Special care should be taken in coping with the physiographic and geological environments when driving tunnels in Taiwan. Difficulties frequently encountered are the following:
1. Squeezing Grounds
Squeezing grounds in fault and shear fractured zones are one of the frequently encountered difficulties in long tunnel construction in Taiwan, which tend to cause failure of supporting systems due to excessive deformation and cave-ins. Excessive deformation would necessitate re-mining of the tunnel cross section, thus imposing impacts such as extra cost, extended time schedule and safety risk on the project. Furthermore, as the ground stability is in critical condition during re-mining, the slightest negligence would lead to major cave-ins.
Squeezing deformation is common to extremely poor rock mass under high overburden in a tunnel alignment. However, in Taiwan, this phenomenon is not limited to tunnels with high overburden. For instance, the Mucha Tunnel No. 2 on the Northern 2nd Freeway had an overburden of only 140 m, yet the crown settlement for this tunnel within the scope of the Wantan Fault reached 1.5 m, with deformation in excess of 10%, displaying a highly ductile behavior of ground. Similar occurrences were observed in the Central Tunnel and the Taniao Tunnel of the Southern Link Railway Project.
   
2. High Water Influx Grounds

The annual rainfall in Taiwan is in excess of 2,500 mm and some strata are porous in nature; as a result, groundwater is very abundant. The manners of existence and occurrence of such groundwater bearing bodies are difficult to detect during the investigation stage prior to tunnel construction. Consequently, in many cases, pressurized water pockets were encountered during excavation which caused high influx of groundwater into the tunnel, leading to extensive damage and threatening the lives of workers in the tunnel and tunnel construction safety. In addition, prolonged submergence of exposed rock in groundwater tends to soften the rock mass, rendering it difficult to excavate.
To a lesser degree, groundwater surge would cause local cave-ins on the excavation face. In a more severe case, large-scale collapse may occur, and the debris spread about by the influx water may inundate tens or even hundreds of meters of the tunnel, forcing the excavation operation to stop.
Construction stoppages due to severe groundwater influx are quite often in Taiwan. The Central tunnel and Taniao tunnel of the South-Link Railway Project, the Sanyi railway tunnel, the Pinglin tunnel of the Taipei-Ilan Expressway, and the New Yungchun tunnel of the Eastern Railway Betterment Project afford examples. Both the last two mentioned tunnels are still under construction. The mishap in the New Yungchun tunnel is the most severe accident ever caused by groundwater in the tunnelling history of Taiwan. Up to the end of December 1998, the maximum water influx into this tunnel was more than 80 tons per minute, and the flooded length of tunnel was 541 m. The tunnelling activity had to be completely stopped. The magnitude of groundwater surge in this tunnel was comparable to that of the mishap which occurred in the Hakodate-Aomori undersea tunnel in Japan.
   
3. Grounds with Toxic and Explosive Gases
Miocene geosynclinal strata in Taiwan comprises at least 3 coal-bearing formations, which may contain gases of various kinds. Tunnelling is thus exposed to the risk of encountering toxic and explosive gases that may seep into the tunnel tube, endangering the tunnel construction. Gases frequently found in tunnels include methane, H2S, and carbon monoxide. Methane is colorless and odorless; it is both asphyxiating to workers and causes explosion when the concentration reaches 6% - 16%; it is therefore a highly dangerous gas.
 
4. Gravel Formations

Gravel formations are widespread in the Western Foothills Geologic Province in western Taiwan, which also present a major obstacle to tunnel construction. Gravel formations are complicated by nature. They are composed mainly of gravel, with mixtures of sand, silt, and clay. The degree of cementation varies from place to place, whereas the diameters of the gravel vary from several cm to over 1 m. The mean diameter is around 5 - 20 cm. Grounds underlain with gravel formations are considered the midway between incompetent rock mass and soil formation. Their mechanical behavior is dependent on their degree of cementation, and grain size distribution.

There is no ready solution when confronted with gravel formations during the construction of a tunnel project. Geologic boring does not amount to anything, since it is impossible to gain even a rough knowledge of such formations through drilling. So, the grain size distribution and the degree of cementation are not known. Their engineering properties cannot be satisfactorily represented by currently known rock mass classification methods.
In general, the degree of difficulty in tunnelling in a gravel formation is governed by the degree of cementation and the groundwater surge from the formation. Under the conditions of high groundwater influx and poor cementation, tunnel excavation and supporting are very challenging tasks. The Pakuashan tunnel now under construction is located, for most parts, in one of the major gravel formations, the Toukoshan Formation. In the beginning, tunnel construction was quite smooth; however, when water influx occurred, the advance became stagnant.
   
5. Incompetent Deposits
In most major cities in Taiwan, tunnels for sewers and mass rapid transit systems are mostly driven in incompetent alluvial deposits composed of loose sand, silt and soft clay, and a common plague is the very high groundwater level. Besides the cut-and-cover method, shield machines, first introduced into Taiwan in the early 1970's, are now widely used in sewer, mass rapid transit system and water pipeline projects. The total length of tunnels completed for these projects so far is over 70 km. With increasing demand, the use of shield machines is also increasing.
Support design for tunnels excavated by shield machine should take into consideration the characteristics of the machine, the pressure from the ground, and the thrust requirement of the machine. Steel support is used at specific locations, sharp turns, and where joining is required, while, precast concrete segments are the main support elements. The use of shield machine saw a gradual evolution from the early open, hand-dig type to the earth-pressure-balance (EPB) type that can cope with special geologic conditions, and finally to the slurry-pressure-balance (SPB) type. Through the use of these different types of shield machines, considerable results have been achieved.