In order to control differential settlement caused by new and old subgrade connections after expressway reconstruction and expansion，this paper proposed a layered treatment structure for the loose steps of the old road with high water content.A layer of construction and demolition waste was laid on the step surface of the old road to wrap the geogrid.The treatment effect was compared with that of sand wrapping and lime mixing，and the digital speckle correlation （DIC） was used to observe the reinforced influence zone，so as to determine the thickness of different layers of the subgrade wrapped by construction and demolition wastes.The particle rotation and displacement were further analyzed，and the reliability and accuracy of the reinforced influence zone determined by DIC observation were verified.The test results show that：① Among the measures to improve the interface strength of reinforced soil in the old road steps，the effect from good to bad is construction and demolition wastes C （particle size of 10?20 mm） > construction and demolition wastes B （particle size of 5?10 mm） > sand > construction and demolition wastes A （particle size of 1?5 mm） > mixed ash of 6% > mixed ash of 4%，and the reinforcement effect increases with the increase in the particle size of the construction and demolition wastes；② The DIC test results show that the area of the reinforced influence zone decreases with the increase in normal stress，and its thickness above the geogrid is greater than t hat below the geogrid.The particles that rotate are mainly concentrated within 3 cm below the geogrid and 5 cm above the geogrid；③ The recommended thickness of the layer wrapped by construction and demolition wastes is determined based on the thickness of the reinforced influence zone：about 20 cm in the middle of the subgrade，about 15 cm in the bottom of the subgrade，and not less than 25 cm in the upper part of the subgrade.