Nozzle bottom underdrains provide a balance of direct support and gravel support.  Most nozzle filtration systems we have seen have a cap gravel of 1/8" X #16 in size underneath the sand for support.  In some instances, sand can be directly on the nozzles depending on the nozzle size openings.  These types of underdrains are done as a monolithic pour, and have a threaded couplings grouted within the pour for these nozzles to thread into the floor.  Let S4 Water provide you with any size nozzle or orifice size.