Best method is non-woven geo-textile under the slabs,
(prevents weed growth and soil-heave),
with gaps filled with resin-bound gravel to ensure
storm water can drain between the slabs into your soil.
Even the best of the geotextiles, Terram, which I use under compactable gravel driveways doesn't stop soil heave completely. Cracking can still occur in inflexible stone surfaces.
Best method is non-woven geo-textile under the slabs,
(prevents weed growth and soil-heave),
with gaps filled with resin-bound gravel to ensure
storm water can drain between the slabs into your soil.