Figure 9.11 shows Mohr circles of total
and effective stress for confined and unconfined compression tests on samples with the
same voids ratio. The Mohr circles of effective stress are identical; they both touch
the lines given by τf = su and τ
f
= σ
f tan φ
c
. The Mohr circles of total stress have the
same diameter (because the voids ratios of the samples are the same) but they are in
different positions, so the pore pressure in the unconfined compression test sample is
negative. It is this negative pore pressure that produces positive effective stresses and
gives rise to the unconfined compressive strength; this accounts for the strength of a
sandcastle and the stability of a trench with steep sides.