Cross passage sinkhole halts work in Perth 27 Sep 2018

Ground loss at the breakout junction of the first cross passage to be excavated on the twin tube Forrestfield-Airport rail link project in Perth, Western Australia, brought TBM progress on the project to a halt and closed the adjacent Dundas Road to traffic.

The inflow of water and material began at about 1pm on Saturday afternoon 22 September and, despite efforts to fix the problem, continued through the night to Sunday morning creating a sinkhole on the surface and requiring closure of the road. The incident caused no injuries or loss of excavation equipment but the TBMs some 3km and 2.5km ahead of the location remain on hold while the situation is recovered and access for MSV multi-service vehicles to the advancing headings can be reinstated.

The cross passage being excavated is the first of 12 cross-passages to be excavated on the 8km twin tube rail alignment and is about 200m from the Forrestfield portals from where the two TBMs working on the project were launched in July and September 2017 (Fig 1). The oval cross passages, of about 5.7m high x 4.7m wide at the widest point, vary in length and lie up to 26m deep to invert level. The ground is supported by blocks of pre-excavation jet-grouting and each cross passage is programmed to take up to two months to complete.

Cross passages under the airport property to link to the Perth Airport Station will be supported with ground freezing. “Installation of the freeze system at these locations is yet to start,” explained David Hynes, spokesman for the state Public Transportation Authority, the client, when TunnelTalk made contact for further details of the incident.

In addition to the 12 cross passages there are three emergency egress shafts and the cross passage connections to the running tunnels from the Abernethy emergency egress shaft have been completed successfully.  

The troubled cross passage is under a cover of about 9.4m to crown level and most of its 4.8m length (measured between centrelines of the running tunnels or 8.2m long measured from crown-to-crown or invert-to-invert) (Fig 2). The jet-grout block had been installed around the cross passage profile and segments at the breakout points in both running tunnels had been removed prior to the incident.

According to Hynes; “the inflow appears to have been from the bottom corner at the breakout interface between the cross passage and the running tunnel.” Despite remedial efforts, groundwater and silt continued to flow into the cross passage and into the running tunnel to create the sinkhole. “No equipment was lost and backfill concrete did not flow in to the cross passage or the running tunnel,” said Hynes in email communications with TunnelTalk.

Due to the ground loss, there has been movement of about 10 rings of segmental lining near the point of the cross passage and additional bracing has been installed to support the tunnel. While recovery work is ongoing, a survey of the ground along Dundas Road and above the tunnel route and the cross passage is also underway.

In statements attributed to State Transport Minister Rita Saffioti: "The Government is monitoring this very closely and working with the contractor, the Salini-Impregilo-NRW JV to review the method used to create future cross passages. I have asked the Public Transport Authority to review the timeline of the project, to ensure worker safety is paramount. My number one priority is to ensure work is carried out safely.”

This is the second incidence of significant ground loss experienced on the project. In February this year (2018), both TBMs were at a standstill while the causes of surface settlement of up to 1m was recorded over the lead TBM heading. The lead TBM was on hold for two months while the trailing TBM waited once within 40m of the lead TBM and until both resumed tunnelling in restarted in April. New operating procedures for the two Herrenknecht Variable Density machines were adopted, including no stoppages on weekends, to assist excavation and progress beneath the watertable and through the loose granular conditions of the ground in the area.

After breaking through into the Airport station box in May, both machines relaunched in late July and are 1.4km and 2km from breaking through into the Redcliffe Station.

As well as progressing the TBM drive and the cross passages, work is advancing on the three emergency egress shafts. After completing excavation and installing waterproofing, the permanent concrete lining between the tunnels and the emergency egress shaft near Abernethy Road has been poured and focus is now shifts onconstruction of the staircase, lift shaft and ancillary building.

At Woods Road, within the Airport West Terminal 3 & Terminal 4 Precinct, construction of the base slab after core excavation is scheduled to commence in September. Once completed the shaft will be connected to the running tunnels with an adit and cross passage.

At Wright Crescent in Bayswater, the diaphragm wall support is complete and after installing the capping beam, excavation of the core within the 9m diameter shaft will commence, supported by temporary lowering of the ground water level. Excavation is expected to take about eight weeks with 10 to 20 truck movements each day for muck disposal. Once the base slab is in, it too will be connected to the running tunnels.

A plant room adjacent to each emergency egress shaft will store fire and emergency response equipment, as well as hydraulic, mechanical, electrical and communications services which are required for tunnel operations.