Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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Fig. 8.2. Yield locus and effective yield locus
The parameter of the yield locus is the bulk density ! b at preconsolidation.
With higher preconsolidation loads, bulk density and material strength increases,
and the yield loci move upwards. Each yield locus terminates at a point E in
direction of increasing normal stress 1. Point E characterizes steady flow, that is
flow with no changes in stresses and bulk density. Two Mohr circles are drawn
determined by two normal stresses 1 1 – the major principal stress at steady state
flow called major consolidation stress, and 1 c – unconfined yield strength of the
sample. Each yield locus gives one pair of values of 1 1 and 1 c . Conducting the test
for several values of consolidation pressure gives a set of pairs of the parameters.
:LWKWKHVHYDOXHVDSORWRI1 c DJDLQVW1 1 is obtained that is used to characterize
flowability of granular material. To characterize flowability, Jenike [74] proposed
to use the ratio of the major principal stress at steady state flow 1 1 to the
unconfined yield strength 1 c . The classification of materials originally introduced
by Jenike was modified by various authors as eg. by Chattopadhyay et al. [34]
(see Chapter 4), or by Tomas to the following form (as cited by Schwedes [152]):
ff < 1 hardened
2 < ff < 1 very cohesive
2 < ff < 4 cohesive
4 < ff < 10 easy flowing
10 < ff free flowing
The flow function gives the value of stress that lets an arch collapse or lets the
material flow through an orifice. The inverse of the slope of flow function is termed
the flow index i:
i = σ c /σ 1 (8.2)
which is used as an index of flowability following Jenike classification.