Landslide investigations, Main North Line Railway, South Island, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The most important section of railway in the South Island of New Zealand, the Main North Line, offers a continuous rail link serving the northern half of the South Island's east coast. The Main North Line, operated by the New Zealand Railways Department, extends 348 kilometres between Christchurch (in the south) and Picton (at the northern tip of the South Island). At some 25 to 30 sections of the line, speed restrictions imposed on rail traffic have reduced Christchurch-Picton travel times by approximately one hour. Many restrictions occur at sections of the line where instability in natural slopes, cuts and fills gives concern to track safety. This study summarises investigations and recommends correction measures for three areas of slope instability through which the railway passes. The Ethelton Slip, an earth movement in a natural slope, is sited 101 kilometres north of Christchurch. The Main North Line passes for 450m across the toe of the landslide. The site covers an area of 30 hectares, with relief between the upper and lower boundaries approaching 200m. Slope angles over the surface range to 45°. At the turn of this century, movement on the railway reached 0.5-1.0m per week. This study has shown that movement is not occurring generally over the site, but is restricted to a small (one hectare) zone above and including the railway. Displacement of this zone reached 13.1cm over a 9.7 month survey period; numerous shear and tension cracks over much of the zone infer movement is deep seated. Material forming the slide mass is a colluvium, derived by deep weathering and erosion of a basic volcanic bedrock intrusive into sandstones and mudstones of Triassic age. Slope failure is believed to have resulted from a process of overstressing within the weathered slope mantle as a result of oversteepening and undercutting at the toe of the slope through river action. Surface drainage and slope regrading is recommended to eliminate water infiltration into the slide mass. If movements persist at the site following these measures, horizontal or counterfort drains could be considered as additional controls. The second area considered in this study, a one kilometre- long railway cutting, causes perennial track clearance and maintenance problems from the numerous mudflow and earthfall-type landslides originating from the batters above both sides of the track. The Hawkswood Cut, located 140 kilometres north of Christchurch, has batters originally rising at 45° and approaching 20m in height. Near the centre of the Hawkswood, instability of the cutting sides has advanced to the stage where slopes are approaching the vertical and remedial attention is a necessity. During high intensity or prolonged rainfall, small earthflows become mobilized from the batters as a result of surface runoff leading to a reduction in the effective normal stress of surficial slope material. Earthfalls, or soil blocks breaking away from the slope faces, are also associated with periods of heavy rainfall. Earthfalls result from seepage pore water pressures exerting on unfavourably orientated, stress-relief-induced fissures. Earthworks to reduce the present slope angles are shortly to be undertaken. A continuous (without benching), near-30° batter is recommended for the new cutting, and surface drainage and slope regrading above the cutting to permit surface runoff is strongly advised. The Mikonui earthflow forms an extensive area of natural slope movement over a distance of 1.3 kilometres from the Kaikoura coast (east coast, South Island), inland to a point 215m above sea level. The Main North Line crosses the toe of the landslide at the coast. The active width of the earthflow tapers from 200m measured parallel to the coast, to 25-30m at the upper boundary of the site. The lateral boundaries of the earthflow are marked by discreet shear zones. The basal sliding surface is coincident with the upper surface of an insitu, late Cretaceous age, Ca-rich bentonite immediately underlying the slide mass. The slide mass attains a thickness of over 30m near the lower boundary of the site. Surface survey stations were displaced a maximum 91.1cm over a 10 month survey period during this study. Slope regrading to eliminate ponds, swampy areas, and hummocky topography, and an extension of surface drains already constructed at the site, are recommended to encourage surface runoff. Gravitational driving forces causing movements in the earthflow may be reduced by the excavation of an area of secondary landsliding above the site. The installation of subsurface drainage should further increase slope stability. Elimination of the earthflow by bridging the railway across the toe of the landslide is likely to be the only long-term, permanent control.