The Geology of Omokoroa-Te Puna Region within the Tauranga Area: A Case Study from the Geotechnical Investigation of Stage 2 of the Takitimu North Link Project

Stage 2 of the Takitimu North Link (TNL) project involves the proposed construction of a new 7 kilometre long four-lane expressway for State Highway 2, connecting in the east to the end of Stage 1 near Loop Road, extending approximately 900m west of the existing Omokoroa Intersection. It is to include several bridges and grade separated interchanges. The geology between Te Puna and Omokoroa is highly variable and complex and includes weathered and unweathered ignimbrite deposits at depth, which are overlain by reworked aged alluvial deposits. The landform along the TNL alignment consists of several broad terraces which are underlain by volcanic tephra deposits. The terraces are bisected by deeply incised gullies with steeply graded escarpments. Geologically recent very soft alluvial deposits are present within the base of these gullies. A Geotechnical Investigation for the specimen design of Stage 2 was undertaken between March and October 2021 to develop a ground model for the site and to provide design parameters and key geotechnical recommendations for the construction of the expressway. This paper presents the geotechnical investigation techniques used during the geotechnical investigation and describes the logistical challenges associated with accessing site locations. It focuses on presenting and discussing the complex ground model that was derived from the investigations and some of the key geotechnical design issues that will require resolving due to the difficult ground conditions anticipated.