Marlborough Road Recovery

The Brief

We were engaged to play a crucial role in providing geotechnical engineering expertise for the restoration efforts at the picturesque French Pass, located in the Marlborough Sounds. 

Challenges Identified

The site, a historically landslide-prone gully, was affected by the severe weather event in 2021. Excessive rainfall led to the oversaturation of the underlying colluvium (loose sediment), resulting in a substantial underslip that impacted approximately 70 metres of the road alignment, subsequently reducing it to a single lane. 

What We Did

Due to the vulnerability of the site, the chosen remediation strategy involved repositioning the road by cutting into the existing hillside and restoring it to its prestorm conditions. 

A 1.7-metre-high timber retaining wall was constructed at the most 'at-risk' point of the site, with increased drainage and concrete-lined swales. Loose and weak soils were removed from the exposed faces and hydroseeded, a planting process and erosion control technique, to decrease erosion and improve the overall site slope stability.