Without some sort of backfill you would be creating a nice cavity under the concrete that will lead to cracks or sinkholes. Probably not an issue on sidewalks, but I expect you will be shortening your driveway life if you run the pipe in an area you often drive over, or where you actually park the car.
Also, if you live in a wet environment you are providing water with an easier path to cross the driveway (increasing the void).
But I guess if this is a "short term" situation (replacing the driveway or moving in the next 5 years it won't be your problem).
If I went with that method I would at least want to make a dam on both sides and flow water with sand/soil/something to try to partially refill the gap. If the refill is low pressure the sediment will partially fall out of suspension (in the void) as the water overflows the dam.
It's fine, just squirt some of this stuff in there:Without some sort of backfill you would be creating a nice cavity under the concrete that will lead to cracks or sinkholes. Probably not an issue on sidewalks, but I expect you will be shortening your driveway life if you run the pipe in an area you often drive over, or where you actually park the car.
Also, if you live in a wet environment you are providing water with an easier path to cross the driveway (increasing the void).
But I guess if this is a "short term" situation (replacing the driveway or moving in the next 5 years it won't be your problem).
If I went with that method I would at least want to make a dam on both sides and flow water with sand/soil/something to try to partially refill the gap. If the refill is low pressure the sediment will partially fall out of suspension (in the void) as the water overflows the dam.