Meeting the challenge in Pakistan’s Lower Himalayas with the use of TBMs

The Neelum Jhelum Hydro Electric project is located in the Muzaffarabad district of Azad Jammu and Kashmir (AJK), in northeastern Pakistan within the Himalayan foothill zone known as the Sub-Himalayan Range. The terrain is rugged with ground elevations that range from 600 to 3,200 m (2,000 to 15,000 ft) above sea level. The project is a run-of-river scheme, employing 28.6 km (17.7 miles) of headrace and 3.6 km (2.2 miles) of tailrace tunnels that bypass a major loop in the river system, for a total static head gain of 420 m (1,400 ft). The headrace tunnels’ twin bores (69 percent) contain the tunnel boring machine (TBM) excavation (Fig. 1).

Geological settings

The entire project was excavated in the sedimentary rocks of the Murree Formation, which is of Eocene to Miocene age. The TBM tunnels are being driven through a zone bounded by two major Himalayan faults that trend subperpendicular to the tunnels: the Main Boundary Thrust, and the subsidiary Muzaffarabad reverse/thrust fault. The lithologies are detailed as follows:

• Siltstones and silty sandstones: Uniaxial compressive strengths (UCS) are 50 to 70 MPa.
• Mudstones: With UCSs in the 30 to 40 MPa range,
• Sandstones: With UCS in the range of 130 to 230 MPa.
• In situ stresses: Overcoring tests in sandstone beds in the TBM tunnels found a tectonically altered zone of high stresses (k up to 2.9) with the major principal stress oriented sub-horizontally and subperpendicular to the tunnel azimuth.

Geological structures and conditions

In addition to the expected rock types, there are certain ground conditions that are expected to be encountered. The TBM was designed to manage these conditions and included facilities to detect these conditions in advance in order to take appropriate measures to successfully negotiate them. These structures and conditions included:

• Unstable rock zones.
• Squeezing and swelling ground.
• Soft ground.
• High water inflows.
• Extensive fault zones.
• Rock bursts.
• High overburden depths (1,870 m or 6,135 ft).

TBM selection

Two open (gripper) TBMs were used to excavate 20 km (12.4 miles) of the headrace tunnel system in two parallel tunnels.

A significant consideration for the TBM selection was the possibility of encountering squeezing ground, with deformations of up to 500 mm (19 in.) on the tunnel diameter. This would exclude many types of TBM designs due to the possibility of becoming trapped within the tunnel. The open (gripper) TBM was determined to be best suited to deal with this potential condition because of the short length of the front shield and its ability to collapse inward at various sections of the front shield, depending upon ground conditions, and still maintain the ability to excavate forward.

The second ground condition that was indicated to be present in the higher overburdens and more brittle rock was rock bursting. Again, the open (gripper) TBM configuration allows for equipment to be installed to detect and mitigate potential rock bursts.

The two TBMs were referred to by their model manufacturing model number, TBM 696 and 697. TBM 697 was launched first and for the majority of the tunnel excavation and was the lead TBM.

Rock support design. The initial design for the rock support consisted of four categories of support designated as Q2, Q3, Q4 and Q5. These support designs were to be installed according to the observed geology or Q class, which was revealed at the rear of the TBM shield as the TBM advanced. The most favorable rock class was Q2 and the least favorable was Q5. The support requirements in terms of quantity of shotcrete and rockbolts, wire mesh mining straps and full steel rings increased with the increase of Q class. The main component — shotcrete — started at 125 mm (5 in.) thickness and increased with class until in Q5 class, where the thickness was 350 mm (13.7 in.).The balance of support installation and rock class is an area that still relies heavily on human intervention, skill and experience to balance support installation with safety and production.

Challenge one — delivery of the two TBMs

With the two TBMs procured and manufactured, the first major challenge was the delivery of the TBMs and all associated parts to the construction site. The two TBMs were manufactured in Germany and China, respectively, and delivered to the port of Karachi in Pakistan. The manufacturer number was used for individual identification as 696 and 697. The TBMs were then loaded onto road transport and traveled 1,777 km (1,104 miles) to the construction site located in northwest Pakistan. The route used major road systems in the south and toward the end of the journey, the route climbed into the lower Himalayas following cliff edge roads and passing through towns and villages, (Fig. 3).

