Driven through a mountainside with 600 m (1,968 ft) of cover in Central Turkey, the Kargi Kizilirmak Hydroelectric Project is one of the most challenging tunneling projects ever completed in the region. A 10-m (32.8 ft) diameter Double Shield TBM was supplied to contractor Gülermak to bore the headrace tunnel that would divert water from the dam to the powerhouse. The entire tunnel, with both TBM and drill and blast portions, is 11.8 km (7.3 mi) in length. The project generates 470 GWh of power annually for project owner Statkraft, an amount sufficient enough to supply approximately 150,000 homes. The extremely challenging ground conditions of this project made it necessary for the machine to be modified in-tunnel. The results of which largely contributed to the creation of the Crossover machines used today.
The expected geology along the tunnel alignment consisted of Kirazbasi complex Kargi ophiolites (including sandstone, siltstone and marl) for the initial 2,300 meters (7,545 ft), followed by 1,000 meters (3,280 ft) of Kundaz metamorphites (including marble, metalava and metapelite), and the remaining 8,500 meters (27,887 ft) consisted of Beynamaz Volcanites (including basalt, agglomerates and andesite). The anticipated strength of the rock was up to 140 MPa. Multiple fault zones and transition zones added to the complexity of the geological conditions.
Almost immediately after launch in early 2010, the machine encountered geology that was substantially more problematic that was described in the geological report. The geology consisted of blocky rock, sand, and clays. 80 meters (262 ft) into the bore the TBM became stuck in a section of collapsed ground. As a countermeasure that was immediately put into place to avoid the cutterhead becoming stuck in the blocky material, crews began boring half strokes and half resets. Despite these measures, the machine encountered a section of extremely loose running ground with high clay content. Another collapse occurred in front of the cutterhead and the weight of the material trapped the cutterhead.
After assessing all the available options, it was decided that a bypass tunnel would be required. Robbins Field Service assisted Gülermak with bypass tunnel design and work procedures to free the cutterhead and stabilize the disturbed ground. Blasting techniques were ruled out due to concern over further collapses caused by blast induced vibration; hence, the excavation was undertaken using pneumatic hand-held breakers. Hopes that the section of bad ground would be an isolated occurrence were quickly proved wrong, and the actual complexity of the geology became apparent. Six more tunnels were needed within the first 2 km (1.2 mi) of tunneling.
Both the contractor and manufacturer worked together to develop and improve bypass tunneling and hand tunneling techniques, resulting in an average bypass tunnel construction time of just 14 days. All tunnels were completed safely and in a timely manner, though there were of course significant delays associated with the downtime. Despite the setbacks of these multiple events, the TBM did succeed in crossing numerous faulted sections that would have trapped a machine with less power. Bypass tunneling was successfully completed and at that point the section of bad ground was believed to be an isolated one.
In order to improve progress in the difficult conditions, the contractor, owner, consultants, and Robbins engineers worked together to generate solutions. The contractor, with the assistance of the field service team, installed a Robbins 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 (9 cm) 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 (98 ft) and, in addition, was able to help detect loose soil seams and fractured rock ahead of the face.
To further mitigate the effects of squeezing ground or collapses, custom-made gear reducers were ordered and retrofitted to the cutterhead motors. They were installed between the drive motor and the primary two-stage planetary gearboxes. During standard boring operations the gear reducers operate at a ratio of 1:1, offering no additional reduction and allowing the cutterhead RPM to reach design speeds for hard rock boring. When the machine encountered loose or squeezing ground the reducers were engaged, which resulted in a reduction in cutterhead speed but the available torque was increased by nearly double. The net effect of the modifications allowed the Double Shield TBM to operate much like an EPB in fault zones and squeezing ground with high torque and low RPM—these methods effectively kept the machine from getting stuck. In addition, short stroke thrust jacks were installed between the normal auxiliary thrust to double total thrust capabilities.
The TBM design changes gleaned from Kargi were not considered to be isolated, and immediately began to inform the design of Robbins’ new line of dual-mode type machines, termed Crossover TBMs. Crossover TBMs are also called hybrid or Dual Mode machines, and are able to “cross over” into distinct types of geology. Crossover TBMs feature aspects of two TBM types, and are ideal for mixed ground conditions that might otherwise require multiple tunneling machines.
Despite the slow progress initially, the Robbins Double Shield TBM made some remarkable advances once modifications were in place, which essentially made it a Double Shield TBM with Crossover features. An advance rate of 600 m (1,986 ft) in one month was achieved in March 2013 and a project best of approximately 723 m (2,372 ft) was achieved in spring 2014, including a daily best of 39.6 m (130 ft) in April 2014. In so doing the TBM significantly outperformed a drill and blast heading progressing from the opposite end of the tunnel. Crews at that heading progressed in relatively good ground conditions for 4 km (2.5 mi), where they achieved advance rates of nearly 300 m (984 ft) per month. The TBM bored 7.8 km (4.8 mi) of the tunnel in total, making its final breakthrough in July 2014.