Consequences of scalping and scalping/replacement procedures on strength properties of coarse-grained soils

Evaluation of shear strength characteristics of coarse-grained soils by performing
laboratory tests resorts often to a scalping or scalping/replacement procedure
due to the presence of oversized particles in comparison with the device size. This
work aims to study consequences of these two procedures on the resulting shear
strength characteristics. Experimental tests are performed on original soils having
gap-graded particle size distributions and on scalped and scalped/replaced
soils by using triaxial apparatuses with different diameters = 50, 100, 200 and
300 mm. Original soils are mixtures of fine particles (sand and glass beads) and
coarse particles (different types of gravel). Different factors that influence the mechanical
properties of the scalped and scalped/replaced soils in comparison with
those of the original soils are analyzed: (i) the compactness parameter controlled
for scalped and scalped/replaced soils, (ii) the fine content of the original soils,
(iii) particle properties of the fine and coarse particles, particularly properties
of the replacement material in comparison with those of the replaced material,
and (iv) the ratio of the maximum particle diameter dmax of the original soil to
the scalping diameter dscalp. It has been shown that the scalping method can
either underestimate or overestimate the shear strength depending on the fine
content of the original soil and on how the compactness state of the scalped soil
is controlled. The scalping/replacement method gives a good estimation of the
shear strength for soils with high fine content ff 60%. This method still works
for soils with low or medium fine content if the ratio dmax=dscalp 2. Otherwise,
it underestimates the shear strength since the void ratio of the scalped/replaced
soil remains higher than that of the original soil despite a great compaction effort.
Choice of the replacement material was found to be important, which can
affect greatly the shear strength of the scalped/replaced soil.