Jacked Tunnel Design and Construction

This paper describes the development of tunnel jacking from its early days of small section tunnels (100 sq.ft. face or less), through to the current day when tunnels with a face area in excess of 3000 sq.ft. are being installed. The development will be illustrated using a selection of design details, diagrams, and construction photographs. Tunnel jacking is a term which refers to the installation of tunnels by pushing them into the ground while excavating from an open face. The tunnels, which are usually of rectangular cross section, are installed beneath a facility which cannot be removed or which the facility owner does not wish to be removed. This technique can be used for relatively small sections (6ft x 6ft) up to large, full size highway sections (80ft x 40ft) in lengths up to several hundred feet. The technique developed from pipe jacking when the circular sections available were either too small or inefficient for the final use of the tunnel. The technique is most often used in soft ground and at shallow depth. It has been used successfully in a variety of ground conditions, including soft clays, granular material, filled ground and mixed ground. It is a specialist technique which requires a clear and precise understanding of the inter-relationship between design and construction, and perhaps for this reason it has been carried out by a relatively small number of contractors around the world. The contractors often have their own preferred methods of installing the tunnels. Some of the methods and/or the equipment used are patented by individual organizations, although there are usually several alternative methods and details which can be used. Although it is not the solution in all situations where a tunnel or underbridge is required beneath a facility, it is certainly an approach which should be considered in many applications and often provides the most beneficial and cost-effective solution in situations when all factors and costs are taken into account.

The paper discusses engineering principles relating to the design of jacked box tunnels as developed by the author on a series of projects carried out in the United Kingdom. Examples of typical projects are presented by way of illustration.

The construction of CA/T Contract C09A4 entails the jacking of three (reduced from four) tunnels beneath eight sets of Amtrak and MBTA commuter rail tracks at South Station in downtown Boston. The tunnels range up to 38 feet (11.5m) in height and 79 feet (24m) in width, and crowns are separated from the tracks by only 8 to 23 feet (2.4 to 7m) of manmade fill. The original design called for the tunneled ground to be extensively treated with a combination of dewatering, jet grouting, penetration grouting and soil nailing, but surface settlements could still have reached as much as 13.5 inches (34cm). There was need to monitor both subsurface movements and rail settlements to protect the tracks, so designers provided for 200 subsurface instruments of eight different types, upwards of 1200 optical survey points on tops of rails, 1200 railroad "hubs" for determining dynamic rail movements, and special software developed to quickly sort and report hundreds of rail elevation measurements made every day. Studied and rejected were a computerized hand propelled Portable Geometry Trolley for measuring static rail elevations and an extensive array of remote reading Electrolevels and Beams for determining both static and dynamic rail elevations. The post-bid deletion of one tunnel and replacement of design ground treatment with ground freezing forced construction managers to continually adjust the mix of instrument types, numbers, locations and installation procedures to stay abreast of developments.