Behaviour of soft rock around shallow double tube tunnels

Numerical models by the Particle Flow Code (PFC) as well as physical model tests showed that in case of a stable system the stresses in a pillar with a thickness of the radius of two parallel tunnels approach the dead weight of the overburden divided by the pillar thickness. These stresses may rise up to 2.2 times the uniaxial compressive strength without pillar failure. The pillar and the outer sidewalls are fairly equally stressed. In case of rock failure, substantial stress redistributions in the outer sidewalls are possible because there is enough rock mass available. In the pillar, however, stress redistributions are only possible to a limited extent, because rock failure causes a reduction of the load bearing pillar cross-section and consequently an increase in the stresses in these areas. The pillar, therefore, has to be supported as quickly and stiffly as possible, whereas the outer sidewalls and the abutments can be supported more softly.