timbering

Timbering is the use of timber to provide support for ground during excavation, tunneling, and other earth-retaining operations. It is also used to describe timber-based formwork and temporary supports in construction. The practice has historical roots in mining and timber-framed building techniques and remains in use where timber is readily available or where flexible, reversible support is advantageous.

In underground or trench works, timbering typically consists of vertical timbers (posts), horizontal members (beams or caps), and infill panels (lagging). Common arrangements include cribbing, where timbers are stacked to form a box-like support, and lined timbering with lagging boards pressed against the ground, supported by cross-braces (struts) and wales to spread loads. The goal is to resist rock and soil pressures while allowing for future retreat, dewatering, or inspection.

In construction, timbering also refers to temporary supports used during excavation and to formwork for concrete pours. Modern practice often supplements or replaces timber with steel sets, precast members, or shotcrete, but timber remains in use in certain geotechnical applications or where timber is economical and conditions are suitable.

Safety considerations include decay, moisture, fire risk, and the potential for sudden failure if supports are not properly installed or maintained. Timbering requires regular inspection, timely replacement of damaged timbers, and alignment with design specifications and local codes.