Excavation: general remarks - Construction Technology

The rules of measurement for SMM7 Section D2 have been drafted with machine excavation in mind as the principal method. Any excavation which the estimator judges can be carried out only or better by hand must be priced within the constraints of these rules. The implications of this for both the estimator and the person preparing the bill of quantities are discussed later in the chapter.

Excavation is measured in eight classes or types of excavation:

1. Top soil for preservation;
2. To reduce levels;
3. Basements and the like;
4. Pits (give total number);
5. Trenches up to 300 mm wide;
6. Trenches over 300 mm wide;
7. For pile caps and ground beams between caps;
8. Benching sloping ground to receive filling.

For the first class, state the average depth. For the remaining classes, give the maximum depth in stages: up to 250 mm deep; up to 1 m deep; up to 2 m deep; and so on in stages of 2 m. In addition, a starting level for the excavation should be provided where the particular class of excavation commences beyond a depth of 250 mm below existing ground level.

The sub classification into depth stages with a given starting level is a blanket requirement of SMM7.



The class 'To reduce levels' has been retained from SMM6 and is assumed to be the same, i.e. the excavation takes place above the starting level. The blanket rule that a starting level is stated when the excavation starts beyond 250 mm below the ground level obviates any unnecessary provision of a starting level for reduce level.

The last class, 'benching sloping ground to receive filling', relates to the practice of excavating a permanent unsupported side slope down which there are horizontal steps which, when filled over with imported material, prevent that material slumping to the bottom of the excavation. Again the necessity to state starting levels and give depth stages can be questioned. If the excavation is a 'hole in the ground' with permanent side slopes stepped and filled over, then there is a starting level and a depth to consider. However, the most common occurrence of this type of work is in large cuttings such as those encountered in motorway construction, where the excavation is entirely above the starting level and this last is the original ground level!

The previous two paragraphs have been critical of SMM7. Whether these criticisms are justified or not, they serve to illustrate that a quantity surveyor will interpret these rules and present the bill for pricing in the firm belief that his item descriptions and all the other

supporting documentation really do represent the work properly. The estimator must be certain that he is pricing the intended work. 

There is no requirement in SMM7 to state in the bills of quantities whether or not the work is to be done by machine or by hand. However, the Measurement Code published with SMM7 does state that 'Information provided in accompanying drawings or in descriptions should identify those circumstances where it may be difficult or impractical to carry out excavation by mechanical means'. The final judgement must be made solely by the estimator. To enable him to make it, the bill should clearly state whether the work is inside an existing building, under water (and whether this is river, spring or tidal) or in compressed air (SMM7, General Rule 7). A study of the drawings and the preambles will elicit further information, as will a visit to the site. However, there are further considerations which are known to the estimator and depend on his experience and the technical capabilities of his company. These are all discussed below.