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