For cables in tunnels, this is the thermal resistivity of the tunnel wall, typically made of concrete.

For cables in troughs, this is the thermal resistivity of the trough wall and cover, made of concrete or polymere.

Symbol
$\rho_{\mathrm{t}}$
Unit
K.m/W
Used in
$H_{\mathrm{tc}}$
$T_{\mathrm{tr}}$
$T_{\mathrm{tw}}$