Cliffsend Jacked Deck Underpass

Opened to traffic on May 23, the underpass is part of a larger project known as the East Kent Access Road Phase 2, which comprises 8 km of divided highway linking two main roads—the A299 and A256—in the County of Kent, in southeastern England.. Kent County Council awarded a construction contract for the project to a joint venture comprising VolkerFitzpatrick, of Hoddesdon, Hertfordshire, United Kingdom, and HOCHTIEF UK who took the lead on the main structures, which included a bridge as well as the underpass. As part of this arrangement, HOCHTIEF was responsible for designing the underpass. In in developing the underpass design, HOCHTIEF engaged Jacked Structures SA, of Geneva, Switzerland and licensed their patented jacked deck concept.. Jacked Structures is a specialist consultant who has extensive experience in all types of jacked structure installation and undertakes conceptual and detailed design of installations.

 

Working with Jacked Structures and Atkins, HOCHTIEF developed a design that had significant advantages compared to jacking a full box structure. HOCHTIEF, the Jacked Structures Group, and Atkins modified the design to include two 3m bored pilot tunnels providing space to install more than 500 concrete piles into the underlying chalk strata. The piles would serve as a foundation and abutments when exposed.. The space within the tunnels then was used for the construction of the slide paths along which the deck sections were jacked into place. The initial deck section being equipped with a steel shield on its leading edge which allowed safe excavation. Outfitted with compartmentalized sections, the shield provided continuous support at the excavation’s face, reducing face height to what would be required for a full box by 50% and limiting the likelihood of major settlement at the ground surface.

A further measure to ensure minimum disturbance was the use of continuous steel sheets which were fed out through slots in the top of shield and anchored at the drive pit to eliminate soil drag.

 

The use of the slide path meant that the deck could be installed to a very high accuracy. All aspects of the work went as planned and the 126m deck was installed under the rail tracks in 63 days.

 

Because of concerns by Network Rail regarding the potential for disruption to its rail operations, continuous automatic monitoring of the railway was conducted to detect any settlement. Throughout the life of the project, construction of the underpass resulted in no interruptions of service, though occasional adjustments had to be made to the track during off-hours to adjust for minor settlement.

Experience gained on this project establishes that even larger and longer structures can be installed using the Jacked Deck concept. These could include metro stations, TBM launch boxes and many other underground structures.