To learn the airproof capacity of filter cakes as opening chambers under air pressure, a series of tests were carried out. The variations of discharged water with air pressure and time were observed, and the relationship between airproof capacity of filter cakes and surrounding air pressure was analysed. The test results indicated that there were three stages as compressed air acting on filter cakes: completely not infiltration, a very small amount of infiltration, and penetration leakage. The certain air pressure between the first and second stages was called the airproofing value of filter cake. And a capillary bundle model was used to explain the mechanism of air tightness of filter cakes. In Nanjing Yangtze River Tunnel, a 5 cm thickness filter cake was formed in gravel sand, and its airproofing value was a little lower than 0.12 MPa. The air pressure used as opening chamber should be equal to the summation of water pressure in sand and airproofing value of filter cake. While the air pressure is larger than the summation, the filter cake would be gas permeable. The slurry formulation and airproofing value of filter cakes obtained in the tests were applied successfully in Nanjing Yangtze River Tunnel.

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The falling-head and rising-tail method for permeability test allows for both inflow and outflow rates to be conveniently measured. This helps to detect any possible swelling or consolidation of specimen during testing and to identify the continuity of flow of water through the specimen. It has been widely adopted to measure the hydraulic conductivity of saturated porous media of extremely low permeability, such as hydraulic barrier soil materials. Centrifuge permeameter test with this method has recently been devised to accelerate water flow and reduce the time needed for testing. The accompanying equation for calculating hydraulic conductivity is, however, found to be simplified because it was formulated by idealizing such a variable-head test as multiple segmental constant-head tests. In this paper, a new generalized equation is established based strictly on the variable head. Moreover, based on curve matching between the rates of flow into and out of the soil specimen, an approach is also proposed to identify the continuity state of flow. The data relevant to this state are extracted for performing the calculation of hydraulic conductivity. The practical application of the proposed equation and its associated approach is illustrated through a reanalysis of recently published experimental data in the literature.