Design Considerations for Base/Subbase

In 1940, the U.S. Army Corps of Engineers were assigned the responsibility for the design and construction of military airfields to support new heavy bomber aircraft such as the B-17 Flying Fortress. Pavement loading from these aircraft was three to five times heavier than any highway or aircraft loading designers had dealt with previously [Ahlvin 1991]. Based on a world-wide review of pavement design procedures, the Westergaard Design Method was chosen based on H.M. Westergaard’s work with the Bureau of Public Roads and design method validation from the Arlington Road Tests.

In the early days of rigid pavement construction, concrete slabs were placed directly on top of the subgrade without any base/subbase layers. This pivotal work on rigid pavement design by the U.S. Army Corps of Engineers led to a much better understanding of the importance of the use of bases and subbases, their uniformity, and degree of compaction. One of the key findings during the implementation of the new design procedure was the importance of bases for concrete pavements. With an increase in traffic loads, volume, and speed, pumping of the subgrade material was observed through the joints and cracks in the PCC pavement. The loss of support due to pumping resulted in an increase in other distresses such as faulting, roughness, and corner breaks. Initially, a sand filter layer was specified to mitigate pumping of subgrade materials. With continued use, it became apparent that the filter layer also acted as a “subgrade improvement” layer, contributing not only to the reduction in pumping but also to the strength of the pavement and its constructability.

The key characteristic of a good quality rigid pavement foundation is not the strength of the support, but rather the provision of uniform support that is free of any abrupt spatial and material changes. Rigid pavement design relies on the structural carrying capacity of the PCC and on the uniformity of support provided by the base layers. As such, the pavement design engineer should not attempt to use the base/subbase layers simply to increase the overall structural capacity of a rigid pavement system or to reduce the thickness of the PCC layer. In most rigid pavement designs, the PCC design thickness is relatively insensitive to the foundation strength and, therefore, slightly increasing the slab thickness is more economical than structurally increasing the thickness of the base layer to achieve the necessary structural capacity. A pavement design engineer should evaluate the potential causes of a non-uniform foundation and design the base or subbase layer to mitigate their effects. The three major causes of a non-uniform foundation are:

• Pumping of the fine particles.
• Frost heave.
• Soil expansion.

These factors must be controlled and limited over the life of a rigid pavement to ensure satisfactory performance. The conditions necessary to cause the above performance issues are summarized below:

• Pumping:
  
  • High-speed, heavy axles capable of deflecting the concrete slabs.
  • Joints with poor load transfer, especially undoweled joints.
  • Presence of water between pavement and subgrade.
  • Fine-grained subgrade or erodible base/subbase materials.
• Frost heave:
  
  • Frost-susceptible soil: Fine-grained soils with low plasticity and high percentage silts are most susceptible to frost heaving, while gravels and sands with fines and sandy/silty clays are prone to moderate frost action.
  • Source of water.
  • Freezing temperatures penetrating the soil.
• Soil Expansion:
  • Expansive soil: Soils sufficiently expansive to cause problems include the American Association of State Highway and Transportation Officials (AASHTO) classification A-6 or A-7 soil groups or the Unified Soil Classification System CH, MH, and OH soils.
  • Degree of moisture change within the soil.