The Western news. [volume] (Libby, Mont.) 1933-current, 1935-04-25

Hudson 1t Is combination suspension bridge
between Rncon hill in San Francisco and Yerba
Buenn island out In the middle of the bay ana
a cantilever bridge between the island and Oak
land Yerba Buena island will be crossed through
the largest vehIcuar tunnel In the world. the
bore being 76 feet wide and IS feet high.
The Bay bridge proper, including the island
crossing will be approximately four and ONE
half miles long but Its total length from the
end OF the western approach to the end OF the
eastern approach will be eight and one-fourth
miles long The bridge will be a doubledeck
structure with six lanes of automobile traffic on
the upper deck and three lanes OF trucks plus
two Interurban tracks on the lower deck.
The building of this bridge Involved some OF
the most difficult engineering problems ever at
tempted by man. Never before has bridge
been reared above piers which were sunk by the
caisson method from the surface OF the water
down through both mud and water to rock bot
tom, sight unseen Never before have suspen-
sion towers reared themselves Gifted by their
own boot-straps' might say until their
hollow steel frames rose sos feet above the wa
fer and their concrete bases sank In some cases
23J feet below water level.
No other bridge yet built has called for so
many PIERS 81 In number. In fact, 1t was the
belief for more than half century that lack or
adequate foundations In the bay for piers, as
well as the impossbllity OF stretching a span
from Yerba Buena to San Francisco would pre-
vent the bay from being bridged. But In 1929
survey of the bottom of the bay, directed by
state engineers, revealed high ridge OF bed
rock extending between San Francisco and the
island which would provide a foundation at
/. higher levels than surrounding bedrock and
make practicable route. The water along this
route Is from so to 105 feet deep and bedrock
lies from 100 to 200 feet below the mud on the
bottom Of the bay,
SO the most serious problem was the work
below the water level and the building OF the
five major piers between the island and San
Francisco Because OF the depth Of the water
and the mud "sandhogs" as underwater labor
ers are called who work In pressure chambers
to clear away the mud to the rock bottom. could
not be used. The solution of the problem was In
a compressed alrfotaton caisson method which
enabled the bridge builders for the first time to
construct their piers from the surface Of the
water down. using the jaws OF huge clamshe
buckets instead Of men to do the excavating.
Each casson consists of cluster of huge steel
tubes Is feet In diameter. held together by an
outer casing Compressed air made the caisson
buoyant as 1t was towed into place and an
chored In the space between the tubes con-
crete was poured forming an enormous box
something like an egg CARTON except that the
openings were round. Each tube was sealed with
an alright cap The concrete poured around the
tubes caused the calsson to sink slowly and rs
II sank additional lengths were added to the
tubes and more concrete poured around them.
When the bottom OF the casson sank to the
mud In the bottom of the bay a steel cutting
edge pushed down through the mud. The caps
were cut off the tubes and dredge buckets were
dropped down through them to scoop out the
mud below the caisson. Gradually each casson
was worked down through the mud to rock bot
tom. In one case 235 feet below the surface OF
the water Thus the piers were built