Ground Control: Mitigating the Inherent Risks of Urban Tunneling
Settlement. It is one of the biggest risks in urban tunneling and the cause of sinkholes, cracked building foundations, and tunnel collapses. Surface subsidence and upheaval have vexed contractors and equipment suppliers worldwide, but there are some proven ways to minimize ground movement, even in low cover tunnels.
Tunneling at Mexico City Metro’s Line 12 provides a good, recent example of settlement prevention. The 7.7 km (4.8 mi) route is being excavated just 7.5 to 14 m (25 to 45 ft) below downtown Mexico City. The 10.2 m (33.5 ft) diameter Robbins EPB is surfacing into eight cut and cover station sites during excavation, all along major downtown streets.
The ground, which consists of watery clays, sand, and large boulders up to 800 mm (2.5 ft) in diameter, makes settlement a concern. Ismail Benamar, Tunnel Manager for contractor Ingenieros Civiles Asociados (ICA), spoke about the measures they are taking to control the ground: “We have an extensive monitoring program to detect displacements and pore pressure on the surface, underground, inside the tunnel, and in the most critical structures next to the tunneling line.” The risk of surface subsidence and vibration is also being controlled by regulating the rate of advance and controlling earth pressure at the front of the machine, as well as the back-fill grouting pressure.
In particularly low cover areas, Benamar cited a reduction in EPB cutterhead rotation as one of the primary means of avoiding ground disturbance. Cutterhead rotation is kept low (around 1.5 RPM) while torque is kept relatively high.
In addition, the machine’s two-component back-filling system is used to further stabilize the space between concrete segments and bored tunnel. The liquid mixture consists of water and bentonite cement plus an accelerant, which are combined in the tail shield and harden rapidly after injection. Because the two liquids are kept separate, high pressure concrete pumps, which can disrupt the surrounding geology, are not needed.
By May 21, 2010, the machine had broken through into Mexicaltzingo Station—the first of eight cut and cover station sites along the tunnel route. Little to no settlement issues were recorded during tunneling.
Taken together, the methods at Mexico City Metro show the best ways to minimize settlement in urban EPB tunneling. Settlement can be due to multiple factors, including over-excavation, improper ground conditioning, inadequate back-filling, or loss of face pressure. Despite the risks, proper machine design, continuous monitoring, and a knowledgeable crew can keep ground under control.
For more about the causes of settlement and tunnel collapse, visit an online forum at tunneltalk.com