Küssnacht Southern Bypass – Support Measures for a Shallow Soft Ground Tunnel Under a Built-Up Area

In autumn 2017, excavation of the bypass tunnel around the centre of Küssnacht, Switzerland, had to be stopped due to obstruction of the works by heavy slurry inflow and a cave-in. Intensive investigation of the causes, reassessment of the existing support concept and a new plan produced by the project engineers and the tunnelling contractors for safe tunnelling under Küssnacht finally enabled the resumption of excavation works in February 2018. As auxiliary pre-support measures in addition to the originally planned pipe umbrella, preventer-equipped grouting and drainage holes were bored and a horizontal jet grouted sealing curtain was provided. The excellent collaboration of all project parties thus created the preconditions for the successful breakthrough on 29 May 2019.

1 Introduction

The first section of the southern bypass around the historic village centre of Küssnacht am Rigi in the Canton Schwyz is to be completed by September 2020. The objective is to relieve the historic village centre of traffic. The project is financed by the Canton Schwyz and the council of Küssnacht. The client is the Canton Schwyz, represented by the civil engineering office. The total cost of Section 1 (incl. land purchasing) is about 126 million Swiss francs (about 120 million euros). The key structure of the bypass project is the Tunnel Burg, which runs through soft ground below...

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