Questions and answers: testing techniques 1892
2c
200
.c u) 150-100such values from test results -which is sometimes straightforward, and
sometimes open to interpretation.; Schematic cross-section
-.- of the direct shear test
I
1(a) Plotting the displacement and stress results for the shear test,
illustrated above, generates the diagram shown below. There is a well-
defined peak strength, followed by a poorly defined residual strength.
By poorly defined, we mean that a residual strength is encountered
at a displacement of about 6 mm, but should the residual strength be
this or the one reached at a larger maximum displacement? Although,
by definition, we regard the residual strength as that reached at large
displacements, this may not be appropriate if the fracture has failed at
some intermediate lower strength - before reaching the higher strength
attained at a larger displacement.
By analogy with the complete stress-strain curve in compression,
the in situ stability of the fracture depends on the stiffness of the
loading system (i.e. the engineering structure for a field project), and
the appropriate value for the residual strength can only be chosen given
the engineering circumstances. Thus, it may be that the appropriate
residual shear strength is either about 260 kPa or about 310 Wa. If a
shear displacement of more than 8 mm is sufficient to cause catastrophic
collapse of a structure, it is of no value to know what the strength would
be at subsequent, higher, shear displacement values.