Soft rock tunnels in red beds frequently encounter anchor failure issues. Improving the mechanical properties of the red-bed soft rock-grout interface is highly significant for ensuring the stability of anchor support. This study employs laboratory direct shear tests in combination with digital image correlation (DIC) technology to investigate the mechanical properties (peak shear strength 

residual shear strength 

and shear stiffness ) of the grout-rock interface (GRI) among three typical soft rocks from red beds (red sandstone

mudstone

and grey sandstone) and two grout materials (ordinary Portland cement (OPC) and early high-strength cement (EHC)) under varying curing periods (6 h

1 d

2 d

and 7 d). The shear behavior and DIC-based failure mode are analyzed for red beds GRIs. The following key findings were observed: (1) As normal stress increases

both the shear strength () and shear stiffness () of the GRI also increase

following the trend: red sandstone > mudstone > grey sandstone. The cohesion values of the red sandstone

mudstone

and green sandstone with the EHC grout-rock interface at 2 days are 2.4

0.9

and 1.2 MPa

respectively. The corresponding internal friction angles are 57.6°

38.0°

and 27.0°

respectively. (2) With an increase in curing age

both and at the GRI increase non-linearly. EHC exhibited superior bonding performance compared to OPC

with  reaching 7.9 MPa at 6 h and 90% of the 7-day  being achieved at 2 d in red sandstone conditions. (3) The OPC-bonded specimens primarily exhibit grout failure near the GRI

whereas the EHC-bonded specimens show rock failure in red sandstone and mudstone conditions

and adhesive failure at the interface in grey sandstone. (4) An empirical model for the shear strength of the GRI in red bed soft rocks is proposed and validated

providing a framework for the rapid and reliable evaluation of anchorage strength in red bed soft rock tunnels.