Abstract: The invention relates to a method of fastening a guard rail (60) to a concrete slab (100) by means of a self-expanding and self-undercutting bolt (1, 1?, 1?) comprising a dowel (10) having expanding lugs (17) and an expansion core (20, 23), the method comprising a phase consisting in drilling a hole (103?, 103?) in the slab, a phase consisting in driving the bolt in to a desired depth (E1+E2) independent of the depth of the hole, a dynamic tightening phase (h) resulting in the formation of the undercut and a static tightening phase of the guard rail. The guard rail bolt comprises the dowel (10) and a counter-dowel (20). The dowel (10) comprises at its fastening end (11) a guard rail fastening head (13) rigidly connected at its screwing end (12) by means of incipient fracture means (15) to a ring (16) provided with expansion lugs (17).