Tunnel Foundation Remediation in an Area of Mild Squeezing Conditions

This paper presents a case study of a tunnel foundation modification design in an area of mild squeezing conditions. During the excavation of the Klokova double tube rock tunnel in Western Greece with a length of almost 3,000 m, a folded flysch zone of about 50 m length was unexpectedly encountered among the hard limestone rock mass. Due to the fast construction pace, two tunnel segments were constructed in the flysch zone using an open final lining cross section designed to provide sufficient support to the limestone. Such an open type section was considered inappropriate to support the soft flysch formation due to the development of a small plastic zone around the tunnel and mild squeezing conditions. As a result, the construction of a horizontal foundation slab connected to the existing footing of the tunnel lining was proposed that would transform the open cross section to a closed section in order to efficiently support the flysch mass pressures. Further to the foundation slab construction, the displacement restraint of the overall foundation was accomplished via micro piles and passive steel anchors under the slab and the original foundation respectively.