EARTH PRESSURES 249 
making many special tests. All these tests indicate that China clay behaves 
as avery fine wet granular material (for definition, see p. 250) ; that is tosay, 
exhibits compactability and dilatancy and cohesion due to the water content. 
As dry, wet and moist sand (as defined later) all exhibit the same phenomena, 
it is probable that all soils exhibit the fundamental properties of granular 
materials. 
Cohesive Granular Material. 
The cohesive materials here discussed are only the granular materials 
rendered cohesive by the presence of water or other liquid; all soils, 
including clay, except when baked dry, come under this definition, also 
freshly mixed cement, mortar, and probably concrete and plaster, before 
they set. Tar-macadam and such road materials are included. 
Just as the old theories of the mechanics of dry granular material, such 
as Rankine’s, are necessarily imperfect because they neglect the effects of 
compactability and dilatation, so the old theories of cohesive granular material, 
arrived at by endowing such dry granular material with a shear strength, ' 
are also imperfect and may lead to very erroneous results. The writer 
has come to the conclusion that there are certainly two (possibly more) 
different types of cohesive granular material of common occurrence which 
possess quite different properties ; they may be called : 
1. Moist granular material, and 
2. Wet granular material. 
1. Moist Granular Materials —The common example of this type is damp 
(not saturated) sand. It has been much studied in agricultural research. 
Each grain is wet and where they touch a little disc of water forms, bounded 
by an annular meniscus. ‘The surface tension on this meniscus exerts 
a small force, drawing the grains together. ‘The magnitude of the forces 
and their dependence on the size of the grain and the amount of water is 
discussed in Fisher’s papers, ‘On the Capillary Forces in an Ideal Soil’ 
(Fournal Agricultural Science, 1926, pp. 492-503 ; 1928, pp. 406-410). The 
“voids ’"—i.e. spaces between the grains—are filled partly by air and partly 
by water. Ifthe quantity of water is sufficient to fill the voids, thus excluding 
air, the meniscuses disappear and the conditions entirely change. If the 
water dries up the meniscuses disappear and the conditions change to those 
of ordinary dry granular material, except when the material sets solid, which 
is notably the case when there is very fine granular material present—i.e. 
colloidal material—which ‘ glues ’ the grains together. 
Compactability —Moist granular material is compactable, like dry granular 
material. During compaction the percentage of voids decreases and more 
points of contact arise, so that the cohesive forces change. As the packing 
gets still closer a state may be reached when the voids are entirely filled 
with water, and cohesion will disappear. ‘The material then ceases to be 
moist granular material. 
Dilatancy.—Moist granular material exhibits dilatancy just as dry granular 
material does. During dilatation the number of meniscuses is reduced and 
the cohesive forces change. Saturated granular material may be converted 
into moist material by dilatation, the free water being sucked into the voids, 
followed by air. This phenomenon was described by Osborne Reynolds 
in his original paper.” 
An admirable material for experiments on this type of granular material 
may be made by stirring a few drops of olive oil into a beaker-full of the 
minute spherical beads known as ‘ glistening dew ’ (vide Proceedings Royal 
2 Videvol. 2 of Osborne Reynolds’ Scientific Papers, Cambridge University Press. 
