Test Pit Inspection
Test Pit Inspection
(OP)
I am planning to excavate test pits for site investigation for a light structure. The soil tends to be granular.
I will not have any test equipment and hope that by visualization I could suggest a conservative bearing pressure.
Typically the ground water level, type of soil (which seems to be till like materials) and presence of organic soils are examined.
What other parameters should I look for in the investigation and how can I assess the compactness of the materials? Thanks for your feedback.
I will not have any test equipment and hope that by visualization I could suggest a conservative bearing pressure.
Typically the ground water level, type of soil (which seems to be till like materials) and presence of organic soils are examined.
What other parameters should I look for in the investigation and how can I assess the compactness of the materials? Thanks for your feedback.





RE: Test Pit Inspection
Visual assessment is fine for characterizing and classifying soils for experienced individuals, but you can't tell much of anything about in-situ physical properties from visual assessment. I would suggest that, at the least, you have a bucket auger and probe rod with you. Do an auger boring before the observation pit to assess the relative difficulty of turning the auger, which will help to characterize the strength of the soil. The probe rod can be useful to help determine the relative compaction of the material, though you can't quantify anything with either.
Keep in mind that you need to meet your standard of care exhibited by other geotechnical engineers in your area. Would they use the same or similar methods to arrive at conclusions or would they bring testing equipment to the site and do a proper program of sampling and testing?
RE: Test Pit Inspection
RE: Test Pit Inspection
RE: Test Pit Inspection
Dig to 4 ft and then stop. Go into the hole (yeah, yeah on the OSHA stuff, but for most soils 4 ft is completely o.k by OSHA. Read up if you need to.) Bring a g-pick, a tape measure, sample bags, pocket penetrometer, marking pen and such. Measure topsoil thickness. Take any/all samples you want. Photographs of the cleaned sidewalls are good (chip away backhoe smear).
Continue digging (yeah get out first). Cast these spoils in a separate pile and then get samples as the new pile changes from one soil to another (if it does).
It's nice to have a stick (folding rule) to probe the trench below 4 ft. You really need a tape measure AND a folding ruler.
You can hold a piece of PVC pipe on the vertical end of a trench (if it's dug that way) and add a ground water observation pipe if you want to).
Just some initial thoughts.
Oh yeah to get some idea of soil strength, there are literally rules of thumb that correlate to undrained shear strength, just like bar probes can correlate to frictional shear strength. NAVFAC DM 7.1 or 7.2 has a table on field methods and consistency or density correlations.
This is rough reckoning so beware!
It can be done though.
f-d
¡papá gordo ain't no madre flaca!
RE: Test Pit Inspection
I wonder if somebody could give me a hint about how to estimate the density of soil without any CPT/SPT/DCPT operation. Though no penetration testing carried out as the cone would be obstructed by cobbles and boulders. Thanks again.
RE: Test Pit Inspection
RE: Test Pit Inspection
RE: Test Pit Inspection
RE: Test Pit Inspection
This tells me, based on what you have said, that the soil is pretty competent. What is your loadings? And if you use minimum 24" wide footings - presumably syrip ftgs - what is the applied pressures. If applied pressures are less than 100 kPa, you should be okay given no other hiccups. What are the typical allowable bearing pressures in materials like this when you do have SPT data? - and by the way SPT data in soils containing gravels and cobbles is a bit unreliable anyway.
Next - use experienced judgment. You say you are a geotech, then you should have this experience as a starting block anyway.
RE: Test Pit Inspection
This tells me, based on what you have said, that the soil is pretty competent. What is your loadings? And if you use minimum 24" wide footings - presumably syrip ftgs - what is the applied pressures. If applied pressures are less than 100 kPa, you should be okay given no other hiccups. What are the typical allowable bearing pressures in materials like this when you do have SPT data? - and by the way SPT data in soils containing gravels and cobbles is a bit unreliable anyway.
Next - use experienced judgment. You say you are a geotech, then you should have this experience as a starting block anyway.
RE: Test Pit Inspection
http://en.wikipedia.org/wiki/Till
RE: Test Pit Inspection
Digging was hard but may be because that boulders of 18" to 40" diameter were difficult to excavate by the backhoe bucket and not probably because the soil matrix was very strong. It seemed that the light brown clayey soil matrix were not sticking to the large size materials tightly as one would see in some till materials. Boulders in till materials are usually become part of the soil matrix but in this case they were abrupt rock lumps of cubic, blocky or elongated.
The applied pressure of strip footing is 75 kPa but it would be desirable if it can be increased to 100 kPa. If I assumed that the SPT would not be obstructed, a SPT of more than 7 would have been expected. So this could mean that it would be loose granular soil. However, in this case I am not sure if the liquefaction of loose materials (with boulder inertia) may be issue.
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RE: Test Pit Inspection
RE: Test Pit Inspection
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RE: Test Pit Inspection
"Boulders in till materials are usually become part of the soil matrix but in this case they were abrupt rock lumps of cubic, blocky or elongated." This seems to support the idea of a slide. Where's the geologist when we need him/her?
RE: Test Pit Inspection
RE: Test Pit Inspection
Sieve analysis indicated silty clayey gravels (11% fines, may be GW-GC or GW-GM). I wonder how these materials behave in a seismic event. Seems they cannot liquefy. Can they.
RE: Test Pit Inspection
If there is landslide rubble, think about what made the slide stop. Could it be reactivated by prolonged wet weather, with no earthquake involved? That might be much more likely than a 500-year earthquake.
RE: Test Pit Inspection
Is it not that when the geological map says:
"landslide rubble, gravel and minor sand up to 10 m thick"
It would mean that the sand fraction would be a minor in the soil, let's say 65% gravel and 20% sand. Or does it mean that we have an independent layer of sand. Thanks.