Excavation wall failure may occur in a number of situations. In a recent report published by the National Safety Council, H. W. Hempel, Stearns-Roger Corporation, Denver, Colorado, noted some hazards to watch for in excavating operations. Here are his suggestions: An excavation or trench in moist sand may appear to be stable. However, drying due to evaporation or wetting due to rainfall causes rapid loss of cohesion. Therefore, excavation walls in sandy soil should be sheeted and shored or they should be sloped to the angle of repose, no steeper than 45 degrees, or 1 to 1. When dry, clay has high cohesive strength. When wet, it has a considerably lower cohesive strength. Therefore, the walls of excavations into clay to depths exceeding 4 feet should be sloped back not steeper than 1 to 3, or if cut vertical, they should be protected with upright planks and trench shores. An excavation wall with hard clay overlooking soft plastic clay can be very hazardous since the plastic material can squeeze out from beneath the hard clay, removing the vertical support from the upper block of soil. Therefore, whenever excavation discloses these conditions, the walls should either be sloped back about 1 to 1 or should be protected with uprights and shoring.