External works take-off: roads, drainage and earthworks from the site plan

This guide follows French practice: French standards (NF DTU), French regulations and French bill conventions. Most sources are in French.

External works items and their units

An external works package (roads and utilities, “VRD” in France) combines three families, earthworks, roads and drainage or utilities, often joined by landscaping. In a remeasurement contract, common in civil engineering, the rates are given by a schedule of rates (BPU) and the quantities by an estimated bill of quantities (DQE); in a lump-sum contract, by a priced bill (DPGF). Either way, the unit and what each rate includes are read in the specification (CCTP) and the bill.

In France, the rules of execution for public civil engineering contracts are gathered in the booklets of the general technical specifications (CCTG): booklet 2 for general earthworks, 27 for asphalt, 31 for kerbs and channels, 70 part I for gravity drainage pipes. Like the building standards, they say how to carry out the work; the method of measurement comes from the contract.

Reading the site plan and the sections

The external works site plan gives the footprints: roads, car parks, footways, islands, green areas, with the line of the kerbs. The drainage plan gives the route of each pipe, its diameter, the numbered manholes, the cover and invert levels from which the depths follow. The long section and the cross sections give the earthworks heights.

These drawings are at small scales, often 1:200 or 1:500: a scale error is costly, hence the value of checking it against a known dimension (see the scale of a drawing). Also identify the phases: firm phase, optional phases, temporary and final roads are priced separately.

Earthworks: simple volumes and their limits

A “simple” volume, area × thickness, is suitable when the thickness is known and roughly constant: topsoil stripping, capping layer, sub-base, local dig-out of soft spots. It is also the method used in exam papers for lowering a platform: stripped area × average thickness.

As soon as the natural ground is irregular, an average thickness is no longer enough. Cut and fill are then calculated from cross sections (average area of two neighbouring sections × the distance between them), or as the difference between a model of the existing ground and the design, in earthworks software.

Two volumes must never be mixed: the in-situ volume, measured in the ground, and the bulked volume, larger, the one the lorries carry. The bulking factor depends on the soil and comes from the investigation or the tender documents: a vocational exam paper listed in the sources gives 1.35. The DQE states which of the two volumes it pays for.

Pavements, asphalt and kerbs

A pavement is measured layer by layer, over its footprint: capping layer, sub-base, surface course. Islands, green areas and large structures are deducted from that footprint, according to the contract. Asphalt is paid per m² for a given thickness, or per tonne: tonnage = area × thickness × density. The density depends on the asphalt mix: take the figure from the contract or the supplier, never one “from habit”.

Kerbs are measured per linear metre, one item per model (T2, A2, P1, CC1 channel…), following their actual line, curves included. Dropped kerbs, small-radius curved kerbs and corner units are often counted separately. The perimeter of an asphalt area is a good basis, provided the entrances and joints without kerbs are taken out.

Drainage, manholes and trenches

Each network has its item: foul water (EU), surface water (EP), drinking water, power and telecom ducts; and within each network, one item per material and per diameter. The length is taken on plan, along the route; the contract says whether it runs from manhole centre to manhole centre or between their walls. The fall of a drain, a few per cent, barely changes the length: at 3%, √(1 + 0.03²) = 1.00045, less than half a thousandth.

Depth, on the other hand, matters. Manholes are counted by type and often by depth band, from the cover and invert levels. The trench volume is length × width × average depth. Booklet 70 part I gives a minimum trench width according to the outside diameter of the pipe and the depth, and manhole excavations at least equal to the outside dimension of the structure plus 50 cm on each side. Take the depth below formation level if the pavement is measured elsewhere: otherwise the capping layer is paid twice.

Worked example

A car park to be built on flat ground, with no excavation other than the stripping: asphalt footprint of 46.00 × 28.00 m, two planted islands of 16.00 × 2.00 m, a 6.00 m entrance without kerbs, and a 300 mm surface water sewer under the central aisle, with four manholes down to the existing network. Thicknesses, asphalt density, bulking factor and trench dimensions are taken for the example.

Earthworks and pavement

Kerbs and drainage

Forgetting the islands adds 64.00 m² of asphalt, i.e. 64.00 × 0.06 × 2.35 = 9.02 t. Measuring the trench from the finished surface, 0.46 m higher (asphalt and capping layer), would add 45.50 × 0.80 × 0.46 = 16.74 m³, already paid with the pavement.

The most common mistakes

• In-situ and bulked volumes mixed together, or a bulking factor taken “from habit”.
• An average thickness applied to sloping ground, instead of cross sections.
• Islands, green areas or buildings left in the asphalt area.
• Kerbs counted along the whole perimeter, entrances and joints included.
• Pipes of different diameters or materials combined on one line.
• Trenches measured from the finished surface when the pavement is paid elsewhere.
• Phases mixed together: it then becomes impossible to price the optional phase on its own.

Doing it in Kotelys

Kotelys measures on the PDF site plan, or a scanned one, at its scale. Each item is a type, with its colour and its total.

• Asphalt, pavement layers and green areas are drawn with the Area tool, islands as deductions (S key). With a thickness given to the type, the volume is written out in full: “1,224.00 m² × 0.40 m = 489.60 m³”. The asphalt tonnage is then worked out in your estimate.
• Kerbs and pipes are drawn with the Length tool, one type per model or per diameter; text repeated along the line (“EP Ø 300”) keeps them readable on the exported drawing. The perimeter of each area, written in the take-off calculations and in Excel, serves as a check for the kerbs.
• Manholes, gullies, street lights and parking spaces are counted with the Count tool.
• One zone per phase or per island, and the reports break down every quantity. The same site plan can carry the site layout plan.

Kotelys does not read long sections or cross sections and does not calculate earthworks volumes from a ground model: trench depths, cut and fill between existing ground and design, and bulking stay in your estimate.

Further reading: external works take-off in Kotelys, and the other guides.

Sources

1. Official bulletin of the French Ministry for Ecological Transition, list of the CCTG works booklets (annex to the order of 7 October 2021)
2. Official bulletin of the French Ministry for Ecological Transition, CCTG booklet 70-I « Fourniture, pose et réhabilitation de canalisations d'eaux à écoulement à surface libre » (version 4.01, May 2021)
3. Éduscol STI, vocational baccalaureate in public works 2020, U22 paper: earthworks volume from cross sections
4. Éduscol STI, vocational baccalaureate in public works 2018, U23 paper: bulked volume of excavated material (factor 1.35)

Thresholds and values described as “taken for the example” are not rules: those of your contract (specification, bill of quantities) always prevail.