Thermal resistance of tunnel wall

The equation for rectangular tunnel is given in the book 'Kabel und Leitungen für Starkstrom' by L. Heinhold, 5th edition 1999.

Symbol
$T_{tw}$
Unit
K.m/W
Formulae
$\frac{\rho_{t} \ln{\left(\frac{Do_{t}}{Di_{t}} \right)}}{2 \pi}$circular tunnel
$\frac{2 \rho_{t} t_{t}}{2 U_{ti} + 8 t_{t}}$rectangular tunnel
Related
$Di_{t}$
$\rho_{t}$
$\theta_{to}$
$\theta_{to_{z}}$