Mountain tunnels are linear structures constructed in inner ground and geological data usually cannot be fully collected in the tunnel design stage. Therefore, standard support patterns are generally used in construction and modified according to measurements on cutting face during excavation. In recent years, the development of auxiliary methods like umbrella method facilitates the construction on severe ground conditions (unconsolidated ground, fracture zone, water inflow, neighboring construction etc.). However, reliable prediction method to the behaviors of ground ahead cutting face has not been established.
In this study, two sites(Rittou Tunnel and Hirayama Tunnel) employing such auxiliary methods for stability of cutting face are depicted. In the future, more and more tunnels will be constructed under critical and severe conditions, such as construction beneath urban area with thin overburden, large cross section and unsymmetrical in-situ stresses. Therefore, applying appropriate tunnel support and auxiliary method is becoming more and more important.
Mountain tunnels are linear structures constructed in inner ground and geological data usually cannot be fully collected in the tunnel design stage. Therefore, standard support patterns are generally used in construction and are modified according to measurements on cutting face during excavation. In recent years, the development of auxiliary methods like umbrella method facilitates the construction on severe ground conditions (e.g. unconsolidated ground, fracture zone, water inflow, neighboring construction). However, reliable prediction method to the behaviors of ground ahead cutting face has not been established.
In this study, two sites employing such auxiliary methods for stability of cutting face are depicted. Rittou tunnel has a cross section of 252m2 for dust collector using top heading & bench cutting NATM method. In Hirayama tunnel, ground estimation system based on behavior prediction to the ground ahead of cutting face using the mechanical data from tunnel jumbo was applied since this tunnel is located in complicated ground condition where geological characteristics vary remarkably. The images of survey process using borehole camera carried out during excavation are shown in Plate 1 and Plate 2.
(Plate in full paper)
Safe and effective countermeasures (auxiliary method) should be adopted to solve the problems generating during construction. At the same time, cost considering the construction conditions (ground condition, restriction condition during construction and construction schedule etc.) is also an important factor.
Characters of a few commonly used auxiliary methods are described as follows:
Long steel pipe forepiling (L>5m) Preceding displacement in front of cutting surface could be inhibited by placing long steel pipe ahead of cutting surface. Recently, applying this method by general construction instruments in tunnel construction becomes possible. It has advantages in construction period and economy and is adopted increasingly.
Injection forepoling (L<5m) This method is an extensively adopted forepoling method, which strengthens the necessary region of preceding ground by injection. In large-section cases, this method sometimes cannot inhibit the preceding displacement.
