We also poor a concrete slab under steps but I always order the concrete and pay the small load fee, hand mixed concrete is not as strong. I've been doing it this way for 12 years and have not had any issues with the block settling, also I stack the block solid all the way up, I dont put it on compacted fill. I excavate 18" and the top 6" is concrete with wire. I also do this under seat walls, bars and grills. Here is the last one I did...The last pic is for DVS.