Abstract
Two kinds of soil (sandy loam and light clay) were used for laboratory experiments to investigate soil settling and water flow in puddled soil. Main results are as follows:
1) Knowledge of “settlement” is necessary to clearly understand percolation phenomena in puddled soil.
2) Percolation provides a stronger settlement than no percolation at all, especially at the bottom part of the soil columns.
3) Instantaneous lowering of drain level during the stable state takes several days for it to reach its new stable state. During this transition, the settlement and percolation velocities are much higher than those in the stable state. The process is reiterated by the succeeding changes in the drain level.
4) The popular method for estimating the percolation rate based on the conceptions of average hydraulic gradient and average hydraulic conductivity is not availabe for this transition period.
5) Observation of the pore water pressure distribution suggests that the large change in the hydraulic gradient near bottom of the column causes the change in the drainage velocity in puddled soil. The observed hydraulic gradient in upper part of the column indicates upward dewatering during several days after the start of the downward percolation. It is also elucidated that permeability of soil near bottom of the columns is much lower than that near the surface.
6) The permeability reduction of puddled soil in these experiments does not depend on the behavior of fine particles, but mainly on the non-uniform settlement or the strong packing in the lower layer.
