What mission at the front is Paul's group sent to perform? What do the men rely on to survive?

Paul and his group are sent to the front to lay wire, which allowed them to communicate during battle. When the English artillery starts firing--an hour before their regular ten o'clock schedule, gunfire breaks out nearby, and Paul observes changes in the men's faces and behavior. They must rely on the earth to survive, as each man buries himself into the dirt during fighting. They frantically sought out holes and small valleys for protection. Paul sees this action as instinctive.... and the only way to save oneself.