Fibre optic sensing innovations on Crossrail

The project

Crossrail is currently the largest construction project in Europe. It includes 10 new rail stations, six of which are under central London, and 42km of new rail tunnels weaving through the city’s congested sub-terrain. The project presented two opportunities for CSIC to collaborate with industry leaders on innovative applications of fibre optic cables to challenge traditional engineering design assumptions in order to save future tunnelling and excavation projects time and money. Crossrail’s strong innovation policy allowed CSIC to set up ‘laboratories’ on site.

The first project, led by Research Associate Nicky de Battista, focused on measuring the additional strains induced in the sprayed concrete lining (SCL) at junctions in the tunnels at Liverpool Street Station. A tunnel’s SCL is thickened at these junctions in order to sustain the stresses caused by the excavation of the cross-passages. Tunnel lining design is based on finite element models but there is a lack of experimental data to calibrate these. By embedding FO cables within the SCL at one of the junctions at Crossrail’s Liverpool Street Station concourse, CSIC was able to map the strain build-up in the lining at every stage of the cross-passage excavation and, for the first time, observe the behaviour of the SCL during the excavation sequence.

The second project, led by Research Associate Zili Li, monitored the deformation of a Diaphragm wall (D-wall) during deep excavation at Paddington Station. As the only train station in the Crossrail project constructed using a top-down excavation, the Paddington site provided the opportunity to evaluate the effect of the excavation of an existing tunnel on D-wall behavior using fibre optic cables for the first time. Fibre optic cables were embedded in diaphragm wall panels allowing CSIC to monitor the changes in strain conditions during three key stages of construction; tunnel, concourse and base excavation. This was the first time FO cables have been used to validate finite element model assumptions about this scenario.

The innovation

On Crossrail, CSIC demonstrated innovative applications of distributed FO sensors to collect new data about commonly used construction techniques with the potential to refine and improve future design.

Both projects used Brillouin Optical Time Domain Reflectometry (BOTDR) embedded in concrete to measure strain and temperature changes within the material at key stages in construction. CSIC’s FO technologies enable strain measurements in the tens of microstrain range in a continuous manner over lengths of up to 10km, offering an unprecedented level of detail on the concrete’s behavior during excavation. 

Impact and value      

An improved understanding of the performance of infrastructure during excavation, margins of safety, and resilience enables better, leaner future design.

While further research is needed, the results of both studies indicate areas for significant potential savings in future designs. The results of the monitoring of the SCL at Liverpool Street Station showed that the effects of cross-passage excavation on the parent tunnel’s lining are localised in the vicinity of the cross-passage openings. These preliminary findings indicate that there are significant savings to be made in materials, labour, and plant as well as environmental benefits associated with reduced material use and improved site safety due to a decrease in working at heights to erect steel reinforcement and spray concrete. Similar studies could translate these findings into real savings for similar projects such as Crossrail 2.

Preliminary results of the Paddington Station monitoring indicate that the measured D-wall displacement is about 60% of the design D-wall displacement. The incremental bending and deflection profiles generated through the fibre optic cable’s continuous strain readings indicated that the effect of the removal of an existing tunnel on the D-wall deflection and ground heave during deep excavation can be significant, but was less than predicted. This research can be used to improve and refine future D-wall design, presenting the possibility of savings in materials and cost through more accurate modelling.

These ground-breaking studies should serve as a catalyst for infrastructure owners and researchers to carry out similar studies on different types of SCL tunnel and D-wall construction techniques.