assumes a rigid wall (at-rest) so it will be additionally conservative if your wall can rotate to achieve active conditions.