Membrane Penetration and Its Effect on Pore Pressures

An apparatus is described to measure the volume changes due to the membrane penetration into the interstices of a specimen of sand or silt with changing pressures on the membrane. These tests confirm the relation between mean grain size and membrane penetration. It is possible to reduce the membrane penetration by up to 85% by spreading the membrane with a thin layer of liquid rubber. This initially liquid rubber is pressed into the interstices of the surface grains by the cell pressure where it gradually hardens. A comparison of undrained triaxial tests with sand shows that treating the membrane with liquid rubber the excess pore-water pressure generated in the undrained test is up to 100% higher than in a test with a normal membrane. So the normal use of membranes will underestimate the excess pore-water pressure and overestimate the undrained shear strength.