Vol. 10 No. 5  October 2002

TRIAXIAL TESTING MACHINE FOR MEASURING VOLUMETRIC CHANGES IN SNOW

Triaxial testing machine

        Triaxial testing of snow has contributed to an understanding of the behaviour of snow under confinement and has facilitated an accurate determination of volumetric change in the snow subjected to different states of stress. DRDO has designed and developed a computer- controlled triaxial testing machine to impart impetus to this frontal technology. The machine can generate a confining pressure up to 0.5 MPa through a liquid called FREEZE-12 (freezing point around -60 C). The pressure can be achieved with an accuracy of 0.001 MPa. LVDT of 10 mm capacity monitors the axial strain with an accuracy of 0.001 mm. A load cell of 1 kN senses the axial load with an accuracy of 1 N . An arrangement of accurate determination of volume change has been designed with the help of a flow meter, which monitors the volume of liquid going in or coming out of the load cell. The load cell has been fabricated from a transparent perspex, which enables visual inspection of sample from time-to-time.

        The strength of snow is pressure dependent at strain rates above 10-5 S-1, where crack nucleation is suppressed. Also at low temperature with other condition remaining the same, snow is able to take more load. The ability of the snow to take more load at lower temperature is established. However, to include the compressible character of snow in its constitutive law, triaxial test data is extremely significant. With higher confining pressure, the volume change of snow is less than that at lower confinement. Strengthening effect due to higher confinement is limited to a certain stress level (roughly up to about 0.3 MPa). It is seen that the effect of pressure melting of snow as a result of high stresses at grain boundaries, also cause softening of the snow and reduces its strength at higher pressure.