Lime stabilisation 101

A very important, and visible, part of building the new rail embankments along the Narrabri to North Star Phase 1 alignment has been the use of a white powdery substance called lime. But what is lime and why is it so important to the construction of Inland Rail?

A section of Inland Rail foundation near the level crossing at Penneys Road undergoing lime stabilisation

Lime is the commonly known name for calcium oxide, a material derived from ground limestone. When water is added back to the material it takes on adhesive properties and helps to bind and stabilise the new mixture. Lime can substantially increase the workability, permeability, and long-term load-bearing characteristics of clay soils.

The use of stabilisation is common practice in construction to improve foundation conditions and earthworks materials, especially when building roads, airports and railways! Depending upon soil material properties and laboratory testing, the stabilisation agent can include lime, cement, slag/lime blends and even bitumen.

The current black clay type soil track foundation and earthworks materials along the N2NS Phase 1 alignment need to be strengthened to provide a stable and uniform support for the extra weight of double-stacked, 1,800m long trains with 21-tonne axle loads.

Lime stabilisation happens in three stages. The old foundation is removed and all the existing embankment materials including the ballast are re-used and blended with lime and any additional earthworks material over a series of layers. Water is added to the mixing process and the blended mix is rolled and trimmed to shape in place with heavy vibratory compaction equipment. After each stage, the new foundation requires time to cure and strengthen before testing is undertaken to ensure its suitability prior to the next layer being placed.

Safety precautions are in place during the spreading of the lime to minimise risk health and full Personnel Protective Equipment (PPE) is used by all workers. Lime should not be handled without the full required PPE. When water is added to the lime, a chemical reaction occurs which generates steam. This is known as slaking.

IMAGE: A section of Inland Rail foundation near the level crossing at Penneys Road undergoing lime stabilisation

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