A Study of the Landslides and Their Relation to Engineering and Planning in the Southern Districts of the Narracan Shire, Victoria
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A STUDY OF THE LANDSLIDES AND THEIR RELATION TO ENGINEERING AND PLANNING IN THE SOUTHERN DISTRICTS OF THE NARRACAN SHIRE, VICTORIA By J. C. Brumley Title page: Oblique aerial view of numerous landslides along the valley of Watkins Creek ABSTRACT Landslides of the slump-earth flow type are common in areas of strongly undulating terrain in the Strzelecki Ranges south of Trafalgar. Failures occur each winter following heavy rain which triggers movement in the clay rich residual soil slopes. The Shire of Narracan has a major problem in maintaining roads which are damaged each year by landslides, and some landowners are troubled by landslide damage to their pasture, fences and dams. The current trend towards subdividing farm land into small hobby farms also poses serious problems when subdivision is planned in landslide-prone areas. This study examines the nature and distribution of landslides, the properties of the soils involved and the factors responsible for slope instability. Detailed studies have been made of the clay mineralogy, soil lithology and classification properties, and the history of land use in the area. Suggestions are made for mitigating the existing and potential landslide hazards. i CONTENTS 1 INTRODUCTION 1 1.1 Background to the Study 1 1.2 Aims of the Study 2 1.3 Investigation Methods 2 2 THE NATURE OF THE LAND 4 2.1 History of Settlement and Land Use 4 2.2 Geology and Geomorphology 5 2.2.1 General Geology 5 2.2.2 Lower Cretaceous Strzelecki Group (Kls) 5 2.2.3 Childers Formation (Tec) 6 2.2.4 Tertiary Older Volcanics (Tvo) 6 2.2.5 Geomorphology 7 2.3 Rainfall 7 2.4 Seismicity 11 3 LANDSLIDE TYPES AND OCCURRENCE 12 3.1 Classification of Landslides 12 3.2 Recognition of Landslides 12 3.3 Landslides in the Study Area 14 3.3.1 Landslide Distribution 14 3.3.2 Roadside Landslides 14 3.3.3 Landslide Types 15 3.3.4 The Age of Landslides 18 4 FIELD STUDIES 20 4.1 Initial Reconnaissance 20 4.2 Site Investigation of Selected Landslides 20 4.2.1 Drilling and Sampling Techniques 20 4.2.2 Field Logging 20 4.2.3 Observations at Selected Landslides 21 5 SOIL ANALYSIS, DESCRIPTION AND CLASSIFICATION 29 5.1 Laboratory and Field Soil Classifications 29 5.1.1 Laboratory Classification Tests 29 5.1.2 Correlation between Field and Laboratory Soil Classifications 29 5.2 Soil Descriptions for each Geological Formation 34 5.2.1 Tertiary Older Volcanics 34 5.2.2 Childers Formation 35 5.2.3 Lower Cretaceous, Strzelecki Group 37 5.3 Clay Mineralogy and Groundwater Chemistry 40 6 CAUSES AND REMEDIAL MEASURES 44 6.1 Causes of Slope Movement 44 6.2 Remedial Measures 45 6.2.1 Approach Methods 45 6.2.2 Landslide Susceptibility and Hazard Evaluation in Relation to Landslide Avoidance 45 6.2.3 Removal of Landslide Material 46 6.2.4 Reduction of Activating Forces 46 6.2.5 Increase of Resisting Forces 47 6.2.6 Current Shire Practice 48 7 CONCLUSIONS AND RECOMMENDATIONS 49 7.1 Conclusions 49 7.2 Recommendations 49 ACKNOWLEDGEMENTS 51 REFERENCES 52 Cited 52 Uncited 53 APPENDICES 57 ii 1. INTRODUCTION 1.1 Background to the Study The study area is a portion of the Strzelecki Ranges bounded approximately by Yarragon, Trafalgar, Moe, Narracan and Mirboo North as shown in Figure 1.1. Figure 1.1 – Landslide Study Area (shaded). Part of the Southern District of the Shire of Narracan The Shire of Narracan has a major problem in controlling landslides which intercept the numerous roads between farms and towns in the area. In a letter dated 5 September 1977 (Appendix 1.1), the Shire Engineer, Mr R D Thege, requested the Department of Minerals and Energy to undertake a study of the area with a view to establishing guidelines for achieving more effective control, prevention and avoidance of local landslide problems. The State Treasury made funds available for the project in February 1978, and field work commenced in May 1978. Mr Thege stated that during 1975, approximately 22 landslides were active, and after the heavy winter rains in 1977 further landslides developed, some of them quite massive. Following the 1975 landslides, the cost of remedial work was estimated to be at least $286,000. The cost of the 1977 landslides is anticipated to be in the order of $200,000 with $110,000 being required to rectify a landslide on the main Trafalgar to Thorpdale road (Figure 1.2). Periodic expenditure of this magnitude on remedial work cannot be expected to eliminate the problem of roadside landslides in the Shire. Continuing improvement on some landslides required recurrent work, and will remain an ongoing cost to the Shire until long-term solutions are found and implemented. The Shire’s list of 1975 roadside landslides is attached as Appendix 1.2. Details of landslide location, nature of damage, estimated cost and proposed remedial work are tabulated. The identification number allocated to each landslide will be used throughout this report. Figure 1.2 – Landslide in fill section of the main Trafalgar to Thorpdale road reduces width to one land. April 1978. The economic significance of landslides is not confined to the chronic damage caused to roads in the area. Landslides also degrade the quality of farmland pastures, damage crops, destroy dams (Figure 1.3). add to the sediment load of streams and occasionally damage buildings, fences and powerlines. No cases of human death or injury due to landsliding in the area are known to the writer, but electric fences are needed to stop stock from becoming stuck in fresh earthflows and in slump fissures. 