Probe rod references? Anyone?

[dirtsqueezer]

I am just curious about documentation for a metal probe rod that I ususally use for near-surface stabiliby of a native cut. I know the concept is based on square area- the weight of my body on a very small area (the tip of the rod) will simulate a much larger weight over a larger area (like a building). I've had engineers in the past mention limitations to using the rod, as when there are larger rocks present. Does anyone know any papers or research that has been done on this? I know it all seems obvious, but it would be nice to know if there were any 'ground rules' one could follow, and cite, if possible. Thanks!

 

[BigH]

dirtsqueezer - I know that this is very crude for "relative density of cohesionless soil":

From: National Research Council Canada "Guide to the Field Description of Soils for Engineering Purposes", Technical Memorandum 37 (1955) -

" . . . it is difficult to drive a 2- by 2-inch wooden picket into dense soil for more than a few inches. A 2- by 2-inch picket can be easily driven into loose soils. If the grains are "cemented" together, density cannot be estimated by this simple method."

This is along the lines of chewing silt, etc. but it is a reference. Canada was at the forefront of geotechnical research back in the 50s and 60s.

 

[3of12]

I have been asked to design some residential foundations near a lake here in Michigan and have been told the top 8 to 14 feet of the site is a layer of fibrous peat. Based on my own personal experience, I believe I have 2 options:1. remove the peat and replace with compacted coarse granular fill or 2. use timber piles tied to the house footing. By the way, the developer wants full basements for the homes and does not want to alter the current grade. Also, there is substantial soil beneath the peat. Any ideas on costs for the 2 methods? and.....Does anyone out there have thoughts on other options? Thank you to all

 

[fndn]

It depends on the kind of probe you're using. If it is the simple 3-5 feet tall with a conical point at the tip, then you're soil strength is a function of tip resistance as well as side resistance. I don't use those since it is hard to tell how much of the resistance is from end bearing and how much from side resistance.

If your cone is higher end like the Geo Cone with encased needle that just measures end bearing and side resistance is not measured, then I have always divided 4 by the cone readings of Kg/sq.cm to get my allowable bearing for 1" settlement. I have compared that with Allowable bearing based on N blows/ft and had identical parameters. I have used such cone on over 2000 lightly loaded structures founded on older alluvium and glacial tills with no problems. The factor of 4 is my way of converting the dynamic reading to static allowable bearing pressure.

If your cone is the dynamic cone with a falling weight at a small distance, then I believe Dr. Sowers had a paper on that since he designed it.

 

[BigH]

3of12 - you should post this in a separate thread - but from your description - see the developer, say hello followed by "No thanks." and take a hike. Basements in fibrous peat - meaning that the water table is high?? In New Jersey, we would preload, make sure we had 5 to 6 ft of sand base down above the peat and maybe, a one to two story residential structure would be built. Piles are the other alternative, like you said, but not with a basement. You said substantial soil beneath - not sure what you mean other than your pile wouldn't be on rock. That's no problem if you have a compact sand below - or stiff clay. I would think that piles (timber) would be more economical than removing and replacing 8 to 12 ft of peat - don't know what the cutoff is any more but it used to be about 9 ft. Do remember the concept of dragdown loads when designing any piles. Any fill to the site will cause it to a degree. See SlideRuleEra's web site for his series of Foundation Know-hows. He has extensive references for timber piles - real piles in real situations.

 

[casimmons]

I don't know how much documentation is out there. The probe rod is a crude test, and it can sometimes be misleading- if you have a dry hard crust at the ground surface the probe rod may not detect softer ground just a few inches below. Your best bet is to develop some rough correlations by doing this "test" along with other tests like the Dynamic Cone Penetrometer, Pocket Penetrometer, SPT, etc.

 

[dmoler]

I am not sure of how one can use a probe rod to determine "near surface stability of native cut". Could you elaborate on what you are trying to acomplish. Is this pushed sideways or something into an excavation side wall?

A good guide, but dense and sometimes difficult reading, for cone penetrometer is "Guidelines for Cone Penetration Test Performance and Design" by US Dept of Transportation Publication FHWA-TS-78-209.

We use the Durham Geo Static Cone Penetrometer model S-214 for some types of soil testing, correlate to compaction sometimes in hard to reach places. Their operating manual can be downloaded at:

http://www.durhamgeo.com/pdf/m_test-pdf/manuals/I-S-212M.pdf

 

[fndn]

Thanks dmoler. This cone by Durham Geo is the one I was reffering to. It used to cost around $800 few years ago. Interestingly, at the time I purchased, Durham would not give out any information on meaning of output parameters. For small lightly loaded jobs and a couple of years of experience with this cone, you will have additional tool to your subsurface logs.

 

[dmoler]

For cleanish sand (<12% passing #200 sieve), we use

35-45 95% compaction per ASTM D1557
45-50 98%
50+ 100%

Rocks will mess up the readings.