From Density to Strength-based Construction of Geolayers for Airfield Semi-prepared Landing Zones

Abstract: During construction of airfield semi-prepared landing zones in contingency environments, compaction of the geolayers to a maximum strength target plays a vital role in ensuring expedient cargo aircraft missions are accomplished satisfactorily. While current practices for the construction of pavement geolayers rely on achieving a maximum dry density at an optimum moisture content to ensure long-term durability, shifting construction practices in contingency environments to target a maximum strength for geolayers results in an optimum structural stability for short-term operations. To ensure the geolayer achieves maximum strength during field compaction, a strength-based testing technique, both in the lab and field, is required for design and quality control. The California bearing ratio (CBR), traditionally used as a design input, can be measured during construction activities; however, the field CBR test is considerably complex and bulky. This paper discusses the effects of shifting the design and construction of geolayers for airfield semi-prepared landing zones from a density-based to a strength-based approach using a small footprint testing device. The data for this study were collected from both laboratory and large-scale test experiments involving varying geomaterials such as crushed limestone, clay-gravel, silt, and lean clay material. From the laboratory data, design curves were created for each geomaterial using the proctor compaction method, CBR, and lightweight deflectometer (LWD) tests. Field data was collected using a nuclear density gauge, an LWD, dynamic cone penetrometer (DCP) and field CBR tests. Pavement-Transportation Computer Assisted Structural Engineering (PCASE) software was used to conduct structural evaluations for airfield semi-prepared landing zones and analyze the impact of constructing geolayers outside the laboratory optimum conditions, as judged by the compaction and strength design curves. The presentation will inform the use of the traditional modified proctor compaction method, the laboratory CBR test, and the lab lightweight deflectometer (LWD) to characterize typical geomaterials in the laboratory, as well as the application of the same test methods during the construction of geolayers. Overall, the results confirmed that geolayers constructed to a target maximum strength can be satisfactorily achieved by using a lab and field test method, such as the LWD to supplant the CBR test method. In addition, PCASE evaluations informed the significant structural improvements of airfield semi-prepared landing zones when designed and constructed based on strength criteria.