Hiway Stabilizers are experts at subgrade stabilisation. Backed by over 20 years experience and the largest fleet of modern, purpose built stabilisation plant in New Zealand, the company offers unparalleled performance in this highly specialised industry.

The Romans undertook lime stabilisation of pavement soils some 2000 years ago, the results of which are still clearly obvious throughout Europe today. The process has since been highly mechanised and reformed but the principles remain the same.

Basic Principles

Lime stabilisation is the most common type of subgrade stabilising. It involves the spreading of lime oxided (CaO) which is then watered (slaked) to form lime hydroxide (Ca(OH)2) and mixed into the subgrade soil. The lime hydroxide reacts with the aluminate and silicate minerals found in almost all clayey soils. The resulting calcium silicate and calcium aluminate hydrates are very similar to those produced by the hydration of Portland cement and produce often significant increases in soil strength over a relatively short time period.

Subgrades are generally stabilised with lime or cement or a combination of the two. Generally cohesive, silty, clayey soils will react well with lime and sandy soils will react well with cement. Some soils will be a combination of silt, sand and clay and could require stabilising with both lime and cement.

Depending on the reactiveness of the soil, strength gains of up to 20 times is often achievable. This strength increase is one of a number of benefits subgrade stabilisation offers and enables a pavement to be designed with reduced depths of subbase and basecourse aggregates.

To determine the reactivity of a soil, testing, which generally involves laboratory CBR tests on the natural soil and with binders added, is undertaken. The results are compared and the reactiveness and CBR strength increases are gauged.

The Process

The subgrade stabilisation process involves the following:

• Spreading a specified application of either lime and/or cement on the prepared subgrade via purpose built precision spreader trucks.
• Applying sufficient water to the binder for slaking/hydrating via purpose built pressurised water carts, preferably with side mounted spray bars.
• Mixing the binder with a purpose built stabilising hoe to a specified depth and to achieve a  specified degree of mixing and soil particle size.
• Compacting the mix with a heavy, vibrating padfoot roller.
• Trimming to final design levels using a grader and smooth drum roller.

Hiway Stabilizers are the most experience New Zealand contractor in this process. Main stabilising plant items include numerous Komatsu GS360 stabilising hoes, precision belt spreaders and pressurised spreaders, pressurised watercarts, and vibrating smooth drum and padfoot rollers up to 21 tonnes static weight (the heaviest in New Zealand).

The Benefits of a Stabilised Subgrade

Subgrade Stability:
A stabilised subgrade is a much more stable and durable subgrade that is resistant to volume and strength changes caused by moisture and temperature variations.

Compaction Anvil:
Stabilising will produce a subgrade layer with increased strength that will act as an anvil when compacting subsequent aggregate layers thereby enabling higher densities to be achieved in the aggregate layers, realising the aggregates full potential.

Reduced Pavement Depth:
By stabilising a subgrade an increased design CBR strength can be used with resulting reductions in overlying aggregate layer depths for an equivalent pavement design life and traffic loading. Reduced aggregate layer depths result in reduced aggregate volumes, associated costs savings, reduced transport costs and conservation of aggregate resources.

Strength and Stiffness:
Aside from the previous items, increasing the strength and stiffness of a subgrade will provide a much more stable platform that is resistant to deformation and failure caused by construction traffic and requires less effort to trim to final levels.

Reduced Plasticity:
Clayey soils have a high plasticity index which is the range of water content that the soil maintains a plastic state. Lime stabilising reduces the plasticity index and increases the workability of the soil and its ability to maintain its desired profile under loading.

Moisture Barrier:
A stabilised subgrade will form a moisture barrier that will protect the overlying aggregate layers from capillary action and the underlying soils from gravity action. Subsoil and water table drains provide an important component in all types of pavements and are critical in removing water trapped by the stabilised layer.

Particle Migration:
A stabilised subgrade reduces or eliminates the migration of soil particles into the overlying aggregate which causes aggregate contamination and can act as a lubricant between the aggregate particles. Also aggregate particles will not punch into the subgrade and effectively become lost to the integrity of the pavement.

Other Benefits of Stabilised Subgrades:

• Reduces the risk of costly subgrade pavement failures.
• Creates a more uniform pavement layer.
• Reduces the soils moisture content and increases its workability.
• Eliminates or reduces the necessity for undercutting and the associated cost and environmental impact that transportation and disposal of unsuitable material involves.
• Is a proven, cost effective and durable pavement component.
• Reduces transport demands on existing roading infrastructure by reducing aggregate and undercutting to waste requirements.
• Provides a superior working platform especially during winter construction programs.

Summary

Subgrade stabilisation should be the first factor considered when designing a pavement and if not designed for should even be considered at the construction phase, which is often the case, as the benefits are numerous, the potential savings in time and cost are significant and a superior pavement will be constructed.

Hiway Stabilizers have the experience and expertise to undertake all types of subgrade stabilisation. Our stabilisation plant inventory is second to none and is operated by highly skilled and efficient crews capable of high production out puts while maintaining the highest quality standards.