Assessing the potential for future first-time slope failures to impact the oil and gas pipeline corridor through the Makarov Mountains, Sakhalin Island, Russia

The Sakhalin Energy Investment Company Ltd (SEIC) Sakhalin II Phase 2 oil and gas pipeline corridor is 782 km in length, and extends from landfall sites in the north of the island to a liquefied natural gas plant and oil export terminal in the south. For c. 120 km this alignment crosses the Makarov Mountains, an approximately north-south-orientated grouping of ridges and valleys formed in Cretaceous mudstones, Tertiary siltstones, sandstones, coal measures, lavas and tuffs. Situated on the NW Pacific rim, the island is prone to seismic activity, and the design and construction of the pipelines' alignment has had to take into consideration a number of existing landslides (more than 400 in total). Additionally, the continuing seismic hazard, combined with slope toe erosion by rivers and high groundwater tables brought about by spring snow melt and summer rains, creates a potential for first-time landslides to occur during the 50 year operational period of the pipelines. To assess where such failures might take place a study was undertaken that combined back-analysis and sensitivity analysis, landslide susceptibility analysis (including factor analysis) and qualitative evaluation. The qualitative evaluation allowed a judgement-based assessment to be included in the identification and prioritization of potential future first-time failure areas. Ground truthing was then undertaken in the areas of highest susceptibility, and generally supported the findings of the analysis. In c. 50% of these cases the potential for landslide initiation was considered to be sufficiently high to warrant intervention, in terms of ground investigation, slope movement and groundwater monitoring, or engineering mitigation. The study highlights the uncertainties that exist when undertaking exercises such as these where geotechnical information is insufficient to carry out rigorous modelling and where broad geological and geographical assessments have to be made in order to yield the required outputs.