Conservation (Landscape) – Mitchell’s Causeway

The causeway structure at Victoria Pass consists of two main stone retaining walls either side of the highway, with other smaller, ancillary retaining walls higher up the pass. The structures were built by hand using convict labour to the design of Major Thomas Mitchell (the colony’s Surveyor General at the time) and the road was opened for use in 1832.

In 2010 RMS maintenance staff observed bowing in the buttresses of the main north wall of the causeway. This triggered an assessment of the stability of the causeway structure. Mott MacDonald inspected the structures in detail and confirmed that there was evidence that the northern walls and buttresses were shifting and potentially migrating downhill.

Due to the highly invasive nature of installation of any effective restraint to the structures against this migration, it was decided to confirm the movement by repeated survey and to develop a long-term strategy for the management of the asset that:

• Ensured an appropriate level of conservation works were carried out,
• Followed the principles of the Burra Charter, i.e, to carry out only as much work on the asset as necessary to mitigate against further deterioration,
• Provides a database and access strategy to inform and enable all future works,
• Initiated a process to ensure that any works carried out were necessary and fully informed, and precisely targeted to the areas needed.

Mott MacDonald proposed works to prevent ongoing destabilisation and deterioration of the structures, and upon approving the commission of the survey and stabilisation works RMS decided at the same time to take the opportunity to carry out additional conservation and restoration works on the causeway.

The final scope of works included:

• Removal of vegetation, rubbish and overburden from the walls and around their bases to enable survey
• Construction of access paths along the base of the main north and south walls to enable survey as well as ongoing monitoring and maintenance
• Repair of the damaged and dilapidated sandstone walls and buttresses (indents, stone replacement, joint repair, and some reinforcement stitching)
• Restoration of original stone parapets along the north wall using new and salvaged stone
• Protection works along the highway kerb to redirect stormwater away from walls
• Interpretation of original (now redundant) drainage systems through walls, and
• Protection of historic inscriptions.

After the review of environmental factors was completed to the satisfaction of the consent authorities the vegetation removal was carried out by RMS and the main works package was tendered. RMA Group were engaged in 2014 and were able to complete the works with minimal variation to the original scope and without significant disruption of road traffic along the great western highway. The works were completed by late 2016. A series of laser surveys have been conducted on the walls and to date have found that down-hill migration of the walls is so far negligible. The survey monitoring and vegetation control is ongoing.

Worked on the project:
Roads and Maritime Services, Mott Macdonald, RMA Group

What the Judges said:

“The historic sandstone causeway at Victoria Pass near Mt Victoria was hand built by convicts. After structural assessments and repair of damaged and dilapidated sections access paths allow exploration of the work of then and now and interpretation materials bring it to life. A very impressive piece of work.”