Vietnam’s Thuong Kon Tum Hydroelectric Project is a 17.4 km tunnel that may well be the longest in the country. The completed conduit was designed to draw water from the Dak Nghe River to supply electricity to the Central Vietnam region via a 350 MW capacity power station. This project encountered numerous challenges, from the geology to the original contractor. A 4.5 m diameter Main Beam TBM excavated a section of the tunnel in granite rock up to 250 MPa UCS. The TBM was originally launched in 2012, though non-satisfactory performance led to the original contractor leaving the site. The revamped project launched in 2016 with a new contractor and Robbins leading the refurbishment and operation of the TBM.
Originally the geology of the project was noted as a granite rock type with strengths up to 120 MPa UCS. However, the available geology of the project was extremely limited with factors being the mountainous jungle above the tunnel making scouting extremely expensive and complicated. In this area rainfall is torrential during monsoon season with annual precipitation averages of 1800 mm. There were also landslides, which blocked roadways and left travelers without detours to their destination. These difficulties led to Onsite First Time Assembly (OFTA) being used to allow shipments to arrive in smaller pieces.
In 2012 the 4.5 m diameter Robbins Main Beam TBM and Continuous Conveyor System were supplied to bore a 10 km section of the tunnel.
In 2016 a joint venture with Robbins and a Vietnamese contractor, Construction Joint Stock Company No. 47 (CC47), was awarded the contract to refurbish the TBM, which suffered damage from lack of maintenance and the tropical climate (ambient air temperatures in tunnel hovered above 30 degrees Celsius, with humidity at a constant 90 percent or higher). The contract was also to excavate the remaining tunnel.
In March 2016, the refurbishment to the TBM and equipment began. This included repair to several kilometers of tunnel conveyor belting and components, rock support systems, and all motors on the equipment. Guidance Systems and VFDs also needed to be recommissioned. With the high humidity being a factor, main bearing conditions were observed and detailed tests were conducted; results showed the main bearing chamber had not been contaminated.
Equipment testing began in April 2016, to adjust the boring conditions to cope with the exceedingly hard rock (tested at up to 300 MPa UCS). Over the next few weeks, the machine’s progress skyrocketed with it being shown the equipment could handle the terrain after sitting idle.
In June 2016, the TBM began boring after the two months of intensive repairs. Day to day operations for the tunnel and training of personnel were conducted by Robbins, while the contractor provided services to the tunnel such as ventilation, water, power, rail lines, ring beams, and other materials. The machine was able to bore 17.5 m per day for three days constantly, eventually surpassing those expectations.
The crews expected to pass through about 10 percent type 1 faults, and 65 percent class three faults. The rock mass was expected to consist of 10% Type 1 (massive, competent), but it was around 75 to 80 percent of the bore. The granitic rock was highly massive with few fractures and jointing. The compressive strength of the rock was estimated at 170 MPa, but it averaged 270-290 MPa.
The disc cutters used were able to keep the TBM progressing. Mostly heavy-duty cutters were used with each disc ring lasting for 500 to 600 m.
Complications arose throughout the bore. The remote environment had arduous tunnel conditions such as temperature and fault zones. There were numerous amounts of faults. Boring through granite, the typical advance rates were between 2.3 and 2.5 m an hour, and between 400-500 m per month. Besides the fault zones there were three major influxes of water, which required a steel sheet to divert water to the invert, then a ring beam and McNally system was installed. The TBM equipped with McNally pockets in the roof shield allowed for steel slats or rebar to be extruded and installed as the TBM advanced. By replacing the roof shield fingers on a Main Beam TBM, the McNally system prevents movement of loose rock in the critical area immediately behind the cutterhead support.
Fracture zones were not that common; the TBM passed through the major faults that discharged heated water at a rate of 600 liters/second draining naturally from the graded tunnel but made the excavation difficult. These fault zones required steel water diversion plates and drainpipes installed to direct the extreme water ingress.
Crews had a probe drill that could be mobilized quickly if grouting needed to be done ahead of the TBM. Regular face mapping was done ahead of the TBM to determine how ground conditions were changing.
By Spring 2018, there was only about 2.8km (1.7mi) of tunnel remaining. With less than two years of operation the tunnel went from 15 percent complete to 85 percent complete. The tunnel conveyor system had an availability of 93 percent, meaning that it only had seven percent downtime. Despite the successfulness of the bore, challenges still arose. Thunderstorms and power cuts wreaked havoc causing production to slow down. The geological report also predicted more fault zones in the tunnel path with an incredibly large zone near the last 500 m of a tunnel.
In the last quarter of 2018, crews hit a large fault zone, 250 m away from where the TBM would meet the downstream Drill-and-Blast excavated portion of the tunnel. Upon encountering the fault, the tunnel was inundated with water and debris with an estimated volume of 1,200 cubic meters. The resulting cavern was unstable and directly in the bore path causing the machine to be stopped.
After filling the cavity and consolidating the surrounding geology with over 22,000 kg of polyurea silicate foam, the machine was able to advance forward. However, a complication emerged. When an excessive amount of loose rock was encountered, investigations discovered that the foam had not reacted and set as expected. The heat in the cavity along with the heat created by the thermal reaction of the bi-component foam had produced an environment that prevented the foam from fully expanding or setting. To complicate matters further, additional new collapses of the cavity could be heard from inside the TBM. The size of the cavern was now estimated to be more than double the original size.
The machine was pulled back from the cavity and a third-party contractor was employed to install a concrete plug between the TBM and cavity by using a concrete formwork and pumping in concrete using the TBM shotcrete system. A decision was made to remove the TBM and install the final lining.
In the last quarter of 2019, the final 250 m of tunneling was completed by a Drill & Blast team. TBM excavation rates were very good for the majority of the boring, despite all the arduous challenges that emerged throughout the process.