A Comparison of Subgrade Improvement Methods

Building upon a solid foundation is one of the longevity keys for airfield pavement performance. For new or replacement airfield pavement on subgrades with good foundational characteristics, the design process is straight forward as defined in FAA Advisory Circular (AC) 150/5320-6F, Airport Pavement Design and Evaluation. However, airfield pavements can be designed to perform on marginal foundations/subgrades. These marginal subgrades often present Engineers and Contractors with certain challenges, which may include the following: optimization the pavement section—design a cost-effective pavement section for the requisite or desired performance life; and constructability—a recognition that though the marginal subgrade may adequately support the pavement, marginal subgrades often need improvement for the interim support of construction equipment and to create a solid working platform. This paper will discuss the subgrade requirements contained in AC 150/5320-6F. The paper will also discuss various subgrade improvement methodologies use in airfield pavement design, including the following: shallow ground treatments for pavements in poor soil conditions, including: cement, lime and fly ash stabilization of subgrade soils; traditional undercut of unsuitable subgrade material; and use of geotextiles and other premanufactured products. Deep ground treatments for pavements in poor soils conditions, including: use of light weight aggregates or flowable fills; preloading/surcharge of subgrade soils to induce consolidation; and rigid inclusions (grout columns) with a load transfer blanket. Finally, the paper will discuss the construction of the aforementioned subgrade improvement methodologies by providing examples of their uses at various airports across the United States.