Figure 13.3 Preparation for construction of pier footings by placement of reinforcing (сооружение фундаментов опор с установкой металлической арматуры)

Bridge abutments and piers must be secure at the foundations in order to support the superstructure. The term “foundation” includes some elements placed in and on the ground to provide support. The methods of foundation construction may be vary to some extent and depend on geological conditions, ground strength, pier types and sizes, loads from the spans and piers. In case of especially weak or swampy soil, the workers drive wooden or steel piles into the ground to support the foundations. Then they cast concrete footings, wait for sufficient concrete hardening, and begin building piers and abutments (fig. 13.3).

Bridge foundations include two large groups: shallow foundations (fig. 13.1d) and deep foundations (fig. 13.1e, f, and g). Shallow foundations are usually monolithic structures on the natural bed. This technology is rather cheap and simple, but it is applicable for solid rock only (fig. 13.1d). The construction of shallow foundations begins with trench excavation. Rather deep trenches, excavated in a dry place for the abutments or piers of the flyovers, viaducts or trestle bridges, require a watertight enclosure or a sheet piling made out of metal bars. It is also necessary if a trench is excavated in a river (fig. 13.1d; 13.3a) as the bed must be pumped dry to permit the pier construction. A pile driver drives a sheet piling to the predetermined depth. Horizontal framing components constructed of heavy timbers, steel, or a combination of the two, hold the vertical piles in place. The structure must be able to resist the horizontal forces from the surrounding body of water when the inside water is pumped out. Then concrete is placed against the casing. The workers spread and compact it. The most difficult work is to pour the concrete for the foundations constructed in the river. Despite working water pumps, some water enters the trench, and the builders have to inject additional grouting mortar (fig. 13.1d) to consolidate the soil in the water.