Lateral stresses which existed on the excavation wall are removed as the excavation is done. The soil in the excavation wall immediately begins to move, however slowly, into the excavation. At the same time the surface of the ground next to the excavation subsides, creating an unnatural situation. The surface of the ground is in tension and some of the weight of the soil in the excavation wall is transferred to the soil back away from the wall face by a phenomenon called shear. The combination of tension in the ground surface and shear stress causes cracks to form back from the edge of the excavation.
Cracks occur from the edge of the excavation.
Cracks occur from about to of the depth of the excavation back from its edge. For example, if an excavation 10 feet deep is dug the cracks may be found somewhere between three to seven feet back from the excavation edge. There may be several cracks. They are usually vertical and they may be the depth of the excavation. When cracks develop, the weight of the soil in the excavation wall is no longer partly carried by the soil back from the excavation's face.