The project is located in the Muzaffarabad District of Azad Jammu & 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 3,200m above sea level. The project is a run-of-river scheme, employing 28.6km of headrace and 3.6km of tailrace tunnels that bypass a major loop in the river system, for a total static head gain of 420m. The headrace tunnels twin bores (69 per cent) contain the TBM excavation.
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 sub-perpendicular to the tunnels: the Main Boundary Thrust, and the subsidiary Muzaffarabad reverse/thrust fault. The Lithologies are detailed below;
¦ Siltstones & Silty Sandstones Uniaxial Compressive Strengths (UCS) is 50-70 MPa.
¦ Mudstones With UCSs in the 30 to 40 MPa range,
¦ Sandstones. With UCS in the range 130 to 230 MPa.
¦ In-Situ Stresses
Over-coring 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 sub-perpendicular to the tunnel azimuth.
GEOLOGICAL STRUCTURES AND CONDITIONS
In addition to the expected rock types detailed above, there are also certain ground conditions which are expected to be encountered and for which the TBM will be designed to manage. Also the TBM should have further facilities to detect these conditions in advance, in order to take appropriate measures to successfully negotiate the identified conditions. These structures and conditions included:
¦ Unstable rock zones.
¦ Squeezing and swelling ground
¦ Soft ground
¦ High water inflows.
¦ Extensive fault zones
¦ Rock bursts.
¦ High Over burden depths (1,870m)
TBM SELECTION
Some 20km of the headrace tunnel system was excavated by TBM in two parallel tunnels. Two open (gripper) TBMs, were used to excavate this tunnel system.
The type of TBM selected was taken after evaluating the most up to date TBM designs and more importantly successful application of TBM technology in same or similar ground conditions worldwide. For example, the successful application of TBMs in the similar ground conditions of the Gotthard Base tunnel in Switzerland; where very similar open (gripper) type TBMs excavated 85km of the rail tunnel system.
A significant consideration for the TBM selection was the possibility of encountering squeezing ground, with deformations of up to 500mm 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 is best suited to deal with this potential situation, due to the short length of the front shield and its ability to collapse various sections of the front shield inwards, depending upon ground conditions, and still maintain the ability to excavate forward.
The second ground condition which 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 same TBM manufacturer responsible for four of the TBMs used successfully on the Gotthard Base Tunnel was selected after tender evaluations to supply the two TBMs for the Neelum Jhelum project.
The designed tunnel excavation diameter is 8.5m diameter giving a total face area of 56.75m2. The tunnel gradient is ascending from SW to NE.
DELIVERY OF THE TWO TBMS
With the two TBMs procured and manufactured the first major challenge was the delivery of all the TBM and associated parts to the construction site. The two TBMs were manufactured in Germany and China and delivered to the port of Karachi in Pakistan. The manufacturer number was used for individual identification the two number being 696 and 697, The TBMs then were loaded onto road transport and travelled 1,777km to the construction site located in north-west Pakistan. The route used major road systems in the south and towards the end of the journey the route climbed into the lower Himalayas following cliff edge roads and passing through towns and villages. The figure, left, shows the route.
The enabling works, i.e., road strengthening and improvements and temporary bridge construction commenced over one year prior to the TBM delivery. There was also a key date in 2012 that the TBM should be delivered by the end of May, prior to the start of the summer monsoon.
INSTALLATIONS AND COMMISSIONING
The project prepared for the arrival of the TBMs with the construction two underground assembly chambers and launch tunnels with average length of 200m constructed from a 2.2km adit and located 1,200m below the ground surface. Each assembly chamber was equipped with a dual 200t/20t gantry crane with a clear lift height of 12m.
TBM POWER SUPPLY
The consequence of having two very 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 infrastructure to meet the TBM requirements. Therefore a complete 19.6MW power station had to be constructed on a hillside near to the TBM access adit. The power station consisted of four no. duty 4MW generators and one 3.6MW standby generator, powered by heavy fuel oil (HFO).
