Highways & National Rail

Highways, Network Rail and London Underground (LUL) Infrastructure Protection Act . . . What’s that all about?

Building works carried out adjacent to the public highway or rail infrastructure will be subject to Highways Agency or Network Rail approvals.  There is statutory requirement for local councils, Network Rail, London Underground (LUL) and others (HS2 etc.) to ensure that the nation’s infrastructure is protected.

Croft is perfectly placed to provide this information and has completed numerous Highways, London Underground and Network Rail related projects.

Each government body has its own rules and guidance.  The simple rule to remember is the Exclusion Zone.  If you are inside the Exclusion Zone the Infrastructure Act will apply.

  • 3m either side
  • 6m above & below

Exclusion Zone

There are some other rules:  Cranage must not oversail a certain zone, consideration for collapsed cranes and scaffolds that can fall onto an Infrastructure asset.

What is the process?

The paper work is similar to that of most government bodies as follows:

  1. Pay their fee….there is always a fee!
  2. Provide an Approval in Principle (AIP, or Form 1 for Network Rail). This document lays out the principles that are to be used.  The codes designed to, the idea of the structural assumptions and applied loads.  Importantly it defines the Category of Design check to be undertaken for the permanent and temporary works, known as CAT I, II & III checks. The design approach requires the approval of the government body.
  1. The detailed designs are undertaken and checked by the client’s consultant team.
    • CAT I – Design is reviewed and signed off by a Chartered Engineer involved on the project
    • CAT II – once the design is completed the drawings and reports are provided to a separate design engineer and an inline design is undertaken again. Internally Croft sets up an internal “Firewall” so that the second design team that was not involved on the project, can provide a fresh opinion on the design.
    • CAT III – this is similar to CAT II but the design must be provided by an independent external company. It provides a different approach, on one project over a network rail tunnel, Mott Macdonald undertook a design led by a geotechnical engineer, that considered the effects differently to our structural engineers.  Both resulted in the same outcome but approached the design from a different angle.


During this stage LUL (London Underground), Highways and Network Rail take a back seat.  They have approved the design in principle.  It is for the design team and the CAT checkers to ensure it is completed in detail.  They will ask for the completed design checks, but not the detailed design information.  A final sign off is sent for approval and typically a one-page signed sheet is received.

  1. Construction Approval - With the design approved, the construction approvals will be needed.   This is where RAMS (Risk Assessment & Method Statements) are provided and approval is gained.  The areas of concern may be construction sequences and loading, cranage, scaffolds, surveys etc.

Some of Croft’s Experience

Completed  Projects

Building over the Thames Link Tunnel Streatham

An interesting project in Streatham required the construction of a four-storey apartment development above a mainline Thameslink Tunnel. Read more . . .

Westminster Highways – Basement under road Oldbury Redevelopment

Oldbury Place was a complex project converting an existing Victorian industrial building into three modern townhouses and incorporating a new two-storey basement under the full premises.  Read more . . .

Kensington Highways for a Vault alteration on Winchester St

The alterations to the vault were fairly minor but during the works the street light needed replacing, as a result of this Highways became involved.  Croft were employed because the original engineer was not confident in dealing with Highways.

The main item was the design of the temporary works and the CAT II check necessary for this.  Croft provided this information and the sequence of the works.

Clearly from the sketch we are not artists but do try and give a 3D image where possible.

On the Drawing board

As we speak Croft is working with Oxford and London Basement Company on the installation of two basements for separate properties against an open cut section of the District Line.  The works require that the basement be placed against the existing retaining wall.