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RE: soils pressure

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Man....I love this forum......

Believe it or not... I had this issues arise recently in a similare situation......cutting a rock hillside.......wanting to leaving minimal distance between the rock and the back wall of the building to install formwork for the concrete wall & a drainage layer.......what pressure to design the concrete wall for ?....others on the job were trying to say the "width" of the cut greatly affected the load that the wall would see....I disagreed.....the geotech settled the issue.... recommended an "at-rest" coefficient (with use with the granular specific weight) due to potential for "compaction" of the granular layer between the wall & the rock cut....even though the rock was believed to be "good" (intact with little potential for fracture).........the "width" of the granular layer was not a factor  (4 ' or 7' no the geotech didn't make any distinction)........rock cut varied from 8-10 ft. to 20 ft......I looked in several "references" for an answer...couldn't find one....it's very difficult to find references that address thing other than the "typical situations".....we all know that soil & water behave a little differently...however we also realize that many things happen not addressed in the "text books".....good'ole engineering judgement....gotta love it.....

Robert Rogers, PE


-----Original Message-----
From: Rand Holtham, P.E. [mailto:rand(--nospam--at)sigmaengineers.com] 
Sent: Friday, April 23, 2004 3:38 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: soils pressure


Please elaborate if you can. Doesn't a 12' tall 1'x1' column of soil not weight say 1200# whether or not there is other soil around it? Isn't the lateral force directly related to the weight of soil above it at a given depth?

Rand
----- Original Message ----- 
From: "Shapton & Partners" <shapton(--nospam--at)nwlink.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 2:10 PM
Subject: Re: soils pressure


Yes, no, no.

----- Original Message -----
From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 11:18 AM
Subject: Re: soils pressure


Doesn't a 1" diameter of water 12' tall exert the same pressure as a 1 foot diameter column? Isn't it the same for soil? doesn't a 12' tall column of soil exert the same pressure if it's 2' thick or 10' thick column?

Lateral pressures are usually based on the cubic foot weight of soil never does the distance behind the wall get factored in (except Caquat forces
etc.) so I think so-called "full" soil pressure would be present as long as loose (then compacted) fill is loaded behind the wall. Use the rock as your wall attach a ledger for your floor then you have an economic solution that your "friends" will be happy with. Engineering is an art in that when you can bring along the reluctant to do the right thing it means so much more than just the design alone.

My thoughts,
Rand



----- Original Message -----
From: "Jesus Gomez" <jgomez(--nospam--at)schnabel-eng.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 8:04 AM
Subject: FW: soils pressure


Joe,

As indicated previously, the earth pressure is dependent on the width of drainage material between the rock face and the wall. A small width (1 to 3 ft), will reduce the pressures significantly. Your assumption of full soil pressure is very conservative (assuming there is no water and that there are no rock pressures).

Jesús Gómez, Ph.D., P.E.
Associate
Schnabel Engineering
510 East Gay Street
West Chester, PA 19380
jgomez(--nospam--at)schnabel-eng.com

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