1. INTRODUCTION, AIMS AND SCOPE

Transcription

1 1. INTRODUCTION, AIMS AND SCOPE 1.1 INTRODUCTION This thesis is concerned with the study of slope instability on the Illawarra Escarpment slopes in the northern half of the Wollongong City Council (WCC) Local Government Area (LGA). The work has led to a better understanding of the factors which lead to slope instability and the frequency with which it occurs. Computer based quantitative methods of assessment of landslide hazard and risk have been researched and developed. The City of Wollongong is situated on the coast approximately 80km south southwest of Sydney (Figure 1.1). The city is nestled in a narrow triangular coastal plain flanked to the east by the Pacific Ocean and to the west by steep slopes. The slopes are topped by sandstone cliff lines and comprise the Illawarra Escarpment, shown in Plate 1.1. The Wollongong City Council area extends from Helensburgh and Garie Beach in the north, to Windang in the south and to Macquarie Pass in the southwest (Figure 1.2). This represents an area of approximately 715 square kilometres. Plate 1.1. Oblique aerial view south along the Illawarra Escarpment. Taken by the writer in December 1990, Scarborough Railway Station and Lawrence Hargrave Drive in the foreground. 1-1 continued on page 1-4

2 Figure 1.1 Study area location plan. Segment of the NATMAP 1:1,000,000 International Map of the World, SYDNEY Sheet, showing the east coast of New South Wales, Newcastle to Nowra area, including Wollongong. Inset Map of Australia showing Study area. 1-2

3 Figure 1.2 The Wollongong City Council Local Government Area. The general Study area is the escarpment slopes north of Wollongong. The 1:4000 Map area considered during this research project is shaded in grey. The Subject area, defined for a spatial analysis carried out during this research project is highlighted with a red border. 1-3

4 Wollongong is located within the Illawarra Region of New South Wales and is the largest city in the region. The Illawarra Region extends from Helensburgh, 45kms south of Sydney, to Durras Lakes located 230km south of Sydney. The Illawarra spans the five Local Government Areas of Kiama, Shellharbour, Shoalhaven, Wingecarribee and Wollongong councils, and covers approximately 8,485 kms 2. The name Illawarra is probably derived from the aboriginal inhabitants name for the area, Allowrie. European settlement of Wollongong commenced in the first decade of the 1800's. By 1841 there were 4044 residents (Mitchell, et al 1984) living between Stanwell Park and Nowra. Since that time the population has increased considerably; 25,000 in 1910, 100,000 in 1955 and 165,000 in In 1996, the population of the Wollongong City Council s LGA reached approximately 183,500 people (IRIS, 1997) and the area is considered to have one of the highest population growth rates in NSW. As the population of Wollongong grows, so does the demand for urbanisation of the sloping areas of the escarpment. This has already led to an increase in the known occurrence and hence appreciation of land instability along the escarpment area. The damage or destruction of a total of 89 houses over the period 1887 to 1996 within the study area can be directly attributed to landslide movement. This number includes 9 houses along a single street, Morrison Avenue in Coledale. Urban development on the slopes of the escarpment would not be considered appropriate if town planning of the city of Wollongong were to start afresh today. However, Wollongong is almost 200 years old and development is well entrenched. Hence, Wollongong will continue to be affected by land instability in the future. Land Instability is documented as having affected the Illawarra since at least January 1879 as evidenced by a newspaper article (Illawarra Daily Mercury, 1879). A technical report (Shellshear 1890), discussed a landslide which occurred in Stanwell Park in May 1889 and the subsequent remedial works. Geomorphological and geological evidence suggest that land instability has been instrumental in the geomorphological evolution of the Illawarra Escarpment and Coastal Plain during the Quaternary period and possibly much of the Tertiary. Natural land instability is acknowledged to have been present prior to European settlement. Land instability induced or exacerbated by man in addition to natural land instability is proving to be problematic following deforestation, land development, construction of transport links 1-4

