In September, 1914, the German commander, General Erich von Falkenhayn ordered his men to dig trenches that would provide them with protection from the advancing French and British troops. As the Allies soon realized that they could not break through this line, they also began to dig trenches.
As the Germans were the first to decide where to stand fast and dig, they had been able to choose the best places to build their trenches. The possession of the higher ground not only gave the Germans a tactical advantage, but it also forced the British to live in the worst conditions. Most of this area was rarely a few feet above sea level. As soon as soldiers began to dig down they would invariably find water two or three feet below the surface. Along the whole line, trench life involved a never-ending struggle against water and mud. Duck-boards were placed at the bottom of the trenches to protect soldiers from problems such as trench foot.
Much of the land where the trenches were dug was either clay or sand. The water could not pass through the clay and because the sand was on top, the trenches became waterlogged when it rained. The trenches were hard to dig and kept on collapsing in the waterlogged sand. As well as trenches the shells from the guns and bombs made big craters in the ground. The rain filled up the craters and then poured into the trenches.
1) Private Livesay, letter to parents living in East Grinstead (6th March, 1915)