1 Fourth International Conference on Geotechnique, Construction Materials and Environment, Brisbane, Australia, Nov. 19-21, 2014, ISBN: 978-4-9905958-3-8 C3051 SUBGRADE SUPPORT: A REVIEW OF SHORT-TERM AND LONG- TERM PERFORMANCE REQUIREMENTS Andreas Nataatmadja, Ph.D Faculty of Health, Engineering and Sciences, University of Southern Queensland, Australia ABSTRACT Subgrade performance is a function of a soil's strength and its behaviour under traffic loading. The subgrade should be stable to prevent excessive rutting and shoving during construction, provide good support for placement and compaction of pavement layers, keep pavement rebound deflections to acceptable limits, restrict the development of excessive rutting during the service life of the pavement and minimise effect of changes in moisture level. In the case of weak subgrades, a capping layer of some thickness may be placed on top of the subgrade to facilitate field construction. This paper demonstrates that a semi-infinite effective subgrade stiffness can in fact be calculated for a combination of a capping layer and a semi-infinite subgrade. Furthermore, this paper discusses the importance of thickness and quality of capping layer, for both short- and long-term loading conditions. The recommended capping thicknesses have been benchmarked against the UK and US requirements. Keywords: Pavement, Subgrade, Capping Layer, Modulus, Elastic Analysis INTRODUCTION During construction, traffic is relatively low and is not as channelised as normal service life traffic. Nevertheless, because the traffic-generated subgrade stresses are relatively high during construction, a suitable capping material of an appropriate thickness may be placed on top of the natural subgrade to provide a stable foundation upon which pavement layers can be compacted. The capping layer (or working platform) itself may consist of several materials/layers. In this case, the effective or the combined strength of the subgrade and its capping layer can be used for the design of flexible pavements. In the United Kingdom, for many years pavement foundations had been designed and constructed using established empirical relationships and a recipe specification [1]. The capping thicknesses vary from 0 mm to 600 mm (Fig. 1). However, irrespective of the thickness, the procedure assumes that all foundations achieve a similar minimum level of performance, and hence, are treated equally. A draft interim replacement specification offering 4 foundation types [2] is currently available but will not be discussed in this paper due to some technical differences [3]. Figure 1 was based on the assumption that a capping layer with as-compacted CBR value greater than 15 would provide an adequate platform for construction of the subbase when compacted to the appropriate thickness. The capping thickness from Figure 1 was used in the UK to limit the permanent deformation caused by construction traffic to a maximum of 40 mm for 1,000 passes of a standard axle (single axle load of 80 kN). Note that this requirement is different from that of the Illinois Department of Transportation (IDOT) Subgrade Stability Manual [4], which specifies, through an analytical study, that the finished subgrade must have a minimum as- compacted CBR of 6% if untreated, or 10% if treated, and a maximum rut depth of 13.0 mm under construction traffic. Fig. 1 Example of capping thickness requirement in the UK [1]. Figure 2 shows the IDOT’s minimum capping thicknesses on top of subgrades of varying CBR