The Thuong Kon TUM Hydroelectric project in Vietnam’s remote and densely forested interior now lays claim to the country’s longest tunnel (17.4km), but the journey was anything but easy. The jobsite – located 142km away from Ho Chi Minh port and a six-hour drive from the nearest city Danang – is so remote that 95km of roadway had to be built or reinforced prior to tunnel construction.

A 4.5m diameter Robbins Main Beam TBM and continuous conveyor system were supplied to bore a 10km section of the tunnel. The completed conduit draws water from the Dak Nghe River to supply electricity to the Central Vietnam region via a 350MW capacity power station.

Though the TBM was originally launched in 2012, commercial circumstances resulted in the original contractor leaving the site. In 2016, the revitalised project and a new local contractor contacted Robbins to lead the refurbishment and operation of the TBM.

En Route to restart

Just getting the equipment to the jobsite was a challenge in an area where rainfall becomes torrential during monsoon season and annual precipitation averages 1,800mm. Landslides often block roadways and leave travelers without viable detours to their destination. Because of these issues, and in part to reduce stresses on bridges, Onsite First Time Assembly (OFTA) was used to allow shipments to arrive at the site in smaller pieces. The OFTA process allows a TBM to be initially assembled on location, saving time and money in terms of shipping costs and project schedule.

Originally launched in 2012, commercial circumstances, combined with difficult geology, resulted in the original contractor leaving the site. The TBM had by then excavated 2,636m in two years in hard and massive granitic rock, with some zones having significant stability problems.

In late 2014, Robbins was invited to the site by owner Vinh Son – Song Hinh Hydropower Joint Stock Company (VSH) to inspect the condition of the TBM and auxiliary equipment, which had been left in the damp and warm tunnel for months. After the initial inspection it was obvious that the equipment was not maintained in a proper fashion during operation, nor had any protective measures of the machine been taken before it was left in the tunnel in a tropical climate. Ambient air temperatures in the tunnel hovered above 30°C, with humidity consistently above 90%. The combination of the previously stated conditions resulted in the TBM and its equipment needing considerable refurbishment before any boring could be restarted.

In 2016, more than two years after the original contractor left the site, a joint venture between The Robbins Company and the Vietnamese contractor, Construction Joint Stock Company No. 47 (CC47) was awarded a contract to refurbish the TBM and auxiliary equipment and excavate the remaining tunnel. Robbins was fully responsible for the refurbishment and TBM operation, including supplying operational crews and eventually rock support in the tunnel as well. CC47 was responsible for the site operation, logistics, and rock support behind the tunneling operation. “The most challenging things about this project were the mountainous region and remoteness of the project,” said Greg Adams, Robbins Field Service Superintendent at the site. “When we started the refurbishment, the machine was in a very poor state – it had been left for 14 months in groundwater and high humidity. This is all bad for electrical components. We refurbished all the electrics, hydraulics, and put a new maintenance plan together.”

Refurbishment work began in March 2016, and also included repairs to several kilometers of tunnel conveyor belting and components, rock support systems, and all motors on the equipment. Guidance systems and VFDs had to be recommissioned.

Due to the high humidity, there was concern about the condition of the main bearing and detailed tests were conducted. Results showed the main bearing chamber had not been contaminated, which allowed the rest of the refurbishment to move forward.

The next phase was testing of the equipment, starting in April 2016, to adjust the boring operations to cope with the exceedingly hard rock (tested at up to 300MPa UCS). “Over the next few weeks the machine performance got better and better. We had to do a test for the contract of three days’ continuous boring, 17.5m per day. This was accomplished and it was a bit of a relief that we could show the equipment could handle such hard rock after sitting idle,” said Adams.

Production Ramps up

The TBM began boring in June 2016 after two months of intense repair work. The Robbins crew carried out the day-today operations for the tunnel and training of personnel, while the contractor provided services to the tunnel such as ventilation, water, power, rail lines, ring beams, and other materials. “We started off, in the contract it said we had to do 17.5m per day for three days constantly, and we managed to do it. Since then it got stronger and stronger,” said Adams.

As per the engineering geological profile of this project, the crew expected to pass supposed to pass through about 10% type 1 faults, and 65% class three faults. The rock mass was expected to consist of 10% Type 1 (massive, competent), while in actuality this was experienced for 75 to 80% of the bore. The granitic rock was highly massive with few fractures and jointing. “The compressive strength of the rock anticipated during the contract was 170MPa, but in actuality it averaged 270-290MPa,” added Madhan.

Despite the incredibly hard and abrasive rock, the disc cutters used at site were able to keep the TBM on the move. “Our Robbins cutters were mostly Heavy Duty cutters on this machine. Each disc ring lasted for 500 to 600m. It’s not just the manufacturing of the cutters, it is the process of assembly in the field, and our cutter technicians. They decide what will work for the ground. It’s teamwork leading to success,” said Madhan.

