Abstract: A modular cast concrete element of this invention, capable of being assembled with other, similar elements into a tubular underground installation (e.g., sewer duct, tunnel liner) has a well opening to each of its axially-facing end surfaces wherein a rod is received that is insertable in a like well in an axially adjacent element to guide one of the elements to a predetermined position relative to the other and lock it in that position. In each well is a securement member having an annular radially outer marginal portion confined in a circumferential radially inwardly opening groove in the well and having resilient radially inwardly projecting teeth. The teeth have radially inner edges on a circle of smaller diameter than the rod and are flexible axially to enable the rod to be easily inserted into the securement member but to engage it under convergent bias for holding it against withdrawal from the well.