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.

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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