I met with Ashraf Doureihi from the North Sydney Council, which is the Council that managed the McMahons Point seawall.
Most seawalls around the harbour are 100 year old heritage sandstone walls. They have stood the test of time but over the years and with increased wave action, a lot of the material held back by the seawall begins to siphon out through the joints.
A typical scenario for a Sydney sandstone seawall will begin showing signs of fill failure by sunken holes being present at the pedestrian level. The Councils use a pre-emptive program to save walls before they collapse, avoiding having to rebuild them from scratch. They have been using the grout penetration process for about 17 years now, which was an in house design. Grout penetration consists of drilling 100 millimeter hole casings 1 meter behind the seawall all the way down to the foundation every 3 meter spacing. They then pump grout underneath using a tremmie and a special mix that has been adjusted through trial and error.
The grout penetration then goes through 3 stages:
1 – Build a concrete toe grout at the foot of the existing seawall at .25 meter height
2- Pump/inject grout into all voids in the foundation
3- Raise tremmie to fill all holes along length.
The average cost of rebuilding a failed seawall is around $12,000-18,000 per linear meter. The grout penetration greatly reduces the cost to around $3,000-6,000 per linear meter and preventing any injury to pedestrians. There is a very low 5% rate of failure in grout penetrated seawalls.
At McMahons point, sections of the seawall had collapsed and the sea bed floor was extremely weak. This was a unique case that needed special attention since the sea floor had no bearing capacity and could not be knocked down and rebuilt. The North Sydney Council installed steel I-beams by hammering them 8 meters deep on average until they hit bedrock. They were tied back 10 meters to concrete anchors installed underground every 5 meters. Then, L-shaped precast concrete panels were attached to the steel beams. Sandstone blocks were then laid on the bottom section of the L and built vertically. Several blocks were left out for environmental purposes as they had no structural significance. They also incorporated a rock armor along the bottom of the seawall to protect against an abundance of wave action.
This 150 meter section of seawall cost around 2 million dollars, or $20,000/linear meter. This cost was justified because of the prominence of this spot in the Sydney Harbour. It had to be built vertically as it is prime real estate and could not take up pedestrian space with a sloped seawall. The site had to closed down for 3 years to raise funds and was built in 4 months, completed in 2006. This project shows a great 12 years relationship with the University of Sydney and the biological strength of seawalls. The site used to have zero marine life and has now blossomed with habitat. It is a great example of an urban vertical seawall that provides biodiversity.
The success of the project hinged on a great contractor and project manager that overcame hurdles easily. One hurdle included encountering a large sewer pipe in the way of the tie backs. This included convincing the Sydney Water Company of the benefits of this seawall/
This project could definitely be adjusted to fit the Alaskan Way Seawall. Ashraf explained that the sea wall depth bears no significance on the project, as the structural bearing relies completely on the steel I-beams, which could be drilled to any depth needed.
Ashraf sketched some images of the different seawall options and said he could provide me with the engineering drawings for the McMahons seawall. Once I return to the States, I will scan and upload said images.
Ashraf Doureihi is the Design & Investigations Engineer for the North Sydney Council.