Dynamic Compression of Highly Compressible Porous Media: With Application of Snow

In this research a new experimental and theoretical approach for examining the dynamic lift forces that are generated while compacting both on fresh snow powder and on soft, but aged older snow is presented. At typical skiing velocities of 10 to 30 m/s the duration of contact of a ski or snowboard with the snow will vary from 0.05 to 0.2 s depending on the length of the planing surface. No one to our knowledge has previously measured the dynamic properties of snow on this short time scale and, thus, there are no existing measurements of the excess pore pressure that can build up in snow on the time scale of importance in skiing. In our experiments using a novel porous cylinder-piston apparatus, we have made the first measurements of the excess pore pressure that would build up beneath such a surface and have also measured its subsequent decay due to the venting of the air from the snow at the edges of the piston. In further experiments, in which the air is continuously drained to avoid a build up in pore pressure, we have been able to separate out the force exerted by the solid phase (snow) as a function of its instantaneous deformation. This relationship is highly non-linear. A theoretical model for the pore pressure relaxation in porous cylinder is then developed using a consolidation approach. One finds that if the Darcy permeability K is chosen in the right range, the theoretical predictions provide excellent agreement with the experiment results. The new experimental and theoretical approach presented herein and the previous generalized lubrication theory for compressible porous media developed in Feng and Weinbaum (2000), have laid the foundation for understanding the detailed dynamic response of soft porous structures to rapid deformation.

From left to right: Mr. Qianhong Wu

From left to right: Mr. Qianhong Wu

Experiment in the Lab

Experiment in the Lab