Project Overview: Landslide-Damaged Road Rebuilt with MagnumStone Blocks

Due to several landslides caused by extreme rainfall, a critical section of the Tapu Coroglen Road on New Zealand’s Coromandel Peninsula needed immediate reconstruction. One of the larger landslides, located on a sharp curve near the town of Tapu, presented the most complex challenge. MagnumStone’s retaining wall system expedited construction to deliver a curved, terraced gravity wall solution that stabilized the slope.

With State Highway 25A closed, vehicles used this narrow gravel road as a shortcut to the coastal peninsula’s eastern communities. Engineers needed a fast, durable, and compact soil retention solution to navigate steep, difficult terrain and restore road access. The design versatility and quick installation of MagnumStone’s reinforced gravity walls, with blocks supplied by Cirtex, were vital to the project’s fast-tracked completion.

Challenges & Solutions: Navigating Curves, Slopes & Space Constraints

The Tapu Coroglen Road project presented several complex challenges due to its remote, location and the aftermath of severe landslides. With such tight site access, equipment and logistics required careful planning to securely rebuild the roadway. Due to its sensitive site conditions, the infrastructure project’s major obstacles included:

• Steep terrain with limited working and staging areas
• A tight curve in the road at the landslide site
• Need to retain both lower and upper slopes to prevent further erosion
• Rebuild the existing road while minimizing excavation and land disturbance
• Construction access complications and delivery logistics

Engineers needed to retain both the lower and upper slopes and limit further excavation, which could have led to additional slope instability. A timber piled wall was built to retain the upper slope, using an Armourflow Panel Drain to convey future runoff. The lower wall required a sophisticated stepped retaining wall design using MagnumStone’s hollow core blocks and gravity extender reinforcement units.

Navigating the roadway’s tight radius and constrained space required design modifications and on-site block alterations once construction began. These were successfully dealt with and completed to ensure the project’s complex structural integrity remained uncompromised. The exceptional teamwork of engineers, installers and MagnumStone’s precast block system achieved an outstanding solution for this difficult landslide repair.

Wall Design: Slope Stability & Roadway Reinforcement

To address steep, contoured retaining wall design challenges along the lower slope, the engineering teams developed an innovative solution. HD Geo designed a MagnumStone gravity wall using the system’s versatile gravity extender units for reinforcement. The wall extends nearly 120 ft. (36 m.) and reaches a maximum height of 24 ft. (7.2 m.). In total, the wall rises a total of 33 ft. (10 m.) with the sloped curve’s base elevation changes with a stepped wall design for every second course.

MagnumStone has a built-in 2.4° batter, with SecureLugs beneath each block resting inside the hollow cores of the blocks below. The stepped transitions aligned differently, as the SecureLugs of blocks above were pressed firmly against the back of blocks below. This created the necessary setback angle between courses to align with the road’s reconstruction.

Drainage piping was placed throughout the blocks’ hollow cores and filled with crush gravel to enhance drainage and wall durability. Each exposed step was then sealed with a 4” (100mm) concrete infill to seal exposed areas. This outstanding slope stabilization project restored Tapu Coroglen Road, demonstrating impressive engineering and the MagnumStone precast retaining wall system’s design versatility.

Installation: Precast Blocks Deliver Fast-Tracked Solution

Installation of the MagnumStone wall required plenty of coordination due to the site’s limited access and steep terrain. After the landslide debris was cleared and the timber piled wall was completed on the upper slope, access tracks were cut into the lower slope to begin wall construction. The base course of MagnumStone blocks was set on an embedded concrete leveling pad for a stable foundation. Due to site accessibility complications, blocks were lowered from above using long chains, and deliveries were staged to streamline logistics.

As the wall was built, the road’s tight radius created challenges aligning the block’s SecureLugs and extender units. The curvature caused the SecureLugs to deviate from the blocks’ hollow cores below, disrupting the system’s interlocking connection. With input from Cirtex’s design team, the installation crew was able to adapt quickly. They trimmed small sections of the extender interlock units which allowed the SecureLugs to connect without compromising the wall’s reinforcement.

With blocks installed, a 360° tiltrotator accelerated the placement and precision of drainage gravel around MagnumStone’s gravity extender units. A drainage bucket and hopper efficiently dropped crush gravel into the hollow core blocks to increase mass and drainage filtration.

The upper slope was then hydroseeded for erosion control, and native vegetation was planted below the MagnumStone wall. A new steel-post guard rail was also installed above the MagnumStone wall. After being paved, Tapu Coroglen Road was officially reopened – restoring safe access to the eastern coast of Coromandel.

Aesthetics: Rugged Textures for Rugged Landscapes

MagnumStone’s natural stone aesthetic facings give the lower retaining wall a look that blends into the local forested landscape. While largely out of public view, the block appearance matches Coromandel Forest Park’s lush, rugged terrain.

Top-of-wall details, like the gradual step-downs using MagnumStone’s half-high blocks, complement the curve’s contoured wall design. Working in tandem with the road’s freshly paved surface, new guard rails and timber wall, the MagnumStone retaining wall soundly fits into the scenic peninsula’s remote countryside.

Project Summary: MagnumStone Retaining Walls Restore Road Infrastructure

To repair a severe landslide on New Zealand’s Coromandel Peninsula, MagnumStone’s proven retaining walls delivered urgent slope stabilization and roadway reconstruction. The project’s remote location, steep terrain, and a sharp road curve posed significant engineering and access challenges. Reinforced MagnumStone walls with gravity extender units provided a fast, durable solution, to build a stepped, curved retaining wall solution.

The blocks’ on-site adaptability, combined with the system’s quick installation, allowed engineers to meet tight timelines and space constraints. This creative wall design by experts at HD Geo, Pinnacles Civil and local MagnumStone producer, Cirtex, successfully overcame complex logistical and site conditions. MagnumStone’s proven precast system complemented the project’s rugged landscape as a long-term solution to safely restore access to Tapu Coroglen Road.

Engineers and architects can save valuable time and resources planning retaining wall projects with MagnumStone’s in-depth Retaining Wall Design Software. Learn more about the system’s block specifications, testing and CAD details.

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This project was successfully completed thanks to Cirtex (MagnumStone Producer), HD Geo (Wall Designer) and Pinnacles Civil (Civil Engineer).