Geology of the Fiordland Area - GNS Science

ENGINEERING GEOLOGYThis section pro ides generalised information to assistgeotechnical investigations and hazard assessments, butis not a substitute for detailed site in estigations. Potentialdifficulties with some rock types are highlighted in thoseregions where infrastructure has been or may be de eloped.One of New ealand’s largest engineering projects, theanapouri power scheme, lies within the map area andengineering geological aspects of this scheme ha e recei edconsiderable attention (Fig. 72).Paleo oic to Early Cretaceous roc sPaleozoic to Early Cretaceous plutonic rocks throughoutFiordland are in general strong to ery strong rocks,capable of supporting ery steep slopes. Quaternary glacialerosion and present-day erosion processes in this region ofsteep slopes and high rainfall have removed most surficialweathered material, so plutonic rocks tend to be freshand hard. Paleozoic to Mesozoic metasedimentary rocksare also generally strong, hard and fresh, and capable ofstanding steeply in large faces, but they ha e additionalpotential failure surfaces with more closely spacedfoliation defects. ariability in rock strength on outcropor excavation scale is strongly influenced by the thicknessand orientation of foliation. nfoliated plutonic rocks tendto be more predictable in their properties than foliated andbanded gneisses. neissic rocks may also be folded, withrapidly varying orientation, and are thus more difficultto characterise from an engineering point of iew. Theanapouri power scheme, particularly the second tailracetunnel, encountered ariable drilling properties because ofchange in rock types on scales of 1 m to 100 m (Fig. 72B).Landsliding on all scales is common within both plutonicand metasedimentary rocks, influenced by the steep too ersteepened slopes, and by earthquake shaking. Failuresurfaces are commonly joints, and in more gneissic andmetasedimentary rocks they include planar foliation defects.Where infrastructure is at risk from landsliding, assessmentof joint patterns and foliation attitudes is critical. Faultzones, where rocks are crushed and commonly altered,ha e decreased strength and hardness, as is seen in thenarrow gullies and guts that de elop preferentially alongthese zones. In underground excavations such as HomerTunnel and the Manapouri power scheme, fault zones wereproblematic as they were groundwater conduits.Cretaceous and Ceno oic sedimentary roc sThese rocks ha e widely arying engineering properties.Sandstone (for example, above the Eglinton valley) andlimestone (at the Te Ana-Au caves) are hard, strong rocks,with widely spaced joints. oth are capable of formingprominent vertical to overhanging cliffs (Fig. 48B), whichmay generate block falls and landslides during largeearthquakes. Conglomerates of all ages tend to erode morereadily, as clasts fall out of the softer matrix. udstone(such as the widespread Waicoe Formation) is soft andweak, e en when fresh, and erodes readily by surfaceflaking during wetting and drying. Mudstone is also proneto landsliding on all scales and creates particular difficultiesfor roading. Landsliding on Waicoe Formation, for exampleon the ilford Road south of the Eglinton alley, requirescareful design of surface and subsurface drainage.Quaternary sedimentsThe unconsolidated gravels and sands on valley floors andflatter land within and east of Fiordland are soft, loose andweak. E en the oldest glacial deposits are still consideredto be engineering soils. Steep batters are prone to frettingand collapse, and roads require drainage works capableof handling high-intensity rainstorms. All unconsolidatedmaterials are prone to ground-shaking amplificationand potential liquefaction during Fiordland’s numerousearthquakes, and peaty soils and peat swamps in particularmay amplify ground shaking to damaging le els.76