<P> There were three standard ways to dig a trench: entrenching, sapping, and tunnelling . Entrenching, where a man would stand on the surface and dig downwards, was most efficient, as it allowed a large digging party to dig the full length of the trench simultaneously . However, entrenching left the diggers exposed above ground and hence could only be carried out when free of observation, such as in a rear area or at night . Sapping involved extending the trench by digging away at the end face . The diggers were not exposed, but only one or two men could work on the trench at a time . </P> <P> Tunnelling was like sapping except that a "roof" of soil was left in place while the trench line was established and then removed when the trench was ready to be occupied . The guidelines for British trench construction stated that it would take 450 men 6 hours at night to complete 250 m (270 yd) of front - line trench system . Thereafter, the trench would require constant maintenance to prevent deterioration caused by weather or shelling . </P> <P> The battlefield of Flanders presented numerous problems for the practice of trench warfare, especially for the Allied forces, mainly British and Canadians, who were often compelled to occupy the low ground . Heavy shelling quickly destroyed the network of ditches and water channels which had previously drained this low - lying area of Belgium . In most places, the water table was only a metre or so below the surface, meaning that any trench dug in the ground would quickly flood . </P> <P> Consequently, many "trenches" in Flanders were actually above ground and constructed from massive breastworks of sandbags filled with clay . Initially, both the parapet and parados of the trench were built in this way, but a later technique was to dispense with the parados for much of the trench line, thus exposing the rear of the trench to fire from the reserve line in case the front was breached . </P>

Why were trenches dug by the two sides to begin with