Deep excavations

Basements, metro stations, underpasses and pump houses in cities need deep excavations next to existing buildings, roads and services. The excavation must be stable, the water kept out, and the ground movement small enough that neighbours are not damaged. Each of these, not just the first, controls the design.

Support systems range from soldier piles with timber lagging, through sheet piles and contiguous or secant bored piles, to diaphragm walls cast in slurry-supported trenches. The wall is held by struts across the excavation, by ground anchors tied back into the soil, or by the permanent floor slabs cast top-down as excavation proceeds. Diaphragm and secant walls also cut off groundwater; otherwise dewatering is needed, which can itself cause settlement of nearby buildings by raising effective stress.

Earth pressures on braced walls differ from those on free retaining walls because the struts prevent the wall from moving as a classical active wedge. Peck's apparent pressure envelopes, derived from measured strut loads, are used to size struts in preliminary design. In soft clay, base heave, the soil under the excavation squeezing upward, can be critical, and in sand with water below, hydraulic uplift and piping must be checked.

Modern practice combines numerical analysis with an observational method: inclinometers, settlement points, strut load cells and piezometers are read throughout construction against trigger levels, and the sequence is adjusted if movements exceed predictions.