caverns in kastic environment

[pelelo]

Hello,

I am working on a project which is located on limestone. Part of the scope of work involves electrical resistivity survey in order to detect the caverns that were not picked up by the borings.

My question is when a size of a cavern is considered "big" or "decent size" that the geotechnical engineer would need to raise a flag in the report and recommend some type of ground improvement. I assume if there are few 4-in in size caverns under the proposed structures, that wouldn't be significant issue for the engineer, however, when are the caverns big enough to recommend some type of improvement?

 

[TigerGuy]

Again, we have the classic, "What is the project? etc" line of questions. You are correct that minor voids are of little concern. There are ways to address sinkholes that open up during construction.

We will need to know approximate sizes for the cavities, depth from bottom of foundation to limestone (unless you will be founding on the limestone), and what type of structure and loads will be involved.

I read a paper by Dan Brown, PE, regarding drilled shafts in the karst areas of Florida. The recommended solution was to drill a test hole, at least 2 shaft diameters in length, at the bottom of the shaft and check for voids. Since down the hole inspections are largely a thing of the past, you could also pre-drill each shaft location to determine limestone properties and voids, but this would be if you are utilizing drilled shaft foundations.

 

[EireChch]

You probably cant really detect anything less than 0.5 in diameter with geophysics.

How many anomalies you find is relative to the size of your geophysics grid. If you do one or 2 lines and find say 3-5 anomalies, then it seems there is potential for more.

If you do 50 lines are find 3-5 anomalies then not so much of a problem.

There is no hard or fast rule. Many things need to be consider before you decide to do grouting or the likes.

What ever you do make sure to do verification boreholes to check any anomalies.