  The occurrence of heterogeneous perturbations of fluid mass density and solid
elastic strain of a porous continuum, as a consequence of its undrained
response is a very important topic in theoretical and applied poromechanics.
The classical Mandel--Cryer effect provides an explanation of fluid
overpressure in the central region of a porous sample, immediately after the
application of the loading. However this effect fades away when the fluid leaks
out of the porous network. Here this problem is studied within the framework of
a second gradient theory and a thorough description of the static and the
dynamics of the phenomenon is given.
