Cut and fill

Cut and fill is the earthwork needed to turn the existing ground into the finished grade on the grading plan. Cut is soil removed where the ground is too high; fill is soil placed where it is too low. Both are measured in cubic yards.

How it works

Every earthwork quantity comes from comparing two surfaces:

• Existing grade (EG), from the survey: existing contours and spot elevations.
• Finished or proposed grade (FG), from the grading plan: proposed contours and spots, adjusted down to subgrade under pavement and slabs.

Where FG is below EG, the difference is cut. Where FG is above EG, it is fill. Cut is measured in bank yards and fill in compacted yards, so the two cannot be compared directly. Once shrink is applied, put both in the same state: more cut than the fill needs means extra soil to haul off (export); less means soil to bring in (import).

Ways to calculate it

• Average end area: cut and fill areas measured on cross sections, averaged between sections and multiplied by the distance between them. Common on roads and linear work.
• Grid method: the site is divided into squares, the cut or fill depth is found at each corner, and each square's volume is its area times its average depth.
• Surface to surface: both grades are modelled as TIN surfaces and the volume between them is calculated directly. This is how most earthwork software works.

Example

Say two cross sections are 50 ft apart, with a cut area of 120 SF at the first and 80 SF at the second (example values):

1. Average area: (120 SF + 80 SF) / 2 = 100 SF
2. Volume: 100 SF x 50 ft = 5,000 CF
3. Cubic yards: 5,000 CF / 27 = 185.2 CY of cut

That is bank cubic yards, soil as it sits in the ground.

Cut and fill are not equal yards

A yard of cut does not make a yard of fill. Soil swells when it is dug and shrinks when it is compacted:

• Use the swell factor to size trucks and haul.
• Use the shrink factor to work out how much cut it takes to make the compacted fill.

A site that looks balanced in bank yards can still need import once shrink is applied. Topsoil stripping, undercut and subgrade depth under pavement change the balance too, so take them off before you compare.

In the estimate

Earthwork is usually priced in Division 31 as separate lines for cut, fill, export or import, topsoil stripping and subgrade preparation. In IntelCost's Earthwork tab, you build existing and finished grade from contours and spot elevations on the plans, draw the work boundary, and Calculate gives cut, fill and net, with the lines filed under Division 31 in the estimate. For a full walk-through, read cut and fill explained.