A model study of mole drain spacing and performance

A model study on the spacing of mole drains and their influence on pore-water pressure heads was carried out using the finite element package SEEP/W for both transient-state and steady-state conditions. The aims were: to optimise mole drain spacing by modelling two drain spacings and to contribute to the understanding of mole drain performance; to compare pore-water pressure heads generated over a 12-day rainfall period by the model with those measured by a multipoint tensiometer in the field; to compare watertables for the two spacings under a range of steady-state rainfalls and hydraulic conductivity of the several soil layers. The drained field soil was three-layered, with a 150 mm thick permeable clay topsoil overlying a quasi-triangular wedge of permeable loosened clay soil over the mole drains that were drawn in a virgin clay subsoil of very low hydraulic conductivity. Mole drains of 0.075 m diameter were installed in the field at a depth of 0.45 m and tensiometer data were available for a spacing of 1.07 m; two spacings of 1.075 and 2.0 m were used in the modelling. There was good agreement between the pore-water pressures generated by the model and those measured by a multipoint tensiometer in the field for the 12-day sequential rainfall period. In the steady-state analysis, steady rainfalls of 5, 12 and 30 mm per day were imposed on a range of hydraulic conductivit- ies of the soil layers for the two drain spacings. The close spacing of 1.075 m was shown by the model to give good control of the watertable while the more distant spacing of 2.0 m failed to control the saturation of the topsoil under a steady-state rainfall of 5 mm per day. For Irish soil and rainfall conditions, a mole drain spacing of 1 m or less is required to provide satisfactory control of the watertable in contrast with 2--3 m spacing commonly employed in the low-rainfall east of UK.