with tests on VBS-A sand, and shows how the friction angle
increases with relative density. In contrast, the friction angle
at steady state remains constant (Fig. 3). The peak friction
angle for specimens in a loose state is close to the friction
angle at steady state, although the difference increases with an
increase in relative density. This behavior is also consistent
with the steady-state theory (Poulos 1971; Vaid et al. 1990).
The peak shear stress–normal stress envelopes in Fig. 4
were nonlinear and the peak secant friction angle in Fig. 5
decreased with the normal stress. This nonlinearity was explained
by Bishop (1966) and Bolton (1986), and is attributed
to soil dilatancy. However, the friction angle at steady state
also decreased with an increase in the normal stress. This variation
was evident for VBS but very limited for OTW. The
nonlinearity of the envelope at steady state is considered to be
due to the crushing of particles during shearing.