We’re building a new driveway. Need to excavate about half a meter of topsoil just to get to ground level, then sufficient depth for sub base and gravel etc. We are coastal and below the topsoil is pure sand. Driveway will be approx 6x30m so huge volumes involved. My question is how deep the mot type 1 sub base needs to be and whether it is sufficient on its own? Our groundwork guys reckon on 400mm mot type 1, then 100mm of a finer sub base, then 50mm gravel. Is this necessary due to pure sand base or overkill? Thanks in advance.