Performance based design for the Crossrail Liverpool Street Station

Two out of three 42 m deep shafts at Crossrail Liverpool Street Station required real time assessment of monitoring data and progressive modification to provide time and cost savings during construction. This paper describes this process which was based on the use of real time site monitoring data collected using ShapeAccelArray, inclinometers and piezometers. At the station’s eastern end, Blomfield shaft involved an excavation into the Lambeth Group with a historically recorded groundwater pressure of more than 200 kPa. Due to limited space, a depressurisation system with further contingency plans specified to deal with variations was devised. These contingency measures were required and progressively modified as the variability in the water pressures were observed as the excavation approached the final few stages. The designer worked collaboratively with the contractor and Crossrail in modifying the depressurisation design based on performance of each piezometer with additional site supports and monitoring points to complete the excavations just before the target date of Christmas 2014. On the western end, Moorgate shaft was delayed by 11 months prior to construction due to the prolonged time required to remove existing piles. To mitigate the delay, the designer pro-actively and progressively modified the design in response to observed behaviour, making use of real time monitoring data of the shaft and adjacent infrastructure as well as advanced numerical modelling. The outcome was that all previously specified temporary propping was omitted and construction stages for casting of ring beams were combined, leading to programme savings, which facilitated an early tunnel breakthrough.