G.B.Nagendra. General Manager - Projects, Konkan Railway Corporation, gives a first-hand experience of the adverse geological conditions encountered during construction of Tunnel T5 of the Katra-Dharam Section of the USBRL Project and the solutions that were adopted.
Tunnel T5 of the Katra-Dharam Section of the Udhampur-Srinagar-Baramulla Rail Link Project in the Himalayan region of India witnessed several geotechnical challenges during the course of its construction. Most of the 6-km long tunnel was constructed using the New Austrian Tunneling Method (NATM). Several challenging ground conditions were encountered - from sheared rock mass to visco-elasto-plastic stratum. Water gushes were also encountered. This article presents two short portions of the Main Tunnel between CH 44+500 to 44+525 comprising of a complicated arrangement of a shear zone sandwiched between dolomite rock mass, fully charged with water, which repeatedly flooded the tunnel in September 2015 and between CH 48+654 to 48+636 where water-bearing sirbon dolomite was sandwiched with shale.
In the 272 kms long Udhampur-Srinagar-Baramulla Rail Link (USBRL) project, the 25 kms long Udhampur-Katra Section and 136 kms long stretch between Banihal to Baramulla are commissioned for traffic, while works are in various stages of progress in the balance length from Katra to Banihal (111 kms) which comprises of 97 kms of tunneling (Figure 1).
The alignment passes through the young Himalayas with tectonised zones including major thrusts and faults including the Main Boundary Thrust (MBT), locally called as Reasi Thrust near Katra and Murree Thrust near Dharam (Figure 2). MBT/Reasi Thrust which is about 450 meters thick in the Paikhad area has affected Tunnels 1,2,3 and 5 of the Katra-Dharam section. This Thrust zone is characterized by the presence of crushed dolomite, which when charged with water starts flowing in torrents.
Location and geology of Tunnel T 5
The tunnel is located in Reasi District, J&K, with the South Portal in village Gran and the North Portal in village Bakkal, which is on the left bank of river Chenab. From the geological point of view the tunnel is located in the Lesser Himalayan Zone. The middle and the south-western part of the tunnel area (Katra /P1 side) is characterized by the presence of Trikuta Formation, while in the North-Eastern sector (T5P2) outcrops of the Khairikot Formation are present, both belonging to the Sirban Group, which is non-fossiliferous and was formed in the Pre-Cambrian Era. Trikuta Formation mainly comprises of hard and thinly to thickly bedded dolomite with thin bands of slates. The dolomite is moderately to highly jointed, interspersed with shear zones, grey to light grey and greyish white and at places cherty and stromatolitic dolomite bands are present. In general, the dolomite of the Trikuta Formation falls under poor to fair category of rock. The Reasi Thrust crosses the alignment in the open area in front of the south portal T5 P1 and the tunnel is aligned sub-parallel to the MBT for nearly 1km from the South Portal.
From the north portal (i.e.Srinagar side), Khairikot Formation is encountered which is constituted by quartzites, dark grey variegated slates and dark grey to light grey dolomite/limestone bands. Over the Khairikot Formation, a patch of Subathu Formation represented by variegated shales/slates exists. The exposure of a thick band of shale Ch.48+000 appears to be pinched either on account of facies variations and /or phenomenon of pinching and swelling of shales. Hence in the zone of the study, shale is not encountered in the tunnel as a continuous band but as inter-bedded with Sirban dolomite. At the basal part of the formation a thick quartzite band is present. Khairikot Formation overlaps with a disconformity over the Trikuta Formation, and is involved in a syncline having the northern limb exposed around the north portal area. Syncline core is constituted by a meanly sheared dolomite/limestone and traces of shale. The sheared material is composed by rock flour with gouge and varies from dry to moist. General strike observed inside the formations ranges from N35W-S35E/N15W-S15E dipping 40°/50° north easterly at the upper limb, to N10W-S10E dipping 25°/35° north easterly at the lower limb of the syncline. The geological section of the tunnel is placed as (Figure 3).
Tunnel T5 is a mined tunnel (Figure 4) and comprises of a Main Tunnel (MT) for railway traffic, a parallel Escape Tunnel (ET) with 16 Cross Passages (CPs) for facilitating rescue of passengers in the unlikely event of an exigency. The finished areas of cross-section for the MT and ET are 40.5 sqm and 18.4 sqm respectively.
