Why haul roads lose wet stability and what to do about it.

A wet-stable road should hold firm after rain. It should resist rutting, deformation and washout, and remain trafficable with minimal recovery work before trucks can move safely again.

“One thing I would say confidently is that wet stability is a real test of whether a road treatment is doing its job,” says Julian Musgrave, Head of Business Development Australia at Bind-X.

A haul road can look strong in dry conditions, but rain shows whether that strength remains when the surface is under moisture and load.

Rain affects a mining haul road in two ways: through the immediate condition of the surface and through the longer-term structural integrity of the pavement. The structural effects are often much more significant, and more expensive, than the visible mud and potholes.

Most mine haul roads are unsealed granular pavements. Their strength depends heavily on maintaining the right moisture content. When too much water enters the pavement, it changes how the aggregate, base and subgrade respond under heavy truck loads.

“Sometimes a stabilising product is not suited to the site material. Sometimes the product loading is too low. Sometimes the treatment depth is too shallow for the truck fleet, traffic volume or expected rainfall,” says Julian.

Drainage also plays a major role. If water sits on the road, runs too fast across the surface, or concentrates in low points, it increases the chance of softening, washout and deformation.

Effective stabilisation depends on good road design, drainage, maintenance, material compatibility, treatment depth and whether the stabilised layer has been designed for the loads and moisture conditions it will face.

On an untreated haul road, the first changes are usually visible at the surface. Friction decreases, reducing tyre grip and increasing stopping distances. Water collects in depressions and wheel tracks, while saturated fines turn the wearing course soft or muddy.

Truck speeds may then be reduced for safety, immediately affecting productivity. Puddles, shallow rutting and potholes can also indicate that water is beginning to enter the pavement and affect the layers beneath it.

When water fills the voids between aggregate particles, friction between aggregate particles decreases. The pavement loses bearing capacity and becomes less able to support haul truck loads. Each wheel pass creates greater pavement deflection, which accelerates the loss of shape and strength.

Rainwater can also infiltrate through the wearing course and enter the base and subgrade. Saturation can dramatically reduce the strength of fine-grained materials, resulting in localised road failures, deep wheel tracks and rutting.

Another common failure mechanism on heavily trafficked haul roads is pumping. Repeated truck loads force trapped water upwards through the pavement. Fine material migrates towards the surface, progressively weakening the pavement structure and making the road rougher.

Flowing water creates a different type of damage. It removes fines and aggregate from the road surface and drainage system. Crowns, crossfalls, batters and ditch lines can deteriorate, while severe storms may wash out entire sections of road.

These failures are more difficult to correct than surface mud or isolated potholes because the road has not only lost its shape. It has begun to lose the structure that supports the fleet.

Not all sections of haul road cause the same level of stress. Julian explains that corners can be problem areas to maintain because trucks continually push material sideways when they turn. Over time, this creates rutting, channel formation and surface deformation, increasing the internal stresses on the tyres. Wet stability problems usually show up first where trucks place the greatest load, movement and shear force into the surface: corners, ramps, intersections, braking zones and loading areas. Corners are especially difficult. As trucks turn, they push road material sideways. If the surface has softened after rain, that force can quickly create rutting and channel formation.

“If I was designing a haul road treatment program, I would generally treat corners more aggressively than straight sections, because they’re often the first areas to go,” Julian shares.

Autonomous fleets can amplify this effect. While autonomous trucks vary their path slightly on straight sections, they tend to follow very similar lines through corners. The same sections of road receive repeated loading, which accelerates wear in concentrated areas.

The result is a surface that becomes progressively rougher and more difficult to maintain. Those localised defects might seem minor at first, but they create exactly the conditions that increase tyre flex, add additional abrasive material onto the road surface, generate heat, and shorten tyre life.

Once those sections lose stability, traction becomes harder to manage. Wheel spin is more likely, especially on ramps and corners. The tyre grinds against the softened surface, displacing more material and increasing deformation under load.

“Once one truck starts slipping, the whole fleet stops. Everything pauses until they can either grade the road or improve the conditions enough for the trucks to operate safely again.”

Even when the road remains passable, wet conditions affect haul truck performance. Tyres sink deeper into the softened surface, increasing rolling resistance and the amount of energy required to keep the truck moving.

As rolling resistance rises, fuel consumption increases and cycle times become longer. Tyre temperatures and wear can also potentially increase, while roughness, rutting and potholes place greater loads on truck suspension and structural components.

A useful rule of thumb in mining is that water almost always translates into higher rolling resistance. Rolling resistance then becomes a direct productivity and cost penalty across the fleet.

