Tie Bars Support Crucial Retaining Walls after Tunnel Collapse

Recently, GEWI®Plus Tie Bars (Tie Rods) were extensively used for reconstructing a precast tunnel in Gerrards Cross, a town located approx. 32km outside London. In order to create space for a supermarket with parking for 300 cars, an innovative solution was chosen which featured the covering of the existing railway line with precast tunnel segments. New fill up to the original ground level was placed to create additional surface area and make use of what is known as air rights.

When the fill material for the new building ground was placed around the newly formed tunnel profile, the tunnel collapsed due to uneven stresses on the pinned joints of the tunnel segments. An approaching train engineer was able to stop his passenger train just in time before reaching the collapsed tunnel.

In order to achieve a stable and safe solution for the reconstruction of the enclosure, two lines of contiguous bored piles were driven on each side of the tunnel to act as retaining walls and tied back to anchor pile walls on the upside of the railway cut slopes.

As the stiffness of the tie bars used to tie the rail facing walls back was of primary concern given the close proximity to the live railway, the designers chose to use GEWI®Plus Tie Bars.

The robust tie bars proved to be the ideal choice for the contractor because they can be adjusted to the correct length by either adjusting the nut at the termination point or adding a coupler and additional section of bar. All in all, DSI supplied 108 Ø 57.5mm GEWI®Plus Tie Bars in lengths of 14 to 18m as well as 60 Ø 43mm GEWI®Plus Tie Bars in lengths of 12 to 16m.

To accommodate the angled anchorages, DSI fabricated articulating wedge bosses with articulations consisting of GEWI®Plus domed nuts. Due to the fact that the diameter of the dome allows the use of anchor plates with larger through holes, greater articulation is ensured. Thanks to the articulating anchorages, construction time was significantly reduced. A total of 336 anchor plates with angle compensation wedge bosses and hi-load domed nuts were used for the project.

As long term settlement of the fill was predicted, settlement ducts consisting of large diameter corrugated sheathing were employed on each of the tie bars. The tie bar is laid along the base of the settlement duct and fill is placed around the duct in layers and compacted. Over the course of time, the fill consolidates and settles without affecting the stressed tie bars, which are free to float within the settling duct.

If settlement ducts were not used, the tie bar would be subject to catenary bending. This would put additional loads on the walls and potentially lead to shearing of the tie bars at the bearing plates.

The durability of the tie bars was ensured through sacrificial corrosion allowance. The corrosion rate of the steel diameter is determined throughout its complete service life, and the bar is produced in a larger diameter so that corrosion can occur without compromising the tie bar’s design load.

After the GEWI®Plus Tie Bars had been installed and stressed to their respective working loads by DSI, the collapsed tunnel was removed and the new tunnel lining reinforced at the crown pin joint. Afterwards, the tunnel was over-clad with a 600mm reinforced concrete deck, and 200,000t of fill were placed over the tunnel. The supermarket on the enclosure recently opened for business.


Tesco Stores Limited and Network Rail, both Great Britain

Costain Group PLC, Great Britain

Peter Brett Associates LLP, Great Britain