Title Determination of the intervention point for rehabilitation of dense graded asphalt wearing courses – the Western Australian Experience. Author Martyn Glover BSc MBA FIPWEA MIEAust Abstract The key to effective asset management of a road pavement, is the determination of an appropriate intervention point when replacing the asphalt wear course. An important tool in this exercise is the deterioration curve or prediction model for the pavement’s point of rapid decline in condition. The sand sub‐grades of the Perth metropolitan area in Western Australia provide some of the best road pavement foundations in the world. When combined with Perth’s Mediterranean climate without any freeze/thaw it is considered that these road pavements have much longer life spans than roads elsewhere in Australia and the world. The published deterioration models for road pavements and the wear courses from around the world have been traditionally used in pavement management systems that are available in Western Australia but in most cases they do not reflect the actual deterioration of the pavement. This paper presents the final findings of a significant study in the Perth Western Australia which develops the deterioration curve based on nearly 10,000 kilometres of pavement monitored in some cases for in excess of 100 years and reconciled against recent ARRB pavement testing. Introduction In Western Australia there has been minimal research on the optimum intervention point for replacing dense graded asphalt wear courses. This has been in part due to our isolation but mainly due to Perth’s meteorology and geology which is very conducive to long lasting and very economic road pavements. Although minimal research has been done, the road data available in Perth and all of Western Australia is exceptional as records have been kept originally by the Local Roads Boards and in the past 50 years by the State road authority, Main Roads Western Australia in a central repository which in the case of this research provides accurate data as far back as 1914. This data includes accurate visual assessments and both construction and resurfacing history of all roads in Western Australia in a consistent format. The initial purpose of this paper is to analyse the available data and develop an appropriate intervention point. All Western Australian Local Governments are represented by the Western Australian Local Government Association. This organisation provides a report to Local Government each year on the level of expenditure on the road network each year and the resultant condition of the network as a result of the expenditure. The secondary purpose of this paper is to use this information to demonstrate the effectiveness of the current intervention strategies on the overall condition of the road network. Study Area The use of asphalt surfacing in Western Australia is predominantly in the metropolitan area of Perth and the larger regional towns and cities. Consequently the study area has been limited to metropolitan Perth extended to include the City of Mandurah. Most of this area is within a geological formation known as the Swan Coastal Plain. However it also includes area on the Darling Scarp to the east of the metropolitan area. Participating Local Governments To provide an appropriate data set all thirty metropolitan Local Governments within the study area were contacted seeking permission to access their data and the following fourteen Local Governments agreed to be part of the project: • City of Armadale, • City of Belmont, • Town of Cottesloe, • City of Gosnells, • City of Joondalup, • Shire of Kalamunda, • City of Mandurah, • City of Melville, • Shire of Mundaring, • City of Nedlands, • City of Rockingham, • City of South Perth, • City of Swan and • City of Wanneroo. There is variability in the subgrade soils across the Swan Coastal Plain consequently it was decided to split the participating Councils into three distinct groups as follows: • The Perth Coastal Group including Belmont, Cottesloe, Joondalup, Melville, Nedlands, South Perth and Wanneroo collectively provide 3,657Km of asphalted roads. The subgrades in these areas are almost exclusively sand. • The Darling Range Group including Armadale, Gosnells, Kalamunda, Mundaring and Swan collectively provide 4,093Km of asphalted roads. The subgrades in these areas are either sand or lateritic soils of the scarp. • The Murray Coastal Group which marks the very southern area of the study area, comprises Mandurah and Rockingham and these provide 1,628Km of asphalted roads. The subgrades are predominantly sand however groundwater in these areas tends to be very shallow. The Swan Coastal Plain The Swan Coastal Plain is a 30 kilometre wide strip on the Indian Ocean coast directly west of the Darling Scarp uplands running from Cape Naturaliste in the south to above the City of Perth. This study will look specifically at the northern section of the Swan Coastal Plain, geologically referred to as the Perth Basin and aligned with most of the Perth metropolitan area. In the area of the Perth basin, the Swan Coastal Plain is about 34 kilometres wide in the north, 23 kilometres in the south, and is bounded to the east by the Gingin and Darling Fault Scarps, which rise to over 200 metres above sea level. Geologically, the scarps represent the eastern boundary of Tertiary and Quaternary marine erosion. The Swan Coastal Plain consists of a series of distinct landforms, roughly parallel to the coast. The most easterly landform comprises the colluvial slopes which form the foothills of the Darling and Dandaragan Plateaus and which represent dissected remnants of a sand covered, wave‐cut platform known as the Ridge Hill Shelf. To the west of the colluvial slopes lies the Pinjarra Plain, a piedmont and valley‐flat alluvial plain consisting predominantly of clayey alluvium that has been transported by rivers and streams from the Darling and Dandaragan Plateaus. To the west of the Pinjarra Plain, the Bassendean Dune System forms a gently undulating aeolian sand plain about 20 kilometres wide with the dunes to the north of Perth generally having greater topographic relief than those to the south. The dunes probably accumulated as shoreline deposits and coastal dunes during interglacial periods of high sea level and originally consisted of mostly lime (calcareous) sand with quartz sand and minor fine‐grained, black, heavy‐mineral concentrations. Apart from a small local area to the south of Perth, the carbonate material has been completely leached leaving dunes consisting entirely of quartz sand. West of the Bassendean Dune System are two systems of dunes which fringe the coastline. The most easterly of these is the Spearwood Dune System, which consists of slightly calcareous aeolian sand remnant from leaching of the underlying Pleistocene Tamala limestone. The most westerly dune system, which flanks the ocean, is the Quindalup Dune System (Safety Bay Sand), consisting of wind‐ blown lime and quartz beach sand forming dunes or ridges that are generally oriented parallel to the present coast, but which may also occupy blowouts within the Spearwood Dune System. These sandy soils have particularly good road sub‐grade qualities which along with the Mediterranean climate contribute significantly to the longevity of road pavements constructed upon the soils. Study Data Sets There are three key data sets that will be utilised for the research. The first is the Roman II dataset which is the Road Management System utilised by 134 of the 140 Western Australian Local Governments. The basic road data recorded in this dataset includes road lengths, widths, construction date and resurfacing dates. All of the participating Local Governments use Roman II. This system and its predecessors have provided asset management tools for Local Government since the 1970’s and the oldest data on asphalt was recorded in 1914 from the Town of Cottesloe. The second dataset is the financial information contained in the annual Report on Local Government Road Assets and Expenditure published by the Western Australian Local Government Association (WALGA). This publication provides information on annual Local Government road rehabilitation expenditure which has been consistently reported in the same format for the past 15 years. The final dataset is a series of field studies conducted by ARRB between 2003 and 2007 around Australia and including sites within the Perth metropolitan area. The study collected all recorded data on the road sections as well as current physical conditions including roughness, rutting, cracking and deflection. Road Pavements The sands of the study area of the Perth including the Safety Bay sand, Tamala Limestone and Bassendean Sand formations have California Bearing Ratios or CBRs in excess of 10.0, consequently the designed pavements are thin. Normally they consist of a relatively thin unbound sub‐base and base course overlaid with an extremely thin asphalt wear course. In consideration of the minimal margin for error due to the ultrathin pavements the Department of Planning in WA decided to adopt a minimal standard for development. The Institution of Public Works Engineering Australia (WA Division Inc) Subdivision Guidelines Edition No 2.2, 2012 (the Guidelines) was the resultant adopted document and it prescribes the design standards for pavement thickness. The Guidelines establish that all pavements are designed in accordance with nationally recognised design documents and they also prescribe the anticipated design life of pavements