Preloading at the South End of Confignon Tunnel

The south cut and cover of the Confignon tunnel in Geneva, Switzerland, lies on deposits of clay and clayey silt of high and varying compressibility. Without improvement of the foundation, settlements under the final loads of the cover are unacceptable for the project. The improvement of the foundation was achieved by a preload drain system. Monitoring settlements under the fill loads and heave at fill removal indicate: (1) maximum settlement values of 34 cm (340 mm); (2) reduction of the compressibility of the foundation to one-third of its original value in the preloading process; (3) values of the in-situ coefficient of consolidation 1.5–2 times larger than the design values derived from the oedometer test results; and (4) a computed settlement 1.5 times the monitored settlement at the location of the maximum settlement. Monitoring the settlements of the concrete structure under the cover loads indicates: (1) a present maximum settlement of the order of 8 cm (80 mm); and (2) a predicted maximum settlement over the 80-year life period of the project on the order of 12 cm (120 mm) with low differential settlements.