TBM AND INSTALLED EQUIPMENT
Cutterhead and front shield
The two TBMs had been selected on the basis of the good performance on similar recent projects. The cutterhead was equipped with 12 cutter head motors giving a total installed power of 4,200kW. The maximum torque at 6,7rpm was 5,675KN to cope with potential squeezing ground of up to 500mm.The front shield was designed to collapse to allow for the maximum ground squeezing of 500mm, leaving only the solid cutterhead.
Crown support equipment (L1)
Each TBM was equipped with the following equipment for support installation in the L1 location which was the 3.5m immediately behind the rear of the cutter head shield. Rock bolts were installed using twin rockdrills mounted upon a circular track system which gave a radial coverage for the top 260° of the tunnel periphery. Each drill could achieve 135° allowing for an over zone at the tunnel crown. The drills could install 3.85m rockbolts. The entire rockbolting assembly had the ability to move longitudinally in the tunnel by 3m, giving greater flexibility in support installation. Wire mesh was installed by way of a mesh erecting device which lifted the mesh into position on the tunnel circumference and then secured by way of rockbolts. In poor ground conditions the TBM could erect full circular steel rings using an erector installed in the TBM shield. Shotcrete could then be applied if required by way of a mobile shotcrete spray arm over the top 200° of the excavated tunnel.
Crown support equipment (L2)
60m behind the L1 section is the L2 section. In this zone a duplicate set of rockbolting equipment was installed, with a slightly reduced radial coverage due to the installation of rail track. Immediately behind the rockbolting equipment is the permanent shotcrete lining equipment, this consists of two spray robots each mounted upon a longitudinal track allowing a 7m travel distance laterally along the tunnel and a radial coverage of 270° of the tunnel crown. The two spray robots could also travel radially around the excavated tunnel one installed on the left and one installed on the right. The combined radial coverage was 270° in total, but each side could overlap the other at the tunnel crown by 15°.
Invert support equipment (Between L1 & L2)
Between the L1 and the L2 crown support equipment was a shotcrete spray robot was installed on a rail car system, which moved laterally along the tunnel 11m. This spray robot had the capability to install up to 100° of the excavated tunnel in the invert. This activity was combined with the installation of the tunnel railway.
Shotcrete supply equipment
Behind the L2 section was the shotcrete operation center. There was provision to operate two complete shotcrete delivery systems simultaneously. The left hand side installation could be used for both the L1 or L2 operations as required.
Ground investigation equipment
Immediately behind the L1 support equipment was installed two separate sets of advance probing equipment. The first installation was immediately behind the L1 support zone and consisted of a high performance drill which had the capability of travelling radially 360° in extreme requirements. However, the full 360° access required a detachable port of the radial track to be installed in the lower portion of the tunnel, which in normal tunnel operations would cause severe hindrance to access for materials and cleaning operations. The normal radial access was 270°. This drilling installation could be used for both advance probe drilling and pre-excavation grouting. The grout mixing and storage equipment was installed immediately behind the shotcrete equipment.
A further 10m behind the first probe equipment was a second probe drill equipped with an extra-long drill feed, this probe drill could not travel radially as per the first probe drill, but did have a small capability to move transversely and could drill probe holes over the top 20° of the excavated tunnel.
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 reach of the TBM shield, as the TBM advanced. The most favorable rock class being Q2 and the least favorable being 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 being shotcrete started at 125mm thickness and increase with class until in Q5 class where the thickness was 350mm.The balance of support installation and rock class is an area which still relies heavily on human intervention, skill and experience to balance support installation with safety and production.
Commencement of tunnel excavation
Both TBMs commenced the planned 11.4km twin headrace tunnel excavation in early 2013. Both TBM launches were in stages to allow the continuous conveyors to be installed after excavation of 100m. The first TBM to be fully installed and operational was TBM 697 and from early 2013 steady excavated and increased monthly production. The second TBM number 696 followed suit and progressed approximately 500m behind TBM 697. The alignment of the twin tunnels encountered a preconstructed access tunnel some 1,700m from the TBM launch location. This tunnel known as Adit 2 had been completed prior to the planned arrival of both TBMs.