After a rain event, the usual response is to send the grader through.

That may make the road look serviceable again and trucks can return to the route. But grading does not always restore the strength of the treated layer.

If the stabilisation has softened, washed out or failed to hold the road material together, the surface may look repaired while still being weak. Once haulage resumes, the same section can deteriorate again.

Julian has described this issue in relation to haul road maintenance more broadly: crews often restore the shape of the road without rebuilding the integrity of the road. That distinction matters after rain.

Where water has reduced bearing capacity, softened the subgrade or caused fines to migrate through the pavement, regrading the wearing course alone does not address the underlying failure.

The biggest determinant of wet-weather road performance is often drainage, rather than road thickness alone. A well-built road must be able to shed water quickly before it infiltrates the pavement or remains trapped under repeated truck loads.

This requires an appropriate crossfall, adequate table drains and functioning culverts/drains, supported by a properly treated and compacted wearing course. A crossfall of approximately 2-4% may be used to help move water away from the running surface, depending on the road design and site requirements.

Where the drainage system is working effectively, the road can shed water and may recover within hours. Poorly drained, untreated roads retain moisture, soften and rut, and may require extensive regrading or rehabilitation before normal haulage can resume.

Drainage features also need to remain functional during the rain event. If table drains, culverts or crossfalls are blocked, damaged or poorly maintained, water will either find another path across or through the road or build up on the road.

The answer is to stabilise the road properly for the conditions it will face. That means selecting a stabilisation method that suits the road material, rainfall exposure, drainage conditions, truck fleet, traffic volume and haul road geometry.

A uniform application across the whole road may not be enough. Straight sections, corners, ramps and intersections experience different forces. They may need different treatment depths, product, product loadings or construction methods.

In the past, a site might have looked to options like bitumen emulsion for high-strength haul road stabilisation. These products can be effective, particularly where the treatment area is limited and long-lasting surface strength is the priority. But product selection is changing.

Sites are now looking more closely at environmental performance, clean-up requirements, rehabilitation obligations and the full life cycle of the treatment, not just whether it can bind the road.

For Julian, today his preference would be a clean biological stabilisation method that can deliver the required strength, while also performing under wet conditions. The aim is to build strength into the road structure without creating unnecessary environmental or operational issues later.

This is how he would approach a road designed for heavy mining traffic. “If I was using Terrabind Ultimate, I might stabilise straight sections to a depth of 150 millimetres, but increase that depth and product loading through corners because they’re higher-stress areas.”

The specific design would depend on the road, material and fleet. Wet performance has to be designed into the road from the start. Engineers should identify high-stress sections early. Drainage needs to be managed. The stabilisation product needs to be incorporated at the right depth and loading for the conditions it will face.

Crews must properly mix and compact the product throughout the treated layer. Choosing an appropriate product is only one part of the system. The road still needs suitable geometry, effective drainage, and construction and maintenance to ensure the pavement remains strong when wet.

There is no single stabilisation product that suits every haul road. Some products perform well in certain material types but not others. 

“The goal isn’t to choose a product because it’s popular or the lowest upfront cost. The goal is to solve the problem in the right way that also aligns with your site’s operational and environmental objectives,” says Julian.

When weighing up stabilisation options, mines should ask:

• Has the product been tested under wet conditions?
• Does it maintain strength after heavy rain?
• Does it resist washout?
• How does it perform through wet-dry cycles
• What treatment depth is required?
• How does it perform under heavy truck traffic
• Does it need different loading in corners, ramps or braking zones?
• How soon can the road return to traffic after application?
• What recovery work is usually required after major rain events?
• How often does it need to be reapplied?
• Does it create equipment clean-up issues?
• What are the environmental, closure and rehabilitation implications?

These questions help you make a decision based on whole-of-life road performance.

Large rain events are already disruptive. The aim is to stop road recovery from extending that disruption longer than necessary.

A haul road that holds together during and after rain can support safer traffic movement, more consistent haulage, fewer emergency grading interventions and faster return to production.

It can also help limit the increase in rolling resistance, fuel use, tyre wear, truck loading and cycle times that occurs when the running surface softens and becomes rough.

That is why wet stability deserves attention before the next major rain event. A stabilised road should maintain strength, shape and trafficability when conditions change.

That outcome depends on treating the road as a complete pavement and drainage system. The product, road material, treatment depth, compaction, geometry and drainage all need to work together.

If it cannot do that, your stabilisation strategy needs to be reviewed before the next storm exposes the same weakness again.