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.]

_h_ = exterior height, _l_ = exterior width,

{ [delta] = depth of cover, that is,
{ _D_{E}_ = earth, and _D_{W}_ = water depth,

[phi] = angle of repose, and, for tunnels _D_{W}_ > _D_{E}_ a depth

_l_ [phi]
= ----- ( 45 deg. + ------- )
2 2

_W_{E}_ = weight of 1 cu. ft. of earth = 90 lb.; _W_{W}_ = weight of 1
cu. ft. of water = 621/2 lb.

Conditions: 1 = normal sand, 2 = dry sand, 3 = supersaturated firm sand
with 40% of voids, 4 = supersaturated semi-aqueous material, 60%
aqueous, that is, 60% water and aqueous material.

_______________________________________________________
| | | | |
Combined | | | | |
assumed | _h_ | _l_ | [phi] | _D_{E}_ |
conditions. | | | | |
______________|________|________|________|____________|
| | | | |
I_{1} | 20 | 30 | 45 deg. | 40 |
I_{2} | 20 | 30 | 30 deg. | 40 |
II_{1} | 15 | 15 | 45 deg. | 40 |
II_{2} | 15 | 15 | 30 deg. | 40 |
III_{1} | 15 | | 45 deg. | 15 |
III_{2} | 15 | | 30 deg. | 15 |
IV_{1} | 30 | | 45 deg. | 30 |
IV_{2} | 30 |