Christchurch Technical Meeting

5:30 to 7:00 pm, Thursday 20th June 2024

BECA office, Level 2, ANZ Centre, 267 High St, Christchurch CBD,

Ōtautahi / Christchurch

Presenters: Mike Thorley from Beca & Alistair Stewart from SRLA (Melbourne)

Dive into Melbourne suburban rail trenches: a story of putting the “I”nternational and “A”wesome into IAH through trans-Tasman hydrogeological collaboration.

Mike Thorley and Alistair Stewart will talk about their involvement in a major suburban rail infrastructure project in Melbourne, Australia. The project comprised the removal of six level crossings by lowering the Frankston line under the local road network into three massive rail trenches (approximately 2 km each), with new suburban rail stations at Edithvale, Chelsea, and Bonbeach. This section of the rail line runs close to the coastline of Port Phillip Bay, the next to the Nepean Highway, and approximately 1 km east of the environmentally protected Ramsar Convention Edithvale-Seaford Wetlands.

The overall size of the inground structures had the potential to significantly dam groundwater movement to the coast and possibly cause groundwater flooding of nearby residential areas. The final rail trench design solution had to comply with Environmental Performance Requirements (EPRs) that formed the basis of the Projects Environmental Framework and approval criteria. This included mitigation against groundwater mounding and drawdown, degradation to groundwater quality, and ground settlement and subsidence. The concept reference design solution included extensive inground trench structures which relied on a passive horizontal ring drain system to divert groundwater around the structures.

Mike and Alistair were faced with providing prompt and solution focussed hydrogeological inputs to the Southern Programme Alliance (SPA) detailed design. Working with geotechnical engineers, construction specialists, environmental and structural engineers, the multidisciplinary team produced a design which ultimately differed greatly from the reference concept design solution, met the legislative requirements, and was affordable and buildable. The adopted solution improved performance against the reference design and EPRs, and eliminated the need for a proposed passive horizontal ring drain system. This was achieved by optimising the size tanked sections (groundwater dams) and allowing groundwater to flow through a “sawtooth” pattern created in the primary sheet pile system, and around wing walls by utilising groundwater ‘mounding’ and ‘drawdown’ across the structure.


Mike Thorley

Mike works at Beca, as a Principal – Hydrogeology, based in Christchurch. He holds a degree in Geological Sciences and a MSc in Geology from the University of Auckland.

Mike has worked in Christchurch for the past 20 years, mainly in consulting across a wide range of projects involving large infrastructure, water supply, groundwater modelling, and supporting resource consent applications.


Alistair Stewart

Alistair is based in Melbourne and is a Principal Hydrogeologist with the Suburban Rail Loop Authority (SRLA). Alistair holds a degree in Environmental Engineering from the Royal Melbourne Institute of Technology (RMIT).

Alistair has over 15 years’ experience mainly in consulting for large linear infrastructure projects both in the Technical Advisor role and within D&C consortiums. Alistair also has extensive mining, oil and gas experience which as seen him work all over Australia.



This presentation will be in-person at Beca Christchurch and available online via MS Teams. Please join us at the Beca Office at 5:30pm for a 6:00 pm start. Snacks and drinks will be provided. Click here to join the meeting via the MS Teams option. We encourage people to attend in person if possible and will facilitate building access from the High St entrance (next to Terrys Café). Please txt Mike Thorley on 027 702 4675 if you get lost or are stranded outside the Beca office