Test Pit Inspection

First of all, I prefer to call them "observation pits" rather than test pits....since they are more for observation than testing.

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?

Buy a geo-tech!!

you should take a soils engineer or an engineering geologist with you to classify the soil and estimate the density in situ. oh, you list your specialty as geotechnical? then you should be able to tell a lot by how well the sides of the test pit stand up. if it is a loose soil, it will ravel into the excavation. sometimes i tap on it with a hammer, or see how well it digs with an army shovel. carefully read the new york city or massachusetts building code and you should get some ideas on the empirical estimation of the bearing capacity once you have made your observations.

have the digger operate with the sun in his eyes or on his back. Direct sunlight on a part of the trench wall is a good thing. It really helps to think of lighting.

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!

2 test pits were carried out to depths of 10 ft. We waited for 45 minutes and the pit still maintained its wall except some planar caving into the pit. There were considerable number of boulders up to 1 m diameter in the soil and no ground water.

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.

The cobbles and boulders screw up the in-place density concept of the soil. With those in place, it is somewhat irrelevant.

A picture is attached from the TP. The rock with blue stain is about 2.7 m below the ground level. I wonder if I assume that the soil would be loose, this would be a safe assumption. As otherwise conventional in-site testing may not be feasible. Thanks.

sorry here is the pic

My first question is: what is the purpose of the investigation? Designing the foundatio I would presume. Was the digging "hard" or easy? You indicate that the material seems to be granular till-like with boulders; groundwater or its evidence was not found.

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.

My first question is: what is the purpose of the investigation? Designing the foundatio I would presume. Was the digging "hard" or easy? You indicate that the material seems to be granular till-like with boulders; groundwater or its evidence was not found.

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.

sounds like glacial till or outwash, but that would need to be confirmed by a geologist. identify the geology and that could help a lot with your evaluation of the site.

BigH

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.

I have never heard of glacial till liquefying. Have others? What seismic zone are you in? Not too many active zones/areas in till country - a bit about Toronto and ottawa - the tills in Ohio would probably be clayey and without the boulders. This is a problem with many bldg codes - they are "universal" and not specific.

sandy soils, high water table, shaking and it can liquify. if I am not mistaken, the 1964 Alaskan earthquake caused significant liquifaction in areas likely consisting of glacial till.

Am not sure if we can call it glacial till. It is more likely to be an avalanche or debris flow product.

Liquefied till? Hard to imagine. Outwash, more likely. Till can range from broadly graded sand-gravel-cobbles-boulders, to the bullet-proof clay seen in northern Ohio, but by definition, isn't till worked directly by the glacier?

"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?

After discarding the boulders and cobbles, here is the grain size distribution curve. Little of adhesion was noticed during excavation. The grains are of angular shape.

According to geologist, the materials are slope deposits consisting of landslide rubble, gravel and minor sand up to 10 m thick, overlying channel gravel and minor sand or till materials up to 10 m thick.

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.

Considering classification and fines content alone (GW-GC/GM), without some indication of density or a CREDIBLE water content, I don't think you can rule out high excess PWP and cyclic mobility (accompanied by sand boils, spreading, differential settlement). However, with boulders and cobbles, flow liquefaction leading to slope instability would seem pretty unlikely; that's seen more often in tailings, hydraulic fills, uncompacted dumped fills, and very loose alluvium on relatively steep slopes. That "minor sand" below the landslide rubble is a little more concerning, if it's below the water table and depending on what "minor" means.

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.