Site preparation

Earthworks

Excavations

The movement of masses of soil is an activity that is difficult to predict, the details of which are normally planned by civil engineers and geologists. Using the results of a soil survey (boreholes), the anticipated quantity of material to be excavated and the strength of the subsoil can be determined. Afterwards, a decision can be made regarding the best type of foundation for the structure.

The earthworks contractor initially removes the uppermost layer of topsoil and vegetation (approx. 30 cm) with a tractor shovel and retains some of this material on site. Afterwards, the actual excavation work begins in stages. If there is room on the site or in the immediate vicinity, excavated material (spoil) is retained for backfilling at a later date because the transport of spoil is expensive and should be avoided wherever possible.

Working with the excavation plant (excavator, tractor shovel, etc.) is a skilled job; the operators have to work to an accuracy of a few centimetres

Once the required depth has been achieved, the base of the excavation is covered with a blinding layer of l ean concrete (grade PC 150, approx. 5 cm). The lean concrete provides a clean base on which to mark out underground services or the foundations. However, on rocky ground the layer of blinding may not be necessary.

The excavation should generally be about 60 cm larger than the outline of the building all round; 60 cm provides an adequate working space for the contractor. The angle of the sloping sides to the excavation (and if necessary stabilising measures) depends on the properties of the soil. The angle must also be chosen to rule out slippage or collapse and hence guarantee the safety of persons working in the excavation. Depending on the weather conditions and the hydrostatic pressure (slope run-off water or groundwater), any water must be drained away according to the regulations.

Fig. 40: Excavation with sloping sides

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