Network Rail undertrack crossings (UTX)

Yes. A pipe, duct or cable taken under the track is an undertrack crossing (UTX), and it has to be designed and approved under Network Rail NR/L2/CIV/044, with the design certified through the assurance process in NR/L2/CIV/003. Because it is third-party work on the operational railway, it also runs under a Network Rail asset-protection agreement. It is worth engaging Network Rail Asset Protection early, as the approvals sit on the programme and take time to work through. We handle the design and the approvals package for you.

Deep enough to control settlement, rather than to a single fixed figure. Network Rail set the depth of a trenchless crossing by the ground movement it allows: the crossing has to sit at a level where the anticipated settlement or heave at track level stays within 5mm, because a moving track is a derailment risk. The depth is proven by a settlement assessment for the specific site and ground rather than read off a table. A crossing with an internal diameter over 2m is also managed as a structure, which pulls in the full structures-approval regime on top.

A UTX design has to be independently checked, and for third-party (Outside Party) work the standard puts it in the highest category, Category III, which means the check must be done by an organisation that is a separate legal entity from the designer. We commission and provide that CAT3 check and hand the certificates to Network Rail. Network Rail reviews and accepts the third party's design and confirms the assurance is in place, but it does not carry out the check itself, so it is on us and the client to appoint the independent checker, with Network Rail's prior acceptance.

They are the certificates in the Network Rail assurance process (NR/L2/CIV/003). FORMA is the Approval in Principle, Network Rail agreeing the design basis before the detailed design proceeds. FORMB is the certificate of design and check for the permanent works, which on a UTX is the crossing itself, the bore or casing and the ducts left in the ground. FORMC is the same for temporary works, used where the temporary engineering near the track, such as a deep thrust or launch pit, is significant enough to put the railway at risk. There is almost always a FORMB, and a FORMC as well when the pits or thrust works by the line need their own check.

It is driven by settlement, because settlement is what threatens the track. Pipe jacking and microtunnelling give the tightest line and grade control and a supported face, so they suit larger or more settlement-sensitive crossings. Pipe ramming and auger boring install a cased crossing with very low settlement, which is why railways have long favoured a rammed casing. Directional drilling can be used, but its steered, fluid-supported bore means the settlement case and the frac-out control have to be made carefully. We advise the method against the settlement risk for the site, and Network Rail scrutinise that choice.

Yes. The whole design hinges on keeping track movement within limits, so monitoring points are set over and beside the crossing and read through the works, with alert and action levels tied to the settlement limit. We set up the baseline and the monitoring regime and read it through the job, so the movement that actually happens is checked against what the design predicted.