To provide adequate resistance against sliding, the base of the wall should be at least 3 ft below ground surface in front and below the depth of frost action, depth of seasonal volume change, and depth of scour. Sliding stability should be adequate without including passive pressure at the toe. If insufficient sliding resistance is available, the designer may increase base width, provide a pile foundation, or lower the base of the wall and consider passive resistance below frost depth. If the wall is supported on rock or very stiff clay, a key may be installed below the foundation to provide additional resistance to sliding. Considerations of the need for the 3-ft depth when dealing with Reinforced Earth walls should be evaluated in that such walls are not as susceptible to frost action as more rigid concrete walls. In any event, it is recommended that some nominal depth below ground line be provided to accommodate changes in natural terrain over the anticipated life of the structure, often 75 to 100 years. Such changes occur as a result of normal soil erosion caused by wind, rainfall, and other natural processes. Of course, in situations where scour may occur, hydrologic and hydraulic evaluations of scour depth must be made.
8.4.2 Settlement and Overturning
For walls on relatively incompressible foundations, apply the overturning criteria of Fig. 8.21. If the foundation is compressible, compute settlement by available methods previously referred to and estimate tilt of a rigid wall from the settlement. If the consequent tilt is anticipated to exceed acceptable limits, proportion the wall to keep the resultant force at the middle third of the base. If a wall settles so that the resulting movement forces it into the soil it supports, then the lateral pressure on the active side increases substantially. Table 8.3 shows the magnitudes of wall rotation required to mobilize active and passive earth pressures for different types of soil.
