A method to predict changes in hydraulic conductivity caused by drainage plows and backfilling of trenches

A method was developed to predict the reduction in hydraulic conductivity around drainpipes due to the laying technique.For the trenchless technique the flow of soil around the body that is forced through the soil is assumed to be a sequence of steady state situations for which the stream lines are the sliding lines defined by Prandtl (1921). From the stream lines and an assumed displacement of the soil mass, a relation is derived between the deformation of the soil, the friction angle and the distance to the plow body.The relation between hydraulic conductivity and deformation was derived from cores placed in a triaxial apparatus. When the friction angle is known, the reduction in hydraulic conductivity can be determined.For the trenching technique, the change in dry bulk density due to settling of the backfill is derived from the relation between the change in density of loosened soils and the original density of the soil before loosening. Using a relation between density and hydraulic conductivity, the reduction in conductivity is obtained.The predicted hydraulic conductivity in the deformed zone caused by a drainage plow and that in backfilled trenches in a silt loam soil and a silty clay loam soil was compared with the hydraulic conductivity derived from measured drain discharges, heights of water table and known original hydraulic conductivities of the undisturbed soil.