Advancement of pavement design for practical use incorporating climatic effects for unbound pavements with thin seals

Background

Every year, Federal and State governments spend billions on road pavement rehabilitation because of road failures happening worldwide. Environmental factors such as moisture and temperature are identified as the significant reasons for these road failures. Unbound pavements with thin sprayed seals, which amount to about 90% of the overall Australian road network, are more susceptible to environmentally driven deterioration coupled with the effects of traffic loads. Unbound granular materials (UGMs) used to construct the base, and subbase layers are severely vulnerable to performance losses due to moisture variations. Subgrade strength is also adversely affected by moisture changes.

Even though it is intended to construct and maintain 100% impermeable sprayed seals it is not practicable to achieve. Past experimental research studies have shown that the saturated permeability of a sprayed seal may vary from 10-5 m/s to 10-10 m/s and depends on several factors such as seal type, age, resealing conditions, etc. Because of the considerable permeability in seals, moisture exchanges between the surrounding environment and the pavement structure occur due to different physical phenomena, as shown in Figure 1. Thus, even after the construction of the pavement, the moisture conditions in each pavement layer vary depending on the prevailing climatic factors such as precipitation, evaporation, air temperature, relative humidity and the water table depth fluctuations during the service life. These temporal moisture variations, that occur due to climatic factor changes, significantly affect the strength properties in both UGMs and subgrades. Therefore, it is essential to account for those temporal moisture variations during the pavement design process.

Different countries have adopted different approaches to incorporate temporal moisture variations into pavement design. The current method used in the United States of America embodies a specialist module known as the Enhanced Integrated Climatic Model (EICM) to predict the moisture variations that occur with time, in the pavement layers, considering detailed climatic input data for a given location. These predicted moisture variations are then considered in the structural design of pavements when determining the material strength properties.

The current Australian pavement design guide (Austroads 2017) has adopted a conservative approach by guiding the designer to evaluate the strength/stiffness parameters of the materials through testing at the highest moisture content likely to occur during the service life, based on annual rainfall and drainage conditions. Nonetheless, the design guide provides guidance on dealing with moisture and temperature related issues in pavements under Section 4 - Environment. In particular, it notes “The moisture conditions in unbound granular pavement materials can also have a major effect on performance. When the degree of saturation of unbound granular materials exceeds about 70%, the material can experience a significant loss of strength/modulus”.

The current design is yet to advance to incorporate temporal variations of moisture due to climatic factor changes during the service life. Currently, a workable model is not available to accurately predict the moisture variations in pavements after construction for Australia. Thus, this research project attempts to address this drawback and advance the current pavement design for the Australian context.

Model development overview

Firstly, a 1D numerical model, which represents the middle 1/3 of the pavement where the rutting is significant, was developed by capturing essential physical processes of coupled moisture, vapour and heat flow through unsaturated media as depicted in Figure 1. Following the Austroads guidelines and state-of-the-art unsaturated soil mechanics principles, particular preference is given to the degree of saturation (DoS) evaluations during the lifetime of the pavement. The parameters for the model were calibrated from laboratory experiments and literature datasets such that it became usable for the field.

The developed model was validated using an experimental dataset obtained from an actual test pavement constructed in New South Wales, Australia. The analysis showed an excellent match with the simulated results, and more than 80% of spot measurements fit with the simulated results with a 0.1 margin. The developed model was then equipped to simulate, predict and evaluate the moisture variation in unbound granular pavements under different climatic conditions in Australia and used to analyse hydraulic pavement performance behaviours. The developed model was further simplified based on the climatic zones by evaluating relative significance of thermal liquid and vapour flow. Figure 2a shows a sample Degree of Saturation variation (Sr), obtained using the developed model under a typical North Melbourne climate for 10 years of operations. Refer to Figure 2b for the geometry of the selected pavement.

The effect of key factors such as water table depth, initial conditions, and UGM properties were evaluated, and certain trends of Sr variations were identified by a number of sensitivity analyses performed. The developed model was utilised to determine the equilibrium suction and the Sr under different climatic conditions, and investigate the equilibration behaviour of unbound pavement with thin seals. The developed framework will be further validated and improved for industry usage with confidence in the capability of advancing current designs to enhance the quality of construction, cost-savings and safety. This numerical model can also be equipped to evaluate the preference of new materials, such as mixtures of recycled materials in pavement layers, innovate new materials and pavement layer configurations and examine the effect of climate change effects on unbound pavements.