Deformable support (yielding support in other words) has been developed mainly in Europe and applied to the extremely weak ground and/or under high ground pressure at tunneling sites. Examples of deformable support include highly flexible materials that are inserted into the gaps of shotcrete lining, steel ribs with sliding gaps and so-called energy absorbing rock bolts applied to mining sites where rock bursts and/or large deformation are likely to occur. In Japan, on the other hand, multiple-layered support has been applied to such geologically weak grounds. However, such strong support could even yield and eventually be broken due to large deformation and high ground pressure of the weak ground. Therefore, we newly developed "deformation-controlled" rock bolt and conducted pull-out tests to investigate its allowable deformation and resistance load. Furthermore, we carried out numerical analysis simulating the use of deformable support in the extremely weak ground under high ground pressure to better understand the advantage and disadvantage of it.
When tunnels are excavated in the squeezing ground due to the weak geological conditions, expansive minerals and/or high overburden, the ground may greatly deform and the primary support such as steel ribs, shotcrete and rock bolt could be broken. In Japan, multiple-layered support, double or even triple-layered of heavy and strong support, has been applied under such conditions. On the other hand, at tunnel construction sites and/or mines overseas, there are cases in which excavation is conducted while allowing and controlling the deformation of the ground in order to reduce the ground pressure acting on the support (K. Kovári, 2009) (Fig. 1) that would be necessary for us for the future tunnel projects.
In this paper, support that only allows deformation is called "deformable support" and support that can limits the amount of deformation is called "deformation-controlled support".
In Europe especially, there are several cases where deformable support has been used at tunneling sites typically where large deformation has occurred. Similar type of support was used experimentally in the 1980s in Japan (Yoshito Ito, 1983), but in recent years it has been merely used. Meanwhile, overseas, the development of new deformable support that could allow the ground to deform to resist high ground pressure has been in progress.