TASMANIA DEPARTMENT OF RESOURCES & ENERGY DIVISION OF MINES AND MINERAL RESOURCES 1991 URBAN GEOLOGICAL MAPPING PROJECT REPORT 1 Engineering geology of the Greater Hobart area by P. J. Hofto, D. J. Sloane, and B. D. Weldon DIVISION OF MINES AND MINERAL RESOURCES, PO BOX 56, ROSNY PARK, TASMANIA 7018 2 URBAN ENGINEERING GEOLOGICAL MAPPING PROJECT GREATER HOBART AREA 3 CONTENTS INTRODUCTION ………………………………………………………………………… 5 Engineering Geology Maps ……………………………………………………………… 5 Previous Reports ………………………………………………………………………… 5 Acknowledgements ……………………………………………………………………… 6 MAP DESCRIPTION AND DEFINITION ……………………………………………………… 7 Soil …………………………………………………………………………………… 7 Workability/rippability …………………………………………………………………… 7 Hazards ……………………………………………………………………………… 7 Landslides …………………………………………………………………………… 7 Erosion ……………………………………………………………………………… 7 Septic Tank Suitability …………………………………………………………………… 9 Groundwater …………………………………………………………………………… 9 PERMIAN SEDIMENTS …………………………………………………………………… 10 Geology ……………………………………………………………………………… 10 Soils ………………………………………………………………………………… 10 Resources and Land Use ………………………………………………………………… 10 Hazards ……………………………………………………………………………… 10 Septic Tank Suitability …………………………………………………………………… 12 Groundwater …………………………………………………………………………… 12 TRIASSIC SEDIMENTS …………………………………………………………………… 13 Geology ……………………………………………………………………………… 13 Soils ………………………………………………………………………………… 13 Resources and Land Use ………………………………………………………………… 13 Hazards ……………………………………………………………………………… 13 Septic Tank Suitability …………………………………………………………………… 15 Groundwater …………………………………………………………………………… 15 JURASSIC DOLERITE ……………………………………………………………………… 17 Geology ……………………………………………………………………………… 17 Soils ………………………………………………………………………………… 17 Resources and Land Use ………………………………………………………………… 17 Hazards ……………………………………………………………………………… 19 Septic Tank Suitability …………………………………………………………………… 19 Groundwater …………………………………………………………………………… 19 TERTIARY BASALT ……………………………………………………………………… 21 Geology ……………………………………………………………………………… 21 Soils ………………………………………………………………………………… 21 Resources and Land Use ………………………………………………………………… 21 Hazards ……………………………………………………………………………… 21 Septic Tank Suitability …………………………………………………………………… 22 Groundwater …………………………………………………………………………… 22 TERTIARY SEDIMENTS …………………………………………………………………… 23 Geology ……………………………………………………………………………… 23 Soils ………………………………………………………………………………… 23 Resources and Land Use ………………………………………………………………… 23 Hazards ……………………………………………………………………………… 23 Septic Tank Suitability …………………………………………………………………… 25 Groundwater …………………………………………………………………………… 25 QUATERNARY SEDIMENTS ……………………………………………………………… 26 Geology ……………………………………………………………………………… 26 Talus, scree and slope deposits ………………………………………………………… 26 Stable and mobile dune sand …………………………………………………………… 26 4 URBAN ENGINEERING GEOLOGICAL MAPPING PROJECT Raised-beach sand …………………………………………………………………… 26 Alluvium, gravel, sand and clay ………………………………………………………… 26 Beach sand, estuarine sand and clay …………………………………………………… 26 Windblown sand sheets ………………………………………………………………… 26 Resources and Land Use …………………………………………………………………… 26 Hazards …………………………………………………………………………………… 27 Septic Tank Suitability ……………………………………………………………………… 27 Groundwater ……………………………………………………………………………… 27 REFERENCES …………………………………………………………………………… 28 APPENDIX 1: Chart of workability/rippability (Modified from Leaman, 1971b) ……………………… 29 APPENDIX 2: Information legend from Engineering Geology maps (Hofto, 1990) …………………… 30 APPENDIX 3: Unified soil classification system (from Moon, 1980) ……………………………… 32 APPENDIX 4: Water bore database summary …………………………………………………… 34 APPENDIX 5: Construction materials database summary ………………………………………… 39 APPENDIX 6: Site investigation reference point summary ………………………………………… 45 LIST OF PLATES 1. Permian bedrock, shallow weathering — Sky Farm Road, Glenorchy …………………………… 11 2. Sandy gravel slope deposits over Permian bedrock — Turnip Fields, Fern Tree …………………… 11 3. Thick sandy clay (SC) soil on Triassic sandstone — St Virgils College, Austins Ferry ……………… 14 4. Podsol, developed on Triassic sandstone — Blackmans Bay …………………………………… 14 5. Tunnel erosion in soils on Triassic sediments — Battersby Drive, Glenorchy ……………………… 16 6. Erosion of exposed banks and cuttings in Triassic sediments — Wellesley Park, Cascades ………… 16 7. Brown clay (CH) soil on dolerite bedrock — Domain ………………………………………… 18 8. Brown sandy clay (SC) soil on dolerite bedrock — Caroda Court, Howrah ……………………… 18 9. Black clay (CH) soil on dolerite bedrock — Nicholas Drive, Lower Sandy Bay …………………… 20 10. Shallow landslide in dolerite soil — Goat Hills, Glenorchy …………………………………… 20 11. Gully erosion in Tertiary sediments — Droughty Point ………………………………………… 20 12. Gully erosion in Tertiary sediments — Droughty Point ………………………………………… 20 GREATER HOBART AREA 5 INTRODUCTION