foundation wall design 3

[pmtrevisan1]

hello everyone...
something has me puzzled.
I have a fdn wall that is 14ft tall. to reduce pressure we are using geo foam above a certain height. question I have is in regards to the triangular soil pressure distribution. I was taught that due to archemedis paradox the pressure at the base is the same regardless of backfill width. well 25 years later I am questioning this. if you only have say a 3ft width of backfill do you still consider full triangular soil pressure or do I truncate it.

fyi the 3ft backfill width is due to bedrock if it's of any importance.

thanks for any insight.
p

 

[EireChch]

For a start i dont know what Archimedis paradox and I have never had to assess a retaining wall in half soil half rock.. I but ill take a shot..

the soil pressure is based on the active wedge (roughly 45 degree upslope from your base). If you have good quality rock, with favorable joint sets i.e. largely spaced joints, dipping away from your wall, then you should have any negligible lateral pressure from rock.

You could determine your pressure assuming the entire active wedge was soil and then reduce the pressure based on the area of soil you actually have (i.e. looking at a section, if your active triangle is 10m2 and your actual soil is only 3.3m2, then you can take the pressure as 1/3rd of if it were all soil). I would take your moment at half the height of soil as apposed to 1/3 the height.

The above is me thinking out loud, others likely have better recommendations and I will be eager to hear them.

 

[SlideRuleEra]

If you only have say a 3ft width of backfill do you still consider full triangular soil pressure or do I truncate it?
Thanks for any insight.

If this is what you are describing, use the full triangular soil pressure:

Using triangular soil pressure means the soil is being considered to be a liquid (even if that is not mentioned), with an appropriate equivalent horizontal liquid pressure. With this assumption, the laws of physics relating to liquid pressure apply... pressure on the wall is the same whether the horizontal distance shown in my sketch is 3 feet or 3 miles.

http://www.SlideRuleEra.net

 

[MTNClimber]

Do some research on the silo effect. Depending on backfill height, material, and backfill width, you may have a reduced earth pressure in the scenario drawn by SlideRuleEra.

Edit: Or just scroll down to fattdad's response on this thread Link

 

[pmtrevisan1]

slide
that us exactly it.
and that is also what archemedis paradox states. regardless of width pressure is the same.

well glad I'm not losing my mind, though my wife may think otherwise.

cheers


p

 

[jwatso33]

Spangler has some guidance on stone columns. Oddly enough we have found that a close approximation to Spangler is to find the area percentage your fill occupies the theoretical failure wedge...then use that as a reduction for the applied passive pressure. Other than that some geometric methods (Trial wedge, etc.) will get you there. This ignores friction along the faces.

 

[aeoliantexan]

Richard L. Handy addresses soil pressure against "fascia walls" in "Soil Engineering, Fourth Edition" by Spangler and Handy, Page 572. If the backfill is narrow, part of the soil weight is carried by friction on the wall and the bedrock surface; therefore the lateral pressure is also reduced. If you can't find the book, I believe Handy published an article in the ASCE Geotechnical Journal.

Be careful; if water gets in the backfill, it won't care how narrow it is.

 

[pmtrevisan1]

so great point about water getting in there.
I'll stick to my original design with full triangular soil pressure

p

 

[charliealphabravo]

The treatment of soil as a fluid is a conservative simplification with respect to lateral pressure on the wall. However, unlike a fluid, soil has a significant internal friction angle. You can reduce the pressure using active wedge theory or conservatively use the equivalent fluid pressure. In either case, the wall should have a drainage system to ensure that you don't get a hydrostatic pressure condition.

 

[pmtrevisan1]

I had thought about wedge theory but in the end went with full liquid head to be conservative at the permit stage. I'll tweak the design with different options for finals.

wedge theory may prove best option.

cheers.

p

 

[Mad Mike]

The problem here, as with so many cases in engineering, is the generic use of "bedrock", which could be anything from a perfectly stable, massive hard rock, to a closely jointed rock that unravels, to a clay shale with sliding planes developed behind the face...the pressures developing behind a wall cladding rock can be enough to put even your conservative design to shame, although those instances are rare.

We recently observed a major failure on one of our projects, where a hard rock cutting that any geotech. on earth would have claimed to be stable, began sliding out of the face on a horizontal bedding plane. Two new houses behind the face cracked in half and were ultimately abandoned. A detailed investigation revealed a major clay dyke (decomposed dolerite, saturated) some distance behind the rock face, which through intersection of the horizontal bedding and some anomalous driving force that I still haven't unravelled, led to failure.

In that case, enormous rock anchors had to be instated on an emergency basis...I very much doubt this is your case, but I can assure you it does happen!!!

All the best,
Mike