Optimization of support in rock pillars between two parallel tunnels

Abstract: When two parallel tunnels are excavated under high to moderate horizontal in-situ stresses combined with minor deformations in the pillar between the tunnels, the in-situ stresses can be redistributed over the tunnels in form of two compressive arches and are transferred from the roof to the abutments of the two tunnels. At additional deformations in the pillar, a new redistribution of the stresses can occur creating one compressive arch over both of the tunnels. This leads to lower vertical stresses in the pillar between the two tunnels. Today, this effect is normally not accounted for in the design of rock support. By accounting for this arching effect, it is possible to design the pillars for a lower load. The aim of this thesis is to analyze how to optimize the rock support (rock bolts) in the pillar by accounting for this potential arching effect in the rock mass. To study this, a sensitivity analyses is performed where the rock mass is represented by three different qualities (very good, good and fair). The geometry of the rock pillar is defined by a height of 6 m and two width dimensions of 3 m and 6 m are used. The in-situ stresses are categorized in three levels: minimal, typical and maximal according to typical conditions in the Stockholm region. Numerical models are analyzed in PLAXIS based on these different conditions. Stress redistribution in the rock is simulated by reducing the modulus of elasticity of the pillar in five stages. Based on the results from the numerical simulations and comparisons against analytical solutions it can be concluded that it is possible to design the pillar for a lower load, which accounts for this secondary arching effect over the two tunnels. The number of bolts could be reduced with at least 1-5 units per meter tunnel for the analyzed cases; thereby reducing the costs and also contribute to a more sustainable use of natural resources.