Correlation of vibracore rate of advance data with N blow counts 1

[tailhook]

I'm a geologist and an environmental engineer but not a geotech. I'm working a project where we're faced with enough cobble and trash to make a geoprobe unworkable, and enough running sands to frustrate a standard split spoon sampling drill rig. I'm after good samples and recovery but a geotech is going to need enough information to design sheetpile bulkheads along a river channel. Has enough research been done yet to be able to use the results of a vibracore rig to generate the standard blow count information a geotech would need for foundation engineering? Is there a publication out there more recent than an old early "80' ASCE document that addresses this?

 

[DRC1]

SPT's are probaby the data the geotech is looking for.It would seem a hollow stem auger with a roller bit should get through the cobbles and drilling mud should stabilize the sand. The cobbles are important as they will effect the drivability of the sheeting. However, the SPT blow counts in cobbles are not terribly useful except to give an idea of the extent and concentration of the cobbles. The running sands are significant because if the sand is loose, as is often the case with loose sands, as that will effect the passive resistanece of the sheeting system. I would suggest talking to your driller and changing your method of attack.

 

[tailhook]

Thanks DRC1 and we may have to go that way but many regulators don't like to accept the results of sampling when mud rotary is used. We've run into several EPA and State types that feel any subsequent environmental sample is suspect and refuse to accept the results of groundwater samples produced by wells installed with mud rotary. My goal is to recover good samples and determine good stratigraphy, but I accept that most geotechs are used to dealing with SPT data for design.

The question to the community is whether there has been enough data developed on sonic (or vibracore) sampling so that geotechs can derive the design data they need from either the vibracore information, by slicing the recovered core and using field tools, or by sending in sections of the core to a lab to generate the design data needed.

 

[Grouser]

It seems like you have two problems you're trying to solve with one borehole. If you need water samples, why not jet a wellpoint down and get the water samples there. You won't need to use drilling mud.

Are the cobbles too large for hollow-stemmed augers to get by? Can the driller use smaller augers, or rotary with casing, to get through the cobble zone? The geotech will probably want both "N" values and samples for grain size distrbution data.

Some of the more knowledgeable forum members like Focht3, SirAl, et al, may have info regarding the coorelation you seek. Wait to see if they post.

Good Luck.

 

[BigH]

If you are in Canada - west coast area, you could consider the Becker Hammer drill rig - it is good for oversized materials.

 

[Focht3]

Sorry I'm "late to the party", so to speak. Busy week!

You have a tough problem - but you need to understand a fundamental concept: good environmental boring equipment - and drilling techniques - make absolutely horrible geotechnical borings. Don't try to make your borings "dual use" - you're wasting the owner's money. Been there, done that.

I don't have any confidence in vibracore readings for this application - debris and cobbles are too variable. The correlation is tenuous at best, and you don't have a particularly "uniform" site -

The cobbles may be hell to get through regardless of the (conventional) equipment used. I spent three or four very long nights on the shoulder of SR 91 aka the Riverside Freeway east of L.A. trying to drill through a layer of cobbles near the maximum scour depth of the Santa Ana River. Got HSA stuck (Mobile 67) at about 95 feet; same thing for a CME 95 using wash rotary. We lost circulation for good at about that depth, too.

How deep do you think the cobbles and trash are? Proportion of each? Are the cobbles natural, or placed by man? You may have to remove some of it and replace it with clean sand in order to drill the borings.

Finally, sheet piles may not be practical for this site...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[ashjun]

Cheers to Focht3; it is not only difficult, but nearly impossible to drive sheet piles in cobbles; most likely they will get damaged during driving.

 

[tailhook]

Thanks people. Thanks to all the input, we're now looking into bringing in a DMT to this project to produce the design data and using sonic to get the borings/soil samples/ and produce the holes for temporary piezomenters for groundwater samples. This rig will also have a cone penetrometer and an option to do some pore pressure discipation testing to get permeability if we need it. I greatly appreciate your time,assistance and willingness to point me in better directions.

 

[Focht3]

Neither DMT or CPT are terribly effective (read: dismal) in evaluating cobbly materials. Field tests aren't worth much in these soils; this is where we engineers get to earn our keep!



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[BigH]

Just purchased Kramer's book on Geotechnical Earthquake Engineering; Prentice-Hall International Series (ISBN 81-297-0193-6). Anyway, he has a couple of nice pages on the Becker Drill/Hammer which we have alluded to as being suitable for gravelly/cobbly soils. It is in Chapter 6, Section 3. Too, he has two references:
Sy and Campanella (1984) "Becker and Std Penetration Tests (BPT-SPT) Correlations with Consideration of Casing Friction," Can Geo Jour Vol 31, No. 3, pp 343 to 356.
Harder and Seed (1986) "determination of penetration resistance for coarse=grained soils using the Becker hammer drill," Report UCB/EERC-86/06, Earthquake Engineering Research Centre, University of California, Berkely, 126pp.