In flexible pavement design, what is the distinct function of the granular base layer that differs significantly from its role in a rigid pavement system?

In a flexible pavement system, the distinct function of the granular base layer, which is typically composed of crushed stone or gravel, is to significantly contribute to the structural capacity by actively distributing traffic loads over a wider area to the underlying subgrade. Flexible pavements, such as those made with asphalt concrete (AC) or hot mix asphalt (HMA) as the surface layer, rely on a layered system where each layer progressively spreads and reduces the stress exerted by traffic. The asphalt layer, being flexible, deforms under load and spreads it conically. The granular base then acts as a crucial load-spreading layer, taking the load distributed by the asphalt and further disseminating it over a larger footprint, thereby reducing the stress intensity on the weaker subgrade to an acceptable level. This structural role is paramount, as the base layer prevents excessive deformation and failure of the subgrade. Additionally, it provides drainage for the pavement structure and helps prevent the intrusion of fine-grained subgrade material into the base. The base layer in flexible pavement is thus a primary structural component responsible for bearing and spreading loads.

Conversely, in a rigid pavement system, which features a Portland cement concrete (PCC) slab as the primary surface layer, the main function of the granular layer (often referred to as a subbase) is fundamentally different. The concrete slab itself is very stiff and possesses high flexural strength, meaning it acts like a beam to distribute traffic loads over a very large area directly to the subgrade with minimal deformation. Therefore, the granular subbase in rigid pavement does not primarily serve as a load-spreading layer in the same structural capacity as in flexible pavement. Instead, its distinct and critical functions are to provide a uniform, stable, and unyielding support for the concrete slab, preventing localized stresses and differential settlement that could lead to cracking. It also functions as a drainage layer, removing water from beneath the slab to maintain subgrade strength and prevent water accumulation. Crucially, it acts as a barrier to prevent frost heave, which is the upward swelling of soil caused by ice formation in frost-susceptible subgrades, and to mitigate pumping, which is the erosion and ejection of fine-grained subgrade material from beneath the slab edges due to water movement under traffic loading. Thus, while both systems utilize a granular layer, the flexible pavement base actively participates in stress reduction through load spreading, whereas the rigid pavement subbase primarily provides a stable foundation, drainage, and protection against subgrade issues like pumping and frost heave for a slab that already performs the main load distribution.