OCR Text |
Show 6 than zero. Since rtc > 0 , Equation 10 is satisfied under this condition for all values of ° j . The slope angle, fi , of " primary absolute stability" is found by setting ' 7", = Q in Equation ( 10) 1- U^ t = ('-*) W"- M ( 13) A plot of Equation ( 13) is shown in Fig. 6. jjpj exists only if M is sufficiently small, i. e. if the snow is sufficiently incompressible, and increases in a uniform manner with the parameter, £\) . If stability is not initially absolute, the situation may improve according to particular conditions. A settling of the snow causes a decrease in W and consequently a decrease in " M . According to Fig. 7, this results in a decrease in TI and therefore an increase in stability. This effect becomes greater as the slope angle decreases and £* J increases. 4. SECONDARY STABILITY Secondary stability is only assured for a slab which has primary stability and which remains stable if it is submitted to a perturbation. The perturbation may be, for example, a local break in the critical layer. The forces sustained by the critical layer are transferred to neighboring zones with a marked maximum along the boundaries of the slab and a possible propagation of the fracture. Two extreme cases are observed for the configuration of fracture lines: a circular pattern and an extended horizontal pattern. The first case is analogous to considering the stress on a cylinder of radius, f0 , and thickness H . The exact solution of this problem is very complicated and depends on the visco- elastic properties of all the layers in the slab. We limit our analysis to the simple case which rests on the following hypotheses. First, the component of the weight of the cylinder which is normal to |