M4 Motorway Upgrade West Pavement Construction and Stabilisation
The M4 Motorway West Upgrade was carried out over a length of 20km between the twin service centres at Prospect and Mulgoa Rd at Penrith in the 15 month period 14th January 1997–16th April 1998. The work was carried out in order to upgrade the existing two lanes in each direction to three lanes, and in the process resheet the road surface thereby achieving an additional life of the pavement of 20 years.
Stabilised Pavements Australia (SPA) was contracted by Statewide Roads to carry out the removal (and reuse) of the existing shoulder material, placement of additional gravel (to bring to levels), and then finally to insitu stabilise the prepared gravels (the subbase layer). Others would perform basic earthworks prior to SPA’s work, and also place at least 160mm of asphalt after stabilisation and minor level correction.
Insitu stabilisation was chosen to rehabilitate and widen the M4 Motorway for a variety of reasons. Firstly, the production was expected to be greater than conventional means of road building under such confined and precise conditions, with up to 3000m2 of pavement stabilised per day. By recycling the existing pavement material (Recycled Asphalt Profilings (RAP) and Breccia), the requirement to extract gravels from quarries was significantly reduced as 70% of the additional pavement was constructed with existing gravels. Similarly, wastage of materials was eliminated, which when compared to conventional methods of pavement construction saved over 50,000 tonnes of roadbase material from being tipped or used as landfill.
Apart from the obvious financial savings, there are considerably more (perhaps rarely considered) savings to many areas of the community. The procedure substantially reduced construction traffic and therefore associated fuel usage, air and noise pollution. Wear and tear on the existing road network adjacent to the workers and on route to quarries and landfill sites was also reduced.
After earthworks, the shoulder of the motorway was first milled. This shoulder material contained a varying combination of Breccia and asphalt. The milling works were carried out by a specially adapted profiler which was able to reach the full depth (up to 400mm) and width (2.6m) of the existing material in just one pass. This material was then taken along the Motorway to the preparation site.
Between the milling and preparation operations, the existing subgrade had subsoil drainage placed and if required, was stabilised or removed and replaced with improved granular material.
Preparation involved placing a combination of RAP ( Recycled Asphalt Profilings), Greystanes Gravel ( a poor, sandy and slightly plastic local gravel) and Breccia (the existing base material from the shoulder) within the specified envelope. The proportions were confirmed by laboratory testing of a matrix of mixes which defined an envelope of acceptable mixes over the following ranges; 25-50% RAP, 20-40% Greystanes Gravel and 10-55% Breccia. As the Breccia and some of the RAP already existed in the millings, the process involved placing the millings from the profiler, a predetermined thickness of Greystanes Gravel, and finally topping up to level with more RAP.
The pavement was then ready for stabilisation. The additive used was 25% Slag, 25% Lime and 50% Fly ash. Preliminary testing showed this mix would reach the required 7 day accelerated strength (which was found to correlate well within a 28 day moist cured test) of 2MPa with an addition rate of only 3% by weight. It has an extended working time, which was considered necessary to allow time to correct levels and improve compaction.
Spreading rates were monitored closely from the spreader cabin with the use of on-board load cells, which were able to show the exact usage of the additive at any moment in time. A variable width spreader was also desirable as the widths of the areas to be stabilised were not always exact truck widths. The stabilisation work was carried out by a large reclaimer stabiliser (either a CMI RS500 or Wirtgen WR2500), as these are considered the only machines capable of the accuracy, depth and production that was required. The choice of these machines resulted in considerable savings in terms of time and money.
The subbase layer was stabilised to a compacted depth of 320±10mm. This was surveyed daily at a frequency of 1 level per 15m2. The compaction (standard-97% for the lower 150mm and 100% for the upper 150mm), was measured at a frequency of 4 tests per 600m2, and in addition to this progressive UCS samples were taken at least daily to ensure that 2MPa was always achieved.
To achieve the required compaction at an 18T vibrating padfoot roller was used in conjunction with a smaller 14T vibrating padfoot roller. The finishing was then undertaken with a 14T vibrating flatdrum roller.
The widening of the subbase layer of the Motorway was completed in approximately 150 work days and involved a total of approx. 110,000m2 of stabilised subbase, as well as many more square metres of subgrade and bridge approaches. After provision of a multilayered rehabilitation of the existing traffic lanes, the M4 is now a three lane (in each direction) Motorway from Concord to the Blue Mountains, and travel times have been drastically reduced.