Using the self-developed digital image correlation method based on the particle swarm optimization algorithm

the spatiotemporal distributions of rotation angles of principal strain axes for wet sandy soil specimens are investigated under uniaxial compression. Using the bicubic spline interpolation

the rotation angles of principal strain axes at any positions are obtained to study the evolution of the rotation angles of principal strain axes with the longitudinal strains before the occurrence of a shear band

at its center

in the vicinity and at both sides of the band. It is found that with the increase of longitudinal strain

the rotation angles of principal strain axes change from a scattered distribution to a stable one; and at last

its range in most regions is -10°-10°. After the occurrence of an apparent shear band at the late stage of strain-hardening

at the center of the band

the rotation angles of principal strain axes tend to be stable or descend slightly

within a range of -5°-5°

while those at both sides of the band still increase. At tip of the band

the evolution of the rotation angles of principal strain axes is complex.