How to perform real-time monitoring when your reference network is moving?: Innovative solution applied to Britomart Station on the CRL project in Auckland

The City Rail Link (CRL) project in Auckland consists of 3.4 kilometre long twin tunnels to create an underground rail line linking BRITOMART and the city centre with the existing western line near Mt Eden. Around BRITOMART the excavation under the historic building involves the underpinning and jacking of the entire structure via 32 interior columns, without causing damage to sensitive heritage ceramics within the building. The construction of this tunnel underneath Auckland's city centre is a major challenge requiring a robust and innovative approach to construction monitoring. After a quick presentation of the project and the context, the authors detail the overall approach of the integrated and real-time monitoring system that has been deployed, which represents a first of a kind in New Zealand. A focus is on the monitoring of the BRITOMART station where the challenge is to monitor in real time, both the fa ades and interior columns of the building without fixed non-moving reference points within the building. The solution that has been deployed is an innovative and unique technology - CYCLOPS EVOLUTION. This technology is based on a group of automatic Total Stations (ATS) that are sharing a group of common prisms to pass the reference network from the exterior to the interior of the building, and distributing the residual errors throughout the monitoring network using a least square based algorithm. With this EVOLUTION algorithm, the reference information (fixed prisms outside the zone of influence) are used for the whole group of CYCLOPS (ATS) whether located inside or outside of the building. The outcome are coherent measurements over the entire structure allowing for direct comparison of measurements taken by different instruments. The system then allows for use of the Observational Method to provide feedback of the differential jacking of the structure in real-time.