A Robbins Double Shield TBM has achieved breakthrough at Turkey’s Kargi Kızılırmak hydroelectric project after overcoming multiple challenges at the site.

The TBM broke through on the project on 5 July 5 after a run through some of the most difficult conditions ever encountered by Robbins field teams. The machine type, selected jointly by the owner, consultants, Robbins, and the contractor Gülermak of Turkey, excavated through 7.8km of complex geological conditions that proved to be a challenge nearly from the outset, the company said.

The machine was launched into poor geology that resulted in delays to the project leading to team members having to find innovative solutions that included major in-tunnel modifications to the machine. In the first 2km of boring a total of seven bypass tunnels were needed to free the TBM from collapsed ground. The cutterhead stalled on numerous occasions as the conditions varied widely from solid rock to running ground. Small and wide faults along the alignment added another level of complexity, as the excavation was located very close to the North Anatolian fault line in Turkey’s relatively recent rock formations.

The contractor, owner, consultants and Robbins engineers worked together to generate solutions to improve progress in the difficult conditions. The contractor, with the assistance of the Robbins field team, installed a custom-built canopy drill and positioner to allow pipe tube support installation through the forward shield. Drilled to a distance of up to 10 m (33 ft) ahead of the cutterhead, 90 mm (3.5 in) diameter pipe tubes provided extra support across the top 120 to 140 degrees at the tunnel crown. Injection of resins and grout protected against collapse at the crown while excavating through soft ground. As a result of successful use of the probe drilling techniques, Gülermak was able to measure and back-fill cavity heights above the cutterhead in some fault zones to over 30 m (100 ft) and, in addition, was able to help detect loose soil seams and fractured rock ahead of the face.

“The cooperation and trust between the contractor, project owner, and Robbins Management, Engineering and Field Service resulted in the correct modifications being successfully installed on the Kargı TBM," said Glen Maynard, Robbins Site Manager.

Despite the slow progress initially, the Robbins Double Shield TBM made some remarkable advances once modifications were in place. An advance rate of 600m in one month was achieved in March 2013 and in more recent news, a project best of approximately 723m was achieved in spring 2014, including a daily best of 39.6m in April 2014.

Once online the Kargı Kızılırmak Hydroelectric Project, for Norwegian-owned Statkraft AS, will generate 470GWh annually. The tunnel will source water from the Kızılırmak River, sending it to a new generating station operated by Statkraft.

Image: The crew at Kargı excavated seven bypass tunnels in the first 2km of boring and successfully completed extensive in-tunnel TBM modifications to combat the unforeseen ground conditions.