Box Jacking Trenchless Technology
Box jacking is a well-established means of engineering culverts or tunnels under rail embankments or waterways to accommodate road or rail traffic. It is a way of overcoming or circumventing geophysical impediments in order to build road or rail networks. Many tunnels of note around the world have been built using the box jacking process and though technology has improved greatly in recent times the engineering capacity has been around for quite a while. From a technical viewpoint the box jacking process begins with the excavation of a launch area or sink pit where a secure backdrop can be constructed to facilitate the hydraulic jacking rams that will push the pre-cast concrete box sections into place. These large concrete boxes are custom constructed on site to suit the size of the tunnel. Once built they are hydraulically jacked into position with an excavator digging out the spoil at the face and removing it for disposal. As the excavation proceeds the concrete section is pushed slowly into position to form the roof, floor and sides of the tunnel.
Box jacking requires high pressures to push the concrete boxes into place and a series of hydraulic rams is used to accomplish the task. Large cutting heads such as are used with tunnel boring machines do not form part of the box jacking procedure. The excavator performs the task of removing the clay soil or rock from the face as the concrete section is rammed slowly forward to form the tunnel casing. The backdrop against which the jacking rams rest is constructed with concrete filled sheet pile shuttering. This ensures a strong enough backdrop against which the jacking rams can operate to jack the concrete sections into place. This procedure is used for larger tunnelling projects and can be performed quite quickly once the concrete sleeve or box has been constructed on site. Box jacking requires a minimal number of operatives and is therefore cost-effective from a labour point of view.
Box jacking as a form of underground tunnelling is preferred in many cases to overhead bridges for different contextual reasons. It can be more expensive than building bridges in given locations depending on environmental and geophysical considerations. Very often where building a bridge might cause an obstruction to traffic, such as river traffic, there is no alternative but to construct an underground tunnel using box jacking techniques to accomplish the task.
The positive features of box jacking a tunnel include non interference with existing ground surface services such as rail or road links and no disruption to pedestrian or automotive traffic.
The non-intrusive nature of box jacking tunnelling makes it an obvious choice as regards observing very sensitive environmental concerns. It complies with all European directives regarding protecting the environment and this is a big plus for the whole concept of box jacking tunnelling for those commissioned with the task of installing new infrastructure to accommodate rail and road traffic.
Box jacking will undoubtedly continue to be the most propitious method of installing underground infrastructure for whatever purpose it is required.
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