Utiku Landslide occurrence on a major highway – lessons learnt from a complex groundwater model

The Utiku Landslide is a large, deep-seated, rock planar landslide ~400 m wide and ~1000 m long near the centre of New Zealand’s North Island. Landslide movement has been affecting State Highway 1 and the North Island Main Trunk railway since the 1960s. Sustained periods of high groundwater pressures on the basal clay layer (~60 m below ground level) are the primary cause of movement. Early attempts to dewater the landslide were of limited success in preventing further movement, and with the intensity of rain experienced in 2022-2023, the landslide movement increased, causing large tension cracks and subsidence across the roading pavement. With cracks affecting road users, and further movement likely to occur if no action was taken, the New Zealand Transport Agency (Waka Kotahi) implemented an immediate solution while a design business case for a long-term management approach was progressed. A key component of any solution is the lowering of groundwater levels to reduce pressure on the slip surface. This paper will present the challenges and value of developing a detailed conceptual model of the subsurface and groundwater to help understand the cause of instability, as well as the movement of groundwater and effectiveness of drainage. The paper will also discuss lessons learnt in translating a complex 3D ground model into a 3D finite-difference flow model, and the benefits of collaboration across wider teams and disciplines for project delivery and outcome.