W:
I just made a discovery on my email server that about 50-messages were routed to my junk email folder without my knowledge. Unfortunately yours was one of them. I wondered why several clients and subcontractors had told me that they had sent an email that never arrived. Any way, my apologies for not responding to your questions sooner. It sounds like you have been able to work through the spring problem.
Karst topographies are not commmon problems here in Michigan for lake and pond projects due to the thick surfical glacial deposits, however, we deal with them on many construction projects because the voids create sinkholes and cause buildings to settle. We prefer to pressure grout the voids using a drill rig, which injects a bentonite/cement mixture under pressure deep into the formation and seals cracks and fissures.
The springs/voids form because the carbonate rock is soluble and "bedded", which basically means layered. Water dissolves the carbonate along the layers resulting in porosity. It's a relentless process. Hopefully in your case, the repairs will last a lifetime. Let me know if you still have any questions.
