1 INTRODUCTION 1.1 Unsaturated Soils Unsaturated soils are commonly found in arid or semi-arid regions, which constitute one third of the earths surface, due to the process of evaporation ex- ceeding precipitation (Barbour 1999). In addition to the natural formation of unsaturated soils, the proc- ess of compaction may also attribute to why a soil may become unsaturated. An unsaturated soil is con- sidered as a three phase system, constituting soil, water and air phases. An additional phase, referred to as the ‘contractile skin’, was proposed by Fred- lund and Morgenstern (1976) thus taking into ac- count the independent properties and interfacial sur- face tension of the air-water boundary. The structure of unsaturated soil generally consists of a collection of aggregates that form the overall skele- ton; voids within the skeleton are consequently filled with air, water or gas. Each aggregate consists of in- dividual particles that are held together by matric suction. 1.2 Loading Conditions Due to the widespread and frequent occurrence of unsaturated soils, their behaviour as a foundation material and their subsequent reaction under varied loading conditions must be considered. Within a rec- lamation scheme intended for urban use, infrastruc- ture such as buildings and roads will generally be required. In regard to loading conditions that would be mainly considered, structural foundations are generally subjected to static loading, whereas dy- namic loading is mainly encountered during seismic activities or under road pavements. 1.3 Progression of Research Academic research within this field of unsaturated compacted fill has steadily increased over the last half-century and interest continues to grow. At pre- sent, a lack of knowledge transfer, between the nu- merous studies carried out in academic research and the practical application of knowledge, exists and has been highlighted through the first European Conference on Unsaturated Soils (2008). The pro- posed research aims to provide an understanding of the specific use of compacted clay fill as a founda- tion material and the subsequent behaviour under various loading, through pioneering laboratory analysis of the soil behaviour specifically the resil- ient modulus and permanent deformation character- istics. 2 EQUIPMENT DESIGN 2.1 Design Requirements The main experimental requirement of the research was to provide a simple system designed to evaluate the performance of unsaturated soils under repeated loading. Repeated loading can be defined as a series of load pulses that are separated by a distinct rest pe- riod, the duration of the load pulses may also vary in Performance of clay fill under repeated loading P. Cairns, V. Sivakumar & R. Thom Department of Civil Engineering, Queens University Belfast, United Kingdom ABSTRACT: Due to the ever-increasing demand for urbanisation, previously backfilled excavation sites are now the target of major reclamation schemes. As the majority of fill material is placed in a dry state, the fill is therefore deemed as unsaturated. Unsaturated soils constitute a large proportion of foundation materials supporting infrastructure and are subject to various loading conditions. This paper describes the development of a simple system for testing unsaturated soils under repeated loading, the equipment consists of a modified triaxial cell with hydraulic loading system. Samples of compacted kaolin clay were tested under undrained monotonic and repeated loading conditions, the results obtained provided key information in regard to the re- silient modulus and permanent deformation of a soil subjected to repeated loading.