Fundamental behaviour of unbound granular pavements subjected to various loading conditions and accelerated trafficking.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The purpose of this study was to examine specific fundamental loading parameters (load magnitude and number of repetitions, tyre inflation pressure and basic tyre type) that influence the behaviour and performance of flexible pavements. The pavement response and performance measurements included continuous surface deflection basins, longitudinal and transverse profiles, and vertical strains in the unbound granular layers and subgrade.
The main criterion for determining the thicknesses of unbound granular layers in flexible pavements is the vertical compressive strain in the top of the subgrade imposed by each axle load. The design procedure presupposes that the primary mode of structural failure is permanent deformation in the subgrade. If the vertical compressive strain at the top of the subgrade exceeds the capacity of the soil, then excessive vertical plastic deformation will occur, eventually manifesting itself at the road surface as rutting.
The first pavement experiment considered only the elastic response of a thin-surfaced, unbound granular pavement over a weak subgrade, to varying wheel loads, tyre inflation pressures and two basic tyre types (bias and radial ply). The axle load had the greatest effect on pavement response and the tyre type had no apparent effect. For some load and tyre combinations, increases in the tyre inflation pressure resulted in statistically significant, slight decreases (at a 5 percent level of confidence) in the magnitude of the vertical compressive strains in the subgrade and unbound granular cover. The slopes of the relationships between strain and wheel load were similar for the actual values measured and the values predicted by multi-layer linear elastic theory, but the moduli necessary to generate strain magnitudes comparable to those measured are unrealistically low.
Two subsequent pavements were tested to study the relationship between elastic response at different cumulative loadings and the structural capacity of each pavement. The magnitudes of vertical compressive strain measured in the unbound granular layers and subgrade are substantially greater than the levels predicted by multi-layer linear elastic models on which the AUSTROADS and National Roads Board (NRB) flexible pavement design procedures are based, for the same number of loading repetitions to failure: the subgrade strain criterion is conservative. However, the AUSTROADS criterion more closely approximated the actual strain values, compared with the New Zealand criterion. Also, in the second and third pavement tests, the vertical compressive strain levels in the unbound basecourse aggregate tended to decrease slightly in magnitude under cumulative loading, while the magnitude of the subgrade strain tended to increase. The basecourse aggregate consolidates under repetitive loading, before reaching a stable condition.
Pavement responses (strain and surface deflection) to instantaneous loads can be a good indicator of future pavement performance only after an initial trafficking period, of at least 200,000 Equivalent Standard Axles. Also, after approximately 50% of the pavement life has been consumed under cumulative loading, pavement response is a poor indicator of future performance.
Applying the rate of rutting concept (rate of rutting is the incremental change in rut depth over an incremental number of loading repetitions), the change in the rate of rut development in the pavement surface can be related directly to the vertical compressive elastic strain in unbound granular basecourses and the cumulative number of loading repetitions at a point in the life cycle. In addition, the development of rutting early in the pavement life cycle can be used to predict the future rutting of the pavement, but the accuracy of any predictions of the number of allowable load predictions to pre-determined permanent deformation (rut depth) would depend on closely monitoring the development of the permanent deformation during the initial phase of the pavement life and carefully selecting the constant 'a' (intercept), which corresponds approximately to the end of the initial phase of the pavement life.
The effects of pure loading alone are less damaging to unbound granular pavements than previously considered; other factors, such as environment and moisture, must have a greater impact than that which has been attributed to them. Multi-layer linear elastic theory does not adequately model thin-surfaced unbound granular pavements to be able to accurately predict performance soon after construction and during initial trafficking. Vertical compressive strains in unbound granular layers of thin-surfaced flexible pavements can be equal in magnitude to vertical compressive strains in the subgrade under such pavements, so the former should be considered in the modelling of thin-surfaced unbound granular pavements.