Projects
Taree Airport Project
Taree Aerodrome Runway, 1988-1998
Overview
 Polymer-stabilisation of a section of the
Taree runway is one of the earlier Australian examples of this type of
rehabilitation treatment. The top 150mm of conglomerate gravel basecourse
was polymer-stabilised insitu in September 1988 and resealed. It continues
to perform well after ten years. This contrasts with the previous runway
performance where localised rutting under the Fokker Friendship aircraft
necessitated frequent patching. The failures typically developed after
periods of rain.
Investigation
A site investigation showed the basecourse to be a conglomerate gravel
composed of smooth rounded coarse stones within a moderately plastic
(Plasticity Index 9) matrix. This suggested that the compacted material
might be relatively strong when dry but that it would lose considerable
strength when wet. This was confirmed and quantified by laboratory
testing. Parallel samples were prepared for Californian Bearing Ratio
testing. Samples were compacted to identical densities at the optimum
moisture content of 6% and also at 7.5%, 1.5% wet of optimum. This small
increase in moisture content reduced the CBR from 60% to 35%. The
mechanism of strength loss is that the clayey fines become plastic and
'greasy' when wet and lubricate the coarse stones allowing them to then
slide relative to each other. The severity of this effect at Taree
appeared to be due to the 'ball bearing' nature of the smooth rounded
stones in the gravel.
The investigation concluded that the rutting under aircraft wheel loads
was due to rain-related instability of the gravel basecourse rather than
to deformation of the underlying shale subbase or clay subgrade. The aged
sprayed bitumen seal was cracked and clearly permeable, particularly to
water that ponded in shallow birdbaths after rain but also to water held
in the coarse surface texture of the seal for extended periods after light
rain. This analysis of the problem lead to a strategy of improving the wet
strength of the gravel basecourse by insitu stabilisation and protecting
it from ingress of surface water with a new two-coat sprayed seal.
Cement stabilisation is commonly used to ensure adequate wet strength
of gravels and this had been tried previously in another section of the
runway. The client's concern that cement stabilisation would again produce
cracking problems prompted the polymer trial.
Instead of improving gravel strength by cementing particles together,
polymer stabilisation is intended to retain dry strength by a process of
'internal waterproofing'. This involves creating a hydrophobic soil
matrix. The treated pavement material remains flexible and does not shrink
or crack. Permeability is reduced so water ingress is reduced. More
importantly, because the polymers are very strongly attracted to clay
particles they compete successfully with water to coat the clays that are
in the gravel. Thus water that does enter the gravel tends not to soften
the clay so the lubricating effect referred to above that produces rutting
is much reduced.
Permeability Tests
These two effects, reduced permeability and reduced effect of
penetrating moisture were separately checked for the Taree gravel in the
laboratory. Firstly, 100mm high compacted samples of both untreated and
treated gravel were allowed to dry back for a few days then allowed to
stand in a tray containing 30mm of water. After 24 hours the capillary
moisture was seen to rise to near the top of the untreated sample and the
sample began to disintegrate below the 30mm waterline. By contrast, the
polymer-treated sample suffered a capillary rise of only approximately
25mm and the sample remained intact below the waterline. The effectiveness
of the waterproofing treatment was also indicated by squirting drops of
water onto the dry top of each sample. The water immediately soaked into
the untreated sample but 'balled' on the treated sample and did not soak
in.
Wet strength
Wet strength was assessed using the triaxial test, a test in which both
the cohesion and friction angle of a compacted material are measured. It
was considered that this test would produce a more accurate and
comprehensive measure of strength than could be obtained from the
Unconfined Compression Test, the test more commonly used to assess
stabilised materials. The results are tabulated below.
Test Specimen
|
Moisture Content (%)
|
Cohesion (Kpa)
|
Friction Angle (Degrees)
|
Dry Density (tonnes/cu.m)
|
| Dry untreated
gravel |
1.6v |
450 |
39 |
2.18 |
| Wet untreated
gravel |
8.2 |
0 |
22 |
2.11 |
| Wet
polymer-treated gravel |
8.9 |
125 |
37 |
37
|
As expected, the untreated gravel had high strength when tested very
dry of the optimum compaction moisture content of 6%. When tested at 8.2%,
the strength dropped dramatically (a cohesion of zero with a friction
angle of 22 degrees corresponds to an Unconfined Compressive Strength of
zero). The wet polymer-stabilised sample had a relatively high strength at
8.9% moisture.
Conclusion
The continued good performance of the polymer-stabilised basecourse
after 10 years indicates both the effectiveness of the treatment and also
its permanence.
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