  A recently proposed model showed how a clay shrinkage curve is transformed to
the soil shrinkage curve at the soil clay content higher than a critical one.
The objective of the present work was to generalize this model to the soil clay
content lower a critical one. I investigated (i) the reference shrinkage curve,
that is, one without cracks; (ii) the superficial layer of aggregates, with
changed pore structure compared with the intraaggregate matrix; and (iii) soils
with sufficiently low clay content where there are large pores inside the
intraaggregate clay (so-called lacunar pores). The methodology is based on
detail accounting for different contributions to the soil volume and water
content during shrinkage. The key point is the calculation of the lacunar pore
volume variance at shrinkage. The reference shrinkage curve is determined by
eight physical soil parameters: (1) oven-dried specific volume; (2) maximum
swelling water content; (3) mean solid density; (4) soil clay content; (5)
oven-dried structural porosity; (6) the ratio of aggregate solid mass to solid
mass of intraaggregate matrix; (7) the lacunar factor that characterizes the
rate of the lacunar pore volume change with water content; and (8) oven-dried
lacunar pore volume. The model was validated using available data. The model
predicted value of the slope of the reference shrinkage curve in the basic
shrinkage area is equal to unity minus the lacunar factor value, and is between
unity and zero in the agreement with observations.
