Balancing Cut and Fill vs Importing Material

Frequently asked questions

Why doesn't a balanced drawing balance on site?

Because three different volumes are involved and only one of them is on the drawing. BANK volume is the material as it sits in the ground, which is what a cut-and-fill calculation from levels produces. LOOSE volume is what it becomes once excavated, when it has bulked — broken out of its in-situ packing it occupies more, which is what lorries and stockpiles are sized on. COMPACTED volume is what it occupies after being placed in layers and rolled to a specified density, which is generally less than the bank volume it came from. So a cut of a given bank volume does not fill an equal fill volume: the placed material occupies less, and the site is short. The factors depend on the soil, so they come from the site investigation, and the calculation is done in the right units at each stage rather than once.

What makes material unsuitable as fill?

Several things, any one of which is enough. Moisture content outside the range in which it can be compacted to the specified density is the most common — too wet is the usual case, and drying it means spreading and turning it and waiting for weather. High plasticity clays are difficult to compact consistently and change volume with moisture, so they are restricted or excluded in structural fill. Organic material decomposes and settles, so topsoil and anything with significant organic content is excluded from engineered fill entirely, though it has value as landscaping material. Oversize particles interfere with layer compaction. And contamination changes the question from suitability to legality. The specification defines acceptability in testable terms, and testing during placement is what confirms it rather than appearance.

Can wet material be used?

Sometimes, at a cost in time and weather-dependence that has to be in the programme rather than discovered. Conditioning wet material means spreading it in thin layers and turning it so it dries, which works in dry weather and does not work in wet, so an earthworks programme relying on it is exposed to a run of bad weather that no amount of plant fixes. Lime or cement stabilisation is the engineered alternative, chemically modifying the material so it can be compacted and gains strength — effective, well established, and a specialist operation with its own testing, safety requirements and design. The decision is economic: conditioning and stabilisation against the cost of exporting the wet material and importing suitable fill, with the programme risk weighted properly, since a stalled earthworks operation delays everything behind it.

How is compaction actually controlled?

By specifying a layer thickness, a compaction plant and number of passes, or a target density, and then testing what was achieved. The density approach compares the achieved dry density against a laboratory maximum for that material — a percentage of the Proctor value — with the material also within a specified moisture range, since the same compactive effort gives different results at different moisture contents. The method approach specifies the plant and passes instead and is used where testing every layer is impractical. Two things decide whether it works on site: layer thickness, because a roller compacts effectively only to a limited depth and a layer placed too thick is loose at the bottom regardless of passes; and moisture, because material outside its range cannot reach the target density however many passes it gets.

What does exporting material involve beyond haulage?

Classification, documentation and a destination that will accept it — and in most jurisdictions excavated material leaving a site is regulated as waste, with a duty of care on the producer that does not end when the lorry leaves. That means characterising the material, which for a site with any history of industrial use means testing for contamination; identifying a permitted receiving site; using a registered carrier; and keeping the transfer documentation. Cost scales sharply with classification: clean inert material is comparatively cheap to dispose of, and contaminated material can be an order of magnitude more per tonne. Several regimes also provide routes for material to be used rather than disposed of — on the same site or another — under a formal procedure, which is worth investigating early because it changes the economics substantially.

How does haul distance affect the decision?

It decides whether moving material is cheap or expensive, and the relationship is not linear because the constraint changes. Over short distances within a site, an excavator and dumpers move material efficiently and the cost per cubic metre is low. Beyond a certain distance the operation becomes haulage-limited rather than excavation-limited — the plant waits for lorries rather than the other way round — and the cost per cubic metre rises with cycle time, which depends on distance, the state of the haul road, gradients, and any queuing at either end. That is what a haul cycle calculation quantifies, and it is what tells you how many lorries are needed to keep the excavator working. It is also why improving a haul road often costs less than adding lorries.

What about topsoil?

It is stripped first, stored separately, and returned last, and it is not part of the cut-and-fill balance at all. Topsoil is valuable for landscaping and is unusable as engineered fill because its organic content decomposes and settles, so mixing it into structural fill spoils both. The storage matters more than people expect: topsoil heaped too deep and left for a long period loses its structure and its biology as the lower part goes anaerobic, so stockpiles are limited in height and ideally seeded if they will stand over a season. Stripping also has to happen before site traffic compacts the ground beneath it, since compacted subsoil under a landscaped area is what produces the struggling lawn and the waterlogged bed two years later.

When should this be decided?

At site investigation, which is before the earthworks are priced and long before anything moves — because the answer depends on information only the investigation provides. What is needed is the depth and nature of the strata, the moisture contents, classification and compaction characteristics of the materials that will be cut, the groundwater level, and any contamination. With those, the cut-and-fill balance can be done in the right volumes with the right factors and the suitability question can be answered honestly. Without them, the balance is a drawing exercise, and the usual outcome is discovering during the earthworks that the material is wet, or unsuitable, or that there is less of it than the levels implied — at which point the remedy is haulage, at a price nobody allowed for, on a programme that has already started.