Chainage-based measurement for road contractors

A building is measured by floor, grid and member. A road is measured by chainage: the distance along the centre line from a fixed starting point. Every quantity on a road RA bill, from embankment to the last bituminous course, belongs to a stretch between two chainages. Get the chainage record right and the bill almost writes itself. Get it wrong and the engineer cannot tell which stretch was paid for.

Reading a chainage

Chainage is written as kilometres plus metres: km 3+200 means 3 kilometres and 200 metres from the start. A stretch from km 3+200 to km 3+450 is 250 m long. Where an alignment has been revised, the project may carry a chainage equation that ties old chainages to new ones; record which system each measurement uses, or quantities will overlap or leave gaps.

Earthwork by cross-section

Embankment and cutting are measured from cross-sections taken at fixed intervals along the road, with levels recorded before and after the work. The area of each cross-section is worked out from those levels, and the volume between two sections is found by the average end area method: the mean of the two areas, times the distance between them.

The arithmetic: (18.40 plus 21.60) divided by 2 is 20.00 sqm, times 25 m is 500.00 cum. (21.60 plus 19.80) divided by 2 is 20.70 sqm, times 25 m is 517.50 cum. Some contracts specify the prismoidal formula instead, which gives a slightly different figure on uneven ground. Use the method your contract names.

The levels behind each area are the evidence. Initial levels should be taken jointly with the engineer before work starts, because they cannot be recreated once fill is placed.

Pavement layers

Pavement is built in layers, and each layer is a separate BOQ item: granular sub-base (GSB), wet mix macadam (WMM), then bituminous courses such as dense bituminous macadam (DBM) and bituminous concrete (BC). Each layer is measured for the stretch where it is complete, as length times width times compacted thickness, unless the contract pays by weight.

For WMM laid from km 3+200 to km 3+450, 7.50 m wide and 0.250 m compacted thickness: 250 times 7.50 times 0.250 is 468.75 cum. Where a bituminous course is paid by the tonne, the volume is converted using the density the contract specifies or the density established by testing, and that basis is printed on the measurement sheet.

Width is not constant

Carriageway width changes at curves with widening, at junctions, at bus bays and at transitions. A measurement that applies one width to a whole kilometre will be cut. Record width at each cross-section, or split the stretch where the width changes, and compute each part separately.

The strip plan

A strip plan is a diagram of the road along its length, with one band for each layer and each side of the road. Completed stretches are shaded by layer, with their chainages. It is the fastest way for an engineer to see what is being billed and what was billed before.

• One band per layer: subgrade, GSB, WMM, DBM, BC, and shoulders where they are separate items.
• Left and right sides shown separately on divided roads or where work proceeds one side at a time.
• Structures such as culverts and minor bridges shown as gaps, so the pavement quantity over them is not double counted.
• Each bill's new stretches marked in a different shade from earlier bills.

Checks engineers make

• Layer sequence. A stretch billed in WMM must have GSB below it billed already. A stretch billed in BC needs DBM below.
• No overlaps. Stretches billed on two bills must not overlap. Chainage ranges make this easy to check, if they are recorded exactly.
• Test results. Each layer stretch has its density and thickness tests passed before it is billed.
• Structure gaps. Pavement quantity excludes the length over culverts and bridges where the contract measures them separately.
• Cumulative length. For each layer, cumulative billed length cannot exceed the length of road in the work order without a deviation approval.

Recording at site

Road sites are long, and the person measuring is often kilometres from the office and from network coverage. Measurements should carry their chainage range, side, layer, width and thickness, a photo with location, and the date of the test that passed the layer. A register that holds only quantities, without chainages, leaves the billing engineer guessing which stretch they belong to.

How the bill reads

On a road RA bill, each layer item in the abstract should refer to a measurement sheet that lists its stretches by chainage, side, width, thickness and quantity, with the bill on which each stretch was first claimed. The engineer then checks this bill's new stretches against the strip plan, and the cumulative columns against the previous certificate, in one sitting. A layer item whose quantity cannot be traced to chainage ranges is the first one cut.

How Tatha Vastu measures roads

In the measurement book, road items are recorded by chainage range, side and layer, offline, with the location and photo attached. Overlapping ranges and out-of-sequence layers are flagged before approval. Cross-section areas can be imported from survey spreadsheets. The road and highway page shows the layer-wise bill, and the measurement guides cover granular sub-base, wet mix macadam and bituminous courses.