The big problem with mortars is not the design, but the way they are fired.
It's hell to find a mortar that is 100% safe to use for people and the environment. Anything with a heat source as an igniter, blackpowder, other explosives tends to go against a whole truckload of laws.
The only safe mortar system I have seen uses a compressed spring to send the contents of a film canister up and over maybe 30-50 feet at the most.
There are many threads about mortars (and designs) you can read here to find more info.