The lining (1) of a tunnel is reinforced without reducing the tunnel bore. Grooves (6, 9) are cut into the tunnel lining (1) following the maximum curvature or skew to that, sometimes with one set of grooves crossing another set of different depth. Each groove (6, 9) is of generally T section and one or more rods (11) encased in a fabric sleeve (12) are inserted through the narrow mouth (6) of each groove (the stem of the T) to be within the enlarged part (9) ( the cross-bar of the T). Grout (13) injected into the sleeve expands it against the groove, and some seeps through to bond to the lining (1). Further anchoring may be achieved by drilling through the lining at the ends of the grooves into surrounding rock (15) to receive expansion bolts which are secured to the ends of the rods (11) or further grouted reinforced sleeves (17) which may be extensions of the main sleeves.