This innovative and ground breaking project comprises 6.2 km of two parallel 7m diameter single track tunnels beneath the centre of Amsterdam. The total metro infrastructure contains 3km of sub surface metro infrastructure, 3km of at grade metro infrastructure, 3 stations and 4 deep stations (over 15m deep). The drilling process is very complex due to the timber pile foundations and the soil characteristics. To avoid possible ground instability it is necessary to measure the pore pressure on critical locations while the drill is in operation.
Deployment
In this urban area there is limited place and time to install subsoil instrumentation. The city of Amsterdam was looking for a solution to measure pore pressures in all ground layers. The points of measurement needed to be in the same vertical axis, just below each other. Furthermore immediaite data visibility was necessary for thorough insight in the drilling process. The only system that could apply to this terms was the GeoBeads monitoring solution. Moreover it was considered the most cost and time efficient solution because only one borehole was needed to install sensors in all layers. Installation was a matter of minutes in stead of hours.
Implementation
Together with the consultants of Witteveen & Bos a risk analysis was done to determine the exact locations in the tunnel route and the depth and layers to do the readings. All systems should be places exactly in between the two future tubes. Chains of GeoBeads sensors were prepared and tested at the Delft location of Alert Solultions. Because all placement were in highly populated areas with dense traffic and hard pavements like roads and train tracks, the installation was a precise planning of several drilling activities within limited time during evening hours. For the drilling a 20 ton track truck was used and during the drilling, measurements were taken to detect maximum deviation of the angle of the sounding rod. All systems where placed within the boundaries of the desired deviation and the passing of the tunnel boring machine proofed that the position of the sensors was as planned.
