Excavation, Earthworks and Ground Treatment Measurement
Earthworks questions test classification, whether a volume is gross or net, and where disposal and filling belong. Excavation is one of the few sections where the measured quantity (the void) and the material actually carted away (bulked) differ, and the paper is designed to catch that.
Classification
- Excavation is classified by purpose - to reduce levels, for foundations, for basements, for services - and by material: topsoil, common material, hard material, rock.
- Depth bands commonly run 0–2 m, 2–4 m and 4–6 m. Work is measured in the band of actual depth, not an average of the trench.
- Surface stripping of topsoil is measured in m² or m³ according to the method, with average depth stated.
Working space, filling and disposal
- Working space for formwork or masonry is measured where the method allows, typically 0.300 m each side, and must be shown in the dimensions rather than absorbed silently into a rate.
- Disposal is either off site or on site, with lead stated. Excavated material bulks by roughly 20–25% for common material; the bill measures the void and the rate carries the bulking.
- Filling to excavations is measured as volume of fill in place, so concrete and masonry occupying the void are deducted from the fill, not from the disposal.
Ground treatment
- Grouting, vibro-compaction and dynamic compaction are measured in m² of treated area with depth or spacing stated; dewatering is an item or sits in the preliminaries.
Worked example
A strip footing trench is 20.000 m long, concrete 0.600 m wide and 0.300 m thick, trench 1.200 m deep, working space 0.300 m each side. Excavation width = 0.600 + 2 × 0.300 = 1.200 m; volume = 20.000 × 1.200 × 1.200 = 28.800 m³. Concrete = 20.000 × 0.600 × 0.300 = 3.600 m³. Return, fill and ram = 28.800 − 3.600 = 25.200 m³. Disposal off site = 28.800 m³, with bulking carried in the rate. Disposal follows the excavated volume, not the fill volume, and the fill is net of the concrete.
Traps that cost marks
- Adding working space on one side only.
- Deducting the concrete from disposal as well as from the fill.
- Measuring a topsoil strip in m³ when the method requires m².
- Ignoring depth bands and pricing all trench as shallow.
- Forgetting that hard material and rock are separate items with their own rates.