Effect of Deep Settlement on Airfield Pavements and Backcalculated Subgrade Modulus

An excessive amount of differential settlement was observed during the construction of taxiways at Changi Airport in Singapore. The pavement exhibited severe settlement (up to 200 mm). The first cracks appeared in April 2018, 11 months after the pavement construction was completed. The cracks stayed in the asphalt concrete layer, but 15 months after the emergence of the first cracks, they extended downward and propagated beyond a subgrade layer. From the analyses performed on the pavement structure using the finite element method, it was found that tensile strain occurred at the surface of the pavement. The amount of tensile strain was higher at the joint between the main pavement and the shoulder pavement than in any other location on the pavement. Fracture analyses were conducted and indicated that cracks would propagate from the top of the pavement due to the tensile strain and move to its bottom. To identify an indication of settlement, heavy weight deflect (HWD) tests were conducted over three pavements experiencing three levels of settlement—severe, moderate, and no settlement. Backcalculation analysis was done for the three pavements using backcalculation programs BAKFAA and BAKSIMPLEX, and the closed-form solution. It was observed that the backcalcuated asphalt modulus was highly affected by root mean square (RMS) %. It is hence less reliable when the asphalt concrete modulus was determined at a higher RMS %, whereas the correlation became stronger when it was determined at an RMS % lower than four. Meanwhile, subgrade modulus backcalculated even at higher RMS % values was less affected by the error because most errors occurred and accumulated near the loading area of HWD. From the statistical analysis (t-test) performed on subgrade moduli backcalculated, it was concluded that the means of subgrade moduli obtained from the three different pavements differed from the hypothetical means of interest at a 95% level of confidence. This indicates that if a pavement undergoes settlement, the indication of settlement could be captured from HWD tests and the backcalculation of subgrade moduli.