Disclaimer: I am neither a soils expert, nor even a digger of holes (apart from a few post holes) I am regurgitating what I have read, and am a hobby geologist.
I would take a guess that water is running horizontally between limestone layers, thus why you have springs. As you are digging, you are exposing this flow. It is possible that there is good news in this: The flow is where you can reach it, rather an straight down into the Mines of Moria.
Around us in western NY the limestone is pretty solid, more like dolomite. It is more or less impervious after you get below the weathered layers. However in some parts, limestone is very porous, full of holes, sinkholes, caves, etc. I suspect you have the first type, where once you pass the weathered layers, you have a solid base. This is based on that you actually have a stream and ground water.
If you have a means to divert the water downstream while you work rather than pump it, I would do so (siphon?). However it sounds like you are in a really tough spot of needing to pump.
I think the only thing you can do is bust out the rock down to solid unweathered layers along the entire length of the dam before backfilling with very tightly compacted clay making a core that connects the clay to reliable stone. When you are done, you have to hope that inflow is greater than outflow, and that there are no other stony escape paths for water inside of the basin.
Good luck with your work. I think it is common to run into these issues where bedrock is just below the surface. In my experience, projects never are as easy as hoped.
Last edited by liquidsquid; 08/21/13 07:46 AM.
