Question
I am building a rock pond, basically under a live oak tree in and around the burnet tx. area. My pond is about four feet deep, and while I am reasonably certain I did not damage a large root in the process of digging the pond, I am worried about the outcomes of a cement slab and liner in my hole obstructing root growth and killing the beautiful large old tree.  My question,  Will a live oak root be able to still live and get water with a cement pond sitting in the ground amongst its root base?

Answer
You can cut up to about a 1/4 to 1/3 of the root system and not kill the tree. The roots extend out beyond the branch but you can use the area under the branches as a guide of how much of the root system has been damaged. Draw a line around the tree at where the branches come to--then remove or calculate what was removed by the pond. IF this area does not exceed 1/4 of the circle than you should be ok. You may get some dieback on the side of the crown where you dug but this is not a very large problem.
The roots that remain will grow around the pond and the pond will not obstruct the growth. Root tend to seek water so if you ever get a leak in the pond do not be surprised if the roots enter the pond through the crack in the cement. The tree will survive --if you begin to see some dieback I would recommend that you fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. This will help the remaining root's health.