What causes landfill settlement to skew your volume surveys

You fly the same cell in March, then again in April. The scale house numbers match what you expected to place that month. But the surface model says you lost more airspace than the tonnage accounts for. Nobody hauled waste back out. What you're looking at is settlement, and if you don't separate it from actual consumption, your monthly number stops telling you anything useful.

Settlement vs. compaction: two different things that look identical on a surface model

Compaction happens at the working face, in real time, as the compactor runs its passes over each lift. It's intentional. You're already building it into your in-place density assumptions, and it's why a freshly placed lift reads lower than the loose waste that came off the truck.

Settlement is different. It happens over weeks, months, sometimes years, as buried waste decomposes, voids collapse, and moisture redistributes through the mass. A photogrammetric surface flown once a month captures both of these as the same thing: elevation drop. The model has no way to tell you which one caused it. A cell that's three years into decomposition can show a meaningful surface drop with zero new airspace consumed, and if that drop gets folded into your fill-rate calculation, you'll think you have more remaining capacity than you do, or less, depending on which way the error runs.

Differential settlement in a waste cell isn't even across the surface

This is where it gets harder to eyeball. Settlement doesn't happen uniformly. Waste composition varies lift to lift and cell to cell. Older refuse near the base settles differently than fresh fill at the working face. Leachate collection trenches, variable daily cover thickness, and uneven subgrade slope all create localized dishing or waviness in the surface over time.

The practical effect: two adjacent flyovers can show a patch of the cell appearing to "gain" airspace while another patch "loses" it, and neither reading has anything to do with waste placed or waste consumed that month. If you're comparing raw cut-and-fill between two flights without accounting for where the cell sits in its decomposition timeline, you're going to chase numbers that don't reconcile with the scale house.

Reading the monthly number without getting fooled by it

A single survey-to-survey comparison conflates settlement with placement. There isn't a clean way to strip one out of the other from a single flyover pair. What works better is tracking the trend across several consecutive flights, flown the same way each time, so a cell's settlement curve becomes visible as a pattern rather than noise. Older, capped sections of the landfill settle on a slower, more predictable curve than the active face. Once you know what that curve looks like for a given cell, a flyover that deviates from it is worth a second look, either a placement error, a cover problem, or a slope that's moving faster than it should.

None of this replaces checking the number against the scale house tons for the month. That cross-check is still the fastest way to catch when a volume change is really a settlement artifact and not consumed airspace.

What makes this workable month over month is measuring the same cell against its own last survey at a consistent resolution and flight method, so every comparison runs off the same baseline and the settlement pattern shows up as a pattern instead of getting buried in survey-to-survey noise. If your permit review is coming up and you want that remaining airspace figure to hold up to scrutiny, that's the problem Landfill Volume is built to solve.