Newly developed MH Box Machine
An efficient tunnelling concept for crossing beneath traffic arteries
The demand for new underground infrastructures for pedestrians and cyclists is increasing along with the rapidly growing traffic load in large and megacities. This makes it necessary to cross beneath a great number of busy junctions, expressways or railway routes at short distances and at a low depth. The high traffic burden at these intersections makes aboveground interventions extremely difficult or even impossible without a great deal of additional work and expense.
Against the backdrop of the above mentioned challenge, Herrenknecht has developed the MH Box Machine as a solution. It is a further development of the MH partialface excavation technology that has been used successfully for more than 35 years. These machines have established themselves as efficient tunnelling equipment when building tunnels and pipelines above groundwater level because of their simple design, low operating costs and excellent performance.
The technology has now been adapted for short crossings beneath traffic arteries and combined with tried-and-tested construction methods. The newly developed, rectangular profile of the shield skin ideally meets the requirements of a pedestrian tunnel. As compared to a round profile unnecessary excavation of earth can be avoided. The stability of the overall structure is clearly increased by the rounded corners and the slight bulge of floor and ceiling.
Movable excavator arm
A hydraulic, telescopic excavator arm that can be pivoted both horizontally and vertically loosens the soil at the tunnel face and transports it to the belt conveyor. The excavated material is then transported to the muck cars that remove the material. Depending on the lining method, either hydraulic jacking cylinders or a jacking frame in the launch shaft press the machine forward at the same time. The steel steelcoated shield supports the cavity created during the entire tunnelling process until it is secured and lined.
A range of measures such as the installation of a support platform or telescopic face plates in the ridge area reduce the repose angle of the geology and avoid uncontrolled slacking efficiently.
Individual lining methods
For mechanized tunnelling with the newly designed MH Box technology two methods are generally available to line the tunnel: pipe jacking or in-situ concrete.
About six metres of tunnel can be produced a day with the conventional pipe jacking method, which works by pushing forward prefabricated pipe segments behind the tunnelling machine using hydraulic jacks in the launch shaft. As usual in pipe jacking, the intermediate pipe joints are dampened by a wooden insert and sealed with rubber profile gaskets.
The so-called in-situ monolith lining is the second option. With this method, the hydraulic jacks in the machine body are retracted after each stroke of two meters to allow the tunnel to be lined with in-situ concrete. First of all, the cavity is lined with a damp-resistant foil and then longitudinal reinforcements and steel mats are mounted on to it. Concrete is then pumped through openings in the mobile inner shuttering mould. After hardening, the jacks brace against the new tunnel section for the next stroke. Despite the relatively complex lining procedure, this method provides for tunnelling performances of one to two metres a day.
Both lining methods only need two shafts at the surface and small job site installation for the construction work. This means that traffic can go on almost without interruption.
High safety even with low overburdens
Due to the adjusted profile form of the MH Box technology, the crossings needed can be ideally built very close to the surface. Special safety measures prevent the surface from moving along with the jacks and stabilize it at the same time. One safety measure is to install a steel pipe or bulkhead arch that supports the surface, making it possible to cross beneath a highly frequented traffic artery without problem. The complete shield of the MH Box Machine can also be designed as a so-called poling plate system. Steel plates are extended forward which adjust the repose angle to the geology and stabilize the tunnel face as well. For monolith lining, a simple load bridge can be installed as a supplementary feature. It has hardly any impact on the traffic and compensates for the load of the vehicles until the concrete has completely dried and the tunnel is fully resilient.
The flexible choice of the lining method and the safety measures at the surface make it possible to adapt to the conditions on site and to carry out a number of projects efficiently.
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