During a shear box test on a densely$-$packed

soil, the particles have to push up and over to

get past other particles before shear can occur;

work must be done to create space for shearing

to occur. In a loosely$-$packed soil, there is

enough void space for the particles to move in

relation to their neighbours; in fact, the

movement caused by the shear may allow the

loose soil to pack more densely. This explains

why:

In densely$-$packed soils, the vertical displacement

is positive while, in loosely$-$packed soils, it is

negative.

In densely$-$packed soils, there is a peak shear

stress that is greater than the ultimate shear

stress, but, in loosely$-$packed soil, there isn't.