Investigation of Subgrade Moisture Flow in an Airfield Pavement

Investigation of Subgrade Moisture Flow in an Airfield Pavement Research Proposal for the Investigation of Subgrade Moisture Flow in an Airfield Pavement System SCOPE OF WORK Historically, airfield pavement design has been based, for the most part, on the selection of material design properties that are at the worst possible environmental condition, regardless of where the site is located. Thus, all unbound materials (base, subbase and subgrades) are classically selected at fully saturated moisture conditions. In recent years, there have been very significant enhancements in the area of Unsaturated Soil Mechanics, to start assessing pavement performance predictions using the real time environmental, seasonal and aging effects to predict actual changes in the unbound material strength and resilient behavior of the materials. A major advance in incorporating such an approach into a national design methodology was recently accomplished in the development of the AASHTO Mechanistic-Empirical Pavement Design Guide (M-EPDG) for highway pavements. This procedure makes use of real time change in pressure (gravimetric and capillary/suction) gradients as a driver in the modeling of moisture changes in a pavement system with a specific groundwater table location and the actual site environmental conditions. In pavement performance test sections used to enhance/verify structural design models; it is common practice to compact the unbound material to a predefined soil moisture and density condition to a specified laboratory-compactive energy. This is accomplished so that estimates of the initial soil strength (CBR, k value) and resilient response can be deduced from laboratory generated properties corresponding to the expected in-situ moisture-density achieved in the construction process. These values are also typically cross verified by field testing. Recently, Federal Aviation Administration (FAA) pavement research engineers have discovered and concluded that significant changes in unbound material strength have occurred, with time even during load performance studies. They have concluded that these changes in moisture, and hence in-situ strength, can only be attributed to a function of in-situ soil moisture flow that is caused solely by the development of thermal gradients within the test sections. Based upon these findings; the Principal Investigator has developed an unsolicited proposal to study the validity and existence of soil moisture changes by thermal gradients at the National Airport Pavement Test Facility (NAPTF). It is rather obvious that this issue is a significant phenomenon that must be verified and eventually incorporated into all future NAPTF structural performance studies. Additionally, if this hypothesis can be proven, and accurate and verifiable models can be developed to predict these moisture changes; a more accurate appraisal of the test section performance will be possible. Thus, these findings would have significant consequences upon the ability of NAPTF staff to obtain more precise structural performance trends with their test section-load studies. Research related to explaining and modeling this moisture flow condition is complex and involves the verification of hydro-thermal coupled behavior of unsaturated soils. While several analytical models for the coupled behavior of unsaturated soils are available, data for validating these models and for evaluating the relevant soil properties are very limited in the literature. Laboratory test methods for characterizing the soil properties needed for these models are generally in the embryonic state. Nonetheless, the coupled hydro-thermal behavior of unsaturated soil has taken on increasing importance in recent years due to its significance within a variety of emerging geotechnical problems. This proposal details the development and execution of a new laboratory and field testing program that will enhance the accuracy of the model to predict real-time environment effects upon the real-time changes in unbound material strength and the resilient (non-linear) moduli for use in the design and performance evaluation/ predictions of airfield pavement systems. Furthermore, the product of this study will enhance our understanding of coupled hydro-thermal moisture flow processes in unbound materials and provide the data necessary for validation and implementation of advanced constitutive models for these processes.