Geotechnical Assessment Dargaville Kaipara District Submitted to: Mr Paul Waanders Kaipara District Council 42 Hokianga Road Dargaville 0310 ENGEO Limited 8 Greydene Place, Takapuna, Auckland 0622 10.05.2019 PO Box 33-1527, Takapuna, Auckland 0740 15601.000.002_01 Tel +64 9 972 2205 Fax +64 3 328 9013 www.engeo.co.nz Geotechnical Assessment – Dargaville, Kaipara District 1 Contents 1 Executive Summary .............................................................................................................. 5 2 Introduction............................................................................................................................ 6 3 Scope of Work ....................................................................................................................... 6 4 Our Approach ........................................................................................................................ 7 5 Statutory Framework ............................................................................................................. 7 5.1 Resource Management Act 1991 (RMA) .............................................................................. 7 5.2 Building Act 2004 .................................................................................................................. 7 5.3 Intent of Current Study .......................................................................................................... 8 6 Study Area............................................................................................................................. 8 7 Geological Setting ............................................................................................................... 10 7.1 Published Geology .............................................................................................................. 10 7.2 Northland Allochthon ........................................................................................................... 11 7.2.1 Undifferentiated Mangakahia Complex (Kk) ....................................................................... 12 7.2.2 Undifferentiated Mélange (KOm) ........................................................................................ 12 7.3 Hukatere Subgroup Volcanics (Mtsi) .................................................................................. 12 7.4 Awhitu Group (Pad) ............................................................................................................. 12 7.5 Tauranga Group .................................................................................................................. 12 7.5.1 Pleistocene Alluvium (eQa) ................................................................................................. 13 7.5.2 Holocene Alluvium (Q1a) .................................................................................................... 13 7.6 Unmapped Units ................................................................................................................. 13 7.6.1 Historical Fill ........................................................................................................................ 13 7.6.2 Colluvium ............................................................................................................................. 13 8 Groundwater........................................................................................................................ 13 9 Active Faults ........................................................................................................................ 14 10 Ground Slope Angles .......................................................................................................... 14 11 Geotechnical Hazard Discussion ........................................................................................ 15 10.05.2019 15601.000.0002_01 Geotechnical Assessment – Dargaville, Kaipara District 2 11.1 Seismic Hazards ................................................................................................................. 15 11.2 Slope Instability ................................................................................................................... 16 11.3 Consolidation Settlement .................................................................................................... 16 11.4 Volcanic Hazards ................................................................................................................ 17 11.5 Sulphate Attack on Concrete .............................................................................................. 17 11.6 Other Hazards ..................................................................................................................... 17 12 Historical Aerial Photograph Review ................................................................................... 18 13 Geomorphological Assessment .......................................................................................... 18 13.1 Stereo-Paired Aerial Photo Interpretation ........................................................................... 18 13.2 Site Walkover ...................................................................................................................... 19 13.2.1 General ................................................................................................................................ 20 13.2.2 Borrow Areas ...................................................................................................................... 20 13.2.3 Rock Outcrops .................................................................................................................... 20 13.2.4 Surficial Deposits ................................................................................................................ 20 14 Geotechnical Hazards Identified in Dargaville .................................................................... 22 14.1 General ................................................................................................................................ 22 14.2 Geotechnical Hazard Rating ............................................................................................... 23 14.2.1 Low Hazard Potential .......................................................................................................... 23 14.2.2 Medium Hazard Potential .................................................................................................... 23 14.2.3 High Hazard Potential ......................................................................................................... 23 14.3 Geohazard Map .................................................................................................................. 23 14.4 Seismic Hazards ................................................................................................................. 23 14.4.1 Seismic Site Classification .................................................................................................. 24 14.4.2 Ground Shaking .................................................................................................................. 24 14.4.3 Liquefaction and Lateral Spread ......................................................................................... 24 14.4.4 Geotechnical Investigation Requirements .......................................................................... 26 14.5 Slope Instability ................................................................................................................... 27 10.05.2019 15601.000.0002_01 Geotechnical Assessment – Dargaville, Kaipara District 3 14.5.1 Slope Instability Potential .................................................................................................... 27 14.5.2 Geotechnical Investigation Requirements .......................................................................... 29 14.6 Consolidation Settlement .................................................................................................... 30 14.6.1 Consolidation Settlement Potential ..................................................................................... 30 14.6.2 Geotechnical Investigation Requirements .......................................................................... 31 15 Combined Geohazard Assessment .................................................................................... 32 15.1 Sulphate Attack on Concrete .............................................................................................. 33 15.2 Other Geohazards ............................................................................................................... 34 16 On-Site Effluent Disposal .................................................................................................... 35 16.1 Factors Affecting On-Site Disposal ..................................................................................... 36 16.1.1 Topography ......................................................................................................................... 36 16.1.2 Soil Properties ..................................................................................................................... 36 16.1.3 Groundwater Conditions ..................................................................................................... 36 16.1.4 Disposal Field Setback Restrictions .................................................................................... 36 16.2