248 REPORTS ON THE STATE OF SCIENCE, ETC. 
Introduction. 
So far as the writer is aware there has never been any thorough investiga- 
tion into the mechanics of granular material. The importance of the 
subject may be indicated by giving a list of some of the subjects for which 
a theory of the mechanics of granular material is wanted. 
1. Foundations (bearing pressures of soil). 
2. Retaining walls, dock walls, etc. (horizontal earth pressures). 
3. Earthworks (railway cuttings and embankments). 
4. Landslides and their prevention. 
(Items 1-4 have been entered without qualia tee since clay 
and other cohesive soils have now been shown to be granular 
materials (see p. 249).) 
5. Cement, mortar and concrete (grading, ramming and measuring 
workability). 
6. Roads (foundations, ballast, ramming and rolling both foundations and 
concrete and surface material). 
7. Silos, bins and hoppers for storing grain, coal, road-metal, etc. Design 
of the buildings and design of the valves and chutes for controlling 
the outflow. 
Dry Granular Material. 
The two physical properties of granular material on which their remark- 
able mechanical properties mainly depend are their compactability and 
dilatancy. 
Compactability denotes the most widely known property of granular 
material, namely, that its specific volume depends on the closeness of the 
packing of the grains. How to produce closest packing is unknown, and 
the primitive methods of ramming and shaking! are still used, though - 
anyone who has experimented with them knows how uncertain their results 
are. Specific volume and closeness of packing are often measured by the 
‘ percentage of voids,’ the term ‘ void ’ being used to denote the spaces not 
occupied by the solid grains; the spaces are still called ‘ voids’ when 
partly or completely filled with water. 
Dilatancy denotes the converse property of all granular material (except 
when very loosely packed) of expanding in volume when its shape is changed, 
i.e. when it undergoes shear strain. 
Dilatancy has been discussed by the writer in his paper on ‘ The Pressure 
Exerted by Granular Material,’ Proceedings Royal Society, vol. 131, 1931, 
p. 53. Dilatancy causes granular materials to move in jerks instead of 
uniformly ; a familiar example is the alternate building up and collapse 
of the cone of sand in the bottom of an hour-glass. The cyclic movement 
of sand slipping down against a retaining wall is described in the writer’s 
paper on ‘ The Pressure on Retaining Walls,’ Proceedings Institution of Civil 
Engineers, vol. 234, 1931-32, Part 2, p. 103. This cyclic motion provided 
the clue to the theory of pressures on retaining walls given in that paper, 
which the writer believes to be the first paper to take account of the actual 
behaviour of granular material. 
The importance of dilatancy has been further emphasised by the writer’s 
recent discovery that clay exhibits this phenomenon. He has been working 
for the past eighteen months exclusively on the mechanics of plastic China 
clay and has tested it in compression, tension, simple shear, shear plus 
end compression, and in compression plus hydraulic pressure, besides 
1 Cf. ‘Good measure, pressed down, and shaken together’ (St. Luke vi. 38). 
