For cables in tunnels, this is the thickness of the tunnel wall, both for rectangular and circular tunnels.

For cables in troughs, this is the thickness of the walls and the cover of the trough.

Symbol
$t_{\mathrm{t}}$
Unit
m
Used in
$Do_{\mathrm{t}}$
$H_{\mathrm{tc}}$
$\phi_{\mathrm{b}}$
$T_{\mathrm{tr}}$