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. Managing axle loads is crucial for minimising damage to soil and infrastructure as higher axle loads lead to increased compaction of soil, subsequently affecting water infiltration, drainage, and root growth.
In geotechnical engineering, understanding heavy axle load is significant in ensuring structural integrity and stability over time.
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. Geosynthetics like geogrids and geotextiles function to distribute axle loads across a wider area. When axle loads are applied, these materials deploy the lateral resistance of the soil, enhancing the overall shear strength.
Applications of geosynthetics to manage heavy axle loads
Geosynthetics play a critical role in managing the impacts of heavy axle loads, particularly in civil engineering and agricultural settings. Their ability to improve load distribution, stability, and overall performance of structures makes them valuable in both transportation infrastructure and earth retaining structures.
Filtration and drainage
Geosynthetics, especially geotextiles, are designed to optimise filtration by retaining the soil particles that allow water to permeate. This property is essential for the proper design of roads and railways under heavy axle load, as excess water weakens the groundwater and causes road structure failure.
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 stabilisation, 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 minimise rutting by acting as a separation barrier, preventing the fill material from puncturing the subgrade.
Erosion control
Geosynthetics is designed to protect the earth surface from erosion, promote vegetation growth and maintain soil stability.This is especially important where heavy equipment operates near water or embankments, as it helps to maintain integrity of the land and prevent further erosion.
Asphalt Overlay
In pavement applications, geotextiles are used in asphalt overlays to mitigate reflective cracking. By positioning a geotextile layer between the old and new asphalt, a permanent moisture barrier is created. This enhances the overlay’s lifespan and supports effective load management on existing pavements.
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
While geosynthetics enhance sustainability by reducing resource consumption and greenhouse gas emissions, their production involves notable energy input and potential environmental degradation from raw material extraction. This duality necessitates prudent consideration in the context of heavy axle loads that exacerbate underlying soil issues leading to increased environmental impacts over time.
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. As contractors face budget constraints and rising material costs, balancing expenses with potential savings becomes a critical consideration. This is especially pertinent when evaluating the cost-effectiveness of using geosynthetics in heavy load scenarios where failure results in significant repair costs.
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. Also understand its relationship with both static and dynamic loads to ensure effective infrastructure management.
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.
In conclusion, the integration of geosynthetics in managing heavy axial loads offers significant advantages in civil engineering by improving stability and performance under high stress conditions. Geosynthetics stand out as a cost effective efficient strategy for optimising the structures subjected to excess axle loads.