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 no suggestion of any imminent failure or public safety risk from the hole.

T + T strongly support the immediate repair of the seawall scour without delay

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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.

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A report this month by engineers Tonkin + Taylor says there is no immediate risk of the seawall failing, but the 1912 sandstone and brick Ferry Building is now heavily reliant on seismic strengthening 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 recommend 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 said there was imminent risk of failure and it needed to be closed for health and safety reasons, AT would act.

"But there is no evidence to suggest that, so we are not."

When asked what would trigger such action, Warburton said: "If an engineer's said there was a risk of failure and it needed to be closed."

It is understood the changes recommended to how ferries berth are being followed.

AT has been closely monitoring the seawall and last sent divers down two 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 two 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 AT wharves maintenance meeting on October 4 last year reported video footage of the seawall showing a cavern about 8m wide by 700mm high by 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 this year Tonkin + Taylor said the hole had got bigger, yet the Seawall was still 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.

Local businesses had not heard of the hole when the Herald contacted them this morning.

A Tank employee felt it was slightly concerning they hadn't heard anything from AT about the hole, but assumed they would be contacted if repairs were needed.

Tui Tours manager Lorraine, who did not disclose her last name, wasn't too concerned.

The tour company sits inside the ferry building, but she felt it shouldn't affect business.

"[AT] are responsible for it."

St Pierre's Sushi manager Oay Bhaita was yet to be informed of the hole in the seawall.

"No, we haven't heard anything about it."

Two local cafes also said the Herald's call was the first they had heard of it. A Quay St coffee shop manager hoped it was minor so it wouldn't affect business too much.

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 mid-2015. The work has not started.

Background

The seawall for the Ferry Building was constructed in 1906 comprising concrete and basalt boulders embedded in sandstone/mudstone.

The seawall is 800mm north of the face of the Ferry Building and about 15m from Quay St.

The seawall measures 65m long with a 14m long return at each end to connect to Quay St.

Based on historic drawings the wall is between 6m and 9m in height. The base is estimated to be between 3.2m and 3.5m wide.

The hole, as of last month, is 16m long and located at the toe of the seawall. It extends 1.2m beneath the wall, eroding the foundation rock. It is up to 4m in height. Other holes have been found in the seawall (no sizes given).

In August last year, the hole was put at 8m long, 700mm high and 700mm deep.

Seawall structure

•The seawall for the Ferry Building was constructed in 1906 comprising concrete and basalt boulders embedded into sandstone/mudstone.
•The seawall is 800mm north of the face of the Ferry Building and about 15m from Quay St.
•The seawall measures 65m long with a 14m-long return at each end to connect to Quay St.
•Based on historic drawings the wall is between 6m and 9m in height. The base is estimated to be between 3.2m and 3.5m wide.
•The hole, as of last month, is 16m long and located at the toe of the seawall. It extends 1.2m beneath the wall, eroding the foundation rock. It is up to 4m in height. •Other holes have been found in the seawall (no sizes given).
•In August last year, the hole was put at 8m long, 700mm high and 700mm deep.