Probe drilling below foundation

[pnmkapoor]

I am working on a project in which footings are to be rested on Limestone formation. Limestone rock starts right from the surface. As we all know that there is a probability of solution cavities formation in Limestone. However, out of 20 boreholes drilling across the site, only 3 boreholes showed presence of small cavities of 0.2 to 0.3m. Before foundation concreting, We are planning to do probe drilling at each foundation location to ensure that foundation is not resting on cavities. However, the cost of probing and grouting is coming quiet significant. Other engineers are of the opinion is that we are over-killing the issue. I feel probing is required anyway to avoid any associated risk on structure performance in future. Could somebody share their experience and provide some information on probing guidelines. Thanks & regards

 

[Focht3]

What are you building - an office building, hospital, church/temple/mosque? Are you constructing drilled piers or spread footings? What are the footing sizes? Are "large" solution caverns present "near" the site? How large, and how close?

Your description doesn't give me any reason to feel that you really have a significant risk of damage from large voids beneath the structure. Please explain your concern.

And where is your site (approximately)? What is the local practice?

 

[BigH]

Focht3 is, as usual, right on. The type of building you have does influence on the level of "insurance" you might want to ensure.

As a case history, I am aware of a project in King of Prussia, Pa, where the geotech consultant found large voids in the rock onto which his foundations were to be placed. They were also structural engineers. What they ended up doing, if my foggy brain is operating on all cylinders tonight, was to do an analysis of their foundation system and see what effect there would be on system if a void appeared under any one adjacent footing. I think they used a strip mat foundation and the nodes were at the columns. Might want to consider this - again, depending on the practical concerns you have.

 

[pnmkapoor]

Thanks Focht3 and BigH for the quick response.
To add further, the structure is a industrial building. The footing size is mostly 4mx6m at about 12m center to centre. There is a severe risk associated to the plant equipment and personnel in case of any distress in foundation in future (this unit of plant process hot molten metal). The location is in mid-east. We could find only one code that gives some information regarding guidelines for probe drilling (Australian Standard AS 4482.1-1997). According to this code we need to have atleast 10 probe holes in every footing to have 95% probability of finding cavities of 2m diameter or more. Hope this additional information gives more idea. Thanks once again. Regards-pnmkapoor

 

[Focht3]

I'm not familiar with the Australian code, but it seems to be a sound approach.

The building itself probably does not justify the use of probe holes. I suspect the molten metal handling equipment is independently supported, and it clearly does justify the use of probe holes. That is, unless the equipment will be supported by very large concrete footings with low maximum bearing pressures.

You didn't answer a very important question: are "large" voids known to exist near the site? How large are they? How close to the site?

Who is your client - the owner? What does the owner say about the risk? (I presume they have some experience with similar facilities nearby, or at least with this equipment and use.)

 

[BigH]

As a follow-up, I once used ground penetrating radar to determine if there were mine workings under Highway 11/17 in the ThunderBay area of Ontario. You might try that out - check out a well known seismic exploration company.

 

[BigH]

oops - forgot to add that once we had a "potential" void, we used an air-track drill to probe. Never found one.

 

[pnmkapoor]

Thanks Focht3 and BigH,
We have decided to go ahead with probing program using air track (similar to BigH's experience)
By the way project is in Saudi Arabia, Alumina smelter plant undergoing expansion. Voids of size maximum upto one cubic meter (during probe drilling)were observed at four locations (4 out of over 450 foundations)in phase-I construction about 10 years ago. This expansion site is as close as 50m to phase-I.
Finally we have been able to convince client that its worth their money to spend on probe drilling to ensure foundation performace and avoid any risk/safety hazard in the future.
It was good to learn that our effort is no different than what's being practised in the industry elsewhere. We contacted a local geophysical company for ground penetrating radar (GPR) but that company who has experience in local conditions advised that GPR would not work under these rock formation (not sure why??).
Thanks once again to both of you for your feedback and for sharing your knowledge. I would come back to share my experience on this later on.
Regards-pnmkapoor

 

[BigH]

pnmkapoor - although I haven't talked to my contact in the GPR field for a few years, I will try to contact him for you. You may contact me at bohica@indiatimes.com . Best regards.

 

[georam]

When I was working in Riyadh, SA, it was very common to probe the foundation Limestone for large structures, particularly if it is a sensitive and huge structure. The area was notorious for having large cavities in the limestone bedrock formation. Why is it a big fuss to probe under the structures ?. Certainly it won't cost as much as the building itself if you have to fix it after the collapse of the cavities.
Just use a percussion drill rig. No core, no sampling. It penetrates the rock with diamond tungsten carbide tips within a few minutes per hole. It won't cost a lot of money and time... and it is worth the effort and keep the mind at ease. Let the drill inspector mark up every feet on the drill rod with chalk, and time it for each feet drilling (or plotted if you want). If it goes into cavity, he will surely know... a sudden drop. If there are no cavities, just backfill the hole with cement. End of the story !

 

[pnmkapoor]

Thanks Georam for your valuable input.
That's exactly we have planned now. You have the very similar experience to what we are trying to do.
Regards