The hay is a good way to go when looking for a bridging agent to seal off seepage in a big hole . I would break up the bales though. Add all the bales you can get in the hole and then add a lot of bentonite on top of it all. One way to look at it is to consider the hay as a fiber material. If you ever played pixie sticks and dropped them in a pile on a table, that is what a bridging agent might look like. And then do a very thick cover of bentonite to seal off the small holes in the pixie sticks. If the hole is large enough you might try doing all this in layers.
