How is subgrade typically prepared in highway construction?

Subgrade preparation in highway construction involves ensuring a stable and suitable base for the structural layers that will be placed above, such as the subbase, base, and surface material. This preparation is primarily focused on compacting the natural ground or any improved layer underneath the pavement structure. Proper compaction is crucial because it enhances the strength and durability of the subgrade, reducing the potential for future settlement or deformation under traffic loads.

Compacting the subgrade helps to remove air pockets and minimizes the moisture content, leading to a denser and more stable foundation. This foundational integrity is essential for preventing issues such as cracking, rutting, and heaving in the pavement layers above. Various methods, including mechanical rollers and vibratory compactors, are often employed to achieve the desired compaction levels based on soil type and project specifications.

In contrast, the other choices do not align with standard practices for subgrade preparation. Digging trenches may be part of excavation but does not specifically prepare the subgrade. Applying a layer of asphalt is a part of the pavement construction rather than subgrade preparation, and adding decorative features is irrelevant to the engineering needs of a highway's structural integrity.