Soil resistivity is a specialized measurement. If you don t have actual numbers for your site, many hand- books such as the EPRI Underground Transmission Systems Reference Book give design assumptions. Ambient soil temperature. Warm soil absorbs less heat. It is assumed that the heat finally goes to the ground surface.
Deeper cable sees more total soil resistance.
A direct-buried cable generally dissipates heat more readily than a cable in a duct. Nearby cables complicate heat flow Adjacent cables contribute heat and may induce additional losses in the cable itself. Closer cables have more effect. Sheath grounding makes a difference. Multipoint grounding increases circulating currents in the sheath.