CLIENT
Middle Harbour Yacht Club (MHYC)
YEAR
2004
SCOPE
  • Landslip investigation

Projects

Harbour Yacht Club Project

Middle Harbour Yacht Club (MHYC), located in Sydney’s Middle Harbour, NSW experienced a major submarine landslip in 2004 whilst undergoing an extensive marina modernisation programme. 

During pile driving operations to replace the old timber piles with steel piles, a combination of jetting and drilling vibrations resulted in liquefaction of the bottom sediments, triggering a major submarine landslip which resulted in a backscarp forming rapidly within the public beach and narrowly missing section of a yacht club building. The submarine failure extended approximately 60m off the shoreline and resulted in the formation of a hole up to 6m deep. Sand debris from the failure extended a further 40m into Middle Harbour.
 
The Douglas Partners investigation concluded that situation had been an ongoing process for several decades and had been caused by the constriction of tidal flows at the nearby southern causeway and abutment of the Spit Bridge. Sediment accumulation had occurred against an old ferry wharf abutment and an old sea wall, constructed in the 1930s. Local tidal flow separations caused a near shore zone of more tranquil water at MHYC, and strong flood tide flows over the sand bar across the entrance to Middle Harbour carried suspended fine sand which was then transported to and deposited into the area. 
 
Dredging of the affected area to remove over 4000m3 of excess sand buildup in the near-shore zone together with underpinning of the existing clubhouse foundations were recommended by DP and have since significantly reduced the risk of any further landslip. The piling work has now been successfully completed and there has been no observable extension of the failure area.

 

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