1 Figure 1.3 – Farm dam breached by landslide at Dingley Dell. July 1978. The recent trend towards more intensive land use by subdividing large farms into smaller “hobby farms” and “rural retreats”, increases the potential of landslide damage, and highlights the need to assess landslide susceptibility in the area. The economic cost of poorly planned changes in land use could be immense. For example, under present conditions of urban land development in landslide and landslide-prone areas of California, Alfors et al. al. (1973), estimates that losses due to landslides will total US $10 billion between 1970 and year 2000. It is suggested that the problem in the Shire of Narracan is as severe as the Californian situation – the same potential for dense urban development does not exist in the Shire. However, as in California, measures involving geological investigations, engineering practice and careful planning can greatly reduce the economic losses and inconvenience caused by landslides. 1.2 Aims of the Study The major purposes of the study is to determine the most practical and economical means by which the Shire and landowners can mitigate the damage caused by landslides. In more detail, the aims of the investigation have been: * To describe the major landslide types * To determine the factors which contribute to landslide activity * To map areas of past and present landsliding * To suggest remedial and preventative measures for landslide control. The investigation has focused on the nature of slope stability on a broad scale over the entire area. A number of landslides, considered representative of their type were selected for drilling and soil sampling, but a comprehensive study of any one particular landslide has not been carried out. Such a study would involve detailed geological and geochemical studies beyond the scope of this investigation. It is hoped that the present investigation will provide a valuable basis upon which detailed site investigation of selected landslides can be carried out in the future. 1.3 Investigation Methods The study has involved the following methods of investigation: * Initial field reconnaissance to examine the morphology and geology of the major landslide types. * Individual landslides which have damaged roads in the area were examined in some detail noting the type of damage, possible causes of the landslide and the nature of any remedial work carried out. * Interpretation of aerial photographs, to delineate landslide areas, to assess the causes of landslides and to study the morphology of selected landslides. * Collection of all known data on local landslide activity, such as geological and engineering reports, Shire records, newspaper articles and historical literature. 2 * Analysis of daily rainfall records in relation to landslide occurrence. * Study of the historical changes in land use, with particular emphasis on deforestation, road construction and agricultural usage. * Drilling and soil sampling at selected roadside landslides. * Observation of groundwater levels during and after drilling. * Study of the surface drainage at selected landslides. * Detailed description of the soil profile encountered in each borehole, paying particular attention to stratigraphy, lithology, soil disturbance, soil consistency, plasticity, moisture content and identification of the failure surface. * Laboratory soil tests to determine soil substance according to the Unified Soil Classification System. * Palynological examination of soil samples. * Determination of clay mineralogy by X-ray diffraction methods. * Chemical analyses of groundwater samples. * Extensive literature search to check for appropriate remedial techniques. The choice of investigation techniques was made within the constraints of time and available finance existing during the study. The methods are considered to be sufficiently comprehensive to provide reliable, integrated data, suitable for the nature and scale of the investigation. Numerous techniques are available for the geological investigation of landslides. Zàruba and Mencl (1969) and Varnes (1958) describe the basic techniques used in landslide investigations. More recently, emphasis has been given to field instrumentation for continuous monitoring of landslide movements. Developments in field instrumentation are described in numerous publications, such as that of the British Geotechnical Society (1974). The scope of the present study does not cover field instrumentation.