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 CASE G7 - Lateral Earth Pressure and Structural Adequacy of Drydock Wall, A. H. Wu
Problem: Determination of earth pressure and the structural adequacy of the walls
of an existing drydock (Mare Island Naval Shipyard, Drydock No. 3)
Symptoms:
Elementary analysis indicating structural inadequacy.
Collection of Facts:
1. The methodology for determining dynamic loading acting on a massive drydock
wall, which retains submerged backfill, has not been well established or verified.
2. Previous analyses did not consider the following which affect the stability
of the wall against lateral pressure:
a. Two rows of closely spaced timber piles support the outer crane rail
surrounding the dock. The configuration of these piles should attenuate the effect
of the postulated seismic induced dynamic earth pressure.
b. The rail support beam of the outer rail is tied to the top of the wall
by a series of struts. The rail support beam, pile caps, struts and piles
constitute a system which will restrain the top of the wall and resist lateral
movement and overturning.
c. Total liquefaction of the soil behind the walls is not likely to occur
and therefore, it should not control the analysis of the structure.
d. It is reasonable to assume tension potential in concrete, when a
seismic safety evaluation against a postulated risk of an existing concrete
structure is made.
Solution:
1. Take concrete core samples at the critical plane to determine the tensile
and compressive strength of concrete. (Approximate cost of sampling and testing is
$15k)
2. Conduct field check to determine if reinforcing steel exists around the
flooding tunnel by using a pachometer (reinforcing steel locator).
3. Install two strong-motion accelographs; one on the drydock and the other at
a free field location in the Navy Yard. (Approximate cost of installation by NCEL
is $6k)
4. Perform periodic surveys of wall movement and rotation during the dock
flooding and dewatering cycles. (May use tiltmeter)
5. Annually and after each earthquake of magnitude five or greater on the
Richter Scale, record cracks, if any, in the wall and floor of the drydock,
pumphouse and tunnels equal to or greater then l/8 inch in width.
The above methodology was implemented and it was verified that the drydock wall
was structurally adequate.
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