North-South Subway Line

Around June 19 and September 10, 2008, water leaked through the diaphragm walls of the Vijzelgracht subway station while under construction, causing settlements of several adjacent, historical buildings. The construction pit was not fully excavated yet, and, as it turned out, due to substandard concrete there was potential for further leaks. In order to proceed, it was decided to freeze the soil. Summum Engineering’s Diederik Veenendaal, while working for Adviesbureau Noord/Zuidlijn via Witteveen+Bos, worked on the safety analysis of the groundfreezing operations, specifically to define requirements for further excavation.

The Noord/Zuidlijn (North/Southline) is a new subway line for the city of Amsterdam, connecting both sides of the river IJ, Amsterdam Central Station, downtown and the business district Zuidas. The downtown subway stations were constructed by placing concrete diaphragm walls, installing groundwater pumps, and excavating the soil within.  In 2008, several leaks at the downtown station Vijzelgracht led to settlements of the surrounding historical buildings. Within the span of a few hours, six buildings settled as much as 23 cm, leading to an immediate evacuation of the local residents. Construction, still in its excavation phase, was shut down.

The underlying cause was substandard concrete, which created the potential for leaks between individually cast segments of the concrete diaphragm walls. It was decided to freeze the soil from within as conventional freezing from the outside was no longer viable. This meant that the soil had to freeze all the way through the diaphragm walls to the outer soil. The same procedure was carried out for the neighbouring station Rokin, which was also at risk. The problem was then to establish a minimum freezing duration, given the uncertainty on the number, size and strength of any further leaks.

Material testing was carried out on frozen combinations of local soil, concrete and the bentonite liquid suspension used during construction, to investigate the worst possible material properties of potential leaks. Both simplified calculations and highly nonlinear finite element modelling was done for worst-case geometries of such leaks, the latter accounting for long-term behaviour and visco-elasticity of the frozen material. Based on the worst-case geometry, thermal calculations by CRUX, and the worst-case material properties established by Deltares and CDM, requirements were established for the contractor to define when excavation could recommence. This depended on the strength of the worst-case leaks, which in turn were governed by the temperature, the magnitude of applied loads (which changes during excavation) and the duration of freezing (as strength increases). A set of simple graphs were produced to allow the contractor to account for a changing construction schedule.

Max Bögl | Contractor
Züblin | Contractor groundfreezing
CRUX | Thermal engineering, ground monitoring
Deltares | Material testing, ground measurements
CDM | Material testing
Diederik Veenendaal, Sjaak van ‘t Verlaat, Peter Bosman, Theo Salet | Adviesbureau Noord/Zuidlijn, Royal Haskoning, Witteveen+Bos | Safety analysis, mechanical engineering