How is a hand probe test evaluated?

[ae11]

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!

 

[TDAA]

The probe is subjective, and will depend on what you are looking for, and how you use it. It generally gives you an indication of loose/soft soils, and the extent they may go to.

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.

 

[oldestguy]

I have one of these in my truck and it is over 55 years old. Not used much now (old age).

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.

 

[TDAA]

I have used it in clean sand. You have to view your result a lot differently than other materials, and your "obtainable information" is more limited. Basically, I expect to see about 6-inches of penn., while holding a "calibrated force". If you end up going 12-18+ inches, you have a loose area.

The real key is the experience behind what you feel. Actual penetration will vary.

 

[Mickney]

I use it very sparringly, but keep it in my arsenal much like my pocket penetrometer. I find it useful when probing for soft soils on slope faces or within footing trenches that have been exposed to inclement weather after they were inspected.

 

[TDAA]

Speaking of slope faces, it is also good at finding out that a tension crack is >3' in depth

 

[kitseng]

In my opinion, it really only gives you an indication, and should be used with proper judgment and experience with the soils

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.

 

[Ron]

Obviously subjective and qualitative; however, good info can be gained, even in sands.

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.

 

[fattdad]

Here's my take. Step 1: Do a geotechnical engineering study to identify the soils, layering, strength and such. That, and engineering analysis, will define the bearing pressure for natural soil and properly compacted on- or off-site soils placed on properly prepared subgrades (i.e., natural soils free from topsoil and disturbance).

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!

 

[WHEngineer]

Like most tools, it depends on how you use it. We typically use a probe rod for footing inspections to give an idea of general soil conditions in the footing. Then, we follow up with dynamic cone penetrometer testing in any questionable areas as well as at least one test in critical or firm areas. This also assumes that if there is fill, we monitored the original fill placement. If not, density tests are also performed to verify compaction. Unfortunately, this methodology is only used on commercial projects as most residential projects don't want to spend the money for testing. We leave those projects to the engineers who charge $70 for a footing inspection.

 

[msucog]

many good points. my probe rod is my best friend...there are many like it but this one is mine. it's quick and can be fairly thorough to find bad/marginal spots. it helps me pinpoint where i need to hand auger and use the dcp. plus, if i'm probing, i'm looking...this is a hard concept to get technicians to understand. the probe rod is just another tool to help provide an overall assessment. there should be subsurface exploration, density testing, hand augers w/ dcp to confirm field conditions are similar to the conditions in the subsurface exploration, thorough probe rod assessment, visual observations, stomp tests, thumb tests, stick/splatter tests, etc etc. also, the probe rod is one of the few tools that will identify loose crushed aggregate backfill...you can search my many rants on this topic regarding rodding the probe up and down creating a funnel several inches deep within the loose #57 stone. and lastly, we need something to lean on while we watch dirt get moved around all day. never trust a geotech that doesn't tote a probe rod! (that is a serious statement there)

good luck.

 

[WHEngineer]

I agree with msucog about geotechs without probe rods. However,my wife can't understand why I leave my probe rod in the truck when we drive on vacation - I feel naked without it!