Back-analysis of monitoring results at Macquarie Park station, Epping to Chatswood rail line

The design of the Epping to Chatswood Rail Line project was conducted nearly in parallel with the initial construction work. The Macquarie Park Station caverns were partially excavated while the design of the other three stations was still underway. This provided an opportunity to use as-constructed performance to refine the design parameters for the subsequent caverns.

Geotechnical monitoring of the initial Macquarie Park Station excavation included inclinometers, extensometers, surface settlement points, endoscopes, convergence points, crown sag points and rock bolt load cells. The geology of the excavation faces was also carefully mapped.

On the basis of the mapping, the geological model for this station was refined slightly. The monitoring results were reviewed and back-analysed. The rock mass moduli and the joint stiffness values of the different rock units were changed to “match” the monitored behaviour of the excavation.

The back-analysis work generally indicated that the original models adopted for design were reasonable. Two “admissible” combinations of slightly revised geotechnical parameters were identified. Models for subsequent design of the other stations were adjusted to reflect the calibrated parameters. The back-analysis work was also consistent with the relatively high in situ stresses adopted for the project.

This paper discusses the back-analysis work undertaken and demonstrates that appropriate monitoring is a useful tool for verifying and refining design models.