Expert: Robert Cummings, P.E. - 3/20/2008

Question
Hi,
We, a state DOT, need to install about 700' of utility pipes by primarily subsurface methods.  We've done the required research to know what's involved, but I'm unsure about the test data needed to provide to bidders sufficient information to make the job biddable.  Are Cone Penetration Tests (CPT) sufficient, or is a log of test borings with actual drilled samples required?  We have drilled samples available for an adjacent location, but I assume that over the 700' run conditions could vary enough that those results won't help much.  The CPT is much faster and easier than drilling, so that's my preferance.
Thanks for any insights you can provide.

Answer
I think it kind of depends on where you are and what geotechnical conditions are extant.  If for example you are in Kansas City where there is a lot of loess soil that will be crossed by the utility, then not much variation is expected and you can probably get away with CPT.  If on the other hand you are in the Basin and Range area of the West and there is a chance of interbedded cobbles and sands, then the likely variability dictates that you should protect yourself with actual soil borings.

Because you apparently feel that the soil samples from an adjacent location may not be fully representative, I suspect you are in the latter category.  However, if you have CPT data from the adjoining location, you may be able to relate the CPT signatures to actual samples and then have more confidence in what CPT by itself will mean on the 700 ft alignment.

CPT will not give you information on the distribution of cobbles or on the role or size of coarse particles which might cause problems with directional control and the withdrawal of cuttings.  Also, it may not give you information on highly permeable conditions which may cause loss of drilling circulation, or of high ground water conditions which could in some cases cause problems with site water control.  It will give you an overall understanding of hardness and drillability.

There are a number of directional drilling contractors at national scale who would be willing to answer your specific questions about your site.