Formwork for excavation, timbering, RCC formwork

Formwork for excavation (timbering)

It is the temporary support of the sides of a trench or pit, to prevent the earth from falling in and to protect the workers. It is used when the depth exceeds about 1.5 m.

For firm soil:

• Stay bracing: vertical poling boards in pairs at 2-3 m intervals, held apart by cross struts, for depths up to about 2 m.

For loose soil:

• Box sheeting: closely placed vertical (or horizontal) boards held by walings and struts, forming a box, for depths up to about 4 m.
• Vertical sheeting: for deep trenches (up to about 10 m) in soft soil; done in stages of 2-3 m with an offset at each stage, each stage having its own sheeting, walings and struts.
• Runners: long boards driven ahead of the excavation in very soft, running soil.
• Sheet piling: timber or steel sheets driven to depth for wide or deep excavations in soft, wet soil.

   |poling|   strut   |poling|
   |board |==========|board |
   |      | waling   |      |

Formwork for RCC construction

Formwork is a temporary mould of timber, plywood or steel into which concrete is cast, and which is removed after the concrete has hardened.

Principal requirements:

• Strong and stable to carry the loads of wet concrete, workers and equipment without sagging.
• Rigid, and correct in shape, line and level.
• Tight joints, with no leakage of cement grout.
• Smooth surfaces, oiled for easy stripping.
• Quick and easy to erect and strip without damaging the concrete.
• Economical and re-usable.
• Supported on a firm base with sole plates and wedges.
• Struck at the proper time as per IS 456.