Challenge two — TBM power supply

The consequence of having two capable TBMs was the need for a dedicated power supply. The TBM construction site was located in a remote part of Azad Kashmir and did not have the power supply to meet the TBM requirements, and regularly experienced up to 16 hours a day of load shedding. Therefore, a complete 19.6-MW power station had to be constructed on a hillside near the TBM access adit. The power station consisted of four, 4 MW generators and one 3.6-MW standby generator, powered by heavy fuel oil (HFO). The full power station is shown in Fig. 5.

Challenge three — Fault zone

Commencement of tunnel excavation. Both TBMs commenced the planned 11.2 km (7 miles) of twin headrace tunnel excavation in early 2013. Both TBM launches were in stages to allow the continuous conveyors to be installed after excavation of 100 m (330 ft). The first TBM to be fully installed and operational was TBM 697 and, from early 2013, it steadily excavated and increased monthly production. The second TBM, number 696, followed suit and progressed approximately 500 m (1,640 ft) behind TBM 697. The alignment of the twin tunnels encountered an existing access tunnel some 1,700 m (5,580 ft) from the TBM launch location. This tunnel known as Adit 2 had been completed prior to the planned arrival of both TBMs.

Fault zone. About 90 m (300 ft) before this adit, the lead TBM 697 encountered an extensive fault zone of sheared mudstone more than 80 m (260 ft) in length. The first indication of this poor ground came when the thrust pressures dropped rapidly and large quantities of soft material came through the cutterhead and onto the TBM conveyor system. A cavity rapidly developed in front of the TBM that was stopped to assess the situation. The TBM was then started and advance was attempted; however, the soft ground flowed into the cutterhead that tripped electrically and stopped. A crew of tunnel workers was sent into the cutterhead to manually remove the buildup material; this operation took eight hours to complete. In an attempt to reduce the flow of soft material into the TBM, all six of the buckets had metal components welded into them to reduce the opening and restrict the free flow of material (Fig. 6a). A further three attempts were made to advance the TBM in the poor ground conditions, with limited success. It was then decided to stop any further attempts and to install a 15-m (50-ft) pipe roof canopy (Fig. 6b) over the TBM shield and in front of the TBM, allowing stabilization by way of ground treatment with grout and chemicals. Once the canopy was completed, a top heading (Fig. 6c) was constructed to access the collapsed area and install further support in advance of the TBM.

After nine weeks, the TBM was started again and slowly advanced through the faulted ground, installing full circular steel rings and 350 mm (14 in.) of shotcrete and breaking through into the adit in early 2014. The trailing TBM 696, having the benefit of the knowledge of the fault zone installed a systematic 15-m (50-ft) pipe canopy every 5 m (16 ft), was able to progress the fault zone relatively smoothly but at a reduced advance rate.

Challenge four — Rockbursts

Good progress. Both TBMs broke through and traversed Adit 2 in January 2014 and entered into a period of good progress, reaching 460 m/ month (1,500 ft/month) in installing the full range of rock support for the ground encountered. This period of good progress lasted up until the beginning of November 2014. During this period, the TBM excavation was performed with tunnel overburdens in the range of 1,150 to 1,350 m (3,770 to 4,430 ft) and the only negative experience was the occurrence of a few rockbursts that resulted in damage to some of the TBM equipment.

Rockbursts. Rockbursts had been expected and mentioned in the geological baseline report and the expectation was that this would occur at the higher overburdens. By November 2014, with 4.7 km and 4.3 km (2.9 miles and 2.7 miles) of the tunnels excavated in the left and right tunnels, respectively, regular rockbursts warranted systematic recording. Rockburst events were categorized by magnitude, from “noise only” to “major rockburst.” The system aimed to correlate timing and distribution of rockbursts and facilitate selection of mitigation measures at the TBM. The total number of rockburts encountered for the two TBMs during tunnel exavation was 1,695. Figure 7a shows the breakdown of rockburts by category.

The description of the rockbursts categories is as follows:

Category 1: Noise only — a slight popping sound is heard and there is no damage to the support or ejection of rock.

Category 2: Noise and weak rockburst — a popping sound is heard and there may be slight damage to the support and surrounding rock.

Category 3: Noise and medium rockburst — loud popping sounds are heard and there may be splitting, spalling or shallow slabbing to the support and surrounding rock.

Category 4: Noise and major rockburst — loud sound similar to an explosion, violent ejection of rock into the tunnel and severe damage to the installed support and TBM.

