20.2.1. Contact Pressure

The tyre pressure is an important factor, as it determines the contact area and the contact pressure between the wheel and the pavement surface. Even though the shape of the contact area is elliptical, for sake of simplicity in analysis, a circular area is often considered.

Tyre pressure plays a crucial role in how a vehicle's wheel interacts with the pavement. Higher tyre pressure usually results in a smaller contact area between the tyre and the pavement, which increases the pressure on the pavement. Conversely, lower tyre pressure increases the contact area, decreasing pressure. To simplify calculations, engineers commonly model this contact area as a circle, even though it’s technically elliptical. This simplification helps in pavement design as it allows for easier calculations of stress and strain.

Think of a rubber ball. If you inflate it fully (high pressure), it bounces nicely off a hard surface because the contact area is small. But if it's under-inflated (low pressure), it flops and makes a bigger contact with the surface, leading to less efficient bouncing. Similarly, in pavements, optimal tyre pressure ensures vehicles can operate effectively without damaging the road.

The next important factor is the wheel load, which determines the depth of the pavement required to ensure that the subgrade soil is not failed. Wheel configuration affects the stress distribution and deflection within a pavement.

The load imposed by the wheels of a vehicle is critical in determining how thick the pavement must be. If the wheels exert too much pressure on weak subgrade soil, it can lead to pavement failure. Different wheel configurations (like single vs. dual wheels) affect how this load is distributed across the pavement, which influences the design and thickness of the pavement layers needed to support the load effectively.

Imagine standing on a soft sandy beach. If you stand on one foot (single wheel), your weight is concentrated in a small area, and you might sink in. But if you spread your weight by lying down (like dual wheels), you distribute the pressure over a larger area, and you don’t sink in as much. This principle is crucial for designing pavements that can support heavy vehicles without failing.

The load-carrying capacity of the commercial vehicle is further enhanced by the introduction of multiple axles. Many commercial vehicles have dual rear wheels which ensure that the contact pressure is within the limits.

Multiple axles help distribute the weight of the vehicle more effectively across the pavement. This distribution reduces the contact pressure at each individual wheel, which lessens the chance of causing damage to the pavement. Vehicles designed with configurations such as dual rear wheels spread the load further, ensuring compliance with stress limits defined for pavement designed to endure.

Think of a team of horses pulling a heavy cart. If only one horse pulls the cart, it has a tough job, and it may struggle or damage the road. But if several horses pull together, they share the load, making the journey easier for each. Similarly, multiple axles spread out the load of heavy vehicles, protecting the pavement.

The damage to the pavement is much higher if the vehicle is moving at creep speed. Many studies show that when the speed is increased from 2 km/hr to 24 km/hr, the stresses and deflection reduced by 40 percent.

The speed of the vehicle significantly affects how much damage it can cause to the pavement. When a vehicle moves at a very slow speed (creep speed), the forces exerted on the pavement can cause more significant damage than if the vehicle were moving at a normal speed. Research indicates that simply increasing the speed of a vehicle can reduce the stress exerted on the pavement dramatically, even by up to 40% in specific studies, thus prolonging the lifespan of the pavement.

Consider riding a bicycle over a bumpy road. If you go very slowly over the bumps, you feel every little jolt, which can be hard on both you and the bike. But if you go faster, you glide over the bumps more smoothly, experiencing less impact. Similarly, vehicles moving at higher speeds have a less damaging effect on pavement than when they crawl along.

The influence of traffic on pavement not only depends on the magnitude of the wheel load, but also on the frequency of the load applications. Each load application causes some deformation, and the total deformation is the summation of all these. Although the pavement deformation due to a single axle load is very small, the cumulative effect of the number of load repetitions is significant.

Traffic does not only affect pavement based on how heavy the vehicles are, but also on how often these vehicles pass over the pavement. Each time a vehicle passes, it causes a minor deformation. However, over time, and with many repetitions of these loads, the accumulated effects can lead to significant damage or deterioration of the pavement structure. Thus, pavement design must account for the total number of load applications it will face over its lifecycle.

Think about walking on a sand surface. Each step you take may leave only a tiny footprint, but if you keep walking back and forth over the same spot, eventually you'll dig a hole, even if each step didn’t create much damage alone. This concept highlights the importance of considering the total traffic over time for pavement design.

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