An overview of Jacked Structure Techniques - Pt. 1

Nov 10, 2010

Where one form of traffic (people, cars or trains) have to intersect and cross existing highways or rail tracks, this creates a hazardous situation. Inevitably even with all the safety measures such as barriers, traffic lights at junctions and crossings there are still an unacceptable number of accidents with injuries and deaths. At best such intersecting traffic streams result in delays and congestion. The segregation of various forms of traffic one from another is going to be even higher priority as our roads and rail tracks carry ever more traffic.

Traditional top down construction methods of providing underpasses and subways is highly disruptive and in many situations not an option. Most segregated intersections need to be installed with only shallow cover to avoid expensive deep approaches which limits traditional tunneling as insufficient cover is rarely available for safe working.
The development of horizontal construction methods for subways, underpasses and other underground structures based on jacking techniques was initially pioneered in the sixties and seventies by the author and others in a number of countries.

Three main approaches were used and are still find use today:
  • Box Jacking
  • Advanced Support Structures
  • Modular Jacking

These techniques and applications are described in this first review. The second part will look at some of the new developments.

Box Jacking

The most basic approach, which is still widely used, is box jacking where the box structure is constructed on a launch pad adjacent to where it is to be installed and is jacked into the ground with excavation taking place in the shield. The graphic at Fig 1 illustrates the concept.
Dealing with a face excavation which can be typically 15m wide and 8m high you need to compartmentalize the shield which is fixed to the front of the structure. The shield is provided with a small overcut to facilitate working and external lubrication. There is also a need to provide thousands of tonnes of jacking capability to push such large sections which in turn needs the provision of suitable reaction. The frictional loads on a rectangular structure are much greater than experienced when installing circular sections so there is a need to introduce anti-drag systems to reduce this and also the migration of soil particles.

Many installations have been undertaken under roads, rail tracks and runways. Fig 2 gives some idea of the size and the large number of hydraulic rams that are used. This structure weighed 8,500 tonnes was 10m wide and 25m wide and requiring up to 14,500 tonnes of jacking force to install over a length of 37m.
Perhaps the most high profile box jacking installation was for the three large underpasses jacked under rail tracks in Boston, USA as part of the "Big Dig" project. These boxes were approximately 24m wide, by 11m high and the longest drive was 109m. These lengths required the use of intermediate jacking stations to distribute the load. Because of the ground conditions which were mainly soft marine deposits with old piles and buried quay walls the ground had to be stabilized by freezing. As the rail tracks ran for the most part at ground level the construction of deep launch pits where the sections could be constructed and jacked from was a key element.
Box jacking where the track is temporarily supported and structures are installed directly below the rail tracks is widely used in Europe. One approach is to install temporary full support beams to carry the track completely over the span plus a margin of the structure below plus a margin.
One variation developed in Italy used below rail tracks is based on installing a supporting grid to the tracks during short possessions. The box is designed to directly slide beneath the grid picking up the load as it is jacked under the tracks. The support strapping system and a completed underpass are illustrated in figs 3 and 4.
A variation from France is the "Aurofoncage" technique which is based on constructing in-situ boxes either side of the track and by use of tendons and hydraulic jacks pull the sections towards each other as excavation progresses until they meet and an-situ junction completes the structure.
The reality for all these systems is that however good the design and execution some small amount of settlement will still occur due to the over-break and soil consolidation. This does not occur suddenly but relatively slowly. For rail tracks on a ballast bed this can be can be compensated by track fettling of the bed.

This widely used approach has some limitations including:
  • Limitations of size and drive length due to the high jacking loads involved
  • Very large exposed face which may be difficult to control where soils are not ideal
  • Constructing the box adjacent to the tracks requires a large launch pad area with a minimum length greater than the length of drive.
  • The large jacking forces involved require substantial reaction arrangements
  • If a box starts to go out of level it is difficult if not impossible to make corrections

Advanced Support Structures

This technique is based on the provision of advanced support structures where a canopy is installed as a initial support to allow the construction of the final structure. Typical approaches are shown in the graphic at Figure 5. The steel tubes which form the canopies can be installed by pipe ramming, micro-tunneling or auger boring.
A good example is shown at Figure 6 of such an approach. This was for the installation of a road underpass in Athens using a rectangular advanced support system.
In this case the tubes were installed using pilot boring techniques. Microtunnelling, auger boring and pipe ramming have all been used for the installation of the canopy tubes. Once the canopy is in place excavation can progress in stages installing framing to the tubes to provide structural support. Once excavated and supported the final stage is the installation in-situ of the structural concrete walls and roof together with the road construction.

Modular Jacking

In recognition of some of the limitations the author when working for a major contractor in UK in the seventies developed and patented the modular method. The concept illustrated in Figure 7 is to create a support or foundation structure such as bridge abutments using interlinked pre-cast components which are jacked into position horizontally. Normally the final deck to the bridge is placed in a very limited occupation from the surface. The deck beams are placed on the structure that has been formed below the occupation and disruption is minimal. Final excavation of an underpass after placing the beams on the supports of course can be undertaken mechanically without any special measures as the structure is in place. A typical example would be an under bridge with abutments and central pier. The disturbance to traffic on the existing road or track is minimal. The system has the advantage that relatively small modular units are being jacked so temporary working space and jacking capacity required is lower than that required for larger boxes.
This method allows for a whole series of operations to be undertaken from the boxes such as foundation improvement, cross strutting and finally stressing the boxes together and infilling with concrete. This concept also minimizes the exposed face at any time and require much less site footprint and launch area as only relatively small jacking pits are needed. Minimizing the excavation faces enables surface settlement to be minimized.
During a short track possession the bridge deck can be slid-in or bridge beams placed to complete the structure. A large number of such jobs have been undertaken in UK. A typical installation is the Wandsworth under-bridge shown at Fig 6.
Although to date the majority of Jacked Structure installations have been in Europe projects have been undertaken in many other countries including USA, India, Canada, South Africa, Indonesia, Taiwan, Korea, Japan and Australia.

In part 2 of this review the author will describe some new developments and techniques...

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