noted, the lower half (or its equivalent, _B D_{1} G B_) of the weight
of this is assumed to be carried by the structure, the upper half being
self-sustaining, as shown by the line, _B_{III} D_{IV}_ (or, for
absolute safety, the curved line), and therefore, if rods could be run
from sheeting inside the tunnel area to a point outside the line, _F
B_{1}_, as indicated by the lines, 5, 6, 7, 8, 11, 12, 13, etc., that
the internal bracing of this tunnel could be omitted, or that the tunnel
itself would be relieved of all loading, whereas these rods would be
carrying some large portion at least of the weight within the area
circumscribed by the curve, _D_{II} I T G_, and further, that a tunnel
structure of the approximate dimensions shown would carry its maximum
load with the surface of the ground between _D_{IV}_ and _F_, beyond
which point the pressure would remain the same for all depths.

In calculating pressures on circular arches, the arched area should
first be graphically resolved into a rectangular equivalent, as in the
right half of Fig. 4, proceeding subsequently as noted.

The following instances are given as partial evidence that in ordinary
ground, not submerged, the pressures do not exceed in any instance those
found by the above methods, and it is very probable that similar
instances or experiences have been met by every engineer engaged in
soft-ground tunneling:

In building the Bay Ridge tunnel sewer, in 62d and 64th Streets,
Brooklyn, the arch timber bracing shown in Fig. 1, Plate XXVI, was used
for more than 4,000 ft., or for two-thirds of the whole 5,800 ft. called
for in the contract. The external width of opening, measured at the
wall-plate, averaged about 19 ft. for the 14½-ft. circular sewer and 19½
ft. for the 15-ft. sewer. The arch timber segments in the cross-section