Principal stress rotation (PSR) significantly affects the cyclic behaviour of subgrade soil. Previous studies on PSR have been generally limited to saturated and isothermal conditions despite subgrade soil experiencing daily and seasonal variations in temperature and suction. This study incorporated temperature- and suction-controlled units into existing hollow cylinder apparatus to conduct cyclic shear tests, both with and without PSR, while maintaining identical cyclic deviatoric stress. The study considered different temperatures (5 degrees C, 20 degrees C, and 40 degrees C) and suctions (0, 10, and 30 kPa). The permanent strain increases and resilient modulus decreases as temperature rises and suction decreases. Furthermore, the incorporation of PSR results in increased permanent strain and decreased resilient modulus, with these changes being influenced by temperature and suction. At zero suction, the permanent strain increases by 130% and 230% at 5 degrees C and 40 degrees C when PSR is incorporated. As suction increases to 10 kPa, these values are 50% and 80%. These coupled effects are likely due to the decrease in the overconsolidation ratio (OCR) with increasing temperature and decreasing suction, with PSR effects being more pronounced at lower OCRs. Furthermore, a new semi-empirical equation was proposed to model these coupled effects on resilient modulus, a critical parameter in pavement design.
