Scanning area for Caves , What is the best approach ?

[dtmilad]

Hello ,

We have a project in an area where the soil consists mainly from limestone rock with present of caves.
We are planning to scan the area using Ground Penetrating Radar.

My Question is whether GPR is suitable for this job and what is the proper depth we need to scan the soil for. The project is 140 residential buildings with a height of around 35 meters.

Another question is what is the suitable way of grouting any subsurface caves.

Please find attached pictures of one cave we found during excavation works.

 

[GeoPaveTraffic]

First GPR will not work. Everyone wants to use it because it is cheap, but it will only see 1 or 2 meters into soil and will not see past any groundwater.

I could not open your photo, don't know what the problem might be, so I don't know what the cave you found looks like. But if you have shallow caves in bedrock; then the area is not suitable for houses.

As for grouting caves, that is almost always a bad idea. The caves likely carry groundwater, if you grout them that water will go somewhere else at least temporarily. You can cause flooding and changes to the surface and groundwater flow in a wide area. The other problem is, depending on the conditions, the water may erode the material beside the grout and reopen the cave in a few years.

At a minimum, you need to get a very experienced geotechnical engineer involved with the project.

Mike Lambert

 

[miningman]

Yep, sets of individual piles beneath each structure MIGHT work but unless you are willing to spend a fortune on probe hole drilling , perhaps on a 2 or 3 metre grid , I cant see this proposal going very far.

 

[IRstuff]

The file is a rar archive containing 6 pictures.

http://www.geophysical.com/gssifaqs.htm

http://gp-radar.com/Limitations-of-GPR.html

http://www.global-gpr.com/gpr-technology/how-gpr-works.html

http://www.gp-radar.com/faq.htm

https://en.wikipedia.org/wiki/Ground-penetrating_radar#Limitations

http://www.uvm.edu/~lewebb/CCLI/various papers/Smith and Jol 1995 GPR.pdf

All imply that ≤ 400 MHz and dry limestone might be a good combination to get well past a few feet.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[cvg]

seismic refraction might be another option

http://www.spectrum-geophysics.com/voiddelineation.html

http://files.dnr.state.mn.us/waters/groundwater_section/geophysics/reports/mystery_karst.pdf

 

[oldestguy]

One way I've done this for heavy structures is to do a bunch of quarry drill holes. Keeping track of penetration rate and watching cuttings is needed.

 

[miningman]

"DRY Limestone""??? That's close to being an oxymoron in Karst topography which is exactly where I would expect to find caves.

 

[Okiryu]

I agree with GeoPaveTraffic. I used GPR and it was good only for shallow portions. If you have groundwater and underground utilities around it will not give good results. Although I haven't used it yet, I heard that electrical resistivity is a better method. Also, doing a grout plug if you have groundwater there, it may create problems in other areas, as GeoPave mentioned.

I saw this link and think it is good solution:

http://www.smiddleton.com/environment/sinkhole.htm

Also, I found Sowers book "Building on Sinkholes" as a goo reference....

 

[miningman]

This gives you an idea of what caverns in karst limestone geology looks like.

http://www.enr.com/articles/38948-iraq-hires-trevi-led-team-for-emergency-repairs-to-mosul-dam

 

[fattdad]

Electrical resistivity is one choice. GPR doesn't get deep enough (just echoing earlier comments). Gravity is another choice.

Seismic refraction doesn't work. Not sure about reflection.

Drill holes are essential, just to bring regional geology to the site and confirm what you have. In core holes, we've used the televiewer to actually photograph down-hole findings and capture structure.

Nothing is easy. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[IRstuff]

"GPR doesn't get deep enough (just echoing earlier comments). "

At ANY frequency? So are all the guys with < 400 MHz GPRs lying about the performance?

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[fattdad]

I'm not calling anybody a liar. We don't use GPR for cave/karst/solutioning detection in Virginia. I manage the geotechnical program for our DOT and serve as both engineer and geologist for our main office. We see more EM resistivity from our consultants in conjunction with confirmation borings.

If you have other perspectives, I'm love to learn more about advances in GPR and case studies that show these innovations in practice.

f-d

ípapß gordo ainÆt no madre flaca!

 

[miningman]

Fatdad, I'm curious. Since IRstuff has previously posted that his proposed GPR systems are recommended for dry limestone, and since I've never encountered dry limestone in my career, what is you experience? Is the limestone in Virginia dry, moderately damp or absolutely saturated which is what I would expect in Karst topography?? I would certainly expect local variations, but generally dry? I dont think so

 

[Okiryu]

miningman, my area (south of Japan) has karst geology. Sometimes we find limestone above the groundwater level. In that such conditions, can this limestone be considered "dry"?

 

[fattdad]

Our typical condition is moist soil (sometimes with perched water), residual soil (low strength ML or MH with water content near the liquid limit) and then limestone. Solution features/voids may or may not be inundated. In Virginia, we don't see the word, "Dry" very often. I'm not going to make some proclamation on water table in Karst locations; however. . .

I'm still interested in advances in GPR that can see down to 30 ft or so.

f-d

ípapß gordo ainÆt no madre flaca!

 

[fattdad]

duplicate post