Navigating temporary works complexities for the Bothamley Park trunk sewer upgrade

This paper describes the design, construction and monitoring of a temporary bored pile retaining system for the Bothamley Park trunk sewer upgrade in Porirua, New Zealand. The original construction methodology for installation of the sewer at Cannons Creek was to form a 15m high cut slope batter to enable access for a directional drilling rig. Deeper than expected rockhead and excessive seepage from the cut face resulted in earthworks being stopped with a 5m cut remaining to final excavation level and an alternative retaining approach adopted.

The site sits at the interface of an infilled gully feature associated with development of the area in the 1950’s and 1960’s, resulting in geological complexities including a perched groundwater regime and a variable rockhead depth across the excavation footprint. Other constraints including a sensitive sewer line and temporary mud pond located at the top of the batter restricted traditional methods such as sheet piling or benching the batter.

A bored pile wall and an observational monitoring system including piezometers was adopted and implemented on site successfully during construction using alert and alarm level thresholds, traditional survey, and regular visual observation. This paper highlights not only the technical aspects of the design and construction process but also the importance of adaptive monitoring and intervention strategies in addressing unforeseen challenges during infrastructure projects.