Incorporating a fly-ash stabilised layer into pavement design

This paper describes a case history where the structural support afforded by a fly-ash stabilised layer was accounted for explicitly during the design of two flexible pavements. Pavements were designed and constructed at two sites in southern Wisconsin employing a layer stabilised in situ with fly ash. One pavement is for a residential subdivision. The other is a test section located in a secondary highway that was recently reconstructed. A control test section employing a conventional cut-and-fill approach was also constructed in the secondary highway. Fly ash was used to increase the strength and stiffness of the fine-grained subgrade at both sites, which was soft prior to stabilisation. Pavements at both sites were designed using the 1993 American Association of State Highway and Transportation Officials (AASHTO) method for flexible pavements so that their structural number would be equivalent to that of the conventional pavement originally called for in the design. Measurements of California Bearing Ratio (CBR) and resilient modulus (M_r) were used with the correlation charts for granular sub-base materials in the AASHTO manual to define layer coefficients for the stabilised layers. Tests were also conducted on specimens collected during construction to verify that the in situ mixture had similar properties to those anticipated during design. The pavement at these sites is being monitored seasonally using a falling weight deflectometer and pavement distress surveys. The monitoring programme has indicated that the pavements constructed with fly-ash stabilised layers provide comparable stiffness to conventional pavements employing a cut-and-fill approach. No signs of distress have been observed in the pavements constructed with a stabilised layer. Thus, assigning layer coefficients for fly-ash stabilised soils based on correlations for granular sub-base materials appears reasonable until layer coefficients specific to fly-ash stabilised soils become available.