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• To: seaint(--nospam--at)seaint.org
• From: Tripp Howard <tripphoward(--nospam--at)yahoo.com>
• Date: Wed, 29 Jan 2003 05:37:22 -0800 (PST)

What type of "on-site" soils do you have?

Tripp --

The 85 pcf to 110 pcf is not a soil density -- it is an equivalent fluid weight.  In other words, it already has the at-rest coefficient factored into it.  They typically give another value for in-place density, often in the range of 120 - 135 pcf.  Using these values, that's saying that the at-rest pressure coefficient is about 0.75 -- pretty high.

To give you an idea of what we commonly see, here's a snip of a geotech report from a project I did a while back:

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Lateral earth pressures acting on the below grade walls will depend on the type of backfill material used.  These walls should be considered rigid and designed for at-rest earth pressures as presented below for a level backfill and a drained condition.

EQUIVALENT FLUID PRESSURES
Backfill Material                  At-Rest (pcf)
On-site soils                          110

Select fill, with LL<35 and PI<15      65

Granular backfill w/ <3% passing
No. 200 sieve and <30% passing
No. 40 sieve, non-plastic          45

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Obviously, for large project like a culvert there is no way possible to economically replace the on-site soil with granular backfill, so we're stuck using the 110 pcf -- and designing a bomb shelter.

-- Joel

-----Original Message-----
From: Tripp Howard [mailto:tripphoward(--nospam--at)yahoo.com]
Sent: Tuesday, January 28, 2003 1:45 PM
To: seaint(--nospam--at)seaint.org

Joel,
You aren't using a lateral load of 85psf - 110psf are you?  What you get from the geotech is the soil density that has to then be multiplied by either the active or at-rest coefficient (usually around 0.33 and 0.50 for a good sand).  This would give you about 37psf - 55psf lateral load on the walls (which correlates very well with what AASHTO prescribes).  From what I understand, the AASHTO pressures already include the active or at-rest coefficient.

Tripp Howard
The recent thread about culvert design prompted a related question.  We have an ongoing quandary at out firm regarding what lateral soil pressure to use for design of culverts, retaining walls, junction boxes, and all sorts of other buried structures.  Take culverts, for instance:  If designing to the AASHTO standard, we can get a geotech report or we can simply use the code-dictated 30 pcf or 60 pcf equivalent fluid weight (depending on the load case) to determine the lateral pressure.  TxDOT, in developing their culvert standards, used 40 pcf for the equivalent fluid weight.  However, when we get a geotech report for a project, we typically get an equivalent fluid weight of 85 - 110 pcf for calculating the at-rest pressure.

So, you see our problem.  If you take the "show me the bodies" approach, the TxDOT values are adequate, since there are hundreds of miles of TxDOT standard culverts all over the state, and there probably isn't one of them that has failed due to excessive lateral soil pressure.  But in Texas we are considered negligent by the PE Board if we design a foundation or a buried structure without a geotech report.  So, we dutifully go get our geotech report, find out that the soil at this site "really" exerts 100 pcf of equivalent fluid weight, and design our culvert accordingly.  We're then caught in the difficult position of having to explain why our culvert looks like a bomb shelter.  This is especially troublesome when the client is TxDOT, and we have to try to explain why we can't use their own standards.

I'm wondering if any of you have faced this, and how you have handled it.  What values do you typically use for lateral soil pressure on these types of projects?  How would you approach our dilemma?

Thanks.
-- Joel
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Halff Associates, Inc.
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Tripp Howard

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