were 10 by 12-in. North Carolina pine of good grade, with 2 in. off the
butt for a bearing to take up the thrust. They were set 5 ft. apart on
centers, and rested on 6 by 12-in. wall-plates of the same material as
noted above. The ultimate strength of this material, across the grain,
when dry and in good condition, as given by the United States Forestry
Department tests is about 1,000 lb. in compression. Some tests[C] made
in 1907 by Mr. E.F. Sherman for the Charles River Dam in Boston, Mass.,
show that in yellow pine, which had been water-soaked for two years,
checks began to open at from 388 to 581 lb. per sq. in., and that yields
of ¼ in. were noted at from 600 to 1,000 lb. As the tunnel wall-plates
described in this paper were subject to occasional saturation, and
always to a moist atmosphere, they could never have been considered as
equal to dry material. Had the full loading shown by the foregoing
come on these wall-plates, they would have been subjected to a stress of
about 25 tons each, or nearly one-half of their ultimate strength. In
only one or two instances, covering stretches of 100 ft. in one case and
200 ft. in another, where there were large areas of quicksand sufficient
to cause semi-aqueous pressure, or pockets of the same material causing
eccentric loading, did these wall-plates show any signs of heavy
pressure, and in many instances they were in such good condition that
they could be taken out and used a second and a third time. Two
especially interesting instances came under the writer's observation: In
one case, due to a collapse of the internal bracing, the load of an
entire section, 25 ft. long and 19 ft. wide, was carried for several
hours on ribs spaced 5 ft. apart. The minimum cross-section of these
ribs was 73 sq. in., and they were under a stress, as noted above, of
50,000 lb., or nearly up to the actual limit of strength of the
wall-plate where the rib bore on it. When these wall-plates were
examined, after replacing the internal bracing, they did not appear to
have been under any unusual stress.