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Sitework & Earthworkaka: prepared subgradeaka: subbaseaka: native subgrade

Subgrade

In Plain English

The compacted soil surface beneath a road, concrete slab, or foundation that everything else is built on top of.

Definition

The native or compacted soil surface upon which a pavement structure, foundation, or slab is constructed. The bearing capacity and stability of the subgrade directly influence the thickness and design of the overlying structure. Weak or unstable subgrades require stabilization, replacement, or the use of geosynthetics before paving or construction can proceed.

Why It Matters in Bidding

Subgrade condition drives the quantity of imported base, stabilization, and undercut an estimator must carry, and it is one of the most common sources of differing-site-condition claims. Underestimating poor subgrade can erase the margin on a sitework bid, so estimators tie subgrade assumptions directly to the geotechnical report and clearly state them as qualifications.

Example

Reading the geotechnical report's soft soils at the building pad, the sitework estimator adds a unit-price line for undercut and structural fill so the bid is protected if the subgrade fails proof-roll at the time of construction.

Related Terms

CompactionSoil StabilizationFine GradingBackfill

Frequently Asked Questions

Subgrade strength dictates base course thickness, the need for undercut, stabilization, or geosynthetics, and overall earthwork quantities. Weak subgrade adds material, hauling, and compaction costs. Estimators should base assumptions on the geotechnical report and state them as bid qualifications so any worse condition becomes a change order rather than an absorbed loss.
A proof-roll runs a loaded truck over the prepared subgrade to expose soft or pumping areas before paving. Specs often require it as a hold point. Estimators should price the proof-roll and include a unit price for remediating failing areas, since the extent of soft spots is unknown until performed.
Responsibility depends on contract risk allocation and the disclosed geotechnical data. If conditions materially differ from the documents, a differing-site-conditions clause may shift cost to the owner. Without that clause, the contractor may absorb it, which is why estimators state subgrade assumptions and carry unit prices for undercut and fill.

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