Case Study 1: Encountering Shear zone and water bursts
New Austrian Tunneling Method (NATM) was adopted in design and execution of the tunneling works. Progress of excavation from P1 side was severely hampered in stretches where highly jointed dolomite/shear zones were encountered. Ingress of water up to 70 litres per second (LPS) was experienced between CH 44+024 to Ch.44+074. The monthly progress in excavation in six months from January 2015 was 34m, 43m, 44m, 51m, 50m and 44m respectively. GDE Class C1 indicating moderately jointed dolomite appeared in July. Progress registered in July and August 2015 were 82 m and 101 m respectively, which raised hopes of maximising production in fair strata.
Summary of Ground conditions CH 44+490 to CH 44+520
CH 44+491 to 44+500: A shear zone with mass bound by gougy material, of about 1 m thickness ran diagonally from left SPL to right crown. The rock mass was characterized as Fair with RMR 42-45; the ground water condition was recorded as wet. The rock mass was assigned GDE support class C1 allowing for full face (FF) excavation by D&B and requiring 150mm shotcrete lining and systematic rock bolting all around. Systematic drainage was used at the crown and probe holes were drilled at the face (Figure 5).
CH 44+501 to 44+510: Beyond CH 44+501 and up to CH 44+510 the rock mass was characterized as Poor with RMR below 40 and the ground water condition was reported as dripping to flowing, mostly concentrated above the shear zone. The rock mass was assigned GDE class C2, still allowing FF excavation with mechanical excavation and local blasting but requiring 200mm shotcrete lining, lattice girders, light fore-poling and systematic rock bolting on the side walls. Systematic and spot drainage was used at the crown and probe holes were drilled at the face. In this 10m section, two cavities were formed on the right side (CH 44+503 & 44+508) above SPL generally associated with the shear zone but were dealt with effectively. Additional strengthening measures were carried out in the vicinity of the shear zone with consolidation grouting and providing of longer anchor bolts (Figure 6).
CH 44+510 to 44+520: From CH 44+510 and up to CH 44+520 the rock mass had deteriorated slightly more but still characterized as Poor with RMR below 40 and ground water conditions between dripping and flowing up to CH 44+518, concentrated mostly above the shear zone. The shear zone had gradually moved closer to the crown. The rock mass was assigned GDE support class C2 and excavation was taken up as top heading only with hydraulic hammer and local blasting on the right and lattice girders were replaced with steel ribs. Systematic drainage and spot drainage was used at the crown and face and probe holes were drilled at the face. The strengthening measures adopted for the previous section were continued in this section as well (Figure 7).
Water bursts at CH 44+520
Monday, 07.09.2015: At about 10:00 in the morning, during mucking operations, a heavy gushing of water, approximately 60 LPS, along with debris, occurred at the left SPL (heading) of the Main Tunnel at CH 44+520. A cavity of 0.6-0.75m dia was seen extending deep into the rock strata ahead of the face to a depth of about 40m almost like a shaft. As the gush stopped abruptly, it was decided to try and divert the water and backfill the cavity.
The shear band as mentioned above followed none of the common joint sets and passed diagonally from upper right crown to SPL in the left side and gradually shifted to the crown and left side crown area. The gouge in-filling in the shear band was generally impervious. No probe holes were drilled through this material fearing that drilling may aggravate this zone further and cause more problems. Judging from the previous 500m of tunneling where water seepage was somewhat similar, large ingress of water not anticipated. Photo 1 depicts the status on 07.09.2015.
Tuesday, 08.09.2015: 76 mm dia Drainage holes were systematically drilled in back chainages, a minimum of three holes were also drilled on the left crown slightly to the right of the existing cavity, one for grouting and the other two for drainage to divert the water. However, only a small portion of the water could be captured and diverted. The cavity was gradually filled by about 7:00 Hrs with about 30m3 of lean concrete. Water ingress was slowly subsiding.
Suddenly, between 11:30 Hrs and 13:00 Hrs, three water bursts occurred from the face at the left SPL, one after another, with a time interval of half an hour in between, due to sudden increases in hydrostatic pressure. The water jet reached a horizontal distance of 15-20m behind the face. Gushing and rushing water brought along with it colloidal to boulder sized rock fragments and backfill concrete. The debris was thrown more than 20m away and reached a height of about 1m at a distance of 10m behind the face. The colour of the water was light grey to greyish white. Photo 2 depicts the status on 08.09.2015.
Wednesday, 09.09.2015: Three outbursts of water under high pressure along with large quantities of debris occurred. (Photo 3). Some debris was removed to enable drainage of water to the tunnel portal by gravity.
