Case Study: St. Aubin’s Harbour Harbour, Jersey

The strengthening of heritage marine structures in Jersey with needling technology
The three structures are the North Pier and South Pier of St. Aubin’s Harbour and the St. Aubin’s Fort Breakwater. Both structures are exposed to waves from the south west and act as breakwaters.

In 2001 the North Pier was assessed as being in a state of instability due to movement of its internal wall and apparent settlement of the interior of the pier.

Large cracks had developed and could be seen in the asphalt surfacing indicating a threat of impending wedge failure of a section of the inner wall.

St Aubin’s Fort Breakwater has in turn suffered a re-occurrence of masonry movements and loss of pointing at its inner, lower deck wall. In 2008 a bulge was also identified in part of this inner wall which prompted concerns about this part of the breakwater’s structural integrity.

The States of Jersey Planning and Heritage Departments, who asked for a ‘tying in place’ solution to be investigated for stabilising the inner wall. Environmental concerns regarding the endangered mollusc species further reinforced this argument.

The chosen ‘stitching’ solution used Cintec anchors consisting of a number of solid stainless bars contained within a fabric sleeve injected with cementitous grout after positioning. The bond of the grout that seeps through the fabric sock used in the anchor between the masonry and the reinforced anchor is very high tying areas of loose masonry blocks together. This ‘stitching’ methodology was also able to provide a foundation solution; the numerous small diameter vertical anchors become mini piles when drilled down through the granite wall stones, through the beach deposits and into the Jersey rock shale beneath. The natural arching of the To resist the inner wall overturning, after pinning the base of the wall with vertical mini piles, requires a form of top restraint in the outward direction. The final design consisted of horizontal ‘passive’ ties from the inner wall to the outer face although inclined raking anchors through the core of the Pier tied into the Jersey rock shale were also considered.

The ‘stitching’ anchor solution provided clear benefits allowing the structure to be anchored and stitched together without changing its character and with minimal impact. The number and spacing of anchors, was optimised to satisfy the onerous load criteria imposed by the sea conditions in such