Probe drilling below foundation
Probe drilling below foundation
(OP)
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





RE: Probe drilling below foundation
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?
RE: Probe drilling below foundation
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.
RE: Probe drilling below foundation
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
RE: Probe drilling below foundation
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.)
RE: Probe drilling below foundation
RE: Probe drilling below foundation
RE: Probe drilling below foundation
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
RE: Probe drilling below foundation
RE: Probe drilling below foundation
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 !
RE: Probe drilling below foundation
That's exactly we have planned now. You have the very similar experience to what we are trying to do.
Regards