Using the fiber Bragg grating sensors

a two-year in-situ monitoring for the subgrade permanent deformation in seasonally frozen regions was conducted. The combined effects of different sites

periods of a year

and different axial loads were considered. The results indicat that: 1) Affected by the air temperature

the subgrade’s temperature varies linearly but astatically with time in the freezing and melting periods. During a cycle of freezing and thawing

the range of ground temperature in the urban district at a depths of 30 cm and 75 cm are -9.0-14.4 ℃ and -1.9-15.4 ℃

respectively. The response of ground temperature to air temperature decreases and the hysteresis increases with the depth. 2) When the subgrade is fully frozen

the vehicle load-induced permanent deformation at both sites are small. However

when it is thawing

the deformation rises even under the same load. The maximum deformation is 4.5 and 4.2 times that of the frozen and normal period

respectively. 3) After two cycles of freezing and thawing

the subgrade has not reached a steady state. The permanent deformation under heavy vehicles can not be ignored. 4) Taking the maximum permanent subgrade deformation induced by a vehicle of 40 kN axial load as reference

the measured values caused by the 80 kN and 250 kN increase 17 and 215 times

respectively. There is a nonlinear relationship between the permanent deformation and axial load. 5) The combination of freezing and thawing cycles with heavy vehicle loading will produce the worst result

which can magnify the permanent subgrade deformation.