phenomena. He believes that, both in experimenting and observing, the
engineer will frequently find what is being looked for or expected and
will fail to see the obvious alternative. He may add that his own
experiments and observations may be criticized for the same reason, and
he asks, therefore, that all possible light be thrown on this subject. A
comparative study of much of our expert testimony or of the plans of
almost any of the structures designed in connection with their bearing
upon earth, or resistance to earth pressure, will show that under the
present methods of interpretation of the underlying principles governing
the calculations and designs relating to such structures, the results
vary far too widely. Too much is left to the judgment of the engineer,
and too frequently no fixed standards can be found for some of the most
essential conditions.

Until the engineer can say with certainty that his calculations are
reasonably based on facts, he is forced to admit that his design must be
lacking, either in the elements of safety, on the one hand, or of
economy, on the other, and, until he can give to his client a full
measure of both these factors in fair proportion, he cannot justly claim
that his profession has reached its full development.

Table 1 gives approximate calculations of pressures on two types of
tunnels and on two heights of sheeted faces or walls, due to four
varying classes of materials.

TABLE 1.--PRESSURES ON TYPICAL STRUCTURES UNDER VARYING ASSUMED
CONDITIONS.

[Illustration: Key to Table of Pressures, etc.]