In changing environmental conditions, the moisture variations play an important role in controlling the performance of unbound layers. The increased moisture decreases the strength of unbound materials and can cause early failures in low volume roads. It is important to investigate the performance of unbound granular materials (UGM) with increasing moisture levels as there is a high likelihood of water infiltrating unbound layers. The better understanding of UGM deformation behaviour as moisture levels increase can facilitate the design process of the materials and layers. This research has aimed to understand the UGM behaviour used in the basecourse layer of pavements in wet conditions. Therefore the objectives of the research can be divided into two main categories: To understand how aggregates behave in increasing moisture conditions for different aggregate gradations and fine material contents; and to investigate how well UGM behaviour can be simulated in laboratory based performance tests compared to accelerated pavement tests. The experimental program was designed in three categories apart from the standard tests carried on the basecourse materials i.e., geological and mineralogical testing, repeated load triaxial testing, full scale accelerated pavement tests. The geology of the source rocks was tested by examining the thin sections obtained from various aggregates. X-Ray Diffraction tests were conducted which helped in finding the clay minerals present in the granular aggregates and fine particles (less than 0.10 mm). The unbound materials were tested in the repeated load triaxial (RLT) test apparatus at optimum moisture drained, saturated drained and saturated un-drained conditions to investigate the behaviour of unbound materials at elevated moisture contents. The materials were laid in the basecourse at Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) in various sections with different sealing options. The pavements were subjected to surface water runoff and standard axle loading cycles. The results show that mineral composition of unbound materials is an important parameter which can affect the performance of compacted unbound granular aggregate materials. The results lead towards the selection of aggregates with specific range of expanding minerals, and particle size distribution with a range of Talbot's constant. The research shows the behaviour of different grading of aggregate under elevated moisture conditions in full scale accelerated pavement testing at CAPTIF. This research developed a new permanent deformation model for unbound granular material based on RLT test data which includes stress variations as well as the number of loading cycles applied on the specimen.