Earthworks and Groundworks Measurement

3 weeks ago

Ashley

11 minutes

Earthworks and Groundworks: What Is the Difference?

These two terms are often used interchangeably, but they describe different scopes of work. Earthworks is the broader term — it covers the large-scale excavation, movement, and placement of soil and rock. Road construction, land levelling, embankments, and cuttings all fall under earthworks. Groundworks is more specific to building construction and refers to the preparatory work carried out on a site before the superstructure begins — site clearance, excavation for foundations, below-ground drainage, and substructure generally.

For quantity surveyors, both categories are measured under the same NRM 2 work section, and the measurement principles overlap. What matters is understanding the types of excavation, filling, and temporary works involved — and being able to measure and price them accurately.

NRM 2 Measurement Rules

Under NRM 2, earthworks and groundworks fall within the Groundworks section of Part 3. The key work items include excavation (to reduced levels, trenches, pits, and basements), filling (imported and arising), disposal of excavated material, earthwork support, and ground remediation.

The fundamental units of measurement are cubic metres (m³) for excavation and filling, and square metres (m²) for earthwork support. All excavation items must state the depth, classified in one-metre increments — for example, excavation 0–1m deep, 1–2m deep, 2–3m deep, and so on. This depth classification is critical because costs escalate significantly with depth: deeper excavations require more substantial temporary works, may encounter groundwater, and demand specialist plant and safety measures.

A key change between the first and second editions of NRM 2 concerns earthwork support. In the first edition, support to excavation faces was deemed included in the excavation rate. In the second edition, earthwork support to all excavations exceeding 250mm in depth must be measured separately, in square metres of supported face area. This is an important distinction that affects how the QS prepares the bill of quantities.

Other deemed-included items under NRM 2 include removal of topsoil and vegetation, site clearance, and surface preparation. These are not measured separately but are considered part of the excavation item unless the specification states otherwise.

Types of Excavation

Reduced Level Excavation

This is surface excavation to bring an area of ground down to a uniform formation level. It is the most common type of bulk earthwork on building sites and is measured in cubic metres. The volume is simply the plan area multiplied by the average depth of cut.

Worked example: A site measuring 40m × 25m requires excavation to reduce the existing ground level by an average of 300mm. All material is to be disposed of off-site.

The in-situ excavation volume is 40 × 25 × 0.3 = 300 m³. However, once excavated, clay soil bulks by approximately 25%, so the loose volume for haulage and disposal is 300 × 1.25 = 375 m³. Converting to weight at a typical clay density of 1.8 tonnes per cubic metre gives 675 tonnes. At current inert waste rates — landfill tax of £4.05 per tonne plus gate fees of around £25 per tonne, plus haulage — the disposal cost alone exceeds £20,000. The excavation itself, at typical rates of £5–£8 per cubic metre, adds a further £1,500–£2,400. This example illustrates why disposal costs often dwarf the excavation cost and must be measured and priced carefully.

Trench Excavation

Trench excavation is measured for strip foundations, ground beams, and below-ground service runs. The volume is calculated from the trench length, width (including working space), and depth. Working space — the room needed for workers to construct formwork, lay concrete, and build masonry — is typically 600mm on each side for excavations up to 3m deep.

Worked example: A rectangular building (30m × 20m) with one internal load-bearing wall running east-west (30m) requires strip foundation trenches. The footing width is 600mm, the trench depth is 1.2m from reduced level, and the soil is clay.

Total trench length is the perimeter plus the internal wall: 2 × (30 + 20) + 30 = 130 linear metres. The actual trench width is the footing (600mm) plus working space on each side (600mm + 600mm) = 1,800mm. The excavation volume is 130 × 1.8 × 1.2 = 281 m³.

Earthwork support is measured as the area of trench face requiring support. For simplicity, taking both long faces of the trench: 2 × 130 × 1.2 = 312 m². After the foundations are cast, the trenches are backfilled with excavated material. Because the clay bulks on excavation (×1.25) and then compacts when returned (×0.80), the volume broadly recovers to approximately the original 281 m³.

In the bill of quantities, these would appear as three separate items: trench excavation (281 m³), earthwork support (312 m²), and backfilling with excavated material (281 m³). At typical rates — excavation £6–£10/m³, earthwork support £15–£25/m², and backfill £2–£4/m³ — the total cost for this element runs to several thousand pounds before the concrete is even poured.

Pit and Basement Excavation

Pit excavation covers discrete excavations for pad foundations, lift pits, machinery bases, and similar elements. Basement excavation covers large below-ground areas. Both are measured in cubic metres with depth classified in one-metre increments. Basement excavation in particular often requires significant earthwork support, groundwater control, and careful sequencing — all of which drive cost.

Filling and Disposal

Filling is measured in cubic metres and must state the material type, whether it is imported or arising from site excavation, and the compaction specification.

Worked example: Following the reduced level excavation above, the site requires a 500mm build-up of imported hardcore, compacted in 150mm layers to 90% Standard Proctor density.

The finished fill volume is 1,000 m² × 0.5m = 500 m³. Because compaction reduces the loose volume by approximately 10%, the contractor needs to import 500 ÷ 0.90 = 556 m³ of loose material. At a bulk density of 1.75 tonnes per cubic metre, that is 973 tonnes of hardcore. At a delivered price of around £35 per tonne, the material cost alone is approximately £34,000 — before labour and plant for spreading and compacting in four layers.

