it will smoother many weeds but not grasses or bind weed. also depends on which species you go for.

Thanks I notice grass doesn't seem to like the canopy of trees,hedges and shrubs much nothing does ..Im stubborn enough to keep on piling the dirt on top of my couch grass to block out the light...I've been told it will be fruitless but for now I'm just hiding it

it wont stop couch grass you'll end up with better couch grass as it will get nutrients from the stuff you are putting on it.
there are many grass species [over 100 native to Britain] and some are more shade tolerant than others.