How is a hand probe test evaluated?
How is a hand probe test evaluated?
(OP)
Disclaimer - I'm a structural engineer, so I'm only somewhat familiar with the geotech side. I've seen geotechnical engineers and testing agencies probing soils with a hand probe (essentially a welded "T" rebar assembly). I understand in concept what there doing here - testing bearing capacity and soil density - but what are they really looking for? What distinguishes a good hand probe test from a bad one? How much force are they using to jam the probe in the soil? Are we talking 50 lbs of force resulting in 1/4" soil penetration, or 15 lbs giving 1/2" penetration? I'm interested in this mostly in projects where there's less likely to be a geotech involved, like in light residential construction. Thanks!





RE: How is a hand probe test evaluated?
I generally use a full body lean into the probe. Depending on my weight that season, the force varies. It also varies as the probe goes into the ground and the position of the rod changes to the body position. I usually use only one steady lean, and do not "jam" it. I may use repeated pushes if trying to find the bottom of some deep, soft materials. As the rod goes in, the resistance will increase along the shaft.
In my opinion, it really only gives you an indication, and should be used with proper judgment and experience with the soils. The probe will act differently in sands vs clays.
Where I worked at one location, we use a piece of #6 bar to get a rough guess at a bearing capacity for residential, but most "pointed" probes (as I have seen) are used as I described.
RE: How is a hand probe test evaluated?
The thing is a step or two better than using my umbrella or stomping my heel to get an idea of how dense or loose a soil is, generally a fill.
It also is useful when rough "testing" a broad area before testing actual density, as with nuke or other method, to first give you a notion as to where the better or weaker areas are.
I will admit that, in a pinch, I have probed the site and then tell the owner to go ahead and build and no other "testing" was done. That is because I knew a lot about the site before that, such as the contractor doing the job, the fill type, etc. and yes experience.
The one place it won't work is in uniform clean sand.
RE: How is a hand probe test evaluated?
The real key is the experience behind what you feel. Actual penetration will vary.
RE: How is a hand probe test evaluated?
RE: How is a hand probe test evaluated?
RE: How is a hand probe test evaluated?
Spot on!
Doing 50 nuclear densometer tests takes a few hours if you are not just backscattering. Doing 1000 'probe tests' takes the same amount of time. The younger guys at my work (including me) tend to probe around a site followed by nuke testing in areas of interest.
RE: How is a hand probe test evaluated?
I was taught early on to use a 1/2" diameter rod, with a tee handle, and to push the rod into the ground with arms extended outward (no body push). With this method you tend to put about 20 to 25 lbf on the rod.
Gives a good qualitative "feel" for the soil.
RE: How is a hand probe test evaluated?
So, the project goes to construction. Sometimes the geotechnical technician is not on site during all aspects of subgrade preparation, fill placement or foundation subgrade preparation. Sometimes there's overnight rain. Sometimes it's hot as heck and the surface gets baked. Using a "geostick" (that's what we call it and ours have a conical tip that's about an inch in diameter) can disclose pockets that are too soft for fill placement, or areas of foundation subgrade that are not properly prepared.
A geostick is not used to measure the bearing pressure. A geostick is used to determine whether the natural soil quality shown by the geotechnical study is also shown in the field during construction. This is qualitative. It's important though.
f-d
¡papá gordo ain't no madre flaca!
RE: How is a hand probe test evaluated?
RE: How is a hand probe test evaluated?
good luck.
RE: How is a hand probe test evaluated?