Successful Remediation of Dual Pipeline Stress Within a Complex Landslide

Intelligent “pipeline pigging” detected deflection of a 200mm high pressure gas pipeline and a 250mm petroleum products pipeline buried in the same trench, where they cross a complex landslide for a length of approximately 40m. The landslide, located in West Auckland, New Zealand, comprises two lobes and is situated within a larger relic landslide underlain by East Coast Bays Formation. Surface and subsurface geotechnical investigations, monitoring and related assessment were used to define the landslide model. Slope stability analysis and remediation optioneering had to consider the urgency of landslide stabilisation, pipelines and personnel safety during pipelines exposure (Pipeline Stress Release Trench), and stability of a nearby dwelling. Stability and health and safety risk concerns were identified for a temporary unsupported Pipeline Stress Release Trench, which required exposure of the landslide failure plane across essentially the full width of the landslide. Remedial works included installation of subsurface drainage (Stage 1) followed nine months later by exposure of the pipelines for stress relief and inspection works (Stage 2). Subsurface drainage comprised 164m of French drains, typically 2.5m deep. Inter-stage monitoring, including groundwater levels, surface changes and drain flows confirmed satisfactory drainage performance prior to Stage 2 works. Excavation and subsequent backfilling of the 3m wide by 48m long Pipeline Stress Release Trench was completed safely via a staged approach using trench shields.