Harnessing the catastrophic inrush of water into new Yungchuen Tunnel in Taiwan

Abstract

The New Yungchuen Tunnel, located in northeast Taiwan, is a single-track railway tunnel with a length of 4433 m. The project area is tectonically active and the bedrocks are composed of slate, metasandstone, schist, marble and amphibolites, etc. In the planning stage, its construction was known to be challenging, according to comprehensive geological investigations and experience of constructing the original Yungchuen Tunnel which suffered from severe collapse, water inrush and squeezing problems in the 1970s. In the early 1990s, the route of the New Yungchuen Tunnel was set to the west (toward the hillside) of the original Yungchuen Tunnel to avoid the potential influence of ground water and unfavorable geological conditions. The top heading method was designed for excavating the tunnel. The support system included shotcrete with mesh, steel ribs and rockbolts. Auxiliary measures such as the use of a drainage tunnel, a pipe roof, cement and chemical grouting were prepared, and tunnel seismic prospecting and pilot drilling were requested during tunneling. On October 24, 1998, when the tunnel has been excavated to 1812 m from the south portal, a mass of water gushed out from the working face at 25 ton/min initially, and later at 80 ton/min. More than 15,000 m3 of collapsed debris came out in the water, burying more than 540 m of the supported tunnel. After extensive investigations the inrush of water was associated with the marble layer, which was covered with opening joints and consisted of fractured rocks. The very high water pressure, 5.0 MPa measured in a place with overburden around 260 m only, originated from the west mountain range and attacked the tunnel through some highly permeable passageways in marble layer. The gouge filled fractured zone or faulty zone in the boundary between the marble and schist, was destroyed and caused the tunnel to collapse. The thickness of marble layer near the original proposed line is about 70 m and diminishes to the east. Following more than sixty discussions on budget, possible environmental impact, tunneling-related and operational risks, and the temporal constraints on the Eastern Railway Improvement Project, the philosophy of ''draining the far and grouting the near'' was proposed. The tunnel route in hazard section was modified to the east with maximum distance of 70 m. Widespread horizontal boreholes with total lengths of 2970 m are installed for drainage. Two bypasses with total length of 1917 m were excavated as testing pilots and drainage galleries. Umbrella grouting with bitumen, cement and chemical materials were used for sealing up the groundwater. Reinforced concrete lining with maximum thickness of 1.0 m was installed to sustain the potential water pressure. Observing facilities for water pressure, discharge, tunnel convergence and surrounding rock displacement, and the support stresses were set up. The monitoring program was implemented not only during construction, but for a period after the tunnel was operational. The tunnel was broken through on September 5, 2002 after extensive investigations and reinforcements. Completely overcoming the effects of the catastrophic water inrush section took approximately four years. The philosophy of ''draining the far and grouting the near'' worked very well. Although the project cost increased from 22.4 to 44.0 million USD, the tunnel was opened on time in May 2003. The improved eastern railway, with a total length of 337 km is contributing the high efficiency of the flow of traffic. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.