Canal Walls

Sections of the canal walls at Port Geographe show signs of deterioration and will require rectification at some stage in the future. In the case of residential lots, maintenance rests with the landowner. However the deterioration is particularly evident in public open spaces (POS), where responsibility for repair and maintenance rests with the Council.  . 

Canal walls are built with reconstituted limestone blocks which are made from crushed limestone bound with a cementing agent. The limestone particles typically have a calcium carbonate content of about 65%. The limestone is inherently soft and susceptible to various forms of weathering. 

At Port Geographe the main cause of deterioration is due to salt weathering (technical term haloclastic weathering).  In the splash zone at the top of the wall, seawater enters cracks and voids in the porous blocks and mortar.  As the water evaporates, salt crystals of various chemical compositions form.  These crystals expand in hot weather causing cracks to open further, thus accelerating the salt weathering and physical abrasion. . 

Most weathering occurs at the eastern terminations of canals, and on the promontories of the small re-entrant coves off the main canals. It is particularly evident with the decorative capping blocks used on the canal walls near the footbridge, where the long fetch in the main canal together with wave reflection off the footbridge piers results in vigorous wave splash.

To assist Council in assessing this problem, PGLOA has documented several examples of deterioration in a report entitled Canal Wall Defects on Public Open Space, Port Geographe, 2020  and can be seen here.

In the first instance Council should replace the highly weathered capping blocks on the Eastern Footbridge reserve, as part of the landscaping scheduled in the 2020-21 budget.  Council could consider using rectangular half blocks in lieu of decorative blocks

In older POS areas (north of the footbridge) where there is no capping, placement of exposure grade capping blocks, or rendering the top of existing blocks with Coastal M4 mortar, should be considered.  

In the newer areas (south of the footbridge) replacement of the weathered rounded capping blocks should be considered.  A special order may have to be placed to manufacture blocks suitable for use in an exposed location.  PGLOA is making enquiries to see if a source of exposure grade capping blocks can be identified