However, where the Pacific Highway (State Highway No. 10) crosses Clybucca Flat between 14 and 20km north of Kempsey, neither of those solutions are practicable.
North of Kempsey, the Pacific Highway traverses a flood plain of the lower Macleay Valley for about 20km. The Highway follows a route to the north of the Macleay River and avoids the low lying swamps as much as possible. At Clybucca Flat which is formed from saturated flood silts, the highway crosses and area regularly inundated by flood waters, which break the river bank at Seven Oaks just upstream from the Smithtown. The water crosses and re-crosses the Highway between 14 and 20km north of Kempsey as the adjacent swamps flood and drain. As the water recedes, it clears from the flooded road along North Clybucca Creek and South Clybucca Creek at 20km north of Kempsey.
The existing highway consists of a bitumen flush seal on a shale pavement 450 to 1000mm deep and rests on the natural silts. With repeated flooding and heavy loading there have been major maintenance problems over a number of years.
Reconstruction of the Pacific Highway at Clybucca Flat on a raised formation in its present location to avoid flooding altogether was not practicable. It is necessary for flood waters to have a free passage across Clybucca Flat as a vital part of flood mitigation in this area.
A deviation provided no solution either, because the most suitable flood-free alignment would require the construction of a deviation approximately 35km long. While a deviation would cater for the needs of through traffic it would still leave the problem of providing a service road across Clybucca Flat for those communities which use the highway to gain access to Kempsey.
With the need to keep the highway at existing levels and close to its present route there remained the solving of the engineering problem – the construction of a stable, long-wearing and relatively maintenance-free pavement, built on saturated silt, where the water table regularly rises to within the water table regularly rises to within 100mm of the surface. Under these conditions, a rigid type of pavement was preferred and investigations into methods of construction commenced.
Investigation of the subgrade of Clybucca Flat proceeded over a number of years. Intensive sampling of material from holes one metre deep was undertaken. Routine testing of these samples indicated that a minimum cover of naturally occurring material one metre thick was required even under well-drained conditions which could not be provided in this flood prone area.
In situ California Bearing Ratio (CBR) tests and Dynamic Cone tests were also carried out. CBR values as low as 1% confirmed the results from soil testing that at least one metre of cover was required for flexible pavement construction.
Trial sections were excavated at this stage to investigate possible construction techniques. Satisfactory compaction of material on top of the saturated silts was found to be impossible as even light rollers caused excessive heaving when attempting to compact 450mm of crushed rock placed over the silt.
In the light of all the considerations associated with the construction of a road across this flood plain it was decided to adopt the design of a rigid pavement using continuously reinforced concrete. This is a relatively new concept in pavement construction. The control of shrinkage cracking by continuous steel reinforcement eliminates the need for transverse joints which have been an undesirable characteristic of cement concrete pavements in the past.
To overcome the inability to compact a gravel base on the silt, a base of mass concrete 130mm deep and 13.4m wide is being constructed. A continuous reinforced concrete pavement 230mm deep and 7.4m wide, with bitumen sealed shoulders of fine crushed rock 3 metres wide, is being placed over the base.
Traffic flow along the Pacific Highway must be maintained during the work. The sequence of the work has been so arranged to provide for two sealed travel lanes for traffic at all times during construction.
The available road width is restricted by Menarcobrinni Lagoon on the eastern side of the highway and on the western side by utility services (water, telephone and electricity) in the road reserve. Construction is therefore being carried out in stages to allow for one lane of traffic in each direction in addition to the working areas. To provide for the free flow of traffic at the commencement of the work it was necessary to widen the pavement on the western side by 3.7 metres over a length of 5500 metres.
In use on the construction are two items of equipment not commonly used on roads in country areas.
The preparation of the sub-grade, 4.3 metres wide, to its design line and grade is being carried out by an electronically controlled single lane trimmer spreader which excavates and trims the sub-grade to fine tolerances ready for placing the mass concrete base. The trimmer spreader is kept on correct line and grade by electronic sensors which are activated by a tight nylon string line accurately positioned along each side of the length to be excavated. The second item of equipment is a concrete spreader-finisher which travels on the steel side paving forms which are accurately located on the excavated sub-grade.
The base course of 8Mpa strength concrete is being laid in three parallel passes, and the 23Mpa strength continuously reinforced concrete surface course in two passes, with the longitudinal joint of the latter, keyed and dowelled.
Pouring of concrete commenced on 19 th August, 1975.
About 20,000 cubic metres of concrete will be required to complete the work and the successful tenderer for supply of this material was B.M.G Concrete North. The firm has established a production plant at Frederickton for the supply of concrete.
Crushed rock for the shoulders will be provided by local contract. The remainder of the work is being carried out by the Department’s own forces.
Longitudinal reinforcement for the continuously reinforced concrete pavement consists of 16mm diameter cold worked deformed steel bars placed at 140mm centres. Transverse reinforcing consists of 10mm diameter bars at 500mm spacing. A total of 500 tonnes of reinforcing steel is required.
Prior to the commencement of the work relocation of water mains, electricity mains and telephone cables was carried out by Macleay Shire Council, the Macleay River County Council and Telecom Australia, respectively.
The work is expected to extend over a period of approximately twelve months and the estimated cost of the project is $2 million.
As the construction of continuously reinforced concrete pavements is relatively new to Australia, special instrumentation to record strain in the steel and temperature in the concrete is being installed. Detailed information about the pavement surface is also being recorded. This information will aid in the design of similar pavements in the future.
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