Paying an Earthworks Progress Claim From Aerial Quantities

Monthly earthworks claims are one of the places where an aerial survey earns its cost in a single flight. The contractor has moved material, the owner or consultant has to certify a quantity, and the two parties are usually working from different records. A dated capture of the whole site gives both of them the same record, which removes most of the argument before it starts.

What it does not remove is the need to agree on how the number is calculated. Most disputes over aerial pay quantities are not about accuracy. They are about which surface the volume was measured above, which day the period ended, and whether the contract pays on material as it sat in the ground or as it sits in the fill.

Why the claim is a good fit for aerial capture

A progress claim has three awkward properties. It is periodic, it covers a large area, and it is contested often enough that the record matters later. Aerial capture suits all three:

• The whole site is captured in one pass, so a claim does not depend on which parts a crew had time to walk.
• The capture is dated and repeatable, so the same method produces this month's number and next month's.
• The raw imagery stays available, so a quantity questioned in six months can be recomputed rather than re-argued from memory.
• Both parties can look at the same orthomosaic and surface, which is the part that de-escalates most disagreements.

The cases this comes up on most are borrow pit depletion, bulk cut and fill, placed and compacted lifts on a road or pad, topsoil and overburden stripping, and material imported to or exported from a site.

The base surface is the decision that sets the number

A volume is never a property of a pile or a pit on its own. It is the space between two surfaces, and the lower one has to come from somewhere. On an earthworks claim it is usually one of these:

• The original ground surface captured before work started, which is the cleanest option and also the one most often missing.
• Last month's capture, which gives the period quantity directly and is what most monthly claims should be measured against.
• The design surface, which answers a different question. It tells you what is left to do, not what was done this month.
• A flat plane or a toe-fitted surface, which is sometimes the only option on a pit with no pre-disturbance record and is the weakest of the four.

Two competent survey crews measuring the same month above different base surfaces will produce different numbers, and both will be right about what they measured. If the contract does not say which base surface governs, that gets settled on the first claim or it gets settled in a dispute.

The practical consequence is that the baseline flight matters more than any later one. A site flown before the first scraper arrives has a defensible original ground surface for the life of the job. A site first flown in month three has an original ground surface that is partly a reconstruction, and every quantity measured from it inherits that.

Period cutoffs, because a flight is an instant

A claim period is a range of dates. A flight is a moment. That mismatch is harmless if it is handled deliberately and expensive if it is not.

The workable arrangement is that the period runs capture to capture. Material moved after this month's flight belongs to next month's claim, whether or not the contractor's own records put it in this one. That is easy to agree in principle and it creates two practical obligations:

• The flight has to be scheduled close to the period end and held to it, because a flight slipped by a week moves money between periods.
• Weather has to have a written fallback. An Alberta site can lose a planned capture date to wind, cloud ceiling or snow, and the fallback has to say whether the period extends or a partial claim is certified against the contractor's own records and trued up next month.

On the last claim the cutoff matters more than on any other, because there is no following period to absorb the difference.

Measure the pit and the fill

Where material comes out of a borrow area and goes into a fill on the same site, there are two independent measurements available of the same work. Taking both is worth the small additional effort.

They will not agree, and the difference is the useful part. The gap between material removed and material placed is accounted for by a short list of real causes:

• Compaction, which is the largest term and is a known property of the material rather than a measurement error.
• Material that left the site, was wasted, or was placed somewhere not included in the fill measurement.
• Stripping and unsuitable material removed from the fill area, which reduces the fill without reducing the pit.
• Stockpiled material in transit, sitting between the two at the moment of capture.

If the gap can be explained by that list, both numbers are credible. If it cannot, something is wrong with a boundary, a base surface or an assumption, and finding out which one in month two is much cheaper than finding out in month nine.

Bank, loose and compacted

The same material occupies different volumes in the ground, in a truck and compacted in a fill. Earthworks contracts pay on one of those states and the choice changes the quantity substantially.

A survey measures geometry. It can measure the pit, which gives a bank volume, and it can measure the fill, which gives a compacted volume in place. Converting between them requires a density or shrinkage factor, and that factor is a material property established by testing rather than something a drone can observe.

So the division of labour is straightforward. The survey reports measured geometry for each area with the base surface and the boundary stated. The conversion factor and which state the pay item uses come from the contract and the geotechnical record. A survey report that applies a shrinkage factor silently has buried a contract assumption inside a measurement, and that is the kind of thing that unwinds a claim later.

The quantity has to be large against the uncertainty

This is the test that decides whether aerial quantities can carry a claim at all. Every surface has uncertainty, and comparing two surfaces carries both. On a large monthly movement the uncertainty is a small fraction of the quantity and the method adjudicates comfortably. On a small movement across a large area it may not.

The check is worth running before the arrangement is agreed rather than after a contested claim. If a typical month's quantity is not comfortably larger than the uncertainty of the comparison, the options are to tighten the survey specification, to measure a smaller and better controlled area, to lengthen the claim period so each quantity is larger, or to use a different method for that pay item.

Being able to state the uncertainty is part of the deliverable. An achieved accuracy demonstrated against independent checkpoints is auditable. A nominal accuracy quoted from a specification sheet is not.

The pay items that look identical from the air

A surface comparison sees ground moving. It does not see pay items. Several things that are paid differently look the same to a volume calculation:

• Topsoil stripping and bulk excavation, which are usually separate items and are separated by boundary and by sequence rather than by appearance.
• Unsuitable material removed and replaced, where the same area went down and then came back up between captures and a single monthly comparison may show almost nothing.
• Imported material, which increases a fill without any corresponding pit depletion and needs a haul record to confirm.
• Stockpiles that are intermediate rather than final, which are real volumes that are not yet a completed pay item.

The fix is boundaries. Each pay area gets a boundary file, quantities are reported per boundary rather than as a site total, and areas that went down and came back up inside one period get captured more often than monthly if the money justifies it.

What the survey settles and what it does not

The survey settles measured geometry: a volume above a stated base surface, inside a stated boundary, on a stated date, with a stated uncertainty. That is a fact about the ground and it is the thing worth buying.

It does not settle whether the work conforms to the specification, whether a lift was compacted to the required density, whether material classified as unsuitable genuinely was, or whether the measured quantity satisfies the pay item as written. Those are contract administration and engineering decisions that rely on test results, inspection records and professional judgement.

Keeping that line clear is what makes the measurement useful to both parties. A quantity that everybody agrees was measured correctly leaves only the contract question to settle, which is the question the parties are actually equipped to settle.

Setting it up so it works from claim one

Four things settled in writing before the first flight prevent nearly every argument that follows:

• The base surface for each pay area, and who holds the original ground surface.
• The capture date relative to the period end, and the weather fallback.
• The pay state for each item, bank or compacted, and where the conversion factor comes from.
• The boundary file for each pay area, and the coordinate system and vertical datum everything is reported in.

None of those are survey decisions. They are contract decisions that a survey depends on, and a vendor who asks for them up front is doing the job properly.