M4 Motorway Upgrade West Pavement Construction and Stabilisation - Project
Report
 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.
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