Cut And Fill Balance On A Development Site

You balance cut and fill by comparing the soil you have to dig out of a site with the soil you need to put back, and the aim is to move as little as possible across the boundary. Your finished levels set that balance, so a small change to the platform level can turn a site that balances into a long line of trucks.

Earthworks look like the simple part of a job. They are also the part where a level set without a calculation behind it can quietly blow the budget before a slab has been poured.

How the volumes are worked out

We build two surfaces. One is the existing ground, taken from the detail survey, and the other is the design surface, with the building pads, driveways, hardstand and batters at their finished levels and the pavement thickness taken off.

Comparing those two surfaces across a grid gives the cut and fill volumes for each part of the site. Then the arithmetic gets humbling. On a 10,000 square metre warehouse pad, moving the whole platform by 100 millimetres changes the volume by 1,000 cubic metres, and that is one small decision on one level.

Bulking and shrinkage

A cubic metre in the ground doesn't stay a cubic metre. Soil loosens when it is dug and takes up more room on a truck, which is called bulking. Rock bulks even more once it is ripped or hammered.

The same soil then packs down tighter than it started when it is compacted as fill, and that is shrinkage. A balance that ignores both will look even on the drawing and leave you short or long on site. So we run the numbers with factors drawn from the geotechnical report, rather than treating every cubic metre as the same.

Material that doesn't count

A balance on volume alone can still fail, because not all cut can go back as fill.

Topsoil. It is stripped and set aside, and it has no place under a slab or a pavement.

Wet or highly reactive clay. It can need drying or conditioning before it compacts properly, and on a tight site there is nowhere to lay it out.

Old fill and rubble. Existing fill of unknown quality has to be tested, and much of it ends up leaving the site.

Rock. Hard sandstone needs crushing before it can be placed in layers, so it costs more to reuse than it first appears.

On paper a site can balance perfectly, then end up carting out clay and importing select fill anyway. That is why the volumes and the investigation need reading together, and the geotechnical report has to reach the civil engineer before the levels are set.

What sets the levels

The finished levels have more than one master. Stormwater has to fall to its outlet, and a site that [falls away from the street](/blog/stormwater-for-sites-that-fall-away-from-the-street) can force the whole platform up or down to get there. Driveways have to meet the council's boundary level within their allowed grades, which is the subject of [driveway grades and basement ramps](/blog/driveway-grades-and-basement-ramps).

Retaining walls cost money too, so a platform that balances the soil but needs a long, high wall on the boundary hasn't saved anything. The right level is the one that works across all of these at once, and that takes a few runs of the model rather than a single answer.

Pads split across cut and fill

A pad that sits half on natural ground and half on deep fill has a problem of its own, even when the volumes are perfect. The fill side keeps settling for longer than the cut side, and that difference shows up as [differential settlement](/blog/differential-settlement-and-what-causes-it) in the slab and walls.

Where a building has to straddle that line, we look at the depth of fill, the supervision level and whether the footings need to go through it. Fill that will carry a structure is placed and tested to AS 3798, as described in [controlled fill and compaction testing](/blog/controlled-fill-and-compaction-testing).

Moving soil off site

Soil leaving a site needs a waste classification, and it goes to a place licensed to take it. Clean natural soil can often be reused on another project as virgin excavated natural material, while contaminated material goes to landfill at a much higher price.

So the test results on the spoil belong in the earthworks estimate. A truck movement also brings traffic management, wheel wash and the erosion and sediment controls that keep the street clean, and none of that shows up in a volume.

Basements and bulk excavation

A basement site doesn't balance, and nobody expects it to. The soil and rock come out, and most of it leaves.

The questions change to how fast it can go, where the trucks queue, and what the spoil is classified as. Those belong to the bulk excavation methodology, and they get decided with the shoring.

How we approach it

We run the earthworks model alongside the stormwater and the pavement design, so a level that changes for one reason gets checked for all of them. When the site is an industrial estate or a large pad, we run a few platform levels side by side and show the client the volume each one moves.

Every level on the drawing is really an instruction to a truck.

ACSES provides civil engineering, including earthworks and site grading, across Sydney. Talk to us about a project.

Building Relationships Beyond Structures