We would like to install a 5-6 foot high, 50 foot long Allen block retaining wall in our sloping back yard after we regrade to create two flat levels. In bidding out this job, we have been advised by one contractor to have a concrete footer poured for the retaining wall. Another contractor has said he will use the gravel/sand bed recommended by the manufacturer. Reason given for using concrete footer is that earth moves. Our soil is heavy clay and is located in southern Ohio. What do you think? Thanks for your reply.