5 (road and rail) and other physical changes during the last 150 years. Major transport links to and from the area comprise; The F6 Freeway via Mount Ousley which is also known as Highway 1, the Princess Highway via Bulli Pass, Lawrence Hargrave Drive via Stanwell Park and the Illawarra Highway via Macquarie Pass. These major roads are controlled by the New South Wales State Government Roads and Traffic Authority (RTA), Dual electrified South Coast Railway Line which extends from Nowra in the south and joins the Sydney system to the north at Sydney Central Station via Stanwell Park and Sutherland, and the Unanderra to Moss Vale single track Railway Line. These are controlled by the New South Wales State Government Rail Access Corporation (RAC), Port Kembla and Wollongong Harbour, and Albion Park Airfield. Land Instability has at times severely affected the operation of both the road and rail infrastructure within the district. Two human fatalities occurred on 30th April 1988 following a debris slide which originated in the railway embankment at Coledale. Following this disaster, tens of millions of dollars have been spent by the Rail Access Corporation (formerly part of the State Rail Authority), on upgrading and remedial works along the railway easement. Furthermore, at different times, all of the major roads linking Wollongong to the surrounding area have been blocked by land instability. As recently as mid February 1997 the Macquarie Pass road (south of the subject area), was affected by a debris slump. The road was closed for several weeks for repair and the remedial works cost about $250, AIMS The main aims of this research are to: Achieve a better understanding of the main factors that contribute to the occurrence of slope instability and its frequency in the study area Contribute to the development of a quantitative hazard and risk assessment framework including the methodology Examine selected landslide sites in detail as case studies In order to achieve the above aims the following tasks were considered essential: Carry out an up-to-date literature survey of national and international 1-5

6 developments in landslide hazard and risk assessment Define terms used in this thesis in relation to quantitative assessment of landslide hazard and risk management Carry out an up-to-date literature survey of landslide hazard assessment and management within the study area Development of geological, geotechnical and landslide databases Development of flexible and accurate computer-based maps of Geology and existing Landslides within the study area using a Geographic Information System (GIS) computer package Monitoring of subsurface pore water pressures using existing piezometers Monitoring of landslide movement, specifically shear displacement at depth, using existing inclinometers Compilation and analysis of monitoring records of existing piezometers and inclinometers from various industry based sources for several landslide sites Carry out stability analyses as part of case studies at selected sites Analysis of daily and antecedent rainfall data over the period January 1977 to December 1996, which is significant for its duration and for the occurrence of land instability within the study area 1.3 SCOPE Research for this project has been confined to the geological and geotechnical aspects of land instability within the Wollongong City Council Local Government Area. The geotechnical investigation data used herein, other than where indicated, has been obtained from various government authorities and industry based geotechnical projects and associated reports which are acknowledged as appropriate No borehole drilling has been undertaken as part of this project During this PhD research project the strategy of a staged approach to the development and implementation of a fully quantitative hazard identification and risk management process is acknowledged as the correct method. Only the following aspects of the proposed risk management process (Figure 2.1 on page 2-19, and Figure 2.3 on page 2-23) are covered in this thesis: Establish Project Scope 1-6

7 Characterise the site Development of methods and techniques for hazard assessment Identify Land Instability Hazards Identify Land Instability Risks Relationships between rainfall and landsliding, and geology and landsliding have been considered in this thesis, but the effects of earthquakes on the initiation and frequency of landsliding have not been considered The hazard and risk of potential new rock falls have not been considered The potential maximum velocity and potential run out area of existing or potential landslides have not been considered. The Wollongong City Council (WCC) has been an external partner for this project. This has enabled access to the WCC s Geographic Information System computer package and the GIS staff and has facilitated the development of the maps. An accurate geological framework and land instability inventory is presented using, as map bases, the best practice available today in the commercially available topographic mapping, the 1:4000 Central mapping Authority (CMA) topographic contour maps overlain the Wollongong City Council cadastre. The maps use the Integrated Survey Grid (ISG) for map co-ordination and the Australian Height Datum (AHD) for elevation. The contour interval is 2m. The map boundaries used by the Wollongong City Council and the CMA 1:4000 scale map series are the same. These map boundaries and the individual WCC map sheet names are shown on Figure 1.3. The area outlined in this figure with a heavy black line is considered during this research project as the map area. The individual WCC map sheet names are correlated with the relevant individual CMA 1:4000 scale Orthophotographic map sheet names in Table

8 WCC CMA WCC CMA B13 Garie U I10 Coledale U C12 Helensburgh U J9 Bellambi W C13 Garie U J10 Corrimal W XD12 Stanwell Park U K8 Bellambi W XD13 Otford U K9 Bellambi W E11 Stanwell Park U L8 Bellambi W E12 Stanwell Park U M7 Wollongong W E13 Otford U M8 Wollongong W F11 Stanwell Park U M9 Wollongong W F12 Stanwell Park U N6 Wanyam Billy W G11 Coledale U N7 Wollongong W G12 Coledale U N8 Wollongong W H10 Coledale U O5 Wanyam Billy W H11 Coledale U O6 Wanyam Billy W I9 Cataract U O7 Wollongong W Table 1.1. Wollongong City Council 1:4000 series map names with the corresponding Central Mapping Authority 1:4000 series map names. Map boundaries are co-linear. 1-8

9 Figure 1.3 Wollongong City Council 1:4000 map sheet index map. Map area for this research project is bordered by heavy black line. Table 1.1 relates these map names to co-linear Central Mapping Authority 1:4000 scale map names. 1-9