Complex sentence: The conditions became so bad that infantry regiments were set to work as labour units to maintain the troops in the front line but the rains and French mortar fire destroyed field fortifications almost as fast as they were built.
Split into simpler sentences: The conditions became so bad that infantry regiments were set to work as labour units to maintain the troops in the front line. The rains and French mortar fire destroyed field fortifications almost as fast as they were built.