The Geodata Project was jointly funded by the City and sampling were also undertaken in areas designated for Councils of Hobart, Clarence and Glenorchy, by the major development or regions with recognised Municipality of Kingborough, and by the Division of geotechnical problems. The maps were produced at a scale Mines and Mineral Resources. The two-year project of 1:25 000, using the Department of Environment and endeavoured to produce applied geological data and Planning 1:25 000 topographic maps as bases. This explanatory notes for use as base information when engineering geological survey aimed to produce maps on planning urban development in the above municipalities. It which units are defined by engineering properties. The must be stated that the information provides a general boundaries of these units generally follow rock-type overview and in no way should be considered as replacing boundaries, but may bear no relation to either geological site-specific information and investigation. structure or stratigraphic boundaries. Many stratigraphic and structural boundaries within main units have been This publication was produced in conjunction with two excluded on the accompanying maps for this reason. 1:25 000 Engineering Geology Maps covering the Greater Recognition of rock and soil boundaries was often difficult Hobart area. Together they provide basic geological and in the field because changes in the character of rocks and geotechnical information and advice for engineers, soil are often gradational and can occur both horizontally planners and local government personnel. and vertically. Other boundaries may be covered by slope deposits or fill, and topsoil subsequently placed there. The Appendices to this report contain summarised information from the Division of Mines and Mineral Faulting of geological units may be of fundamental Resources published and unpublished reports and importance to the engineer. However, the position of a databases. The databases contain summaries of fault is often only inferred from the field relations of the geotechnical, construction material and groundwater rocks, and therefore its position can only be mapped information, as well as reports concerning site-specific approximately. A fault may be shown on geological maps investigations. as a single line, but may in fact be a series of shears. If the fault occurs in brittle rocks it may result in broken zones, Increasing regulation within the building industry and while in clays very little disruption may be apparent. The planning authorities, and the ever present question of bearing strength of fault-zone material over the width of liability, has meant that more emphasis will now be placed the fault should be determined, and if necessary, variations on specific building codes. For example, Australian made to foundation specifications as differential Standard 2870-1986, Residential Slabs and Footings, compaction may occur. Although Tasmania is seismically outlines general design principles for foundation quiet, there is always risk involved when building on major conditions commonly found in Australia. The building fault zones. authorities, in this case the cities and municipalities, are expected to provide preliminary advice on site As the enlargement of maps showing approximately classification. The easiest way to provide this advice is by determined boundaries may increase the inherent errors of producing an engineering geology map which shows site mapping and promote unjustified confidence, the fault characteristics. It must be stressed however that these maps positions shown on the accompanying engineering present only preliminary site classification and geological maps may only be approximate. Only major information, and in no way negate the need for faults are shown. The position of these faults has been individual site investigations by competent specialists. transferred from the appropriate 1:50 000 Geological Atlas map. These latter maps should be referred to for more ENGINEERING GEOLOGICAL MAPS complete fault information. Engineering Geological Maps have much in common with geological maps, as both present information about the PREVIOUS REPORTS geological environment. However engineers or planners may require more specific information. Conventional The Tasmania Department of Mines Geological Atlas geological maps group units of similar age and origin. Series maps and Explanatory Reports for Hobart (Leaman, Engineering geological maps commonly include 1972, 1976), Sorell (Gulline, 1983, 1984) and descriptions and qualifications of specific physical and Kingborough (Farmer, 1981, 1985) provide the base engineering properties of rocks and soils, as well as geological information for the engineering geological information