Fault zone
However some 90m before this adit the lead TBM 697 encountered an extensive fault zone of sheared mudstone over 80m 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 and the TBM was stopped to access the situation. The TBM was then started and advance was attempted however, the ground being so soft flowed into the cutterhead and the cutterhead tripped electrically and stopped. A crew of tunnel workers was sent into the cutterhead to manually remove the buildup material this operation taking eight hours. A further three attempts were made to advance the TBM in the poor ground conditions with failure. It was then decided to stop any further attempts and to install a top heading over the TBM shield and install a pipe roof canopy 15m in front of the TBM and carry out stabilisation by way of ground treatment with grout and chemicals.
After nine weeks the TBM was started again and slowly advanced through the faulted ground installing full circular steel rings and 350mm 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 15m pipe canopy every 5m was able to progress the fault zone at a much reduced advance rate
Good progress
Both TBMs broke through and traversed adit 2 in January 2014 and entered into a period of good progress reaching 460m per month in a installing the full ranges 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 over burdens in the range of 1,150 to 1,350m and the only negative experience was the occurrence of a few rockbursts which resulted in damaged to some of the TBM equipment. ROCKBURST
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 4.7km and 4.3km 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 TBM during tunnel exavation was 1,695 and the figure above 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 no damage to the support or ejection of rock
¦ Category 2: Noise and weak rockburst – a popping sound is heard and there may be some light 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.
The figure above also shows that the majority of rockbursts are classified as Category 2, but even this category of rockburst was responsible for delays whilse repairs were under taken. The photo, left, shows the typical aftermath of a category 1 and category 3 rockburst at the front of the TBM ROCKBURTS 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 that the rock between 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 31 May 2015 severe rockburst was a local change in strike of the rock strata from perpendicular to the tunnel alignment to parallel. This hid the rockburst-prone sandstone beds behind siltstone beds. In order to detect future hidden sandstone beds, side probe holes were drilled at 5m 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 31 May 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 fiber-reinforced 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 per cent of rockbursts were detected at the front 10m of the TBM. Therefore, to effectively mitigate rockburst, most of the shotcrete had to be applied as soon as possible, i.e., at the L1 zone. Once the TBMs encountered regular rockbursts, up to 62 per cent of the shotcrete was installed at the L1 zone. Shotcrete is a versatile material that can be installed as a stiff element, as a closed-ring shotcrete lining or as a yielding element, e.g., isolated shotcrete panels. Shotcrete installation at the tunnel axis is particularly important, due to the high horizontal in-situ stresses present there, to cover support elements and prevent damage from the gripper shoes.
Full ring steel support
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 spacing’s ranging from 0.9 to 1.6m. 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 and serve damage to rockbolts, mesh, 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.
TBM over cutters
The excavation diameter of a TBM is the cutter head 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. Regular replacement of gauge cutters keeps the cutterhead diameter close to the original design. The remainder of the TBM including the back-up gantries is designed to fit within this diameter with a clearance envelope, with a requirement for full circular steel rings and thicker shotcrete more space 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 larger diameter cutter wheels (from 17 to 18 inches). Both methods were employed to increase the tunnel diameter by 100mm.
SEVERE ROCK BURTS OF 31 MAY 2015
The severe rockburst referred to as the 31 May event occurred on TBM 696 (trailing TBM) at approximately 11.35pm on 31 May 2015. The magnitude of the event and consequent damages to the TBM, ancillary equipment and rock support were without precedent on the project.
