Excavation & Site Prep: The Hidden Foundation of Every Concrete Job

Why the dirt is the project

Eight out of ten concrete failures we are called to assess trace back to subgrade, not the slab itself. The concrete sitting on top is doing exactly what concrete does, it cracks, settles, or heaves because the dirt under it moved. Curing matters; subgrade matters more.

Site prep is where good contractors quietly spend 30-40% of project budget and bad contractors cut every corner they can find.

Step 1: Strip and cut to grade

Topsoil and any organic material (roots, mulch, decomposed wood) must be stripped completely and hauled off site. Organics decompose, leave voids, and guarantee future settling. The cut depth depends on slab thickness, base depth, and existing grade, typically 8 to 14 inches below finish.

On sites with existing pavement or structures, strip-and-cut overlaps with selective demolition.

Step 2: Evaluate the native subgrade

Bedrock crews probe the cut subgrade with a 1/2-inch steel rod and observe the resistance. Soft spots get over-excavated and replaced with engineered fill. On larger commercial jobs, a geotechnical engineer specifies a proof-roll test with a loaded dump truck, soft areas pump under the load and are flagged for remediation.

Expansive clay soils get a capillary break (4-6 inches of clean stone) and sometimes lime stabilization. Wet sites get under-slab drains piped to daylight or a dry well.

Step 3: Base course

The base is the structural cushion between native soil and the slab. Standard Bedrock spec: 4 inches of compacted 3/4-inch crushed stone (also called CA-6 or Class 5) over a non-woven geotextile fabric. For driveways and patios, this is non-negotiable.

For RV pads and shop slabs, double the base depth to 8 inches and add a 6-mil polyethylene vapor barrier directly under the slab.

Step 4: Compaction is not optional

Crushed stone must be compacted in 2-inch lifts using a vibrating plate compactor (residential) or a ride-on roller (commercial). Target density is 95% of standard Proctor, verifiable with a nuclear density gauge on commercial jobs.

Skipping compaction is the single most common shortcut in residential concrete. The slab pours fine, looks great for a year, then settles 3/4 inch in one corner the second winter and cracks diagonally across.

Step 5: Drainage and slope

Every flatwork project needs a positive slope away from the house, minimum 1/4 inch per linear foot. Sites that drain toward the house need French drains, channel drains, or curbed swales installed before the pour.

Steep sites pair flatwork with retaining walls to create stable, drainable benches.

Cost reality

Excavation and prep on a typical 600 sq ft driveway: $1,500-$3,500. On a 2,000 sq ft commercial slab with proof-roll and engineered fill: $8,000-$25,000. When a contractor quotes flatwork without a separate site-prep line item, they have either bundled it (fine) or skipped it (catastrophic).

Frequently asked questions