The thermal resistivity of the soil has a significant impact on cable ampacity of buried cables, especially when directly buried without backfill. The lower the thermal resistivity, the higher the cable ampacity for the same conductor temperature $\theta_c$.
If the thermal resistivity of soil is unknown, the values from the reference operating conditions in IEC 60287-3-1 are recommended. Or else, the IEEE Standard 442 provides values for some materials and a paper by O.E. Gouda from 2010 lists values for some tested type of soils.