Cut, Fill, and the Contractor's Balance Number
A 4.6-acre office site is ready to bid. The contractor's average-end-area takeoff from a 20-ft grid shows 2,400 cy of cut and 1,900 cy of compacted fill. Six inches of topsoil must be stripped from the 1.5-acre building and pavement area and stockpiled for respreading; the geotechnical report classifies that material as unsuitable for structural fill. Proctor testing shows the on-site silty clay shrinks 12 percent when placed and compacted to 95 percent of standard Proctor. There is no room on site to waste soil: import runs $28/cy delivered, and export runs $34/cy plus $1,200 per haul cycle. The proposed finished floor is 2.0 ft above existing grade at the rear property line, where runoff presently sheet-flows onto a neighboring parking lot. The contractor wants to start stripping on Monday and is asking you for a balance number.
1. Does the site balance? Show the calculation and state the net import or export volume, including any assumption you must make about the takeoff.
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The takeoff was measured to subgrade, so the 6-in strip is inside the 2,400 cy of cut and must be subtracted before any of that material can go to fill.
- Topsoil: 1.5 ac x 43,560 = 65,340 sf x 0.5 ft = 32,670 cf / 27 = 1,210 cy bank.
- Structural cut available: 2,400 - 1,210 = 1,190 cy.
- Fill demand in bank measure: 1,900 / (1 - 0.12) = 2,159 cy, because 12 percent of the excavated volume is lost to compaction.
- Net: 1,190 - 2,159 = -969 cy, so roughly 970 cy of import (about 1,100 cy loose).
Two cautions before anything is ordered. A 20-ft grid is typically plus or minus 10 percent, which is about 240 cy on this takeoff, so no import decision should rest on a single number. And the 1,210 cy of stockpiled topsoil will not fully respread if it is handled twice; plan on 10 percent loss and a small topsoil import. Ask for a supplemental cross-section check through the building pad before Monday.
2. The contractor proposes lowering the building pad 1.0 ft and steepening the rear slope from 3:1 to 2:1 to close the deficit. Evaluate the proposal and recommend a course of action.
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Quantify first. A 25,000 sf footprint lowered 1.0 ft reduces fill demand by 25,000 cf / 27, about 926 cy, which nearly cancels the 969 cy deficit, but only if the finished floor can drop. Check three things before approving it: finished floor at least 1 ft above the seasonal high water table and above any flood protection elevation; positive gravity flow from the pad to the storm system; and the resulting grade at the rear line. Dropping 1 ft also cuts the differential to the neighbor from 2.0 ft to 1.0 ft, which helps the off-site drainage complaint.
The 2:1 slope is the weaker half of the idea. At a 2-ft differential it saves only 2 ft of horizontal width compared with 3:1, it is at the practical limit for safe mowing, and in silty clay it needs permanent reinforcement and a flatter shoulder. Recommendation: authorize stripping and stockpiling, keep 3:1 or 4:1 at the property line, take the volume from the pad if the floor elevation checks out, and hold the import order until cross-sections confirm the grid.