The original design of the Green Hart tunnel between Hoogmade and Hazerswoude Dorp in the High Speed Railway from Amsterdam to Brussels was based on two single tubes. The French/Dutch consortium Bouygues-Koop however offered a provocative alternative. Instead of two single tubes with many expensive transverse connections they offered for ? 435 million a mono tube with an outrageous diameter of 14,5 m based on Design Construct. The profit in comparison to the original design with the twin tunnels was about ? 100 million. The approximately 8600 m long tunnel section is situated below the typical Dutch farm landscape with a minimum of environmental impact. The shield driven part is 7160 m long and the ramps at both ends are about 700 m long. On the bottom of the mono tube a cable duct is situated between a back-fill and on top of it a base slab with anti derailment kerbs. A partition wall with door openings separates the two high speed tracks fit for 300 km/h. The local geology consists of a 12 m thick Holocene layer on top with very soft clay and peat and a Pleistocene sand layer below in which the tunnel is embedded at a depth till 35 m. The water pressure is at a maximum of 3,5 bar. The ramps are built in a traditional open trench with sheet piling of steel, concrete foundation piles and immersed concrete. The entry shaft in the Bospolder at Hoogmade, the emergency shafts at Acht-hoven, N11 and Bent, and the exit shaft at Westeinde close to Hazerswoude Dorp are realized with Diaphragm walls and immersed concrete. The shield driving was done from the north to the south.
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