Queensland’s Gold Coast is known for its surf beaches, elaborate inland canals and waterways. Revetment walls are a common feature in the region, playing an important role in protecting the boundaries of waterfront properties from the effects of erosion that can occur as a result of tidal movements and boating activity.
Over several years, canal water washed away fine sandy soil behind a revetment wall at a residential property in the Gold Coast suburb of Mermaid Waters. As a consequence, the property’s waterfront footpath and garden retaining wall began to subside, rendering the waterfront footpath unsafe for use.
The construction company managing the remediation works, contracted Mainmark to deliver a suitable solution for backfilling the affected area behind the revetment wall, which spanned 27m in length.
The solution needed to be able to withstand the wash out effects of the canal tides and remain firmly in place, providing long-term stability and strength to the revetment wall which supported the concrete footpath and waterfront fencing.
Mainmark’s Terefil® was identified as the ideal solution to support the property’s embankment and provide a suitable base for the concrete footpath. Terefil is an advanced and light-weight polymer modified cementitious filler that is impervious to water and delivers structural stability without adding significant weight to potentially overstressed subgrade soil.
Mainmark was required to replace the eroded sand fines behind the revetment wall with a lightweight void fill solution that would not erode or wash away. The solution also needed to have substantial compressive strength to support the wall and new footpath.
Due to the waterfront location, the Terefil solution needed to be applied efficiently within a 2-3 hour timeframe to avoid the fluctuating tides, while ensuring minimal disruption to the homeowner.
In order to prepare the site for the application of Terefil, preliminary works were undertaken by the construction company to remove the subsided section of the concrete footpath slab, as well as the waterfront fencing and garden retaining wall that had been impacted by soil erosion.
The hydro excavation methodology was used to excavate the loose sand from behind the revetment wall to a depth of 1m to expose the base of the wall.
The excavated area was lined with a geotextile fabric to provide solids control and prevent the Terefil solution from entering the canal system. The geotextile fabric has a high thread count to allow the canal water to pass through easily while preventing any erosion of the solid fill.
Terefil was then pumped through an application hose, and directly applied into the excavated area during low tide to fill the void behind the revetment wall.
Approximately 32m3 of the highly flowable Terefil was placed within an hour. Once in place and set, Terefil is environmentally inert, and continues to gain strength over time.
With a unique self-levelling ability that enables rapid and easy placement, Terefil has minimal shrinkage (typically less than 0.3%), ensuring the void could be completely filled with zero bleed, to avoid any water from ponding on the top surface of the grout as it sets, which can result in the formation of permanent gaps.
The location of the revetment wall presented a number of site access challenges. However, the Mainmark team was able to overcome the issue by using long hoses to apply the highly flowable Terefil solution.
Mainmark was able to complete the project within a single day, allowing the construction company to proceed with reinstating the footpath, fencing and garden retaining wall, to complete remediation works.
Project Resources & Downloads
Gold Coast Waterfront Revetment Wall and Pathway Stabilised with Terefil®
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Mainmark Ground Engineering Pty Ltd
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