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• To: seaint(--nospam--at)seaint.org
• From: Tripp Howard <tripphoward(--nospam--at)yahoo.com>
• Date: Tue, 28 Jan 2003 11:45:04 -0800 (PST)

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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