A coupler device for joining together two segments of a tunnel or shaft lining ring, the device being characterised by a double ended male dowel having an externally grooved cylindrical portion adjacent to both ends. This dowel engages in recesses cast into the segment. A reverse tapered sleeve is cast in the recesses retaining an expandable collar formed of a plurality of arcuate segments, these arctuate segments having complimentary grooves on their inner surface to mate with the grooves on the dowel and a tapered external circumferential portion to abut the reverse tapered sleeve. As the dowel is inserted the arcuate segments, being retained elastically within the reverse tapered sleeve, lock the male part of the device into the recess preventing the device from pulling out.