This paper describes the use of the Observsational Method (OM) on three Crossrail station excavations. Firstly, at Tottenham Court Road, Western Ticket Hall excavation where a code compliant design was started. By the third excavation stage, it was realised that movements were less than predicted and an OM was introduced. This enabled the lowest level of strutting to be eliminated, leading to savings in both programme and costs. Secondly, at Canary Wharf Crossrail Station, an OM approach was envisaged before the start of construction, but was only designed during the final excavation stage when time was available for back-analysis. A saving in costs and programme was achieved. Thirdly, at Moorgate shaft (part of Liverpool St Station) where construction commenced with a standard codecompliant design. A client-led redesign was carried out after installation of the diaphragm wall with an innovative verification process using a proactive OM approach to enable programme savings. When unusual deflection were observed from the shaft east wall due to structural uncertainties and construction activities, the OM and verification process provided a flexible framework within which to reassess ongoing movements and effects on an adjacent tunnel to ensure construction could proceed safely.

The case histories described demonstrate the benefit of adopting the OM for excavations in order to achieve savings in time and cost, and to react to unexpected movements during construction. These case histories fit well within a new framework for OM applications developed as part of the CIRIA C760.