This interesting sequence is from Tony Langley's collection. The series begins with building trenches as part of a comprehensive frontline system of defensive fortifications. Note that the work includes building rail lines and installing communications wiring.
Below shows a trench network in depth, including obstacles, barbed-wire entanglements communications trenches, and dugouts.
Last we see artillery deployed to support the front line against an infantry assault.
Source: Contributing Editor Tony Langley