Surcharge Loads from Building Next to SOE 1

[structgeotec]

What is the rule of thumb to apply surcharges on a SOE coming from a building adjacent to the excavation?
How much surcharge comes from the building. say a 3 story building

 

[Guest090822]

What is an SOE?

 

[MTNClimber]

If there's not enough information, I would assume the typical allowable bearing pressure that a geotechnical engineer would provide in their soils report.

TheRick109 - SOE stands for a Support of Excavation system, also known as shoring. This would be sheetpiles, soldier pile and lagging, drilled shafts wall, ect.

 

[Guest090822]

The rule of thumb "design guidance" I've seen in the past is the 1:1 slope rule for surcharges. If the distance to the surcharge loading from your wall is at least as far as the wall is tall, then some designers ignore the surcharge loading. You could look at the surcharge as a strip loading too and make a judgment call whether or not to include it. As for the magnitude, MTNClimber is correct, there isn't enough information to go on. You will need the estimated applied bearing pressure from the designer of the foundation and assumptions used.

 

[r13]

There is no "rule of thumb", do your own calculation.

 

[Guest090822]

There is no "rule of thumb", do your own calculation.

I respectfully disagree. I know a state DOT Structures Design Division that used the 1 Vertical to 1 Horizontal rule from 1972 up until around 2010 for surcharge loadings in their internally developed substructure software. I'm not aware of any abutment, retaining, sheeting, or any other type of wall failure under this assumption in that state.

In the book "Cranes and Derricks" by Shapiro, they recommend a 1V:1.5H slope to be outside a "failure wedge" of the soil and note the same 1:1 slope for some soils for surcharge loadings from cranes.

This simple rules of thumb "design guidance" actually makes sense if you look at it from a soil failure wedge perspective.

I like the strip-loading model too, but even that has it's flaws since the surcharge loading could be several hundred feet away from a wall and the wall still theoretically sees surcharge loading.

It often comes down to a judgement call unless applying 1-2 psf is really that important!

 

[r13]

No vertical load should be included in the lateral earth pressure calculation for subject located beyond the theoretical failure plan of the retained soil.

 

[PEinc]

TheRick109, I don't have my copy of Cranes and Derricks with me but can you check on the 1V:1.5H that you typed? To get beyond the theoretical failure plane, the slope would be more like 1.5V:1H, not 1V:1.5H.
FYI, I am involved in a claim now where an expert claims that building surcharge needs to be considered even though the existing building's closest shallow foundations in sand are at a 1V:2H to a 1V:3H slope from the bottom of the SOE wall.

http://www.PeirceEngineering.com

 

[Lomarandil]

an expert claims that building surcharge needs to be considered even though the existing building's closest shallow foundations in sand are at a 1V:2H to a 1V:3H slope from the bottom of the SOE wall.

OK, PEinc, you can tell the expert that I've considered it for him and I'm done now.

I agree with Rick -- my practice has been that 1V:2H is no problem (in reasonable soils), 1V:1.5H justifies a few checks but is usually fine, and that practically, 1V:1H or steeper is possible.

----
just call me Lo.

 

[Guest090822]

To get beyond the theoretical failure plane, the slope would be more like 1.5V:1H, not 1V:1.5H.

I agree, if you are using Rankine theory, the failure wedge for an active loading results in an angle from the horizontal of 45 + Phi/2. That's why the 45-degree rule of thumb "design guidance" (1:1) has been used because it's conservative.

The 1V to 1.5H is even more conservative since that's for the passive case of 45 - phi/2.

From Cranes and Derricks:

And from the 1972 New York State DOT Wallrun user manual (note in this example the 1:1.5 is the backfill slope) :

Don't get scared of "experts". I've put many an expert in their place including the professors writing AASHTO codes. Sometimes they even forget basic engineering principals because they are too busy complicating things!

 

[r13]

If both 1:1, or 1.5:1 are considered the so called "rule of thumb", then it shall be called "rule of thumbs" Adding differences in type of soil the surcharge is situated, how many thumbs will be there, go figure. There is not much to be afraid, if the judge understands the matter/principle.

 

[Guest090822]

retired13 - I'll reword "rule of thumb" to "design guidance" how's that? The OP was just looking for some back of the napkin guidance to avoid crunching the numbers. However, I get it, I worked with a lot of engineers that can't help themselves and just have to do the math.

 

[r13]

Rick,

Don't read too much into my previous response, it was sort of joking around. My personal feeling about the "rule of thumb" is, it should be a knowledge/experience learned from hands on practices, rather than asking for handout from others. OP's handle suggests he is a "geotechnical", I just want to remind him that there are (theoretical) tools ready in his tool box (knowledge received from school) for his use, and for him to develop his own "rule of thumb" later.

 

[Guest090822]

retired13 - It's all good. I was actually agreeing with your point that if 2 or 3 scenarios come into play then it isn't just one "rule of thumb"

 

[Bobby46]

The AASHTO Guide Design Spec for Bridge Temporary Works also references the 1.5H:1V requirement in Section 4.3.3