Soil-pipe-atmosphere interaction under field conditions

Unsaturated clay deposits swell and shrink as a result of seasonal variations in climatic conditions and affect the performance of underground infrastructure systems. This paper investigates the response of a 150-mm PVC pipe buried in a highly plastic clay deposit under field conditions. A numerical model was established to integrate the daily climatic conditions. To accurately model field performance, the developed numerical model incorporated the use of a bimodal soil water characteristic curve (SWCC) and the simulation of the hydraulic characteristics of a cracked soil structure. The model results showed that the response of both the soil and the pipe was influenced by soil properties (native and backfill), the depth below the ground surface, and most importantly, the surface boundary condition (sealed versus unsealed surface). Although the pipe trench was covered by a pavement structure, the backfill material consisting of granular materials (i.e., mixed concrete) showed a noticeable variation in volumetric water content (VWC) with time. It was also observed that the effects of the atmosphere extended below the pipe burial depth to a depth of 4 m in the clay soil surrounding the trench. While the pipe trench was backfilled with a non-expansive material, the pipe experienced fluctuating displacements throughout the year. Finally, the model was utilized to give insight into the actual flux transferred through the pavement structure to the backfill material surrounding the pipe. It was predicted that approximately 30% of the surface net flux permeated the pavement structure and interacted with the underlying backfill materials.