The San Francisco call. [volume] (San Francisco [Calif.]) 1895-1913, 1899-04-02

rotted and sank into the mud. Even to-
day, when driving piles for a foundation
it is no uncommon thing for the contract
ors to find their progress stopped by the
ttmbers of some old ship, buried many
feet below the surface.
1t can easily be understood how this
mass of decomposing material has allow
ed itself to be compressed by the weights
placed upon it. Then, until the seawall
was built in 1877, there was nothing to
prevent the waves of the bay washing
out the filing, just as they are doing to-
day east of Mission, where the protecting
wall is still lacking. Even as it is, the
seawall does not form an absolutely per-
feet barrier The tide finds its way
through the interstices of the loosely laid
stones and bears seward each day a
certain proportion of mud. The concrete
wall erected opposite the new ferry depot
affords a better safeguard against the
action of the waves, and if this were the.
only source of trouble would undoubtedly
check the subsistence of Market street.
But we must go still deeper to get at
the root of the evil. 1t is not merely. the g,
loosely made ground on top which is at
fault The trouble is that this filling,
unsatisfactory though 1t be, has no better
foundation to rest upon than soft mud.
1t is an actual fact that the bedrock
underlying San Francisco Bay has never
been struck. The metamorphic sandstone
which crops out on the side of Telegraph
Hill and rises again on the other side of
the harbor dips sharply under the water.
For countless centuries each rapidly run
ming tide has been bringing down its load
of mud from the upper reaches of the bay
and depositing it along the beaches
wherever the contour of the land checked.
for a while the rapid rush of the current.
The Yerba Buena indentation must have
received it's full share of this sediment.
soft, blue, greasy stuff, for no man with
certainty knows its depth.
Experts differ on the point Baron Uh-'
lg thinks the bedrock may be anywhere
from two to three hundred feet below the
city level. 1--WE cannot say accurately"
he explains, for as far as we have yet
gone down, we have struck nothing but
blue mud, gradually stiffening into blue
clay. In 1872, when taking a survey for
the proposed seawall, we made boring
two hundred feet, but never encountered
rock. In 1890, at another place on the
water front, we went down $50 feet with
the same result"
The contractors, who are always busy
driving piles on the water front, can give,
in spite of all their experience, no more
definite information. I'VE have never
struck bedrock anywhere. said a member
OIL one of the largest firms engaged in this
business. THE longest piles we have ever
driven have gone to a depth of eighty-five
feet. but we have never encountered any
thing but mud"
The experience gained in building the
seawall illustrates the unstable character
of the ground. To make this wall it was
necessary to dump huge scowloads of
rock into the bay, and to keep on dump
ing until the stone at last ceased TO set
tie. Often at the end of the, days work
the contractors would leave the wall
standing several feet out of the water.
When they returned to labor in the morn
ing the loosely piled rock would have
sunk to a depth of twelve or fifteen feet.
In one instance strip of the wall about
800 feet in length sunk bodily out of sight,
and the whole work had to be recom+
menced However, patience and an mex
haustible supply of rock triumphed in the
end, the mud was compressed Until it
would yield on longer and the seawaH
sank to it's final bearings
No man can say when the period of sub
sidence will cease-when the underlying
clay shall have been so firmly compressed
that it will yield no more.
Meanwhile, unless some change happens,
it may become necessary to follow the
example of Chicago. and to bodily raise
this low lying portion of the city to a safe
height above high water mark.