Does anyone else think...soil pressure
Does anyone else think...soil pressure
(OP)
That soil pressures are already really high to be in the factored load combination as 1.2D + 1.6L + 1.6H?
When I was in school a decade ago we used 45 pcf for at rest pressure, now I'm seeing 65 pcf in a lot of soils reports for cohesive soils. It seems like this already has a healthy factor of safety on it, and should get a 1.0 or maybe a 1.2 load factor.
Anyone else think I'm right, or do I alone carry the banner of crazy...
When I was in school a decade ago we used 45 pcf for at rest pressure, now I'm seeing 65 pcf in a lot of soils reports for cohesive soils. It seems like this already has a healthy factor of safety on it, and should get a 1.0 or maybe a 1.2 load factor.
Anyone else think I'm right, or do I alone carry the banner of crazy...






RE: Does anyone else think...soil pressure
However, you must follow your geotechnical report and the governing code.
DaveAtkins
RE: Does anyone else think...soil pressure
RE: Does anyone else think...soil pressure
I think it much more likely that it is designed closer to the OT limits than to the concrete strength limits for varioius reasons (not the least of which is that if you need 0.47 in^2/ft, you are probably going to put in #7 @ 12" and call it a day. You just increased your capacity by almost 30%.
RE: Does anyone else think...soil pressure
RE: Does anyone else think...soil pressure
There are enough of us old guys in the engineering community to help you carry the banner of crazy. You can't work in this business for too long with all the code and design changes before you start second guessing yourself at every turn. Anyway, here's my take on your subject.
Rankine's theory states that the pressure exerted by an earth backfill against a retaining wall can be computed with reasonable accuracy on the basis of theory only for conditions rarely encountered in practice. That said, theoretical earth pressure calculations can rarely be justified for a particular retaining wall because the physical characteristics of the backfill are usually not sufficiently known. Regardless of that, Rankine's theory has been widely used to design most retaining wall structures in the US. Go figure.
Assuming you are using Rankine's theory, 45 pcf for at-rest pressure suggests that the soil density would only be 90 pcf, considering that Ko conservatively runs about 1/2 the overall soil density for a cohesionless soil. Typical granular soil densities are about 130 pcf, which would explain the 65 pcf at-rest pressure you've been seeing in your geotech reports and would be appropriate for designing walls on unyielding foundations (ie. mats or pile supported).
The 45 psf pressure appears more appropriate as an active earth pressure, which would be about 1/3 the overall soil density for cohesionless granular soils. Perhaps this pressure is what you remember from school?
If you are into cohesive soils however, you definitely will not want to not use a cohesive soil backfill and should advise your client to replace it with a clean sand and gravel. You also would be wise to review long term settlement estimates with your geotech based on the intended earth surcharge. If the expected settlement cannot be tolerated by the wall (or mat), then a deep seated foundation might have to be considered.
I hope this has been helpful
PE in New England
RE: Does anyone else think...soil pressure
Thanks. That was pretty good looking back at Rankine. Don't you think that's pretty well known quantity and that the service load already basically gives an expected upper bound.
My problem is not the 65, but rather the 1.6 LF in ASCE. I think it should be closer to 1.2. Those load combinations just seem to be as much "feel" as science.
RE: Does anyone else think...soil pressure
however if you substitue in graded aggregate base for foundation or backfill materials, i suppose you'll get a much more "desireable" response from the geotech as far as recommendations go. but, i could see where even that cannot fix all issues on a jobsite. and the cost to use all GAB will also be shocking to the owner. and believe it or not, i've seen contractors screw even that up (building with GAB)...they are amazing creatures (those funny contractors that is).
RE: Does anyone else think...soil pressure
But most problems with retaining walls occur because of skimpy design and construction errors. There is another current thread here about a leaning wall. Turns out the bars are in the wrong face. So msucog's acceptance of concrete and steel as "highly controlled" is not always appropriate either.
RE: Does anyone else think...soil pressure
RE: Does anyone else think...soil pressure
but back to hokie's point: i can assure you that no matter how much you inspect something, the contractor will quite often do his best to screw it up after you walk away (even if it's perfect when you looked at it). this is why i fully support a thorough special inspecdtions program...at least it provides some confidence that things are close to what it should be at that point in time...but again, once you walk away, it's a coin toss.