Earthwork support

Earthwork support is measurable to all faces of excavation except the following.

• Faces not exceeding 250 mm high.
• Sloping faces where the angle of inclination is less than 45° to the horizontal.
• Faces which abut existing structures. 

In SMM7 measurement is classed in depth stages and in distances be opposing faces. It appears that the intention is to give gross areas of support where the total depth of excavation falls within any depth stage.

Up to 4 m between opposing faces it its possible to span across with struts of various kinds, e.g. timbers wedged in or proprietary telescopic struts of steel. Over 4 m it is usual to set up raking supports to the lower level of the excavation. although if this is below water level it may be better to use steel work for the shoring, and this will add considerably to the cost.

Other special conditions must be given in the bill. i.e. where the face is curved. below water level (there does not appear to be any provision for support partly below water level). next to roadways. next to existing buildings or in unstable ground. and finally where the support is left in place. The estimator is left to decide how to actually achieve the support and he will take into account the following additional considerations: 

• The most suitable material must be chosen. for example: solid timbers for waling, poling hoards or strutting; steel sheet. plywood, etc. for support; proprietor) struts spacing of all members considering the soil type:, proprietary close sheeting system.
• The number of uses must be estimated, i.e. the number of times each component can be reused before being discarded. Figures are quoted from 5 to 15 uses.
• Men working in the excavations require protective clothing.
• Outputs for excavation round timbering are generally halved.
• The proportion of various classes of excavation below the water table must be considered.
• Pumping. although given separately in the bill. must be realistically assessed.