/summon command is only usable on entity, like mobs, FallingSand/FallingGravel, arrows, etc. The biggest problem with that command is that you can only summon one entity at the time. If you want to summon multiple entity without repeating the command for each entity, you should use command block loop.
Anyways, the most simple way to cover your "sculpture" is to use the fill command following those simple step:
Cover your "sculpture" with a layer of sand, using that type of command:
/fill ~ ~20 ~ ~25 ~20 ~25 minecraft:sand
Replace the sand you just place with grass, using that type of command:
/fill ~ ~20 ~ ~25 ~20 ~25 minecraft:sand 0 replace minecraft:grass
Be careful if there is sand blocks in you "sculpture", you better use gravel if you don't want them to be replaced.