Giant hole in seawall
AT insists damage to structure supporting ferry building no risk to public as it prepares to start $ 1m repair job
Auckland’s Ferry Building may have to close after a huge hole caused by propeller wash from ferries was discovered in the base of the seawall supporting the historic building.
Documents obtained by the Weekend Herald show the hole was 16m long and up to 4m high in August. It had doubled in size since its discovery in October last year when experts told Auckland Transport “it should be looked at immediately”.
Last night AT chief executive David Warburton said there was absolutely no suggestion of any imminent failure or public safety risk from the hole.
AT has started design work to repair the damage and plans to begin work next month and complete the job before the summer holidays. The repair work is expected to cost less than $ 1 million.
A report this month by engineers Tonkin + Taylor says there i s no immediate risk of the seawall failing, but the 1912 sandstone and brick Ferry Building is now heavily reliant on seismic strengthening done in the late 1990s for stability.
“T+ T strongly support the immediate repair of the seawall scour without delay. Further regression and undermining of the foundation soil may result in movement and damage of the seawall and supported property,” the firm’s senior geotechnical engineer Cliff Edwards said.
Edwards said if the wall continues to scour then stability will “decrease rapidly” and the Ferry Building and Pier 1a may have to close.
“As soon as possible, we recom- mend that ferry services at Pier 1a berth bow- in or avoid prop or bow thruster wash directed toward the seawall face,” he said.
T+ T also recommended that until the wall is repaired it be monitored for movement fortnightly and more detailed work be done to get a better idea of the “wall’s failure mode” and confirm foundation soil strength, backfill soil and load factors.
In response to findings in the engineer’s report, Warburton said if the engineers stated there was an imminent risk of failure and it needed to be closed for health and safety reasons, AT would act.
“But there i s no evidence to suggest that, so we are not.”
When asked what would trigger such action, Warburton said: “If an engineer said there was a risk of failure and it needed to be closed.”
It is not known how the immediate work could affect ferry services, including any changes to berthing procedures.
AT has been closely monitoring the seawall and last sent divers down t wo weeks ago for an update. No worsening to the scour was discovered since August.
The engineer’s report said scouring of the seawall was picked up t wo years ago by divers during installation of piles opposite the Ferry Building. It was the result of ferry “prop wash” following the relocation of the Pier 1a ferry berth closer to the Ferry Building.
The report said a survey by commercial dive engineers in August found a hole 16m long at the base of the 65m- long seawall. The hole extended 1.2m below the wall, eroding foundation rock, and is up to 4m in height. Other holes were found in the concrete seawall, constructed in 1906.
The foundation material was found to be “extremely weak” with one sample weathered to a residual soil with a significant number of wormholes, the report said.
The minutes of an Auckland Transport wharves maintenance meeting on October 4 last year reported video footage of the seawall showing a cavern of about 8m wide x 700mm high x 700mm deep. There was no damage in 2010, the minutes said.
Last October, construction firm STF said it considered the “damage to be serious and that it should be looked at immediately”.
Warburton said he was not aware of any further dive checks on the seawall between October last year and routine maintenance in August this year.
He said the hole last year was inconsequential given that Tonkin + Taylor said the hole had got bigger but the seawall was not at risk of imminent failure.
AT had acted prudently in terms of any danger to life or limb and the use of public money, Warburton said.
Auckland Mayor Phil Goff said he had received an assurance from AT chairman Lester Levy that the council body is acting on the recommendation from Tonkin + Taylor to immediately repair the seawall.
In August 2014, the Herald reported the downtown seawall between Princes and Marsden wharves needed seismic strengthening at a cost of $ 40m.
The council patched a section between Princes and Queens wharves costing $ 700,000 and planned to begin the upgrade proper in mid2015. The work has not started.