Heavy axle loads
What is a heavy axle load?
Axle load is the amount of weight a single axle can bear. When the weight on any one axle exceeds the standard or legal limits given, it’s referred to as a heavy axle load. Effectively managing axle loads is essential to reduce damage to both soil and infrastructure, as elevated axle loads result in greater soil compaction, which in turn impacts water infiltration, drainage, and root development.
Impact of heavy axle loads
Heavy axle loads have a significant impact on various infrastructure functions and vehicle performance. They are;
Effects on soil compaction
Heavy axle loads contribute greatly to soil compaction, especially in wet soil conditions. Compaction occurs when soil particles are pressed together, reducing porosity, thereby limiting water and nutrient supply. The effects of compaction are greater with machine operation at lower velocities, because longer load duration to the soil increases the compaction severity.
Influence on infrastructure durability
Heavy axle loads accelerate pavement deterioration, increase maintenance cost and reduce pavement life. For example, the energy required to operate machines in compacted soil increases dramatically, affecting both handling efficiency and vehicle performance.
Rupture and tearing
If the axle laid is excessive on an unpaved road, the heavy vehicle ruptures the geosynthetic product. This leads to tearing or deposition, leading to loss of efficiency.
Low drainage efficiency
Heavy axle load amplifies stress on the geosynthetic material leading to deformation, causing low drainage efficiency and amassing of water within the soil layers.
Role of geosynthetics in managing heavy axle loads
The modern geotechnical design practices require that structures founded on soil should perform satisfactorily under various kinds of loads. These loads are of dynamic and static nature and in cases where the axle load exceeds the tensile strength of the material, it results in failure, leading to structural instability, soil displacement, thereby, catastrophic failure of the entire structure. The geosynthetic reinforcements reduce the settlements and increase the load-bearing capacity of the subgrade soils.
Applications of geosynthetics to manage heavy axle loads
Geosynthetics reduce the impact of heavy axle loads in civil and agricultural projects. Improved load distribution and soil stability support the performance of transport infrastructure and earth retaining structures.
Filtration and drainage
Geosynthetics, especially geotextiles, are intended to optimize filtration by retaining soil particles that facilitate water permeability. This attribute is critical for the sound design of roads and railways under significant axle loads, as an overabundance of water compromise the groundwater and cause the failure of road infrastructure.
Separation and stabilization
Geosynthetics prevent different soil layers from intermixing, which is crucial in road and rail construction where the integrity of layers is paramount. In stabilization, geogrids are frequently employed to increase the bearing capacity of the trackbed over weak soils, effectively accommodating the stresses from heavy machinery. They enhance load-carrying capacity and minimize rutting by acting as a separation barrier, preventing the fill material from puncturing the subgrade.
Erosion control
Geosynthetics are used to protect the ground from erosion, support vegetation growth, and keep the soil in place. This becomes important in areas where heavy machinery operates near water bodies or embankments, helping to hold the land together and limit erosion.
Asphalt overlay
In pavement works, geotextiles placed between old and new asphalt help reduce reflective cracking. This layer acts as a moisture barrier and extends the life of the overlay while improving how loads are handled by the existing pavement.
Rail track bed improvement
Geosynthetics are also critical in rail infrastructure, particularly for enhancing the performance of trackbeds over soft ground. They improve drainage, mitigate lateral migration of ballast, and stabilise the transition zones from rigid to flexible foundations and help maintain track stability and performance under heavy loads.
Challenges and considerations in using geosynthetics for heavy axle loads.
Environmental impact
Geosynthetics enhance sustainability in civil engineering by minimising aggregate usage and reducing greenhouse gas output during construction. However, the manufacturing process involves high energy input, and polymer extraction can introduce environmental challenges during material sourcing. In areas exposed to heavy axle loads, these factors require careful evaluation, as increased soil stress can intensify long-term environmental effects.
Material durability
Geosynthetics material must withstand the stresses imposed by heavy axle loads, which otherwise lead to fatigue and failure. Factors such as cyclic stress ratios, load application rate, and environmental circumstances influence the durability of geosynthetics. In-depth testing is vital to ensure that these materials can maintain their integrity.
Economic considerations
While geosynthetics lead to long-term cost savings through improving lifespan of structures, the initial investment will be substantial. Cost-effectiveness needs to be evaluated in heavy load applications, since failure of geosynthetics in these conditions lead to high repair expenses and construction delays.
Strategies for mitigating heavy axial load
To alleviate the detrimental effects of heavy axle loads, several strategies are employed. One of the strategies is to manage axle loads by keeping them below 10 tons, which help localise compaction within the top 6 to 10 inches of soil. Moreover, equipping vehicles with dual tires or tracks enhance flotation and reduce compaction impacts. Proper tire inflation is also pivotal, as under-inflated tires aggravate soil compaction by exalting the ground pressure exerted on the soil.
Difference between axle load and radial load
Axle loads are those that can withstand a force in the same direction as the axis, also known as thrust loading. Axle loads cause deformation of the material, influencing its tensile strength and stability. Radial loads, on the other hand, are made to withstand forces that are perpendicular to the direction of the axis. They are often used in applications involving drainage and landfill liners. These cause alterations in pressure distribution and affect the ability to retain fluids.