Planking and Strutting

In case the depth of trench in soft/loose soil exceeds 2 metres then we should resort to stepping, sloping and/or planking and strutting of sides.  In case of loose and slushy soils the depth at which these precautions are to be taken shall be determined by the Engineer in Charge according to nature of soil. 

Planking and strutting shall be 'close' or 'open'  depending on the nature of soil and the depth of trench. The type of planking and strutting shall be determined by the Engineer-in-Charge. Guidance can be taken from IS:3764-1966 for designing the shoring and strutting arrangement. 

i)  Close Planking and Strutting 

Close planking and strutting shall be done by completely covering the sides of the trench usually with short upright boards generally of section 250 mm x 38 mm Figure.

The boards shall generally be placed in position vertically in pairs, one on either side of cutting. These shall be kept apart by horizontal wallings of strong wood at a maximum spacing of 1.2 metres cross strutted with ballies. 

Where the soil is very soft and loose, the boards shall be placed horizontally against the sides of the excavation and supported by vertical wallings which shall be strutted to similar timber pieces on the opposite face of the trench. The lowest boards supporting the sides shall be taken in the ground for a minimum depth of 75mm. No position of the vertical side of the trench shall remain exposed. 

The withdrawal of the timber members shall be done very carefully to prevent collapse of the trench. It shall be started at one end and proceeded systematically to the other end. Concrete or masonry shall not be damaged while removing the planks.

Close Planking & Strutting with Vertical Poling Board 

ii) Open Planking and Strutting 

In case of open planking and strutting the entire surface of the side of the trench is not required to be covered. The vertical boards generally 250 mm wide and 38 mm thick shall be spaced sufficiently apart to leave unsupported strips of 50 cm average width. The detailed arrangement, size of timber and the distance apart shall depend on the nature of the soil. In all other respects, specifications for close planking and strutting shall apply to open planking and strutting. 

Open Planking and Strutting