Adoption of stabilisation saves project building in TAS

Stewart Geeves, AustStab Vice President, provides an overview of how stabilisation proved beneficial to a challenging project in Tasmania’s Central Highlands. Stewart Geeves, AustStab Vice President, provides an overview of how stabilisation proved beneficial to a challenging project in Tasmania’s Central Highlands. 

In October 2017, Tasmania’s Department of State Growth called tenders for a unique project in one of the state’s more isolated regions.

The work, located in Tasmania’s Central Highlands, involves the construction of a pavement overlay and sealing of sections of the Lake Secondary Road between Liawenee Canal Bridge to the start of the sealed section at the Great Lakes lookout at Haulage Hill, representing about 10 kilometres of overlay and seal. The main objective of the project is to bridge the currently sealed sections of road to create a fully sealed secondary highway to facilitate boosted tourism in the area along with accompanying safety upgrades the works will bring.

As the site is in the Central Highlands of Tasmania, most of the work is located at around 1000-metres altitude. As such, it is prone to snow and ice from around May to October, although snow can fall at any time of the year. This significantly reduces Tasmania’s already restricted construction season to around December to March. Many of the roads were constructed by Tasmania’s Hydro-Electric Commission in the 1960s when the level of the Great Lake was raised considerably from that constructed in the 1920s. Today, the road is bordered by large sections of Tasmania’s World Heritage Conservation Area, which brings into play a variety of environmental issues including a variety of threatened flora, fauna, weeds and vegetation.

Andrew Walter Constructions (AWC) was identified as the preferred tenderer for the project. Through the tendered price of around $7 million was significantly more than the client’s budget of around $5 million. For the project to go ahead, significant savings had to be made. AWC is a long-standing member of AustStab and has been carrying out road pavement stabilising activities in Tasmania for over 20 years. The experience of AWC’s project managers, engineers and operators, was sought in seeking cost savings for the project.

As this was an isolated location, delivery of materials was a significant cost. Major cost savings could be identified by modifying the original scope to incorporate stabilisation of the existing gravels, allowing the use of local subbases and reducing the quantity of base course required to complete the project. In addition, the adoption of stabilisation offered significant time savings, critical in view of the reduced construction season.

Prior to works commencing in early January this year, extensive investigation works were undertaken by AWC in November/December 2017. This included systematic test pitting along each section to determine existing pavement depths and to gather samples of the existing pavement materials for laboratory testing. This allowed an appropriate stabilisation treatment to be identified and necessary depths of treatment and overlay to be determined.

In general, the test pitting identified an existing highly plastic decomposed basalt or dolerite base course over a rocky subbase and very tightly packed rock subbase layer. As the old road was unsealed in an alpine area the existing cross-falls were generally high (typically five  to six per cent) and to reduce these crossfalls without using additional material for shape correction, it was decided to reclaim and reshape the existing profile and base any design considering a reduced depth of existing pavement at the crown. After extensive discussions with the department it was agreed that lime stabilising the existing pavement gravels after reshaping with two per cent lime at a 150-millimetre depth prior to a 100- or 150-millimetre, Class 1 base overlay was the most economical solution. Pavement and drainage works commenced early January 2018.

Project Engineer Adam McKeown was tasked with the project and, together with an experienced team of supervisors and operators, successfully carried out the first half of the project before inclement weather shut down works. Adam has been pleased by the progress of the works, saying: “The target for the first season was three kilometres of sealed road. Currently we are on track to complete nearly double that, which is a credit to everyone involved and will surely put a smile on the face of all relevant stakeholders.”

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