Figure 7a shows that the majority of rockbursts are classified as Category 2, but even this category of rockburst was responsible for delays while repairs were undertaken. Figure 7b shows the typical aftermath of a Category 3 rockburst at the front of the TBM.

Rockbursts counter measures

Longitudinal relief holes. Drilling of longitudinal stress relief holes ahead of the tunnel face will fracture the rock mass, thereby releasing stress and reducing rockburst potential. Holes are drilled with the probe drill and should be closely spaced enough so the rock between the cracks or fractures to relieve the stress. The holes can be concentrated in highly stressed parts of the rock mass.

Radial relief holes. Radial stress-relief holes reduce the likelihood of rockbursts by shifting the tangential stress peaks away from the excavated perimeter. The holes must be large enough and closely spaced enough so the rock between the holes cracks and breaks. This creates a stress-relieved zone around the excavation perimeter. Fewer holes are required in fractured rock.

Horizontal side-wall probe. A significant contributor to the May 31, 2015 severe rockburst was a local change in the strike of the rock strata from perpendicular to the tunnel alignment to parallel. This hid the rockburst-prone sandstone beds behind siltstone beds. To detect future hidden sandstone beds, side probe holes were drilled at 5-m (16-ft) intervals on both sides of the excavated tunnel at tunnel axis height. This activity began on all TBM tunnels in siltstone after the severe rockburst of May 31, 2015.

Installation of shotcrete at the L1 zone. Reinforcement of the rock mass begins with installation of rock bolts and wire mesh, used routinely on the TBM. Steel fiberreinforced shotcrete can contribute significantly to the energy-absorbing capability where rock conditions require less support. It is preferable to apply most of the shotcrete at the L2 zone to allow quicker installation of initial rock support and faster resumption of excavation. However, 94 percent of rockbursts were detected at the front 10 m (33 ft) of the TBM.

Full ring steel supports. The original purpose of full ring steel supports was to support the tunnel at faults, large overbreak areas, and soft and squeezing ground. These supports are time-consuming to install and can be installed at spacings ranging from 0.9 to 1.6 m (3 ft to 5.2 ft). The spacing directly influenced the daily advance rate. Initially these supports had been installed in large overbreak areas adjacent to sandstone beds. As the tunnel advanced, rockbursts commenced, and Category 4 events caused major equipment damage to rockbolts, mesh and mining straps. The full ring steel supports, however, remained mostly intact even when dislodged. These elements remained rigid but certainly prevented more extensive damage to rock supports and equipment and, most importantly, provided a degree of protection to TBM personnel.

Challenge five — Severe rock bursts of May 31, 2015

The severe rockburst, referred to as the 5/31 event, occurred on TBM 696 (trailing TBM) at approximately 11:35 pm on May 31, 2015. The magnitude of the event was equivalent to a magnitude 2.4 earthquake on the Richter scale and consequent damages to the TBM, ancillary equipment and rock support were without precedent on the project. Figures 8a and 8b show the same location at the L2 zone of the TBM during normal operation and then after the 5/31 event.

The physical damage and losses were sudden and unforeseen and extensive. The rockburst occurred when the trailing TBM 696 was in mid-stroke. Visible damage was observed along the tunnel for 63 m (207 ft), with the most severe damage to the TBM, excavation profile and permanent rock support in a 22-m (72-ft) section some 28 to 50 m (92 to 164 ft) behind the shield. The maximum impact of the 5/31 event occurred at a tunnel location that was excavated 10 days earlier. The time lag between excavation and occurrence of the rockburst was highly unusual since rockbursts normally occurred in the region of the TBM cutterhead while excavating. The TBM was completely blocked by ejected material in two locations (Fig. 9a) and at these locations the whole TBM buckled and twisted 800 mm (31 in.) counterclockwise by the rockburst. Invert heave was evident throughout the 22 m (72 ft)-long most affected zone, with many of the steel ring beams sheared (Fig. 9b), displaced into the tunnel and lifted above the invert, along with the track and sleepers.

In some areas, the ring beams were also heaved out of position, causing massive secondary damage to the adjacent shotcrete.The severe rockbursts caused millions of dollars worth of equipment damage to a 60-m (197- ft) section of tunnel, as well as significant damage to the tunnel lining in the neighboring TBM 697 tunnel.

