The model specification for tunnelling was produced by the British Tunnelling Society with the Ground Board of the Institution of Civil Engineers and was first published in mid-March 1997.

At its public launch at the BTS meeting on 20 March 1997, the chairman of the drafting sub-committee, Ged Pakes (Gerard Pakes Consultants), announced that the aim of the document was to establish common standards for the design and construction of tunnels in the UK. It was based on good practice and was not unduly prescriptive. It comprised a series of modules so that additions, deletions or modifications could be made to the specification to suit the needs of a particular project.

Although the model specification quickly gained national and international recognition, it was always acknowledged that the document could be improved and that additions to incorporate the latest technology and practice would be required, hence the recent publication of a new edition.

While the first edition drew heavily on the experience of corporate and individual members of the BTS, the new edition has taken into account the comments, criticisms and suggestions of the specification users.

It is still intended as a model specification, to be adapted and modified as necessary. The word model has been removed from the title so that it conforms to other ICE specialist specifications.

At 144 pages in length, the new edition is only four pages longer than the original document. The format and style, the six main sections and all the sub-sections of the original, including the numbering of items within sub-sections, have been retained in the new edition.

However, two new sub-sections, ‘Installation of Waterproof Membranes’ and ‘Compensation Grouting’, have been added. The text within many of the sub-sections remains unchanged from the original.

Other sub-sections have been subject to minor amendments, such as the inclusion of references to new standards or other documents, changes in the order of clauses to improve the logic, deletion of verbose or superseded text and correction of typographical errors.

Significant changes affect a relatively small number of sub-sections. Among the changes are new specifications for a master ring to be retained to enable the tolerances of precast concrete lining segments to be checked throughout the period of manufacture, and there is a call for stricter tolerances on the manufacture of expanded and special (formerly trapezoidal) segments. Engineers are now permitted to inspect jacking pipes at the place of manufacture.

The section on gaskets includes descriptions of the manufacture and installation of elastomeric gaskets for tunnels, hydrophillic gaskets for shafts and gaskets for pipejack joints and there are updated methods for the testing of rockbolts. There is no longer a requirement for explosives to be handled exclusively by shotfirers.

The groundwater section has been partially rewritten and expanded and that on break-outs from shafts and tunnels expanded to better reflect best practice.

There is a new description of the procedure for installing waterproof membranes, including the requirements for testing welded joints. The maximum allowable pressure of hydraulic equipment on TBMs and shields has been increased to 500 bar and the pressure control system on slurry and EPB machines must be capable of achieving, within ±10%, the required pressure.

A revision requires a specific method statement to be submitted to the engineer when tunnelling under or near to existing structures, and additions in the pipe-jacking section cover the detailed use of bentonite.

Sub-section 359 has been renamed ‘Ground movement monitoring’ rather than ‘Settlement monitoring’, and some sub-clauses have been reordered and rewritten to improve the original. The numbering system in the new edition contains errors.

Lining ring build tolerance has been made more onerous. The tolerance for lipping between adjacent segments has been reduced from 10mm to 5mm. Whereas the original document stated that the difference between maximum and minimum measured diameters in any one ring shall not exceed 2% of the diameter of the ring, the new edition states that in diameters greater than 2m, the difference shall be within 1% of the theoretical design diameter of the ring. No recommendation has been provided for tunnels with diameters less than 2m.

Contractors are now required to employ systems to monitor and protect sensitive structures that are likely to be subject to significant stress changes imposed by grouting.

The dewatering section has been revised and expanded to reflect current practice and a new section covers compensation grouting. There is an addition to control and record entry and location of personnel underground, and another to provide limits of infiltration into completed tunnels and shafts.

A new clause caters for the use of steel fibre reinforcment in precast concrete linings. References are made to American and Japanese standards for the monitoring and testing of segments. A possible future addition to this section would be the use of non-metallic fibre reinforcement in precast concrete.

Although the new edition is not substantially different from the original document, the additions, amendments and deletions have improved it considerably. Further improvements can always could be made, but the drafting committee is to be commended for its effort in producing the new edition.