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 63m, with the most severe damage to the TBM, excavation profile and permanent rock support in a 22m section some 28 to 50m behind the shield. The maximum impact of the rockburst occurred at a tunnel location which was excavated some 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 cutter head often while excavating. The TBM was completely blocked by ejected material in two locations (see figure, top left – left portion), and at these locations the whole TBM had been buckled and twisted 800mm anticlockwise by the rockburst. Invert heave was evident throughout the 22m-long most affected zone, with many of the steel ring beams sheared, 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 (see figure, top left – right portion).This severe rockbursts was equivalent to a magnitude 2.4 earthquake on the Richter scale. This disabled one TBM 696 for seven and a half months, causing millions of dollars of equipment and severe damage to a 60m section of tunnel as well as significant damage to the tunnel lining in the neighbouring TBM 697 tunnel.
Further rockburst counter measures
The 5/31 event required further counter measures other than those mentioned previously. The two principle additional counter measures adopted were:
Tunnel separation – The separation between the twin TBM tunnels was increased. The lead TBM 697 alignment was changed on recommencement of excavation and over an 800m section gradually increased the tunnel separation from 25m to 47m. This was to reduce potential rockbursts interaction between the twin tunnels
Microseismic (MS) monitoring – This equipment can provide near real time indication of the possibility, intensity, and location of rockbursts, was installed on both TBMs
Microseismic monitoring
MS monitoring supplied by IRSM of China was installed in both TBMs with the aim to develop a proactive monitoring system for rockbursts. The MS equipment consists of various sensors installed close to face of the TBM and the recoded data transmitted to the surface and processed. The spatial organization of MS sensors on the network is the most influential factor for monitoring quality. The first sensor array is positioned about 10 – 30m away from the working face. This is a triaxial velocity sensor, the other two sensors array as are uniaxial velocity sensors.
MS monitoring techniques involving three-dimensional monitoring of MS events due to microcracking in rocks have been used for monitoring rockbursts for many years. The JDI software is advanced microseismic data visualization and analysis software package created by ISS. JDIs salient features include full threedimensional interaction, integrated event filtering and display, arbitrary surface of the three-dimensional and linear contours, diachronic analysis, and non-uniform rational B-spline surface, visualization of external spatial data digitization, distribution, and event dynamics. The warning of rock burst risk focuses on its location and intensity, not on the time of rock burst occurrence. After the monitored data is transmitted to the IRSM-MS team, the expression of rockburst warning results is:
Probability of extremely intense, intense, moderate, slight or no rockbursts. The probability ranged from 0 to 100 per cent. This information was incorporated into a daily report issued each morning before excavation commenced on each TBM. Additional information contained in the daily report was the time span of the previous day’s events and the graphical trend of number of events and average energy release per event. When the trailing TBM 696’s MS monitoring system overlapped with the lead TBM, a cross correlation for the same tunnel location was also included.
Towards the highest overburden and tunnel completion
The 31/5 event had occurred at an overburden of approximately 1,325m and the maximum overburden of 1,870m was still some 2km ahead, Thus the recommencement of the lead TBM 697 some eight days after the 31/5 event was undertaken with all precautions and counter measures as was the recommencement of the TBM which experienced the full force of the 31/5 event some seven and a half months later. 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 the figure on page 21.
The design incorporated continuous full circular steel rings, rockbolts, heavy duty wire mesh, systematic shotcrete installed at L1 with a final lining thickness of 350mm 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 upon monthly progress The use of the MS monitoring data and other site information enable the TBM project team to optimize all aspects of precautionary measures and excavation operations and as such quickly start to increase monthly production up to 364m/ month please refer to the figure, top right. The daily trend analysis and operations recording started to reveal that the MS activity and rockbursts occurrence did not continue to increase as the tunnels headed for the highest overburdens in contracts the overall microseismic activity started to decrease. Further investigations were undertaken to record the actual in-situ stresses and within the actual rocks which firstly experienced 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 which was not related to overburden. The results of the stress investigation and the twin TBM tunnel alignment are shown are shown in the figure immediately above. In the last 1km 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
