Residential Subdivision Grading in Coastal Oregon

Case Study 1: Residential Subdivision Grading in Coastal Oregon

Scenario

A 12-hectare residential subdivision near Newport, Oregon, required site grading to accommodate 42 single-family lots on a steep, forested slope (average 12% gradient) with sensitive riparian buffers along Salmon Creek. Constraints included strict erosion control requirements (Oregon DEQ), a maximum allowable cut depth of 3.5 m to preserve shallow bedrock, and a mandatory 1:1 cut-to-fill balance to minimize off-site soil hauling due to limited local disposal capacity.

Given Data

• Contour data: LiDAR-derived DEM (0.5 m vertical accuracy), provided as GeoTIFF file OR_NW_SalmonCreek_DEM.tif
• Grid size: 5 m (selected for higher fidelity in gullied terrain)
• Cut-off level: +12.8 m NAVD88 (engineered datum aligned with proposed street centerline elevation)

Calculation

Using the Earthwork Volume Estimator:

1. The tool rasterized the contour data into a regular grid at 5 m spacing (2,400 cells).
2. For each cell, elevation was interpolated bilinearly from the DEM.
3. Cut volume per cell = max(0, elevation − 12.8) × 5 × 5
4. Fill volume per cell = max(0, 12.8 − elevation) × 5 × 5
5. Aggregated across all cells:
   • Total cut volume = 18,642.37 m³
   • Total fill volume = 17,915.81 m³
   • Net volume change = +726.56 m³ (net cut surplus)

Result and Decision

The net surplus of 727 m³ exceeded the project’s 500 m³ haul limit. To comply with environmental constraints and avoid off-site disposal, the design team revised the cut-off level to +12.6 m (lowering by 0.2 m), reducing cut volume by 1,120 m³ and increasing fill by 980 m³. Recalculation yielded total cut = 17,522 m³, fill = 18,495 m³, and net change = −973 m³ — enabling full on-site balancing using imported engineered fill from an approved quarry within 10 km.

Lesson

Small adjustments to the cut-off level (±0.2–0.5 m) can significantly shift earthwork balance—especially on undulating terrain—and are often more cost-effective than modifying horizontal alignment or adding haul logistics.