The reliability in the operative field of the parabolic diffusion equation is investigated by solving the rigorous one‐dimensional equation of groundwater flow in deforming soils. The dependence of the hydraulic conductivity on the specific weight of water is included. The grain velocity is correctly expanded first. This expansion leads to a nonlinear integrodifferential term. An iterative finite element technique of solution is then developed. The true time‐dependent pressure head is compared to the standard one. The entire range of realistic variations for the formation parameters is carefully explored. It turns out that the usual equation gives satisfactory results in the vast majority of applications. The conditions underlying the approximated theory become critical only when the flow field is to be determined in highly compressible units for strong boundary pressure variations. In this case the solid material movements can no longer be considered small. The pressure head changes are faster than it would appear from the standard solution, and the consolidation process is more rapid than that in the classical Terzaghi's the
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