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| | The PLS Method is one of the Pre-lining methods, where prior to excavation and ahead of the cutting face around the periphery of the tunnel, a concrete shell element is built in the form of an arch shell with a length of 3m and thickness of 40cm, with the tunnel excavation constructed afterwards (in 3m sections). The concrete shell element is formed so that the slit cutting is filled up immediately with concrete, therefore almost no ground loosening occurs in using this method. In addition, as the concrete shell has high stiffness it can also act as a support, so that the use of shotcrete or rock bolts is not necessary. This method only requires the construction of the concrete shell and excavation, and its special feature is this very simple cycle. | | |
Case of Yoshii Tunnel
New equipment for the PLS (pre-lining support) method uses a double-chain cutter (excavation depth 3.8m) attached to the arch frame to cut out the slit to form the shell, a concrete pressure pipe set in the back of the cutter and the end-form. The equipment used for the Yoshii Tunnel had a weight of approx. 115t and was able to excavate through the mudstone, which had a uniaxial strength of around 10 MPa.
As the concrete shell also acts as a support, there were requirements for confirming the absence of voids in the concrete filling (it was necessary to maintain the concrete slump and to adjust the support time), the self-standing ability of the concrete edge after passing the end form (it was necessary for the concrete to be self-supporting immediately after adding additives) and on confirming the required observed strength at early times during the tunnel excavation.
| | (Presented by Japan Highway Public Corporation)
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| The maximum excavated cross section exceeds 300m2, making a challenge for constructing a very large section road tunnel the likes of which have not been seen before in Japan.
Prior to the construction of the main tunnel, a parallel working drift (excavated section 41m2, length 347m) was started, The large section tunnel (in this part the tunnel's effective excavated section gradually increases from 97 to 227m2) passes directly underneath a welfare facility building (reinforced concrete structure, 1 basement, 2 above-ground stories), and so it was necessary to tightly control the influence of the tunnel excavation.
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In response to this problem, long section face reinforcing methods (FIT method) and long steel pipe foreshoring support (Trevi tube method) etc. were adopted, thus restraining the ground ahead of the cutting face by forming cores of the ground and so minimizing the influence to the surroundings
From these results, for the very large section tunnel part, the initial designs for a multiple-drift method, top heading and bench method and a side drift heading method etc. were reevaluated, and finally the side drift heading method was adopted for use on the construction.
This method enables to excavate tunnel with a section of over 300 m2.
(Presented by Osaka Prefectural Road Public Corporation) |
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| | The long face reinforcement method is executed using a drill jumbo from the inside of the excavated section.GFRP(Glass Fiber Reinforced Plastic) tubes which can be easily cut, serving as a casing, are pushed into holes. They then reinforce the ground ahead of the face, together with grouting.
The method is applicable even in hazardous weak ground, where a borehole in the ground cannot be self-supporting The lightweight GFRP tubes have a high tensile strength and a sufficient stiffness to effectively reduce displacements ahead of the face.
A ring bit attached to an inner rod is inserted into the GFRP tube (outer diameter 76mm, inner diameter 60mm, length 3m each). After drilling, the ring bits and the GFRP tubes are left in the holes, and the rod with an adapter is removed. The GFRP tubes are then splice connected with each other to the form a designated length to form a reinforcing member.
This method is applied in long face-piling and long forepiling work in tunneling projects. Other applications are as foot reinforcement ahead of the face and as drainage work. The longest ever recorded face reinforcement by this method is 30m.
From the viewpoint of applicability, economy and effectiveness, its excellent capability has been proven in over 100 cases over a 4 year period (1999 - 2003)
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| (Presented by KFC Corporation)
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