Footings, pile caps and retaining walls

Abstract

Introduction For a traditional building or bridge structure, the vertical forces in the walls, columns and piers are carried to the subsoil through a foundation system. The most common system consists of footings. In soft soil, because the bearing capacity is low, piles are needed to transfer the forces from the superstructure to deeper grounds where stiffer clay, sand layers or bed rock exist. The wall or column forces are each distributed to the piles or group of piles through a footing-like cap – a pile cap. In civil and structural engineering, slopes often need be to cut to provide level grounds for construction. To ensure stability at and around the cuts or to meet similar requirements, the use of retaining walls for the disturbed soil and backfill is sometimes necessary. The design of reinforced concrete footings and pile caps is generally governed by shear or transverse shear for wall footings, and transverse or punching shear for column footings and pile caps. Retaining walls behave like a cantilever system, resisting the horizontal pressures exerted by the disturbed soil or backfill (or both) by bending action. The analysis of the forces acting above and below typical wall footings and their design are presented in Section 12.2. The treatments for footings supporting single and multiple columns are given in Section 12.3 whereas Section 12.4 deals with pile caps. Illustrative and design examples are given to highlight the application of the analysis and design procedures. The horizontal soil pressure analysis and the design of retaining walls are discussed in Section 12.5, which also includes illustrative and design examples. Wall footings General remarks There are two main types of wall loading to be carried by the footing – the concentric loading as illustrated in Figure 12.2(1) and the eccentric loading as shown in Figure 12.2(2). Concentric loading leads to uniform pressure on the subsoil, which normally requires a symmetrical footing – see Figure 12.2(1). On the other hand, eccentric loading, depending on the ratio of M * and N *, produces two possible pressure distributions: • the entire footing sustains pressure from the subsoil – see Figures 12.2(2)b(i) and b(ii) • only part of the footing sustains the subsoil pressure – see Figure 12.2(2)b(iii).In the above discussion, the self-weight of the footing is included in the loading.