An Overview of Jacked Structure Techniques - Part 2
Dec 22, 2010
Part 2 describes some of the new developments which open up a wider range of applications.
These methods have proved very effective in non-disruptive installation of underpasses and subways below rail tracks and roads.
However the methods described in Part 1 have some economic and technical drawbacks and Part 2 describes some of the new developments which open up a wider range of applications.
The methods developed and patented are based on the concept of breaking down the installation activities into modular activities which has a number of advantages;
- The exposed excavated face at any stage is much less than a full face box jack greatly reducing the risk of face collapse and settlement
- The launch pit requirement is greatly reduced. In developed locations the space is not available to cast in situ the required length of box. Where pits have to be excavated to significant depth the costs of providing them can be greater than the cost of the construction and installation of the box.
- Modular units can be precast and brought to site. The quality and accuracy can be of a higher quality than possible with site construction.
- The jacking loads are greatly reduced and this in turn means less jacks and reaction provision
- The accuracy of the line and level of the finished structure can be up to 10mm
- Greater spans and drive lengths can be undertaken
This concept in its basic form is to at the first stage install man entry jacked foundation boxes or tunnels which are fabricated with removable lids. These foundation units are designed to ultimately carry the permanent dead and live loading. Once installed a suitable slide track is installed in the boxes to very close accuracy. Precast arch sections are dropped into place at the launch area and jacked in along the track with lids being removed within the shield as excavation takes place. A compartmentalized shield is fitted to the leading unit to control the face as excavation proceeds.
There are a number of variations on the basic concept some of which are illustrated in this article.
Another variation is to use a flat arch and jack it on the top box of abutments using the modular system described in Part 1. An example is shown for an underground metro station later in this article.
Another development is the jacked deck which can be combined with a variety of foundation configurations. One such system is the modular system where the top box is modified internally to allow a slide track to be installed. These top boxes are designed with a removable section which exposes the track as the deck is jacked into place. The jacked deck which can be formed from a complete cast on site structure or formed from precast beams is fitted with a compartmentalized shield where excavation takes place. The sequence is illustrated.
The application of jacked arch and deck methods provides safer, faster and less disruptive ways of installing large underground structures such as metro stations, launch boxes and parking compared to current methods such as Sprayed Concrete Lining (SCL) techniques.
Specifically advantages include:
- Continuous support of the excavation with a full structural lining as excavation proceeds eliminates the possibility of collapse
- The risk of injury or death to workers is virtually eliminated
- Any surface settlement is minimal and adjacent buildings, property and the public are protected
- Use of precast units allows higher quality construction
- The methods are applicable to constructing both shallow and deep structures
- Jacked arch and jacked deck structures can provide greater clear spans over much longer drive lengths than can be achieved by jacking boxes
- Jacking precast elements to form the structure can be undertaken from a small launch excavation
The jacked arch and jacked deck technologies have a capability of installing underground stations. The graphic of a station is based on driving in both directions from a central excavation which would form the access concourse. The length of the station is determined by platform length required to accommodate the number of carriages. A platform length of 140m is shown which will accommodate up to 8 carriages. Longer lengths would be possible. Current dimensions for spans and lengths for stations can easily be accommodated. The jacked arch and deck alternative graphics are based on a 12m width between platforms which would accommodate twin tracks with wider trains than the London underground.
Launch boxes are constructed to provide an underground structure from which TBMS are launched to construct the running tunnels. They then provide the basic structure within which the permanent station is constructed. Typically these will be 22-24m span and around 200m long. The height will be determined by the diameter of tunnel to be driven but for a twin track tunnel this could be 6/ 7m diameter so internal depth of box could be up to 8-10m.
We would drive these from a central access shaft in both directions using the jacked deck method to give the clear spans required. The various alternative configurations for jacked arch and deck concepts described and illustrated would be modified to provide the required span, length and height.
Both the jacked arch and jacked deck could be used to create underground parking below streets, buildings, parks etc. It is envisaged that a series of interconnecting bays would be created either using an arch or a jacked deck concept. To provide parking bays either side of a central access road would require typically a clear span of 10-12m for each bay.
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