Strengthening a Critical Wellington Transport Corridor 

Tucked into Wellington’s steep hillside terrain, Grosvenor Terrace provides an important connection between Thorndon and Johnsonville. As part of Wellington City Council’s wider resilience programme, Retaining Wall 237 required strengthening to improve its seismic performance and reduce the risk of failure during a major event. 

Working alongside WSP, Contech delivered a comprehensive strengthening solution while maintaining traffic flow, protecting nearby residential properties, and minimising disruption to the local community. 

Delivering Complex Works in a Constrained Environment 

The project involved strengthening the existing retaining wall through a combination of ground anchors, reinforced concrete piles, new concrete buttresses, drainage improvements and targeted concrete repairs. 

The works were undertaken directly adjacent to an active road corridor and residential properties, with limited working space and challenging access conditions. Careful staging and detailed planning were essential to ensure Grosvenor Terrace remained operational throughout construction. 

The scope of works included: 

• Site establishment and pre-construction surveys 

• Traffic management implementation and staging 

• Vegetation removal and ground preparation 

• Installation and testing of proof and production ground anchors 

• Construction of reinforced concrete piles and concrete buttresses 

• Concrete repair works to the existing retaining wall 

• Installation of sub-horizontal drainage 

• Reinstatement of surrounding areas and footpaths 

Specialist Ground Anchoring Solutions 

A critical element of the strengthening design was the installation of post-grouted ground anchors to improve the stability of both the retaining wall and the surrounding soil mass. 

Operating within a restricted footprint, Contech utilised scaffold-mounted drilling rigs to undertake the anchor installation works safely and efficiently. Once installed, the anchors were pressure grouted and post-grouted using tube-a-manchette techniques to maximise bond strength and long-term performance. 

Each anchor underwent proof and acceptance testing to verify compliance with the design requirements. 

Improving Seismic Performance 

In addition to the anchoring works, reinforced concrete piles and new concrete buttresses were constructed and integrated with the existing retaining wall structure. 

These elements increased the wall’s structural capacity and enhanced its ability to withstand seismic loading. 

Drainage improvements were also incorporated through the installation of sub-horizontal drains to relieve groundwater pressures behind the wall, contributing to its long-term stability in Wellington’s hillside environment. 

Working Within a Live Urban Environment 

Maintaining access throughout construction was a key project requirement. 

Grosvenor Terrace remained operational for the duration of the works, supported by staged traffic management measures including traffic lights and lane closures. 

Contech worked closely with Wellington City Council, transport operators, residents and other stakeholders to coordinate activities such as bus stop relocations, parking adjustments and pedestrian access arrangements. 

Through early engagement and ongoing communication, the team was able to minimise disruption while maintaining a safe working environment for both the public and site personnel. 

Building Resilience for the Future 

The Grosvenor Terrace Retaining Wall 237 strengthening project has significantly improved the resilience of this important transport corridor. 

By upgrading the existing structure to meet modern seismic performance expectations, Wellington City Council has reduced the risk of wall failure and improved the network’s ability to remain operational following major events. 

Drawing on specialist expertise in retaining wall strengthening, ground anchoring and construction within constrained environments, Contech delivered a practical and durable solution that provides lasting value to both the client and the wider community.