UNDERGROUND ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 261

F UNSEEN LEAKAGE
It is remarkable that the groundwater is such a common factor in failure cases. It
may gradually change the local conditions, particularly when large pore pressure
drops are involved. The collapse of a temporary retaining wall of a building pit in
Rotterdam in 1995, is such an example.
A grout-column retaining wall protecting a deep building pit near a river
collapsed, unforeseen, after months of satisfactory performance. A new metro
station was being built around an existing metro tunnel while train services
continued uninterrupted, at more than 10 meter below groundwater level.
Investigations after the event revealed that a combination of several factors
gradually caused a critical situation: leakage, consolidation, erosion, piping, pump
failure, and column fracture. Pore pressures had been measured continuously. They
reveal the unseen gradual process of local deterioration, which took place along the
interface of the existing metro tunnel and the subsoil. The leakage and a possible
small deformation of the deeper clay layer due to the deep pumping system (head
drop of about 12m), may have given place to regressive erosion of the sand under
the existing tunnel (piping), which was founded on piles into the sand layer
underneath the clay. Small water and sand inflows would hardly be noticed during
the excavation and construction works (open pit, it rained regularly).
combi wall building pit
at +3.50m
jetgrout wall top
-11.50 m
-17.50 m
jetgrout wall bottom
existing pile foundation
open slit under existing railway tunnel
weak spot in the jetgrout wall
Figure 15.11 Building pit metro station Wilhelmina Pier, Rotterdam