Ground response of closely spaced twin tunnels constructed in heavily overconsolidated soils

• We examine the influence of pillar width on twin side-by-side tunnels in hard soils.
• A comparison of the ground response for a variety of pillar widths is provided.
• The findings differ from previous studies that show a minimum 1D pillar width.
• In hard soils the pillar can be as narrow as 0.5D without significant interactions.

As part of the City of Edmonton’s light rail transit expansion, twin 6.5 m diameter oval shaped tunnels were constructed using conventional tunnelling methods. The geology of the site consists predominately of a hard, fissured cohesive till unit. The tunnel face construction was sequenced as top header, bench and invert excavations. At its narrowest, the pillar separating the twin tunnels was less than 1.5 m across or 0.23 tunnel diameters. Typically, the minimum pillar spacing required to reduce the interaction between twin, side by side tunnels is one tunnel diameter. Because the tunnel construction was within an urban environment, there was considerable concern with ground loss and excessive surface settlements. This study demonstrates that within similar materials, a pillar width of 0.5 tunnel diameters sufficiently reduces the tunnel interactions and minimize the risk of damage to nearby structures.

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