Once the area was deemed safe to enter and the recovery plan had been developed, work commenced on June 16, 2015. The most urgent activity was to start the debris removal from the top of the TBM to allow the replacement of tunnel rock support and also uncover the full extent of the rockburst zone. The exposed ground was then heavily supported with full ring steel supports, rockbolts and shotcrete. The whole recovery program took 7.5 months. Figure 9c shows the same location as Fig. 9a after the removal of ejected rock and the installation of heavy rock support, to a height of 8 m (26 ft) above the original excavated tunnel, prior to the May 31 event.

Challenge six — Height overburden and tunnel completion

The 5/31 event had occurred at an overburden of approximately 1,325 m (4,350 ft) and the maximum overburden of 1,870 m (6,135 ft) was still about 2 km (1.25 miles) ahead.The recommencement of the lead TBM 697 eight days after the 5/31 event was undertaken with systematic counter measures. The tunnel rock support had been reevaluated and a special rockburst support lining designed and implemented on a permanent basis. The general design is shown in Fig. 10a.

The design incorporated continuous full circular steel rings, rockbolts, heavy-duty wire mesh and systematic shotcrete installed at L1 with a final lining thickness of 350 mm (13.7 in.) being installed at L2. Both forward longitudinal and radial stress-relief holes were drilled in and around the sandstone beds encountered during excavation. All these precautionary measures and heavy tunnel support directly impacted monthly progress specific to TBM 696, which experienced the severe rockburst event. The event and the subsequent detailed investigation resulted in new TBM operational procedures aimed at predicting and investigating future similar geological situations. The most important of these measures was the adoption of probing through the side wall of the tunnel to detect parallel, hidden sandstone beds.

The use of the microseismic monitoring data and other site information enabled the TBM project team to optimize all aspects of precautionary measures and excavation operations and quickly start to increase monthly production up to 364 m/month (1,200 ft/m) (Fig. 10b).

The daily trend analysis and operations recording revealed that the microseismic activity and rockbursts did not continue to increase as the tunnels headed for the highest overburdens. In contrast, the overall microseismic activity started to decrease. Further investigations were undertaken to record the actual in situ stresses and within the actual rocks that first experienced the Category 4 rockburst and then, secondly, at varying overburdens to ascertain if the ground stresses were related to overburden. The details of these findings indicated an area of elevated horizontal stresses that were not related to overburden.

In the last 1 km (0.6 mile) of each TBM tunnel the occurrence of rockbursts reduced to virtually zero and progress increased accordingly. The lead TBM 697 broke through and connected with the dam site in October 2016 and the trailing TBM followed suit and broke through in May 2017.

Challenge seven — TBM overcutters

The excavation diameter of a TBM is the cutterhead width plus the protrusion of the gauge cutters. As the cutterhead turns and excavates the tunnel, the gauge cutters wear faster, leading to a reduction in the tunnel diameter. With a requirement for full circular steel rings and thicker shotcrete more space was required for these elements. Overcutting was achieved by extending the cutters located on the cutterhead periphery using shims to increase the effective width of the cut and by replacing the gauge cutters with large-diameter cutter wheels (from 431 mm to 457 mm of (17 to 18 in.). Both methods were employed to increase the tunnel diameter by 100 mm (4 in.). However, overcutters are normally deployed as a short-term measure to address a local problem. On this project they would need to be installed for a much longer period, close to 5 km (3.1 miles) or half of the proposed TBM alignment. The perceived problem with using the overcutters for a long period includes alignment control, both horizontal and vertical, and rapid and excess wear of the TBM shield components. Figure 11 shows the horizontal alignment for the lead TBM before and after overcutter installation. The graph shows the difficulty of maintaining the horizontal alignment within the expected tolerances after the overcutters were installed. The main points of deviation are numbered and the explanation is shown on the righthand side of the graph. Control of the vertical alignment (Fig. 12) experienced no such control issues, the difficulty being the horizontal alignment control. The second concern of excessive wear of the TBM shield was also monitored and additional wear was detected on the grill bars, but this did not prove to be a hindrance to the TBM operation.

Conclusions

This paper outlines seven significant challenges the TBM tunnels faced on the Neelum Jhelum Project, from delivering the TBMs to the site up to the unusual circumstances of the 5/31 rockburst. These tasks were made much more difficult by the remote location of the project site, making both changes and or modifications to the TBMs or delivery of new products or equipment all the more taxing.

The successful recommencement of tunneling and modification of operating procedures and subsequent tunnel completion was only made possible by the full support of the client and close collaborative work among the employer, contractor and engineer.