ABSTRACT
In the Rijnlandroute twin bored tunnel project, the break-out procedure was based on an arrival into an inundated shaft. Although an identical procedure was adopted for both tunnel drives, at the second drive the temporary watertight seal failed shortly after dewatering the shaft. The leakage generated soil transport from around the tunnel lining into the shaft, and by this a significant disturbance of the soil support finally leading to huge deformations and structural damage of the tunnel rings close to the arrival shaft. An exhaustive investigation campaign was implemented to define the residual support stiffness of the disturbed soil. Subsequently, a soil improvement was executed whereby the efficiency was checked by using the Multichannel Analysis of Surface Waves (MASW) technique in combination with common cone penetration tests. After checking the available space and the required clearance profile, the most appropriate secondary lining was developed in order to make this part of the tunnel fully compliant again. This meant that all criteria regarding structural strength, water tightness, fire resistance and durability needed to be met. The article will describe the investigations made, the design options considered, and the execution process implemented.
