15.1 Grading, Cut/Fill, and Earthwork Balancing

Grading as a WRE Sitework Problem

The Principles and Practice of Engineering (PE) Civil: Water Resources and Environmental (WRE) exam is an 80-question, computer-based test administered by the National Council of Examiners for Engineering and Surveying (NCEES) in a 9-hour appointment (about 8 hours of working time plus a scheduled break). The April 2024 specification lists Project Sitework as one of the larger topic groups, covering excavation, embankment, grading, cut and fill, site layout, erosion control, adjacent facilities, safety, retaining walls, curves, and construction methods.

There is no published raw passing score; NCEES converts your score to a scaled value compared against a fixed ability standard.

For WRE candidates, grading is rarely a pure dirt-moving question. Finished grades decide where runoff goes, whether inlets collect water, how much cover a storm pipe has, and whether disturbed slopes can be stabilized before the next storm.

Grading is the process of shaping existing ground into a proposed surface. Cut is material removed from above proposed grade; fill is material placed below proposed grade. A balanced site is one where usable cut, after conversions and losses, approximately equals required fill. Raw cut and fill numbers on a plan sheet being equal does NOT mean the site balances.

Earthwork Volume States

Use the volume state named in the problem. Two factors drive conversions: swell (loose volume exceeds bank because of added voids) and shrinkage (compacted volume is less than bank because field compaction is denser than natural ground). If excavated material swells 20 percent, then 1,000 BCY becomes 1,200 LCY for hauling. If borrow shrinks 12 percent from bank to compacted condition, 1,000 BCY produces 880 CCY of compacted embankment.

A useful identity: a load factor converts bank to loose (LCY = BCY x [1 + swell]) while a shrinkage factor converts bank to compacted (CCY = BCY x [1 - shrinkage]). Mixing the two, or applying shrinkage as a markup, is the single most common earthwork error.

Earthwork Balancing Workflow

1. Compute raw cut and fill. Use the method implied by the data: average end area (V = L x [A1 + A2] / 2) for roadway-style sections, the grid (borrow-pit) method for site pads, or contour-area planimetry for rough grading.
2. Separate usable and unusable material. Topsoil, muck, organics, debris, and wet unsuitable material may be stripped, wasted, or stockpiled rather than placed as structural fill.
3. Convert to the needed condition. Compare compacted fill demand to compacted yield from on-site cut. Do not compare BCY directly to CCY unless the problem states they are equal.
4. Account for special quantities. Trench bedding, pipe displacement, pavement aggregate, overexcavation, and retaining-wall backfill all shift the balance.
5. Check grading intent. Verify positive drainage, reasonable side slopes, inlet low points, emergency overflow routes, and minimum utility cover.
6. Decide borrow, waste, or haul. A mass diagram or cumulative cut-fill table shows whether material can move forward economically or whether off-site borrow or a disposal area is needed. On a mass diagram, rising segments are cut, falling segments are fill, and a horizontal balance line sets the free-haul and overhaul limits.

Worked Earthwork Example

A detention-basin site needs 9,600 CCY of compacted fill. The grading plan shows 8,000 BCY of suitable on-site cut with 10 percent shrinkage from bank to compacted condition.

Available compacted fill = 8,000 x (1 - 0.10) = 7,200 CCY

Borrow still needed = 9,600 - 7,200 = 2,400 CCY

If the borrow pit material shrinks 12 percent from bank to compacted, required borrow = 2,400 / 0.88 = 2,727 BCY. The classic PE trap is computing 2,400 x 1.12, which treats shrinkage as a markup instead of recognizing that compacted volume is smaller than bank volume, so you must divide by (1 - shrinkage), not multiply by (1 + shrinkage).

Grading Checks That Matter for Drainage

For WRE, the finished surface must move water intentionally:

• High points divide drainage areas and define where sheet flow separates.
• Low points should coincide with inlets, swales, culverts, or a defined overflow path; a low point with no outlet is a trapped basin.
• Minimum slopes for positive drainage are typically about 0.5 percent on turf and 1 percent on pavement; flat paved areas may need careful spot elevations because a small rim error creates ponding.
• Maximum slopes are limited by erosion and mowing/maintenance; vegetated slopes steeper than about 3H:1V often require armoring or turf reinforcement.

Field compaction is the other half of the answer. Fill placed wet of optimum moisture will not reach target density no matter how many roller passes are added; the practical response is moisture conditioning, drying, removal and replacement, or geotechnical direction. The standard Proctor test sets the maximum dry density and optimum moisture content used as the field acceptance target (often 90 to 95 percent of maximum dry density for structural fill).

On the exam, choose the answer that preserves design intent, verifies quantities in the correct volume state, and respects both drainage and compaction constraints rather than only minimizing haul cost.