Thursday, 10.09.2015: A foot trek was made to the top of the tunnel to observe the surface condition of the alignment above the face CH. 44+520. It was found located very near to Madi Khad crossing and the possibility that water might have migrated toward the tunnel through inter connecting sheared zones and slope induced fractures was deliberated. At 12:00 Hrs, during drilling operations for drainage holes, sporadic bursts of water mixed with debris continued, the elevation of the water seepage in the cavity rose and debris of crushed dolomite/gouge/concrete fragments spread up to CH 44+420. The discharge during the bursts was approximately 150 LPS. It was planned to insert three perforated drainage pipes of 200mm dia into the cavity but the attempt was not successful due to the force of the running water and debris brought with it.
Friday, 11.09.2015: As more debris got accumulated in the tunnel near the face, the water seepage was moving up and had reached the crown. As more loose material got dislodged from the shear zone behind the face and the debris inside the tunnel was blocking the water passage, the primary shotcrete lining was breached about 16m back at CH. 44+504 and the water gushed from the crown with tremendous force. The discharge was 180 LPS.
Saturday, 12.09.2015: By the midnight, the inflow of water from Ch. 44+504 brought such a huge amount of debris that reached almost up to the crown. (Photo 4). Discharge of water was approximately 600 LPS from the location, consequently seepage from the face at CH 44+520 had stopped.
Sunday, 13.09.2015: The overall discharge of water into the tunnel from the crown was not reduced but water was migrating towards back chainages through weak shear zones which was evident from the discharge through drainage holes. An outburst occurred at around 15:00Hrs after which the discharge stabilised at 300 LPS from CH 44+504.
Mechanical Behaviour and Concept of Treatment
In the dolomite of Trikuta formation of Sirban group, shear bands and seams are monotonously encountered in a group or in isolation and are the main cause of instability in T5P1 tunnel- basically caving and rock fall behavior. The extension of the zone is not always regular or linear and can take various forms of dilation and pinching. In fact, the cross section area of the present cavity in the sheared zone is less than 5.0m2, however it could be more in different locations depending on the rockmass quality. The rockmass of the zone suffers intensive deterioration during shearing whereby the rockmass is mylonitized, sugar-cubed, turned densely jointed. The situation of the sheared zone also provides very suitable venues for the subsurface water to accumulate and to serve as saturated zones. Weathering, chemical or physical or both occurs if the terrain is of limestone and dolomite. Generally the central part of the sheared zone is filled with thick whitish gouge which may act as an impervious layer. During chemical weathering whitish precipitates occur through the joints and leaves the rockmass degraded both in density and strength and turn the entire rockmass whitish in appearance. In the presence of high hydrostatic pressure, the heavily crushed material in the sheared zone behaves as a ravelling and flowing ground.
Hydraulic conductivity of this zone is high with Lugeon value greater than 75. The infilling in shear bands were impervious but the surrounding shear band contains high hydraulic conductivity. Under hydraulic pressure, the strata discharges itself together with the infilling as and when openings are created. RMR class at these chainages is almost IV (Poor). Support class is determined based on RMR as C2.
The nearest Cross Passage CP-4 was located at CH 44+331 and the location of the next CP would be at CH 44+600. The face of the Escape Tunnel was adjacent to the CP-4. The following were deliberated:
- Circumvention of the stretch between CH 44+504 to CH 44+540.
Figure 8: Option 1(a) to circumvent the Water Damaged ZoneFigure 9: Option 1(c) to circumvent the Water Damaged Zone by additional CPs at CH 44+400
- Diversion of water from shear zone to the Escape Tunnel vide a Dewatering Gallery to intercept the shear zone of Main Tunnel. (Figure 8). This would mean construction of nearly 275 m of Escape tunnel prior to commencement of excavation for the Gallery.
- Construction of an additional Cross Passage at Ch.44+440 to expedite (a) above.
- Construction of additional Cross Passages at CH.44+440 and CH 44+600 to expedite tackling of the Water Damaged Zone from both sides. (Figure:9)
- Construction of a By Pass section on the RHS of the Main Tunnel between CH 44+440 to CH 44+600. Face logs had clearly indicated moderately jointed dolomite on the RHS. But this would mean additional expenditure in forming a By Pass for 160 m length. (Figure:10)
& CH 44+600Figure 10: Option 1(d) to Bypass the Water Damaged Zone between CH 44+400 & CH 44+600
- To follow the straight-forward method of tackling the Water Disturbed Zone head-on by:
- stabilisation of the tunnel in the back chainages
- chasing of the seepage water
- sealing the shear zone
- strengthening the affected portion of the tunnel.
