The works involved repairs to address a partial failure of the wall, which occurred in December 2013.
The existing seawall consists of masonry blocks of psammite with cement mortar and extends over a total length of approximately 140m. The existing gravity retaining wall varies in height along its length between a minimum of 0.15m and 3.93m. The wall provides support between the road, which is the only access to Scotstown and the rock beach area below.
The works involved repairs to address a partial failure of the wall, which occurred in December 2013. This resulted in a localised breach in the wall, approximately 40m in length, with backfill partly washed out exposing the services located behind the wall. The remaining intact sections of the wall of 70m long to the east of the breach and 30m long to the west were identified as being in poor condition. Vertical cracks and bulges on the face of the sections of wall were evident in addition to significant losses of mortar to the joints.
The failed section of the seawall was replaced with a new cantilever retaining wall constructed in place of the existing partially collapsed wall. The height of the new wall along the 38.6m collapsed section varies between approximately 2.7m and 3.9m. This section of wall was founded on concrete blinding.
On either side of the cantilevered section of wall, 3m lengths of 700mm thickly reinforced concrete facing were constructed directly against the front face of the existing wall. Shear ties were used to connect the existing masonry and new concrete walls together. These sections of wall were founded on a concrete plinth, dowelled to the existing bedrock. The works were detailed to avoid breakout of the existing bedrock as far as is reasonably practicable, however, works still required a small amount of rock preparation dependant on site conditions.
The works entailed excavation adjacent to the existing roadway, with a requirement that one lane remained open to traffic at all times. A pressurised PFI sewer was in close proximity to excavations required for the works and further investigations were carried out to ascertain the precise locations and take this main into consideration in the temporary works design, protecting this main as necessary. One lane of the road and adjacent footpath on the opposite side from the wall remained open at all times during the works and therefore traffic management was a project requirement. The site boundary was enclosed with temporary fencing to exclude the public from the site.
The works were preceded by a condition survey of the wall and wider site. Existing road drainage was maintained through the wall and further weep holes were installed close to base level. The remaining sections of unstrengthened seawall are due to be remediated in future works phases in a similar manner to the 3m strengthening sections constructed at either end of the new wall as part to of this project.
This project is similar to the Manchester Road Retaining Wall Project as it involved the following elements:
- Construction of a retaining wall
- Working adjacent to live services
- Footpath and roadworks
- Traffic management