NR NR/L3/CIV/140/124N ISSUE 3 PDF

General

This Section of the Model Clauses has been drafted to specify requirements for tunnels to be constructed using a shield with excavation using hand tools. Other methods of forming a tunnel such as timbered heading, thrust boring or pipe jacking are not covered although some of the Clauses may be applicable.

The requirements of this Section are considered suitable for tunnels up to 4 metres internal diameter and where ground conditions are reasonably consistent. For larger diameter tunnels, or for work in difficult ground conditions such as soft silts or running sands, the requirements may need to be enhanced. Conversely, where ground conditions are considered good, it may be appropriate to relax some of the requirements.

Temporary tunnels under the Permanent Way are normally associated with portal bridges constructed alongside the railway and slid into place on previously prepared foundations and slide paths. The use of temporary tunnels enables the foundations (either piles and pile caps or strip footings) and slide path to be constructed with minimum interference to the track above.

The tunnels are normally considered to be Temporary Works although the lower part of the tunnel will probably remain following installation of the bridge.

There are two ways of installing temporary tunnels of this nature:

(i) mining through the embankment using a shield, with segmental concrete lining to the tunnel. In this method it is only the shield which moves under the railway,

(ii) by jacking through the embankment a tunnel made of rectangular or circular precast concrete units. In this method the entire tunnel moves under the railway.

For both methods a temporary speed restriction, and Track Monitoring Procedure to NR/CIV/LI/045 is required whilst there are construction works under the tracks. There is little to choose between the two methods with regard to disruption of the Railway. However, the jacking method can take up more room with its thrust pit and jacking equipment, and this may extend the slide path or delay the portal construction. It also tends to be more expensive. As a consequence the mining method tends to be more commonly used.