R. C. Rice

The Pit Bailing Method for Hydraulic Conductivity Measurement of Isotropic or Anisotropic Soil

ABSTRACT WITH the pit bailing technique, a hole is dug (normally with a backhoe) to below the groundwater table. After equilibration, the water level in the pit is rapidly (instantaneously) lowered and the subsequent rise of the water level is measured for calculation of the hydraulic conductivity K around the pit. The pit bailing technique was originally developed for septic tank leach field design. However, the method is also useful for in-situ measurement of K for other purposes. This is particularly true for stony soils, where augerhole and other methods are difficult and often fail. Another advantage of the pit method is that the volume of soil on which K is determined is larger than for the augerhole and piezometer methods. The equation originally given for the pit method was based on the Thiem formula, which limited the method to situations where the pit bottom was at or close to the impermeable layer. The geometry of the flow system for the pit method, however, is similar to that for the piezometer method if the water table coincides with the top of the piezometer cavity. Thus, the equation for the piezometer method can also be used for the pit method. This extends the applicability of the pit method to partially penetrating pits in soils that are underlain by either impermeable or very (inifinitely) permeable material. Geometry factors for the piezometer method presented by Youngs were extrapolated to the case where the static water table coincides with the top of the cavity. The resulting values then can be used for the pit method. The validity of this approach was demonstrated in a large sand tank where the average K-value yielded by pit-bailing tests on holes of various diameters and depths was 31 m/day as compared to an average value of 37 m/day obtained with one-dimensional and seepage meter methods. In anisotropic soils, K in horizontal and vertical direction can be calculated from the K values obtained from two pits with different geometries.