Assessment of the hydraulic and filter performance of different drainage stone aggregates to elucidate an optimum size range for use in clay-textured soils

On poorly drained grassland farms in Ireland, stone aggregates remain the only in-field drain envelope material used by contractors. A variety of aggregate sizes and lithologies are currently in use, but their performance in clay-textured mineral soils is unknown. In practice, this may result in ad-hoc system performance and a varied lifespan due to sediment ingress. The aim of this study was to evaluate the hydraulic and filter performance of a range of aggregate gradations in clay-textured mineral soils. Nine aggregates (three replicates of each) were examined in laboratory units containing clay-textured soil, with a perforated drainpipe surrounded by an aggregate envelope ranging in size from 0.7 to 62 mm and a constant 0.4 m head of water above the soil surface. To determine the hydraulic performance of the envelope, the discharge rate of water through the drainage pipe outlet was measured over 38 days. To determine the filter performance, sediment loss, sediment settlement in the drainpipe, and ingress of sediment into the envelope were measured. The results indicated that only aggregates in the 0.7-19 mm size range performed adequately from both the hydraulic and filter perspectives and were deemed suitable for use with a clay-textured soil. Discharge appeared to be inversely related to aggregate size, with larger discharges being measured in the smaller aggregate sizes and smaller discharges measured in the larger aggregate sizes (exception: Aggregate 2). For all aggregates examined, discharge was greatest at the start of the experiment before reducing over time. When the cost of the aggregate material is also considered, aggregates in the lower size range are 18-50% more expensive than aggregates in the higher size range. Aggregates with particle sizes ranging from 0.7-19 mm are recommended for in situ field testing in clay-textured soils.