Formwork and timbering for trench excavation

Timbering (formwork) of trenches is the temporary support of the sides of an excavation with timber or steel, to prevent the earth from collapsing and to protect the workers. It is needed when the trench is deeper than about 1.5 m or the soil is loose, wet or surcharged.

Timbering in firm soil

• Stay bracing: vertical boards (poling boards, about 200 x 40-50 mm) are placed in pairs on opposite sides of the trench at about 2-3 m intervals and held apart by one or two horizontal struts. Used in firm soil for depths up to about 2 m.

Timbering in loose soil

• Box sheeting: vertical (or horizontal) sheeting boards are placed close together and held by horizontal walings and cross struts, forming a box. Used in loose soil for depths up to about 4 m.
• Vertical sheeting: used for deeper trenches (up to about 10 m) in soft soil. The trench is dug in stages of 2-3 m, each stage having its own closely placed vertical boards, walings and struts, with an offset of 250-500 mm at each stage.
• Runners: long, thick boards are driven about 300 mm ahead of the excavation in very soft or running soil.
• Sheet piling: timber or steel sheet piles are driven for wide or deep excavations in soft, wet soil.

   ground level
  |<--- trench width --->|
  |poling   strut   poling|
  |board  ====o====  board|
  |   |  waling      |    |
  |   |========o=====|    |
  |   |              |    |
  |___|______________|____|

Components: poling boards, struts (props, adjustable with wedges), walings, runners, and cleats. Precautions: inspect daily, keep spoil heaps at least 1 m from the edge, fix struts tightly, and provide ladders and drainage.