A major landslide has influenced a 300 metre length of the South Coast Railway track for the past fifty years. The landslide was intermittently active and an eductor dewatering system and its predecessor, a dewatering main, operated at the site since the early 1970's. Operational difficulties and high maintenance costs led to the consideration of alternative systems to reduce the risk of instability at the site. Major landsliding which occurred along other sections of the South Coast Railway, resulted from significant and long duration rainfall events during the La Nina occurrence of the late 1980s. This exacerbated the situation. The landslide at Coalcliff encompassed a volume 650,000m3 of colluvium overlying remnants of the Stanwell Park Claystone, with a slide surface occurring at a dip of only 2 degrees towards the Tasman Sea. The railway track straddled the scarp of the slide. Artesian piezometric pressures within the Stanwell Park Claystone provide the de-stabilising mechanism in association with a residual strength condition of the claystone. Significant interconnection of piezometric pressures within the rockmass was observed during site testing, which also included downhole micro-flow testing. The identification of these high piezometric pressures and the recognition of their interconnection within the rockmass, led to the development of a gravity drainage system which incorporated deep wells and directionally-drilled interceptor drains, with an outfall through the cliffline above the Tasman Sea. The details of the investigation process and the subsequent hydrogeological model, and the philosophy of the gravity operated de-watering system are presented herein.
The site is known as the Coalcliff Slip. This study area relates to a coastal terrace and side-fill embankment, situated beneath the South Coast Railway (SCR) - part of the Illawarra Line - several hundred metres north of Coalcliff Station and nominally between chn 58.450km and 58.900km south of Sydney.