Limit of thickness of soil layer

Thickness of soil layer providing a thermal resistance equivalent to $h_{amb}$, when the laying depth approaches the radius of the pipe, $L_{cm}$ → $D_{ext}/2$.

Symbol
$e_{limit}$
Unit
m
Formulae
$\frac{D_{ext} \left(e^{\frac{2 k_{4}}{D_{ext} h_{ext}}} - 1\right)}{2}$
Related
$D_{ext}$
$h_{ext}$