What Is Subgrade in Construction?

March 13, 2024

The subgrade is the underlying layer of natural or improved soil in a construction project. Its primary function is to evenly distribute the weight of the structure and any traffic loads, preventing uneven settlement that can weaken the building or pavement over time.

Preparing a quality subgrade is essential for the long-term durability of concrete slabs and pavements. Failure to do so can lead to structural issues and costly repairs. The subgrade is the foundation upon which the rest of the pavement will be built, so it is important to use proper ground improvement methods and compaction to ensure it can adequately support the concrete slab above it.

A poor subgrade can result in uneven settlement and heaving, which can decrease the lifespan of the concrete slab. Settlement creates voids and gaps that weaken the load-bearing capacity, while heaving can cause cracking and displacement. Both conditions increase the vulnerability of the concrete to damage from freeze-thaw cycles and chemicals.

The type of soil used for the subgrade can have a significant impact on its strength and stability. Subgrade soils should be relatively granular and have low organic content. Sensitive clays and silts that liquefy or expand with moisture changes should be avoided, as well as soil with a high montmorillonite mineral content. Subgrade soils should be compacted to their optimum moisture content to achieve the highest load-bearing capacity.

In situations where the in-situ subgrade is poor and can’t be upgraded with better fill material, a capping layer may be needed to protect the subbase from moisture. This can be accomplished by excavating and replacing the subgrade with affordable gravel.

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