Internal bulge as a manifestation of conditions of constrained dilatancy
Resistance to internal bulge determines the bearing capacity of soil under the lower end of piles, trench foundations and other types of deep supports. The conditions of internal bulge are best modelled in triaxial compression instruments during sample-squashing tests. Such tests are usually conducted with a gradual increase of vertical pressure relatively to lateral pressure which is kept constant throughout the experiment. A constraint of dilatancy results in the fact that an increase of lateral pressure on the sample simultaneously causes mobilization of additional resistance to its squashing. The change in the ratios of principal stresses with elastic constraint to dilatancy leads to a decrease in the measured values of the angle of internal friction. Deformation of samples during constrained dilatancy tests always took the form of barrel-like billowing. Parallel to the increase of vertical pressure this billowing grew more pronounced together with the growth of the sample volume.