First discoveries and explanations

Lines of principal pressure during developing flow 

Since the state was to pass from filling (close to an active stress state) to passive, the pressure just below an abrupt step is easily determined as the Janssen value multiplied by the ratio of passive to filling values of lateral pressure ratio K /Ki). The ratio of peak symmetrical discharge pressure to symmetrical filling pressure is a very widely used variable, and its origins can be seen here to have some foundation in mechanics. This ratio is so important in silo design that it is given a symbol and defined as 

It was noted above that the ratio for a typical bulk solid is of the order of 9, 

The most widely used switch theory for vertical walls was that of Jenike et al. (1973), which still underlies the flow pressure rules in the Australian Standard AS 3774 (1996), leading to a high ratio of design pressures for discharge to those after filling. This type of theory is still commonly expounded (Drescher 1991) as a formal part of silo pressure behaviour. 

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