Compact Soil and Prepare a Stable Subgrade

Job Overview

This project qualifies you to prepare a stable subgrade within the footing excavations produced in Project 05, using approved fill material, controlled lift thickness, moisture conditioning, and compaction equipment matched to the soil type and work area. The finished work product is a subgrade that meets the project's specified elevation, material, and compaction requirements; that is free of organic material, debris, frozen soil, mud, and standing water; that has been verified by density, proof-roll, penetration, or another required method; and that is fully documented and ready for the aggregate base or foundation work that follows.

You will review the required subgrade elevation, material specification, compaction requirement, and test method; inspect the exposed excavation for unsuitable material; place approved fill in controlled lifts; moisture-condition each lift within the permitted range; compact each lift with equipment suited to the soil and work area; check elevation and surface condition at each lift; verify compaction; correct failed areas; shape the final lift to the required line, grade, and drainage condition; protect the finished subgrade; and document every material, lift, test result, and correction.

This is the sixth project in the site preparation sequence. It begins with the inspected footing excavations produced in Project 05, and it does not authorize disturbing the bearing surface, the excavation dimensions, or the Project 03 layout controls preserved through Project 05. The verified subgrade produced here becomes the controlling surface for the gravel base and vapor barrier installed in Project 07.

Skills You Will Demonstrate

• SP-031 — Identify suitable fill material and unsuitable soil conditions before placing fill in the Project 05 footing excavation.
• SP-032 — Place approved fill in controlled lifts at or below the specified loose thickness.
• SP-033 — Compact each lift using equipment suited to the soil type and work area, following a consistent pass pattern.
• SP-034 — Verify compacted density, elevation, and surface condition at each lift against the project specification.
• SP-035 — Prepare the bearing surface by correcting failed areas and shaping the final lift to the required line, grade, and drainage condition.
• SP-036 — Inspect and document the finished subgrade for readiness before aggregate base or foundation work begins.

Where This Skill Fits

Project 05 produced footing excavations matching the approved plan, with a clean, undisturbed bearing surface, an inspected condition, and a completed excavation record. That excavation, and the benchmark and dimensions it preserves, control this project. Fill placement and compaction in this project proceed only where the approved plan requires fill; where the Project 05 bearing surface already meets the required elevation, this project's inspection and documentation steps still apply, but no fill is placed.

The verified subgrade produced here becomes the working surface for the aggregate base and vapor barrier installed in Project 07 and, for slab construction, the surface built upon in Project 11. A compaction deficiency or elevation error missed here is not caught again until the pre-inspection review in Project 16, by which time it is concealed under aggregate, vapor barrier, or concrete.

Scope and Intended Use

This project covers:

• reviewing the required subgrade elevation, material specification, compaction requirement, drainage condition, and test method before fill is staged
• inspecting the exposed excavation for organic material, debris, frozen soil, mud, pumping, soft spots, and standing water
• removing unsuitable material to the approved limits and obtaining approval before filling overexcavated or weak areas
• inspecting imported or reused fill for compliance with the project specification
• establishing grade stakes or elevation references that remain visible during placement
• placing fill in lifts at or below the specified loose thickness and conditioning moisture within the permitted range before compaction
• compacting each lift using equipment suited to the soil type and work area, following a consistent pass pattern that covers edges, corners, and confined areas
• checking compacted elevation and surface condition, and performing or arranging the required density, proof-roll, or penetration verification
• correcting failed areas and shaping the final lift to the required line, grade, and drainage condition
• protecting the finished subgrade and documenting material, lift thickness, equipment, moisture, test results, and corrections

This project does not authorize the learner to:

• place fill in an overexcavated or weak area without first obtaining approval, regardless of how minor the correction appears
• compact a lift thicker than the specified loose thickness, or skip a lift, to save time
• compact fill outside the permitted moisture range without first conditioning it, or proceed with a lift the project specification identifies as too wet or too dry
• declare a lift acceptable without performing or arranging the required density, proof-roll, penetration, or other verification method
• disturb, relocate, or re-square any Project 03 control point or Project 05 benchmark while placing fill or operating compaction equipment
• proceed past a stop-work condition, such as unresolved unsuitable soil or a failed verification result, without the direction of the responsible party

Any condition requiring a soil report, an engineer's determination, a revised fill specification, or a decision outside the approved subgrade plan must be escalated before work proceeds in that area.