S. P. Bagli, Chief Technology Officer, Strata Geosystems (India) Pvt. Ltd., India

Preamble

The pavement is a structural entity which takes up cyclic stresses from various types of vehicles that are deemed to ply along the selected route over its designated life. The pavement surface is required to take up both vertical and horizontal stresses. These stresses develop strains in the components of the pavement, which, when exceeded beyond certain magnitudes, can cause rutting and fatigue. Horizontal stresses and corresponding strains are also generated by vehicle traction. The quality of the riding surface is a key factor to give comfort to the driver and the passengers and the pavement is designed to cater to these stresses and minimise strains over the designated life.

Schematic Flexible Pavement Crust Section
Figure 1: Schematic flexible pavement crust section (after Jorge Zornberg4)
It is significant that paved roads have been found in the ancient Indus Valley Civilisation settlements of Harappa and Mohenjo Daro, the first human settlement clusters, inhabited around 4000BC. During the early part of the previous century, cities like Bombay had cobblestone paved roads with light traffic including animal carts and human hauled barrows and carts. Ever since, the pavement structure has become more and more high-tech with increase in traffic intensities and a multitude of combinations of axle loads. These pavements fall under two broad categories:

a) Flexible Pavements,
b) Rigid Pavements.

This paper addresses flexible pavements. A schematic of flexible pavement components is shown in Fig. 1.

There is great scope of application of geosynthetics as structural entities in flexible pavements. Flexible pavements reflect deformations of subgrade and from within the layered components to the pavement surface. Flexible pavements carrying higher traffic loads invariably have asphaltic surfaces. Pavements for lighter traffic as in rural areas may be unpaved. Conventionally, these pavements do not incorporate any geosynthetic or any other type of reinforcement.

Conventional Pavement Designs

The principles of design of a flexible pavement are based on stress distributing characteristics of the individual component layers of the pavement and their deformations under the corresponding stresses. Stress is maximum at the pavement surface directly under the wheel and is equal to contact pressure under the wheels. Vertical stresses also induce horizontal tensile stresses. The various layers of the pavement distribute the stress progressively over a larger area in the form of a truncated cone right down to the lowest layer where the load is spread over a larger area. Hence the stresses are greatly reduced. The uppermost layers, which experience maximum vertical stresses as well as horizontal tensile stresses, need to be most resilient with a robust stress-strain relationship. The lower layers take up lesser magnitudes of stress and there is no direct wearing action. Therefore material of the lower layers of a pavement section may be with lower strength and stress-strain characteristics.

Conventionally, the highway designer considers the characteristics of various components of the pavement crust and their respective thicknesses to carry the design traffic over the design life of the structure. With the introduction of geosynthetics, the designer sees advantage. It may either retain the thickness of the pavement crust but increase the life of the structure, or reduce the thickness(es) of the various components of the pavement crust, thereby achieving economic solutions as well as effecting conservation of the environment by using less natural resources, while maintaining the life of the pavement structure.

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