15 m diameter tunnelling under Netherlands Polders

In order to become part of the Trans-European network of high-speed rail links, Netherlands is working on the transport of the future: a high-speed link (HSL) between Amsterdam and the Belgian border, connecting the Dutch capital with Antwerpen, Brussels and Paris. A solution with a bored tunnel was chosen to minimize the line's impact on the characteristic fenlandzGM4PIHNgpiv landscape of the Groene Hart (the Green Heart of Holland, situated between The Hague, Amsterdam, Utrecht and Rotterdam). Seven kilometers long (8.5 km including approaches), under the Dutch polders, the tunnel will allow the line to follow the fastest route between Amsterdam and Rotterdam while leaving the countryside largely undisturbed. Designed as a single tube, with a bi-directional train circulation, this tunnel, almost 15 m diameter was constructed with the help of Aurora, the specially designed world largest confining tunnel boring machine (TBM). Yet the realization of such an impressive tunnel required an accurate and efficient risk management policy. This papers deals with the treatment of one particular risk, due to the specific hydro geological conditions of the project, that is to say boring under polders. Aurora, as a slurry shield-boring machine, generates excess pore pressure (EPP) in the soil during the boring process. This EPP phenomenon is originated by the dissipation of the confining slurry overpressure in the cutter head chamber amplified by an accumulation process taking place in a captive aquifer as in the Groene Hart tunnel. This phenomenon was early identified as a potential risk for the project, as the generation of excess pore pressure may induce local instability in the impermeable peat layer and thus connections between the brackish captive water table and the free water table. As the first aquifer piezometric level is higher than the ground level, there is a risk of flooding the polder situated above the tunnel. Even if the communication between the two aquifers is not sufficient to get to this point, the inflow of brackish water in the free water table is a risk as the lands above the tunnel are mainly intended for agriculture. Therefore, a first measurement campaign was implemented very early in the tunnel progression and showed that the phenomenon was of such a magnitude that the risk could not be neglected. A second instrumented plot was thus implemented, to be able to improve the knowledge and understanding of the phenomenon, qualitatively as well as quantitatively. The plot instrumentation was formed of 11 settlement points, 19 pressure cells and two extensometers. The measurement period extends from February 4th to March 11th (2003) (i.e., they cover the boring of about 600 m tunnel length). The experimental results associated with 3D transient numerical modeling, by allowing a better understanding of the phenomenon, gave the tool to identify the risk level and determine the required mitigation measures. This analyze of the phenomenon proved to be accurate as Aurora passed the identified risk area without any trouble, and the last measurements in the risk area are concordant with the predicted settlement and pore pressure levels. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.