Trench for basement bath plumbing failure

[traveler]

We put a bathroom in the basement 2 years ago, had to trench the drain under the concrete floor and put in a lift station. It all went fine...until the concrete in the trench started to heave. We have clay soil and a high water table, it seems the water pressure caused the concrete to soften and fail. pretty sure we used the wrong type of concrete. Doesn't matter, what do we do to make it right. So far i've pulled all the fixtures, flooring (luckily it was just vinyl) and I've used a chipping hammer to remove almost all the material we used to patch the trench. it's been suggested we (obviously) use different concrete, possibly "pin" it into the slab. That will be tough, because when we cut the trench we deeply scored it and when we busted it out it took chunks of both sides out from underneath, so there really isn't much to drill into. My contractor buddy (who I should have had helping the first time) thinks that by useing the right material and making sure there is room under the slab on both sides for the new material to go under, the new material will be locked in and not be able to heave. last, should there be plastic or?? below the new pour to keep the ground moisture off the concrete? Any ideas will be greatly appreciated.

[Surface Tension]

Your on the right track. Yes it'll lock in if you fill the void and not just a layer on top. Use the premix cement and not a motor mix. Also don't mix it too dry or too wet and it'll be fine. If there wasn't plastic under the slab I wouldn't bother. You could moisten the slab around the patch. There may have been a blanket of sand or gravel under the slab and you could do that but it should be packed. Really an area that small isn't going to make much difference.