Main procedures for rock tunnelling construction include excavation, mucking, support＆lining. Various excavation method, construction machine and material are applied for rock tunnelling. How to select proper technology, tunnelling method and efficient mechanical to suit the geologic-condition and job site, depends on experience and acquaintance with aspects of geology, geomechanics, safety, environment, and economy. To compare and analyze those data collected and select the optimal combination, is the final goal for the tunnel engineer.
The Verenia tunnel project in Switzerland is also introduced in this paper. Its excellent performance, can provide a good reference for Taiwan‘s tunnelling construction field.
Tunnelling has gradually become the critical path of large-scale public infrastructure projects in Taiwan. The progressive rate of tunnelling usually controls the completion schedule of the whole project. A significant difference of progressive rate of tunnellling exists between domestic and abroad,and even that of the foreign contractors in Taiwan. Study on improving the progressive rate of tunnelling by local contractors is worthwhile.
The study is mainly concerned with drill and blast construction method for long mountain tunnels. The method of study includes collecting of data, both domestic and abroad, and integration of existing study results, establishing of a basic background data bank of the study. Comparisons will be made between local tunnel construction techniques and progresses by both local and foreign contractors, and those of foreign countries, and a review will be made on the factors affecting the progress of domestic tunnel construction projects. Finally, from the point of view of existing construction regulations, contract documents, construction management and construction technique, a proposal on measures to be taken to improve tunnel construction operation and procedure will be outlined.
The Pinglin tunnel of Taipei—Ilan expressway,including two main tubes and one pilot tube,is approximately 12.9km in length. The diameter of the main tubes and pilot tube are approximately 13m and 5m respectively. The Pinglin tunnel was excavated approximately 850m with drilling and blasting method from the Tou–Cheng portal firstly. Tunnel boring machine method will be adopted to complete the remaining section of the tunnel. The Chinyin fault with 60m wide is the most important geological structure in the drilling and blasting section. The dialation behavior of the fault gouge material is very sensitive to the water,therefore the excavation sequence and support systems should be carefully studied. Some reinforced measures and more anticipated rock deformation also have been considered in order to avoid the excavated section intruding into the designed section. Systematic monitoring is also a very important task during the excavation. According to the monitoring results,the placing of the invert shotcrete to close the support system is the most effective and economic method to reduce the deformation of the rock mass after excavation.
Tunnelling Projects in Northern Second Freeway, mostly located in the hilly area near around Taipei Basin . The geological conditions, topographical features, underground water conditions are not favorable to tunnelling construction. There are four cases discussed in this paper, to provide experience solving tunnelling problem to be encountered in the near future.
The project consists of a headrace tunnel about 5,525m in length, an underground power plant about 50m long. A TBM is being employed for driving the headrace tunnel in order to reduce environmental impact during construction and promote tunnel techniques in the local construction industry. The TBM, with a diameter of 4.53m, is of the open main-beam type of Robbins Company,which is considered suitable for the mixed ground at the site. The advantage of TBM is that it enables the installation of rock supporting elements, shotcrete and rock bolts behind the cutter head at a distance of only 3.5m from the face. Rockfall,which is a common problem in the local geology,can therefore be minimized. The TBM was launched from the downstream portal of the tunnel on May 2,1996,and is advancing moderately at an average rate of 12 meters per day . The experience gained in TBM use will be helpful to the labor-short tunneling industry in Taiwan.
Two penstock tunnels of the Mingtan Pumped Storage Project have the length of 423m and the inclination of 48-degree each. Due to the urgent schedule, the pilot tunnels with a cross-sectional area 5.8㎡were excavated by the use of raise climber. The rocks encountered are sandstone and siltstone. The longest length excavated with this method in upper penstock tunnel reached up to 258.7m, with which this agency has made not only a successful construction in different geology for the second time but also a new record in Taiwan. The enlargement of penstock tunnel was then undertaken downwards after completing the pilot.
More than 1000 k㎡ mountain area in southwestern Taiwan (approximately 3% of the total land area) is covered by massive mudstone stratum. There are several important reservoirs in this region which make the conditions more difficult for tunnel construction. The adverse hydrological and geological conditions have caused many tunnel collapses during excavation.
The mudstone has a certain degree of strength if it is fresh and dry. However, the water in the reservoirs will infiltrate into the mudstone by the intercalated sandstone which is highly permeable. Consequently, the mudstone will disperse into soil-like soft material with very low uniaxial compressive strength varing from 5 to 23 kg/c㎡. During tunnel construction, the excavation face will become unstable due to the water inflow from the sandstone layer and the softened mudstone will deteriorate causing serious collapses. This paper describes the lessons learned from the Wu-Shan-Tou reservoir water intake tunnel. The effects of water in this special geological formation are discussed. The appropriate methods for overcoming this adverse condition are suggested for tunnel design and construction.
This paper briefly presents the “on-going design” practice of NATM (New Austrian Tunneling Method) for the mountain tunnels. “On-going design” process during construction, rock mass classification, in-situ observation and geomechanical measurements are the topics used to illustrate the way of NATM practice. All the four topics have one thing in common.: they are dealing with ground behavior.
Some high lights of these four topics are presented in the paper such as:
1. Ground behavior instead of geologic conditions is the basis of NATM rock mass classification. This is quite different from other current classification schemes such as RMR system(Bieniawski) or Q system (Barton,Lien and Lunde). 2. Geomechanical measurements and in-situ observations provide very useful daily information for an experienced site engineer to evaluate the tunnel behavior. With the help of daily construction process records, the site engineer is generally able to control the tunnel stability by means of the construction process adjustment on a daily basis. Consequently the site tunnel engineer can get rid of tunnel disasters or minimize tunnel failures provided he is authorized to do so. 3. The construction process adjustments include changes in round length, cycle time, quantities of support elements, sequence of excavation and installation of support elements, time and distance of invert closure after bench excavation … etc. For the time being, it has to be emphasized that experienced site tunnel engineers and skilled labors are needed in order to construct tunnel safely and economically.