Feasibility of Using 4th Power Law in Design of Plastic Deformation Resistant Low Volume Roads

Abstract

A low volume road (LVR) structural design has two phases: first one is selection of appropriate construction materials and second is the determination of layer thicknesses under the certain traffic and environmental conditions with considering the subgrade bearing capacity. Pavements are prompted to serve the traffic without reaching the terminal serviceability index over its design life. Rut accumulation (plastic deformation) is the most common pavement deterioration type of flexible pavements. Therefore the main goal of the design is prevent rutting. Many low volume road design manual assume that plastic deformation occurs only in subgrade. Construction of overlying layers by selective high performance materials according to the related material and construction specifications is the reason of this assumption. In fact, the assumption is not much reasonable especially for with no, or thinly overlaid low volume road pavements, where the major structural strength is comprised of unbound granular pavement materials and where the principal distress mechanism is rutting in the aggregate layers. Subgrade bearing capacity and the traffic are the main input parameters in the design stage of low volume roads. Subgrade bearing capacity is expressed with California Bearing Ratio (CBR) or Resilient Modulus (Mr). The "traffic" term is determined by Equivalent Standard Axle Load (ESAL) repetitions which is often admitted as 80 kN single axle load. Although it is not too difficult to determine an axle load for an individual vehicle, it becomes quite complicated to determine the number and types of axle loads that a particular pavement will be subjected over its design life. For calculation of Load Damage Factors of different vehicle types, which have various axle load and configurations, a generalized fourth-power law has been used for more than a half century. The objective of this study is to indicate the limitations and difficulties faced on reliably applying a power law relationship in design of LVRs with no, or only thin seals. If a power law relationship to be used due to its simplicity, several parameters must be considered in selection of the power value. Such as stress dependent behavior of unbound granular materials and the selected distress type.