Difficult site conditions continued throughout the bore. “This is a very remote place with pretty tough conditions in the tunnel. Heat in the tunnel was 37 or 38°C. When the machine was boring the shield and front were around 40°C and upwards [but] we went in and did the work,” said Adams. He added that the fault zones were another challenge: “On this project we had considerable amounts of faults. We bored through granite, and typical advance rates were between 2.3 and 2.5m an hour, between 400 and 500m a month. We had fault zones, not all containing water, not all in the same predicted position as geology is not a perfect science. We had three major influxes of water. To deal with this we used a steel sheet, which diverted the water to the invert. We then installed a ring beam and McNally system, and then just carried on forward.”

The TBM, equipped with McNally pockets in the roof shield rather than roof shield fingers, allowed for steel slats or rebar to be extruded and installed as the TBM advanced. The McNally Roof Support System was designed and patented by C&M McNally and licensed for use on Robbins TBMs in certain markets. 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. The system has been tested and proven on projects worldwide – including the world’s second deepest civil works tunnel, the 2,000m deep Olmos Trans-Andean Tunnel in Peru – to increase advance rates while still maintaining worker safety on Main Beam machines in difficult rock conditions.

While fracture zones were not common, the TBM passed through two major faults that discharged heated water at a rate of 600 litres per second, which drained naturally from the graded tunnel but made excavation difficult. These fault zones required installation of steel water diversion plates and drain pipes to direct the extreme water ingress. “Our JV partner installed flood pumps to pump out water fast and avoid flooding. Where once we faced water levels that were half a locomotive in height, it was later tackled with continuous pumping of water,” said Madhan. Crews also had a probe drill that could be mobilised quickly if grouting needed to be done ahead of the TBM. Regular face mapping was done ahead of the TBM to determine how the ground conditions were changing.

By Spring 2018, the tunnelling operation had achieved spectacular results: only about 2.8km of tunnel were left to go. In less than two years, the tunnel had gone from 15% complete to about 85% complete. The tunnel conveyor system had an availability of 93%, meaning that it only had 7% downtime.

“We had English-speaking expats, Chinese, Thai, Vietnamese, Indian and Philippine guys on the crew. Each of them had their own profession, enabling us to have an electrician, mechanic, operator, hydraulic, geologist and welder on every shift. We also employed Vietnamese translators to help us teach the local crew members about ground support methods. We needed to show them how ground support should be best installed,” said Adams. Training sessions included how to drill and install rock bolts, wire mesh and ring beams.

The training sessions made a significant difference: “We worked on faster ring beam building methods, reducing time from seven to eight hours per ring beam build to three hours per ring beam. Because of this we got more utilisation time for boring; less downtime,” said Madhan.

The positive working environment was appreciated by all parties involved: “We have seen that the TBM machine is very suitable for boring hard rock. With the professional operation of the Joint Venture team, and especially from The Robbins Company, with their expertise and experience, the tunneling project was quite stable and the results were good,” said Nguyen Truong Son, Deputy of Project Management for owner VSH.

However, challenges were expected to continue. Thunderstorms and power cuts plagued the jobsite, resulting in loss of production. The geological report also predicted more fault zones in the tunnel path, including a particularly large zone within 500m of the end of the tunnel.

Cavern Encounter

In the last quarter of 2018, just 250m from where the TBM would meet the downstream Drill-and-Blast-excavated portion of the tunnel, crews hit a large fault zone. Upon encountering the fault, the tunnel was inundated with water and debris with an estimated volume of 1,200m3. The resulting cavern was unstable and directly in the bore path, causing the machine to be stopped.

After discussions with the project owners, design consultants, and BASF ground treatment specialists, a decision was taken to fill the cavity and consolidate the surrounding geology with a polyurea silicate foam. An array of self-drilling anchors was installed using the TBM probe drill and over 22,000kg of dual-component foam was injected into the cavity. The machine was then advanced forward; however, an excessive amount of loose rock was encountered. Investigations were carried out and it was 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.

Because of the complications the parties involved decided on a new strategy. “This was a very complicated geological feature, which was difficult to define. It obviously had to be filled with some form of concrete and this would take considerable time,” said Robbins president Lok Home. 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 concrete formwork and pumping in concrete using the TBM shotcrete system. The plan was for the contractor to drill through the plug with a portable drill rig and to fill the cavity with OPC grout.

Since time was of the essence and lining of the bored section of the tunnel was on the critical path, it was decided to remove the TBM and commence installation of the final lining. The remaining 250m of tunnel would be completed by the Drill & Blast (D&B) team coming from the upstream portal. Further definition of the fault could be completed after the TBM was removed and the plan to cross the fault could be well developed by the time the D&B team reached the fault. Robbins was preparing to cross the fault with the TBM, but with good access from the opposite side the project owners decided the schedule would be better served by removing the machine.

“I was disappointed that the fault crossing became so complex and could not be expeditiously crossed with the TBM,” said Home.

As of September 2019 only 18m remain of the upstream drill and blast tunnel section. Despite the difficult obstacle at the very end of tunneling, the project is nearly complete, and TBM excavation rates were very good for the majority of tunneling. “I want to commend the excellent work of the Robbins field service and contractor teams, and the good cooperation with the project owner,” said Home. “This was a challenging project and they worked together to provide different solutions. I hope this project becomes another example and case study of the difficult conditions that can be encountered and overcome during excavation in long and remote tunnels.”