Gravel fill

Hey all.

I dug a trench that I had to lay my telecom conduit in and it is 400mm deep.

I will be pouring a concrete path over the top of it.

If I backfill with gravel will that be ok or should I just throw the dirt back in.

Do I need to do anything special to the gravel ie mix in cement?

My soil is highly reactive clay.

I'd just be backfilling the trench with coarse river sand. You could return some of the dirt to the hole first and then top with river sand. Water so that the sand is packed down. I wouldn't have thought it necessary to fill with gravel.

Throw the soil back in. No need getting gravel, etc.,.

Agree with course river sand. Then just a bit of the soil and turf you removed.

Given that your soil is highly reactive and that your trench runs to the edge of the building it is effectively a water channel that could deliver water under your house and create a risk of foundation damage. Your other service trenches should have clay plugs to prevent this and you should do the same. I don't see why you would use granular fill that conducts water. Just put back clay you excavated and use compaction and consolidation technique to make sure nothing can flow in the trench.
Water coming under house in service trenches is one possible cause of slab heave.