Weak ground conditions are common in urban areas. In such ground conditions certain potential for instabilities in the working area exists. Therefore, additional support elements supplement the standard support system to achieve stable conditions during construction. The most known additional pre-support system is the face bolt, ensuring face stability. A second group comprises different pre-support techniques, which primarily support the working area. This group of presupport methods comprises the spiling method (forepoling) and the pipe umbrella method (longspan forepoling). By using 3-dimensional numerical calculations, these two methods are compared regarding their effectiveness as support element in weak ground tunneling. Failure can be controlled with both methods. The pipe umbrella method has advantages at frequently changing ground conditions or unexpected changes of the ground properties.
The need of infrastructure increases the number of tunnels constructed in urban areas. Cities are often situated on flat areas with fluvial deposits prevailing. Weak or frequently changing ground conditions are common. Due to the housing on the surface, both stability requirements and settlement limitations rule the design of the support system.
Instability of the face and unsupported span are common problems in weak ground. Additional support is used to ensure stability. These support methods include the spiling method (forepoling), the pipe umbrella method (long-span forepoling), the pipe roof method, jet-grouted columns and freezing techniques. Besides the fact that these support techniques increase the stability conditions at the heading during construction, they also reduce the excavation-induced deformations depending on their stiffness and mode of load transfer to the surrounding ground. Therefore, it may happen, that predefined maximum settlement amounts and not the stability criteria govern the decision for the most effective support system.
Logically, additional support elements increase time and costs for construction. For this reason, these support elements must be chosen painstakingly. By unexpected changes of the ground's quality in urban areas, damages may appear. Repairing these damages do commonly cost a lot more than the tunnel construction itself. Furthermore, such unexpected events have a negative influence on the public opinion.
While damages induced by increased surface deformations are often uncritical, the extent of loss is usually more serious in case of tunnel collapses.
For this reason, the overall stability of sequentially excavated tunnels shall be investigated by using numerical calculations. The focus in this comparison is the influence on the stability of 2 forepoling methods; spiling and pipe umbrellas.
Proctor and White (1964) discussed in their book "Rock Tunneling with steel supports" the use of wooden "spiles" as forepoling method for traversing weak and raveling ground. Since that time, different forepoling methods have been developed; all characterized by the installation prior to the excavation with the goal of stabilizing the working area.
In today's tunneling, solid and hollow steel bars (or tubes) substituted wooden spiles (Figure 1) representing the simplest forepoling method. These bars do normally have an outer diameter up to 50 mm.