Excavations may be unsupported or may require some kind of supporting structure that is designed to prevent the sides of the excavation from collapse. The steel sections are driven or vibrated into the ground before excavation, each section interlocking with the adjacent pile. Piles are drilled and concreted before excavation and after excavation form the wall of the excavation. Two concrete guide walls are constructed at the top of the trench to guide the clamshell bucket that is used to excavate the soil. There are two boundary conditions: the total head is equal to the elevation head, and the amount of water released from the pores of the soil as the water falls is equal to the flow across the free surface. The location of the free surface is found from the value of total head at the nodes of the element that is cut by the free surface.