  Following Lazard, we study the $N$-series of a group $G$ and their associated
graded Lie algebras. The main examples we consider are the lower central series
and Stallings' rational and mod-$p$ versions of this series. Building on the
work of Massuyeau and Guaschi-Pereiro, we describe these $N$-series and Lie
algebras in the case when $G$ splits as a semidirect product, in terms of the
relevant data for the factors and the monodromy action. As applications, we
recover a well-known theorem of Falk-Randell regarding split extensions with
trivial monodromy on abelianization and its mod-$p$ version due to
Bellingeri-Gervais and prove an analogous result for the rational lower central
series of split extensions with trivial monodromy on torsion-free
abelianization.
