SFAIUI (and that's only a little) Pirelli specify a maximum
surrounding (ambient) temperature for their SWA (around 5deg C IIRC) &
that it should be surrounded by something that will take the heat away.
See
http://www.pirelli.co.uk/en_GB/cables_systems/energy/catalogue_pdf/lv...
IIRC Pirelli specify depth min 450mm & surrounding the cable in soft
sand (which should be moist, or at least not liable to dry out).

Modern SWA is not laid in ducts normally - presumably you'd have to
derate it to take account of the reduced heat conduction. How much all
that mattters in your case depends on whether and by how much the cable
is overrated for your full load & what proportion of the time it is
carrying full load amps (ie the duty cycle).

HTH