RETROFIT OF TUNNELS

Because of the uneven and mountainous topography of Japan, tunnels are used extensively both in the railway and highway systems. There are about 4,600 railway tunnels in Japan with a total length of about 2,900 kilometers. There are an additional 6,500 highway tunnels with a total length of 1,800 kilometers. In addition, tunnels are used extensively in hydroelectric projects, for water supply, and for access to underground storage facilities. A large number of these tunnels were built prior to, and soon after, World War II. They are reaching the end of their design lifetimes, particularly their supports and linings.

Furthermore, the prevailing strata is of low strength and intersected by numerous faults, causing significant deformation and stresses on tunnel linings over the years. In general, about 45% of the tunnel system needs repair. The major types of defects are deterioration of linings, displacement of linings, insufficient head room due to deformation, and excessive leakage of ground water into tunnels. Although conventional methods of repair and retrofit including the use of rock bolts, shotcrete based linings and extensive steel framing have been used, there has been a great interest in the development of rapid retrofit methods, especially for railway tunnels, which are closed to traffic only between 1 a.m. and 4-4:30 a.m. daily.

For strengthening and lining retrofits, carbon fiber sheet products have been applied to existing tunnel linings and inner surfaces, in a manner similar to their application on columns. The surface is first prepared and primed, then coated with an epoxy on top of which the sheet form is placed and impregnated with epoxy (Figs. 4.38 and 4.39).

Fig. 4.38. Application of resin on FORCA tow sheet applied to a tunnel lining.

Fig. 4.39. Use of an overhead lift for retrofit without significant traffic disruption.

This method is often preferred over others because of the ease and speed of application, as well as the ability to conduct retrofit operations with minimum disruption of traffic. Figures 4.38 and 4.39 show retrofit activity in the Ichnose Tunnel in Shikoku. Two plies of FORCA FTS C-200 carbon fiber sheet material were used to repair an existing concrete lining. The sheets were applied with fibers in the longitudinal and transverse directions with the transverse (or circumferential) layer applied first.

Figures 4.40 and 4.41, from work on the Daiichi Yamaki Railway Tunnel in Tottori Prefecture, show the steps in placement (circumferential reinforcement followed by longitudinal) of the tow sheet. In this case, the carbon fiber material was used to retrofit the arch of the tunnel, which had been weakened by excessive soil movement above the crown.

Fig. 4.40. Application of sheet material in the transverse direction along the arch in the Daiichi Yamaki Railway Tunnel.

Fig. 4.41. Application of a second layer of reinforcement perpendicular to the first, and along the length of the Daiichi Yamaki Railway Tunnel.

Figure 4.42 gives details of the placement of FTS-C1-20 FORCA tow sheet (carbon fiber) along the inner lining in a section of the Nikkureyama Tunnel which is part of the Joshinetsu Expressway in Gunma Prefecture. The retrofit was conducted in December 1992 by the Sanshin Construction Co., Ltd. in collaboration with Tonen Corp. to repair a cracked portion of the concrete arch and to prevent further damage and exfoliation of concrete due to increase in rock pressure from an expansive mudstone layer. It was also intended to strengthen the lining to support the anticipated increases in rock pressure over the next 20 years. The area of retrofit measured 9 m across the arch and 40 m in length. Before application of the single layer of tow sheet, the existing surface was repaired by filling cracks with epoxy injection and grinding the surface to provide a flat surface for adhesion. At the point when the retrofit was undertaken, the arch had deformed substantially but appeared to have reached an equilibrium stage with no further deformation expected in the short term.

Fig. 4.42. Detailed schematic of cross-section and performance level estimates of the Nikkureyama Tunnel.

Figure 4.43 shows the details of the retrofit strategy used in August 1993 on the Ningyotoge Tunnel in Okayama Prefecture. The arch region of a section had been weakened by swelling and deformation of the side wall, which was in contact with a layer of metamorphic rock. In most areas, 2 layers of reinforcement were applied as shown in Figure 4.43. But a third layer was added in select areas where significant cracking had been seen. The entire system, consisting of FTS-C1-20 carbon sheets and FR-E3 resin, was coated with a fluorinated paint after cure. A primer capable of being applied to a wet surface was used on the substrate after grinding off irregularities and filling crevices and cracks with mortar. As in the case of the Nikkureyama Tunnel, the retrofit was done primarily as a means of enhancing capacity of the arch to absorb increased bending moments and to prevent further cracking and exfoliation of concrete.

Fig. 4.43. Cross-sectional details of the Ningyotoge Tunnel (construction by Chugoku Shoko/Nihoku Construction Co.).

Figure 4.44 provides another example of the application of tow sheet for the retrofit of a railway tunnel, using two layers of carbon fiber sheet, one along the radial direction and the other along the longitudinal direction. The retrofit was needed due to an increase in load on the tunnel from road construction above it.

Fig. 4.44. Cross-sectional details for retrofit of a railway tunnel necessitated by construction of a road on top of it.

To date, almost all retrofits of tunnels using composite reinforcing sheets have been carbon fiber based, using the FORCA tow sheet manufactured by the Tonen Corp. A list of applications is given in Table 4.9.

The names of some tunnels were not easily available and the dates of completion for others may be in dispute. The Tonen Corp. reported that 11% of its tow sheet sales/usage in 1996 was in the area of tunnel repair.