It is of great importance to develop a rational forepiling method and to establish its guidelines in order to construct a tunnel safely in worse geological conditions. In this paper, cylindrical loading tests focused on the steel-pipe arrangement in forepiling were carried out in order to enhance the cylindrical shell theory proposed as the rational design method. Furthermore, the rationality was verified by the measurement data generated by actual excavation in large cross section tunnel in urban area.
In recent years, mountain tunnelling method has been frequently adopted in urban areas with little cohesion mainly for economic purposes. Moreover, as the cross section of mountain tunnel is getting larger, the development of the technique to control the deformation and keep the cutting face stable becomes very important. As a result, construction techniques have been advancing, but no rational design methods have yet been established, thus the development of a simple design method based on the understanding of supporting mechanism is heavily demanded. Until now, MGF method, in which steel pipes with shorter length of 5–7m are driven at greater angles of 14–15 degree, has been developed as the rational one with greater supporting effect at lower cost (Kitamoto et al 2001). Increased supporting effect, which was supposed to be led by thicker improved zone shown in Figure 1, was verified by CMT (centrifugal model tests) and actual construction work. In the paper above, cylindrical shell theory was proposed as the better design method to evaluate the forepiling deformation accompanied by tunelling. This theory's rationality was also verified by CMT, meanwhile the Importance of evaluating the stiffness of integrated zone composed of steel pipes and improved soil was recognized through actual construction design. In this paper, first, the outline of the cylindrical shell theory is presented. Second, cylindrical loading tests are carried out to evaluate the stiffness of the integrated zone, and lastly, lastly, their results are used to account for the difference of crown settlement between two sections in actual tunnel construction.
Forepiling is made up of steel pipes and improved soil, and they uniformly shows the ability to control ground deformation. Therefore, forepiling should be evaluated as the integrated shell shaped zone, not as its components separately. In case of tunnelling in shallow ground, forepiling can be thought to be applied on nearly isotropic load in the radial direction as shown in Fig. 1, thus it can be modelled by the shell structure with isotropic load when there is little problem about the whole settlement due to the lack of resistance around the ground at the lower bench.
At a railway tunnel construction, the MGF method was applied in order to verify the superiority to the conventional method, in which longer steel pipes were driven at greater angle of 4–5 degrees as shown in Fig.3. This tunnel was located at the foot of the terrace in Diluvial age and has earth cover of less than 10m.