Geotechnical Query 45 Degrees?

FaberCastel

Hey, long time lurker, first time poster.

Just hoping for some clarification on lateral earth pressures.

What I would like to know is the reasoning behind the 45 degree rule of thumb for excavation / retaining wall zone of influence.

Am I correct in assuming the following?

The failure plane as described by Rankine, is 45°+φ/2, where φ = internal friction angle.

As such, is the idea behind the 45° rule of thumb to assume φ = 0, resulting in a very conservative failure plane of 45°?

This 45° line also governs the proximity of surcharge at the elevated surface

I've attached a sketch to explain what I'm raving about.

If any of the more informed posters could comment I'd be grateful.

Thanks

Victor

It is only a rule of thumb, the slope to be used will vary from material to material. With a rocky material, you could go for a very steep slope, depending on how fractured the rock is, while some fine materials you might go with a quite shallow slope. Mositure levels are another factor. http://en.wikipedia.org/wiki/Aberfan_disaster

You can of course somewhat stack the odds in your favour by using geo-textiles or other reinforcing methods.

Of course, in practical terms, depending on height, you might look at other options like reinforced earth.

FaberCastel

Thanks for replying Victor.

I know different material have different friction angles and you can design near vertical embankments with the aid or geogrids, etc.,

I suppose the key issue I was trying to get at is the thinking/theory behind the 45° rule of thumb.

Is it as I speculated a very conservative calculated failure plane, where φ = 0 and therefore the calculated failure plane as per 45°+φ/2 = 45° ?

I'm just trying to figure out if you knew the properties of the material ( a medium dense to dense, sand, φ = 35°), could you adopt the larger angle 63° instead of generic 45° rule of thumb in terms of proximity of surcharges? (See attached)