Project Case Study.

Residential property bordering Fendalton Stream
143b Straven Road

Structural Strengthening and Earthquake damage repair
Architect / Engineer – Soil & Rock
Project Cost: $500K


The Challenges.

We were contacted in 2019 to review the proposed design for replacing the retaining wall at 143B Straven road. The retaining wall was severely damaged during the 2011 earthquake and was held up in an insurance claim for 7 years. The wall was temporarily braced by EQC and was heaving from the amount of pressure the lateral movement had caused.

The wall is 3 meters tall and runs approximately 25 meters along the stream. There is a damaged property directly across the stream with a lower rock retaining wall that is original from 50+ years ago.

The retaining wall had a weak timber balustrade and an old deck alongside it and pathways and a garden. The driveway next to the wall was also damaged and separated from the earthquake and needed replacing as well.

Fulton Hogan Construction was engaged originally to help replace the wall and peripheral items. They submitted a vague proposal and charged the client many fees for the initial design and consent. This was all before an insurance settlement had even been reached. The owner wanted a second opinion and contacted Fitz Consulting to look things over.

There were many complexities to this project:

The area that was available to remove and replace the wall was extremely narrow and hard to access.

The stream would need to flow while the work was undertaken, which required temporary piping, pumping, and environmental management.

The design called for 14 Steel piles. The piles were 420mm “I” beams that were 9 meters long. They needed to be Galvanized and embedded into the ground at a depth of 6 meters below ground level. The cost of supplying the Steel Piles alone was $125,000.

The stream itself had old retaining boards that needed to stay in place as well as being removed along our new wall and not disturb the water quality.

We had to utilize the neighboring property to gain access to the rear of the property, which required temporary bridging and planking for a 24-ton digger. ECAN (Environmental Canterbury) conducted daily monitoring and required us to submit 3 water samples a day.

Redesign the entire garden area and custom design a new balustrade to run along the new retaining wall.

Adhere to the Covid-19 restrictions put in place following the 2020 level 4 lockdown.

Conduct a feasibility report that included: photos, videos, and detailed descriptions of every neighbor's property condition, before our project beginning. This would protect us from any scrutiny or claims made that we damaged anything during the undertaken work.

The Solution.

Before any work could begin, we engaged the only company we knew that could handle a project with all of these complexities... Hunter Civil is that company. Graham Hunter and his wealth of knowledge and experience laid the groundwork, and he scoped the project, unlike anyone else before him. Of the 4 contractors who looked at the project, Graham streamlined the approach and made it economical to achieve the design presented by Soil & Rock Engineers. With the help of Hunter Civil’s QS and our sub-trades, we submitted a realistic price to replace the damaged property items and helped the owner with his insurance settlement, and began the project just before Covid-19 struck NZ.

Scope of Works:

Temporarily bridge the stream with large wooden sections with steel beam under supporting, without disturbing the neighbors retaining wall.

Dismantle the existing wall in sections while installing the steel beams every 1.5 meters along the property.

Vibrate the beams into the ground, to a depth of 6 meters without disturbing the owner's house or any of the closely located neighboring homes, 9 in total.

Install a 1meter round, 40-meter long pipe into the stream to allow a continuous flow of water and protect the stream from any debris that would fall during the construction.

Install 200mm lagging boards in between the slot of the “I” beams and backfill behind the boards with compacted gravel and concrete.

Install nova flow piping behind the wall and divert it to a new sump along the stream wall.

Per ECAN requirements, native grasses and planting were required along the embankment of the steam that was not part of the wall.

Remove the entire garden, driveway, pathway, neighbors fence, and front entry brick wall for access.

Redesign: The garden, Balustrade, front entry and pathways.

The Conclusion.

The project started in Feb 2020. We successfully bridged the stream, installed the stream pipe, and began removing the old retaining wall and installing the new wall. Then Covid-19 struck. We had to go tools down for 2 months while the country went into lockdown. Upon our return to work, we were required to adhere to the guidelines set by the govt for contact tracing and proximity within the workplace. This added to our Administrative obligations, but we persevered.

Work continued for the next 6 weeks while the bulk of the wall was replaced. The time of year was closing in on the “Trout” spawning season and potentially could have delayed the project if our pipe disturbed the spawning process. It didn’t, and the project ran smoothly, even with the false accusations made by the unfair neighbor. Our feasibility report supported our claims that the damage he claimed we made was pre-existing.

Once we passed all of our inspections and ECAN was happy with our proactive approach to protect the stream and its surroundings, we started phase 2 of the project. Rebuilding: The deck, Front entry, Driveway, and garden area. With the help of our Landscape Designer and Structural Steel designer, we began Phase 2.

We built a custom Mondo composite deck with built-in Raised Garden beds. Once completed, we set out and poured exposed aggregate concrete pathways and driveway. We rebuilt timber fencing, which was painted all black, and brought in new compost for planting a new garden.

The front entry required a new brick/timber wall to match the house and tie the front of the home together.

Finally, our architectural steel company custom-designed a smart design for installing a balustrade along the new retaining wall. It included a 500mm wide metal flashing to cover the top of the wall for the 25 meters and then face mount welded supports to the wall. This was the base for the balustrade to be mounted upon.

The photos better explain the balustrade, but it is very impressive.

The project stayed within budget, and Hunter did an excellent job considering all the complexities involved with a custom project like 143b Straven Road. We love to take on projects like this and look forward to the next one.