SINKHOLES

[Okiryu]

In the US, what are the most common exploration methods for checking sinkhole potential? Exploration depth is within 10 meters below ground surface. I am in Japan and my local GPR contractor does not have the equipment to explore to that depth. What can be other options?

Thanks.

 

[thaidavid40]

We often use electrical resistivity test surveys in this area (North Alabama, because of a lot of karst activity here), and sometimes acoustical soundings also.

Thaidavid

 

[SlideRuleEra]

For an industrial plant site we used a large number of small-diameter soil borings. Samples were not taken for these. The drill operators quickly became very good at detecting a void by observing and logging the drill's progress during the boring. These were in addition to traditional soil borings, with samples recovered and tested.

Tried GPR, but did not get meaningful results.

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[fattdad]

I would agree on the use of electrical resistivity. I would also agree for some projects, there's nothing better than driling a bunch of holes. Air track drilling to get the top of rock and also whether the rock is integral is very effective.

Of course, regional geology, local observations and a review of landforms is always important!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Thanks for your replies. I am checking with a local contractor and appears that they can do electrical resistivity tests.

 

[Mccoy]

ERT is arguably the favourite method to investigate the potential existence of cavities, but often it should be followed up with borings to move from the realm of probability or possibility to the realm of certainty, unless there has been a specific prior validation in that specific geologic setup (that is, borings found cavities where ERT suggested cavities more often than not).

http://www.mccoy.it

 

[Okiryu]

Local contractors said that in order to do ERT the site does not have to have surface or subsurface structures which may affect data collection. So we need a bare landscaped or undeveloped area to get accurate data. We have already done some borings and found caves. Our site geology is mainly limestone or decomposed limestone gravels. We have surface layers of clays or cohesive fill. The site is planned for airfield pavements and other airfield related buildings. Perhaps for the airfield pavements the main point is to have good drainage systems, good sealed joints, etc. But I was looking for a method to explore the entire area for sinkholes.

 

[BigH]

I know of a project in southern-central Indonesia where the contractor had 8 drills working during foundation placement and drilled at least one hole per foundation location! (thermal power plant)

 

[BigH]

P.S. - SRE might have known it too . . . but . . .

 

[fattdad]

BigH, we've seen that too, right here in Virginia! An air-track can get lots of data quickly. Not very refined data, but it will tell you if the rock has integrity - RIGHT THERE!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

BigH, what is SRE? Also, I just heard that there is some method here in Japan which consist of inserting a camera or laser into the borehole to check the size of the cave/sinkhole....

 

[BigHarvey]

Microgravimetry is another possibility. Destructive drilling with drilling parameters recording (instantaneous speed) is a cheap an efficient soltions. Each time you have speeds greater than 400 meters per hour you can suspect decompressed soils or voids.

 

[Okiryu]

BTW, what do you think that can be the best type of deep foundation for this type of geology? We have been using pre-drilled methods to drive pre-cast concrete piles... what is your experience in your area?

 

[SlideRuleEra]

BigH - You are right. We drilled one hole where each 4' diameter caisson was located. There were several hundred caissons that size under the power block (turbine / generator building, boiler, precipitator and scrubber). Year round water table is a few inches below ground surface. The numerous other probes were distributed over the complete plant site, about 1 square mile.

Okiryu - "SRE" is just the initials of my Engineering Tips user name.

...what is your experience in your area?

We had sedimentary limestone with voids about 30' below the surface. There was much harder limestone (no voids) at a lower elevation. For the first generating unit on the site, we took our geotechnical consultant's recommendation and used drilled caissons that relied only on skin friction between the caisson and the hard limestone - no reliance on end bearing support. The assumption was a void could (theoretically) be located inches below the caisson tip. During construction we did encounter a lot of voids. A couple thousand point bearing timber piles were used elsewhere on the site where loads wee lower.

Ten years later, it was time to construct a second generating unit on the same site. This time, our geotech consultants (a different firm) recommended point bearing driven concrete piles. We had not had any failures of the existing point bearing timber piles. The recommended loading for the concrete piles was 50 tons. After discussions, I "covered our bets" and agreed to the point bearing concrete piles, but dropped the allowable loading from 50 tons to 40 tons.

Since I retired, two more units have been built on the site (for a total of four generating units) - using driven pile. No problems with any of the piling or caissons.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Okiryu]

SRE, sorry I did not know that...I was looking at Google and nothing appeared !! 50 tons (445 kN) is close to our typical capacities for 300mm to 350mm diameter pre-cast driven piles installed in very dense gravels (SPT N > 50) in this type of geology. Do you remember what size were your piles?

 

[SlideRuleEra]

Okiryu - It's been a while. I believe the piles were 300mm (square), maybe 250mm. For sure they were not 350mm. Remember, our long term experience is with a design load of 40 tons (356kN).

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