Case Study: Site Excavation for New Construction in Corvallis

Project Overview

In spring 2025, Cojo Excavation & Asphalt was contracted to perform complete site preparation for a new 18,000-square-foot commercial building in Corvallis, Oregon. The 2.3-acre parcel on the south side of town had been vacant farmland for decades, and the developer needed it transformed into a construction-ready building pad with all underground utilities in place.

This case study walks through the project from start to finish, including the challenges we encountered with Corvallis's notoriously heavy clay soils and how we kept the project on schedule and on budget.

Site Conditions

The Property

The parcel was flat former agricultural land typical of the southern Willamette Valley. Vegetation consisted of grass, blackberry, and a few small trees along the property edges. No structures existed on site.

Key site characteristics:

• 2.3 acres total, with a 1.4-acre building pad and 0.9 acres of parking and landscape
• Silty clay soil (CL classification) to a depth of 8+ feet per the geotechnical report
• Seasonal high water table at 4 feet below grade
• Minimal slope — existing grade varied less than 2 feet across the entire site
• City of Corvallis jurisdiction with standard commercial permitting

The Challenge

The geotechnical report identified three issues that would drive our approach:

1. Moisture content: Soil moisture was at 28%, just below the 30% threshold where the geotech said the clay would become unworkable. Spring rains could push it over at any time.

2. Unbalanced earthwork: The design required raising the building pad 18 inches above existing grade for drainage (a common requirement in flat areas with high water tables). This meant the cut areas would not generate enough fill, requiring either import of material or creative site balancing.

3. Compaction specification: The structural engineer specified 95% compaction for the building pad area. Achieving 95% compaction in clay soil at elevated moisture content requires careful moisture management — often the most time-consuming part of clay earthwork.

Our Approach

Phase 1: Clearing and Stripping (Days 1-2)

We mobilized a D5 dozer and a 320 excavator to strip vegetation and topsoil from the work area. Topsoil was stockpiled in the northeast corner of the site for later use in landscape areas.

Approximately 1,800 cubic yards of topsoil was stripped to an average depth of 8 inches. This organic layer had to be completely removed before any structural fill could be placed.

Phase 2: Mass Grading (Days 3-10)

This was the critical phase. The grading plan called for:

• Cut: 2,800 cubic yards (lowering parking areas and creating drainage swales)
• Fill: 4,000 cubic yards (raising the building pad and transition areas)
• Net import needed: 1,200 cubic yards

Rather than importing expensive structural fill, we worked with the civil engineer to adjust the parking lot grade slightly, reducing the fill deficit to 800 cubic yards. The remaining import was 3/4-minus crushed rock for the upper 12 inches of the building pad — a better structural material than the native clay regardless of the earthwork balance.

The moisture challenge: We were racing the weather. Our plan was to excavate cut material, spread it in 8-inch lifts, and compact it before it could absorb additional moisture. On day 6, we got 1.2 inches of rain overnight. The exposed clay absorbed it like a sponge, and we lost two full days waiting for the surface to dry enough to resume compaction.

To prevent further delays, we adjusted our approach:

• We worked in smaller sections, completing each area fully before moving to the next
• We used a sheepsfoot roller for initial compaction (better penetration in wet clay) followed by a smooth drum for final passes
• We covered completed sections with plastic sheeting overnight to prevent additional moisture absorption

Phase 3: Utility Trenching (Days 11-15)

With the building pad at rough grade, we trenched for all underground utilities:

• Sanitary sewer: 380 linear feet of 8-inch PVC at 2% grade to the city main in the adjacent street
• Storm drainage: 450 linear feet of 12-inch PVC with 3 catch basins and connection to the city storm system
• Water service: 120 linear feet of 2-inch copper from the city main to the building pad
• Power conduit: 200 linear feet of 4-inch PVC conduit from the transformer pad to the building
• Communications conduit: 200 linear feet of 2-inch conduit alongside the power run

The clay soil made trenching challenging — walls stayed vertical (a benefit of cohesive soil) but the trench bottoms accumulated water from the high water table. We used a 2-inch trash pump running continuously during pipe installation to keep trench bottoms dry enough for proper bedding compaction.

All utility trenches were backfilled with imported 3/4-minus crushed rock in the pipe zone (12 inches around the pipe) and native material above. Each lift was compacted and tested.

Phase 4: Fine Grading and Erosion Control (Days 16-18)

With utilities in and backfilled, we performed finish grading across the entire site using our GPS-equipped D4 dozer. The GPS system was essential here — the parking lot grade required a consistent 2% cross-slope for drainage, which is difficult to achieve by eye over a large area.

Final activities included:

• Fine grading the building pad to within 0.1 foot of design elevation
• Grading parking and drive areas to design cross-slopes
• Installing silt fence along the south and east property lines
• Placing erosion control blanket on the detention basin side slopes
• Stabilizing the construction entrance with 6 inches of crushed rock

Results

Schedule Performance

• Planned duration: 15 working days
• Actual duration: 18 working days (2 weather days + 1 additional compaction day)
• Total calendar time: 26 days from mobilization to final inspection

The 3-day overrun was entirely weather-related. Our production rates on working days actually exceeded the plan because the GPS grading eliminated rework on the fine grading phase.

Quality Results

Compaction testing by the geotechnical engineer confirmed:

• Building pad: 95-98% compaction across all test points (specification: 95%)
• Utility trench backfill: 92-96% compaction (specification: 90%)
• Parking area subgrade: 93-97% compaction (specification: 92%)

No test failures. No rework required.

The civil engineer's as-built survey confirmed all grades within 0.1 foot of design, and all utility inverts within 0.05 foot of plan elevation.

Budget Performance

The original contract amount was $158,000. Two change orders were executed:

1. Additional export hauling for unsuitable clay removed from the sanitary sewer trench (+$4,200)
2. Extra crushed rock for soft spots in the parking subgrade identified during proof rolling (+$3,100)

Final contract value: $165,300. The 4.6% cost overrun was within the owner's contingency budget and reflected genuine unforeseen conditions, not scope changes or errors.

Lessons Learned

What Went Well

• Early mobilization: Starting as early in spring as soil conditions allowed gave us a weather buffer that we needed
• GPS grading: Eliminated fine grading rework entirely, saving an estimated 2 days
• Proactive moisture management: Covering completed areas with plastic sheeting after the rain event prevented further moisture damage
• Good communication: Daily updates to the developer and weekly coordination meetings with the civil engineer kept everyone aligned

What We Would Do Differently

• More aggressive moisture monitoring: We could have started checking soil moisture content daily earlier in the project to better predict workability windows
• Pre-order import material: The crushed rock import took 3 days to schedule delivery. Having it pre-staged would have saved a half-day of idle time

Why This Project Represents Typical Corvallis Conditions

If you are planning construction in Corvallis or the southern Willamette Valley, expect similar conditions:

• Heavy clay soils that require careful moisture management
• High water tables that affect trenching and excavation
• Flat terrain that requires raising building pads for drainage
• Cooperative city permitting — Corvallis has a well-organized building department
• Limited dry-season window — plan earthwork for June through October when possible

View more of our commercial projects or check our service area coverage.

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• Excavation Cost Estimator: Site Prep Pricing for Oregon Projects
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• Willamette Valley Soil Types and What They Mean for Your Project