Engineering geology of the Northland allochthon, Silverdale, North Auckland, New Zealand

Abstract

The Northland Allochthon rocks in the Silverdale area comprise Hukerenui Mudstone, Whangai Formation siltstone/calcareous siltstone, and Punakitere Sandstone of Mangakahia Complex with minor Mahurangi Limestone of the Motatau Complex. They show significant variation in terms of their lithological and physical properties, structural features and slope movement processes. The main aims of this study were to investigate the influence of mineralogy, microfabrics, defects and structure on physical and engineering geological properties, topography, geomorphology and slope movement mechanisms and discover which units are most susceptible to failure. Methods involved the analysis of rock mass lithologies and their weathering products at micro- to macroscopic scales, in the laboratory and the field. Local/outcrop scale to regional scale geomorphological mapping and field observations were made to analyse topography and slope movement characteristics. Extremely weak to weak multi-coloured clay-rich Hukerenui Mudstone with exotic rock inclusions is highly sheared, closely fractured, and broken formation structure is common. Isolated Hukerenui Mudstone pods were found in other lithologic units. Slope movements occur on continuous clay seam/ shear planes at the soft residual soil/highly sheared transition zone boundary. Quartz, feldspar, mica are within a matrix and turbostratic smectite-rich microfabric. Extremely weak, massive, quartz-rich micaceous, Punakitere Sandstone includes mudstone/ clay shear zones zones/fault gouge and septarian concretions. Dominant smectite plus illite, kaolinite and chlorite occupy matrix and turbostratic microfabrics. Quartz and variably calcite-rich Whangai Formation rock mass comprises extremely weak to moderately strong closely fractured intermixed siltstone/calcareous siltstone rock fragments with polished and slickensided clay and/or calcite infilling. Soft to very stiff residual silts and clays overlie a transition zone soil and rock mixture. Dominant smectite clays occupy matrix and turbostratic microfabrics. Strong, extremely fractured Mahurangi Limestone is composed of tightly interlocked rock fragments calcite or clay infilled fractures. Thin residual soils and transition zone are typical. Residual clay soils become soft near the transition zone as water content increases. Intact rock strengths increase with increasing calcite and decreasing smectite content. Slaking due to high smectite contents and rapid chemical weathering rate precludes formation of rock outcrops. The highly to completely weathered transition zone is where shear planes or shear zones associated with translational slope movement occur. The often sheared and contorted transition zone fabric is considered to be the primarily the result of slope movements with laminar microfabrics on polished defects resulting in very low residual friction angles. Rock mass defects including folding, faults and dominant fracture sets may provide structural control on stability of slopes where continuous defects or rock fabrics can act as release surfaces for slope movements occurring within the transition zone. Land gradients and geomorphic features can be correlated with underlying lithologic units. The gentlest slopes tend to be underlain by Hukerenui Mudstone and the steepest underlain by calcareous Whangai Formation or Mahurangi Limestone. Smectite-rich Hukerenui Mudstone was the weakesUieast stable lithologic unit and calcareous Whangai Formation siltstone and Mahurangi Limestone were the strongest and most stable.