Hydraulic characterization of diaphragm walls for cut and cover tunnelling

Underground linear excavations often encounter problems when crossing aquifers. A viable option for shallow tunnels is the “cut and cover method”. Still, when enclosing diaphragm walls display open joints, even this method risks flooding, which affects construction operations, and soil dragging, which may lead to subsidence and affect nearby buildings outside the enclosure. Characterizing the state of the enclosure prior to excavation would be desirable. We propose in doing so by analysing the response to groundwater pumping during dewatering. We use numerical modelling and analytical methods. The steady state heads along the enclosure and the variations in the flow behaviour during pumping depend on the state of the diaphragm walls. Monitoring of the heads is therefore proposed during drainage of the enclosure to characterize the diaphragm walls. An analytical solution in steady state and two transient state methodologies are presented. These methodologies are implemented to evaluate the state of the diaphragm walls used in the construction of a High Speed tunnel at Bellvitge near Barcelona, where large openings caused significant sediment drag, which provoked sinkholes outside the enclosure.

► The “cut and cover” method is used to build shallow tunnels.
► Open joints are common in the diaphragm walls and can cause serious problems.
► It is important to know the state of the diaphragm walls prior to excavation.
► Groundwater response to pumping depends on the properties of the diaphragm walls.
► This study presents methodologies to evaluate the state of the diaphragm walls.