Design and construction of deep excavation shoring system in swelling Bringelly shale in Sydney

This paper presents a case study of an approximately 20-metre-deep excavation into Bringelly Shale at the St Marys metro station, which is part of the Station Boxes and Tunnelling works for Sydney Metro – Western Sydney Airport. A review of the site geology, ground model and geotechnical design parameters is presented. The swelling characteristics of Bringelly Shale are summarised based on site specific testing results and compared against published data. An attempt is made to appreciate the radial displacement development of ground anchor hole on the bond strength. A coupled groundwater and stress-strain modelling was carried out using Plaxis 2D to optimize the design after anchor pullout testing at the project site. A prediction is made with respect to the ground responses to the retention system using soldier piles and ground anchors during staged excavations. An instrumentation and monitoring plan has been developed and implemented during the construction to validate the performance of the retention walls. The reduced bonded strength of ground anchors in Bringelly Shale with swelling potentials with “weak” rock layers was recommended based on site specific proof testing results. The monitored lateral movement of deep excavation at the St Marys metro station site is ranging between 1 mm to 4 mm per metre of excavation as compared to 0.5 mm to 2 mm per metres for excavations in Hawkesbury sandstone and Ashfield Shale. The findings of this paper are considered to be useful for practitioners who will be involved in future projects dealing with Bringelly Shale having swelling potential.