Retaining wall backfill angle 1

[wikidcool]

I'm curious how some of you address this topic. When doing cantilevered retaining walls without a soil report, the default values in the code are pretty crummy. When specifying backfill material, using the gravel mixes with the lower pressures helps a lot (30 psf/ft) vs the native soil (45-60 psf/ft). I don't see the construction side of these very often, but when excavating to build the wall, I'm assuming they would cut the soil at a steeper angle than the design failure plane, correct? If so, you have this area between the excavation cut plane and the design failure plane that isn't really "backfill", yes?

If I want to use the lower pressure gravels as backfill, do I need to detail a requirement that they excavate at least as far as the failure plane? Or does that happen anyway since the soil wouldn't be stable during construction if they didn't? I've never seen anyone address this, so maybe I'm just inventing things to worry about (wouldn't be the first time!) I'm talking about reasonable height walls (4-10 ft).

 

[DaveAtkins]

Just to be on the safe side, show the angle you want excavated on your detail.

DaveAtkins

 

[oldestguy]

There may be local codes, especially needed to keep workers safe when installing forms,etc. dependent on height of cut.

 

[GeoPaveTraffic]

To use the lower pressure, the native soil should be excavated to the failure plane and backfilled with granular material.

As I responded to your other post, the soil doesn't care what the code says.

Mike Lambert

 

[dik]

I usually come back about 2' from the heel of the wall, and up 4' and then back at a 1:1 slope. I usually put a 'sock' covered drain tile and weep holes in the wall, and backfill with clean 1" stone separating native soil from backfill with a nonwoven geotextile.

Dik

 

[WARose]

For all practical purposes, you'd need to have at least about 12' back from the face of the wall and then start your slope up. As for the angle of the slope, that's a call by the geotech. It will be based on the angle of internal friction, but there will probably be a safety factor on that......especially when you tell the geotech that heavy equipment will be running right next to it.

 

[wikidcool]

Thanks everyone for the input.

WARose - No geotech's available to consult with on these. These are small projects using the lowest default code values for the soil (1500 psf bearing, 100 psf/ft lateral, etc.)

What's everyone's experience in the field with these 10 ft and lower cantilevered type walls - does the excavation work usually extend beyond the native soil failure plane, and therefore the wall is usually resisting actual "backfill"? Specifying a minimum amount of excavation in the details make sense, but I've seen a lot of retaining wall details from people smarter than me that don't - trying to figure out why.

 

[dik]

WarRose: Usually a couple of feet for placing footing formwork... more is better.

Dik

 

[BridgeSmith]

"Specifying a minimum amount of excavation in the details make sense, but I've seen a lot of retaining wall details from people smarter than me that don't - trying to figure out why."

Usually, we specify 1.5' to 2' level area behind the heel, as Dik said, for setting formwork. The 'backfill' is typically the native soil, which is what we design the wall to resist, so the extent of the excavation doesn't matter.

We don't run into clay very often, but when we do, we generally lay it back at about a 1 to 1 slope, starting from the back of the level area behind the heel, and backfill with crusher run base material.

 

[Bobby46]

A "legal" excavation for OSHA is a 1.5H:1V (approx. 56 degrees from vertical) for most soils. The theoretical failure plane is approximately 30 degrees from vertical for most granular soils. Assuming the excavation is made "legally" and without shoring, the native material would be excavated beyond the theoretical failure plane.

 

[wikidcool]

In our area when there's no geotech, we have to use the lowest class 5 soils for design. Out of the 4 USC soil types listed under class 5 soils in the CBC/IBC presumptive soil bearing table (CL, ML, MH, CH), MH and CH aren't suitable as backfill per Table 1610.1. So while it would normally be conservative to design for the native soils, it seems to me that without a geotech, these soils really should be removed and replaced with a material known to be suitable (and the footing results using the higher pressures are often pretty unreasonable anyway).

Per Bobby46's comment, it sounds like "most" of the time the excavation to build the wall will remove the native soil in the wedge area so the wall should be holding only the backfill material when completed. Which explains why the building code would refer to it specifically as backfill type and not generically as retained soil type.

Thanks so much for everyone's input. Depending on how I approached this idea, I was getting wildly different results with my design and it was driving me crazy. I'm feeling better about it now.