Disposal of surplus excavated material is one of the most significant cost items in groundworks. Current UK landfill tax rates are £4.05 per tonne for inert waste (uncontaminated soil and rock) and £126.15 per tonne for standard waste (mixed or contaminated material). Add gate fees of £20–£30 per tonne and haulage costs, and disposal can easily become the largest single cost element in a groundworks package. The QS must classify the material type accurately — the difference between inert and standard rates is over £120 per tonne — and should challenge assumptions about disposal volumes, particularly where material could be reused on site instead.

Earthwork Support

Earthwork support is the temporary provision — shoring, trench sheets and props, sheet piling, or other methods — used to prevent collapse of excavation faces. Under HSE regulations, all excavations exceeding a certain depth require either physical support or a safe system of work such as battering (sloping the excavation sides back to a safe angle).

Under NRM 2 (second edition), earthwork support is measured in square metres of supported face area. The item description should state the depth of excavation and soil conditions, allowing the contractor to select and price the appropriate support method. For the strip foundation example above, the 312 m² of earthwork support might be priced at £15–£25 per square metre depending on soil conditions and the contractor’s chosen method — timber trench sheets, hydraulic props, or steel trench boxes.

The choice of support method has implications beyond the earthwork support item itself. If battering is used instead of vertical support, the excavation volume increases (because the sides are sloped) while the earthwork support quantity reduces to zero. The QS needs to understand this trade-off when reviewing tenders and assessing the overall cost of the groundworks package. The temporary works coordination requirements under CDM 2015 also apply — a Temporary Works Coordinator should be appointed for significant earthwork support, and designs must be prepared and checked by competent persons.

Ground Conditions and Risk

Ground conditions are the single biggest source of uncertainty in earthworks measurement and pricing. What lies beneath the surface is never fully known until excavation begins, and unexpected conditions — rock, groundwater, contaminated land, or made ground — can transform the cost of a project.

Rock excavation is measured as an extra-over item to normal soil excavation. The cost of breaking out rock can be two to ten times the rate for soil excavation, depending on the rock type and method required (hydraulic breaker, drilling, or blasting). Because rock boundaries are rarely known precisely before work starts, rock excavation is often dealt with via provisional sums or remeasurement.

Groundwater adds cost through dewatering (pumping water from excavations) and the need for more robust earthwork support. Excavation below the groundwater table is measured as an extra-over item under NRM 2. Dewatering systems — sump pumps, wellpoints, or deep wells — are either measured as specific items or included as provisional sums. An Environment Agency abstraction licence is required for dewatering in England and Wales, adding both cost and programme risk.

Contaminated land requires specialist assessment and remediation. Excavation and disposal of contaminated material is measured separately, and the disposal cost is dramatically higher — standard landfill tax at £126.15 per tonne versus £4.05 for inert material. Contaminated land is almost always dealt with via provisional sums because the extent and nature of contamination is rarely certain at tender stage.

For all these ground risks, the QS plays a critical role in advising on the appropriate measurement and procurement approach. Where ground conditions are uncertain, a remeasurement contract may offer better value than a lump sum, because contractors will otherwise include substantial risk premiums in their pricing. Our guide to risk management tools and techniques explores how these uncertainties are identified, quantified, and managed through the project lifecycle.

Common Measurement Challenges

Bulking and shrinkage catch out inexperienced measurers. Excavated soil occupies more volume than it did in the ground (bulking), and compacted fill occupies less volume than the loose material delivered (shrinkage). Typical bulking factors range from 1.05 for sandy soil to 1.50 or more for hard rock. If these factors are not applied correctly, disposal quantities will be understated and fill import quantities will be insufficient — both leading to cost overruns.

Sloping and irregular sites make volume calculation more complex. Rather than a simple area-times-depth calculation, the QS may need to use grid survey methods (dividing the site into squares and averaging spot heights) or cross-section methods (calculating the area of cut at regular intervals along the slope). Modern survey tools — GPS, laser scanning, and drone photography — provide accurate topographic data, but the QS still needs to understand the principles behind the volume calculation.

Existing underground services complicate excavation. Water, gas, electricity, telecommunications, and drainage runs may cross the excavation area, requiring hand-digging near services, temporary diversions, and careful coordination. These costs are difficult to measure precisely and are often included in the contractor’s rates or dealt with as provisional sums.

Working space is easily overlooked. The excavation volume must include allowance for the working space needed to construct foundations, not just the structural footprint. As shown in the trench excavation example above, the working space (600mm on each side) can triple the trench width from 600mm to 1,800mm — trebling the excavation volume and cost. Preliminaries items such as temporary access roads, hardstandings for plant, and site establishment also interact with the earthworks scope and should be considered alongside the measured excavation items.

Getting Earthworks Right

Earthworks and groundworks are among the highest-risk elements of any construction project. The volumes are large, the ground conditions are uncertain, and the cost implications of getting the measurement wrong are significant. For the QS, this means reading the ground investigation report before measuring, understanding the NRM 2 rules for depth classification and earthwork support, applying bulking and compaction factors correctly, and pricing disposal at current landfill tax and gate fee rates.

The worked examples in this article illustrate the calculations involved. In practice, no two sites are identical, and professional judgement is always required — particularly around ground risk, provisional sums, and the choice between lump sum and remeasurement approaches. But the measurement principles are consistent, and mastering them is essential for any QS working on projects with significant below-ground work.