Are flexible PVC couplings rated for direct burial?
I just replaced the main stack in my house.  Went from cast iron to pvc.  I dug out about 10 foot in my basement to connect the pvc to the clay piping that goes out to my front yard.

I used a bigger version of this coupling to marry the pvc to the clay and it is buried underground and under concrete.  

Inspector told me that they only start allowing these underground a few years ago in my area.  He also told me to go get a thick trash bag and he came back with some rubber-band like material.  He had me wrap the coupling with the trash bag and seal the bag with his wraps.  He said the 20 cents and 5 minutes should keep it from rusting out an extra 20 years.