EARTH PRESSURES 251 
conditions supervenes : (i) either the suction reaches the maximum value 
which the surface meniscuses can bear, in which case they break and air 
enters the mass ; as the shear strength can get no greater the mass fractures, 
the air enters the plane of fracture and the break looks like a break in stone ; 
or (ii) the dilatation reaches its full value, the suction ceases to rise and the 
strength also ceases to increase ; the mass then yields freely in shear ; it is 
not broken intwo. Both types of failure have been produced experimentally 
in compression test-pieces ; the first is typical of non-plastic materials and 
the second of plastic materials, such as clay. 
At first sight the conception of simultaneous dilatation and compaction 
may appear paradoxical and unnecessary, but further consideration leads to 
the conclusion that it is the obvious way of accounting for the observed 
facts that the suction rises and the strength increases while the volumes 
cannot change. An interesting comparison may be made with the experi- 
ment quoted on p. 54 of the author’s Royal Society paper (Joc. cit.), in which 
an attempt was made to shear dry sand in a closed vessel; dilatation 
prevented any motion till the sand crushed under a very large stress (the 
sand in that experiment had been closely packed so that further compaction— 
except by crushing—was impossible). 
This brief outline of what happens when wet granular material is sheared 
leaves out of account many factors which probably play important parts. 
The shape of the grains, and possibly their mechanical deformation, is 
believed to be important in determining the plasticity of clay. Again, the 
grains in clay are so small that forces which may be called molecular or 
electric must play an important part. What the relative importance of the 
different factors may be has not yet been determined ; all that the writer 
claims is that China clay has been proved to exhibit compactability and 
dilatancy and must be classed as a wet granular material, whatever other 
properties it may have. The fact that China clay turned out to exhibit 
dilatancy, contrary to expectation, was the reason that the whole series of 
researches just completed at the Building Research Station turned ‘out to 
be inconclusive. 
Conclusion. 
Now that the finest ground sand and the much finer China clay have 
been ‘proved to exhibit compactability and dilatancy the writer has no 
longer any doubt that cements, plasters, mortars and concretes will all be 
found to exhibit the characteristic properties of granular materials, and 
that when mixed with water they will be found to belong to the classes of 
moist or wet granular materials as defined above. 
It would be difficult to name any fundamental research that has such a 
close connection with buildings and roads as the investigation of the 
mechanics of granular material. The writer hopes that the work may be 
successfully completed at the Building Research Station. 