The following is the salient steps adopted in the treatment of WDZ
- Post grouting from CH 44+450 to CH 44+520. The objective was to improve the bearing capacity of the tunnel in the section and to divert water through it without causing any damage. Grouting was carried out with the following scheme:
- Diameter of holes (mm) : 45 – 51
- Length of holes (m) : 4
- Spacing of grouting holes (m): 1 both ways
- Maximum pressure (bars) : 10
- Starting Water Cement ratio : (1-0.70):1
- Setting Time (hours) : 24
- Top/mechanical packers required.
- One section grouting/injection.
- Strengthening of tunnel from CH 44+500 to Ch.44+517
- From 03.11.2015, the discharge from the crown at CH 44+504 reduced to around 150 LPS. The work of strengthening the above stretch was commenced. The following is the steps taken:
- Shoring with sand bags to maintain a heap of debris up to the crown at CH 44+504.
- Forepoling above SPL with 76 dia SDAs @300 mm spacing.
- High pressure one stage OPC horn grouting through SDAs to create an arch above the tunnel and also consolidate the debris at this section to prevent them from flowing.
- On 15.11.2015, the water discharge migrated to CH 44+517 as an early outlet was found. The opportunity was seized. Debris was heaped up to the crown at that location by wheel loaders and manually. A bulkhead abutment was created across the tunnel by sand bags to hold the heaped debris in place. The following were the steps taken:
Figure 11: Option (2) to treat the Water Damaged Zone between CH 44+450 & CH 44+520
- All steps at (i) above up to the vicinity of Ch.44+517.
- Provision of wire mesh 150mmx150mmx6 mm over the damaged shotcrete near CH 44+504.
- Providing of ISHB 150mmx75 mm ribs to profile @ 350 mm spacing from CH 44+500 to 506. (Photo: 5). Shotcreting the ribs and anchorage using 9 m long 32 mm dia SDAs radially at spacing of 1.5 m centres.
- Provision of drainage holes to allow the water to be discharged when the section at CH 44+517 – 520 is sealed.
- Backfilling with M30 grade shotcrete of the hole created by water at crown as adequate drainage holes were in place to facilitate drainage of water upon sealing of the section at CH 44+517 – 520.
- From 03.11.2015, the discharge from the crown at CH 44+504 reduced to around 150 LPS. The work of strengthening the above stretch was commenced. The following is the steps taken:
- Sealing the shear zone between CH 44+517 to CH 44+520
- Installation of a 32 mm dia SDA with perforation to target injection in the shear zone and with anchorage of atleast one metre in firm rock above the shear zone.
- SDA perforation configuration: 3 mm dia holes with 150 mm spacing with only one hole in each section of the SDA.
- Injection of two-component polyurethane material through the SDAs. The chemical was a low viscosity, hydrophobic resin that generated a flexible foam after reaction, which provided excellent adhesion in both dry and wet condition. The MDI (Methylene Diphenyl Isocyanate) based resin was free of solvents, VOC, CFC, Halogen, and Phthalates with a foaming time of less than a minute. The Foam Factor was specified between 6 and 20.
- Injection packing type: Top mechanical packer
- Injection type: Single stage, non-stop injection for normal intake up to a pressure of 10 bars and lasting for further 5 minutes.
- Repeating the above steps till complete sealing is achieved. (Photo:6)
- Strengthening of the portion between CH 44+517 to CH 44+520 and beyond till end of shear zone
- Checking of previously drilled drainage holes and providing of drainage holes ahead of the face.
- Installation of overlapping 6 m long 76 mm dia SDAs spaced at 300 mm centres as forepoling umbrella. Grouting the SDAs at high pressure up to 60 bars to create an arch above the tunnel and also to consolidate loose debris.
- Injection of two-component polyurethane as in (c) above in the sheared mass till 1 m above the crown.
- Back filling of voids in back chainages with M30 grade shotcrete followed by cement grouting.
3D Monitoring (3DM) using Bi-Reflex Targets (BRT) and Total Station was done. Convergence Chords (CC), Convergence Rates (CR), Displacements in X, Y, and Z directions, and Displacement Vectors (DV) were measured for assessment of post-excavation behaviour of the surrounding ground of the tunnel. There was practically no convergence either in the longitudinal, transversal or vertical directions.
Treatment of shear zones accompanied by huge ingress of water is complicated. The shear zones are neither regular in size and shape nor consistent w.r.t to in-fillings. The crushed material is in a high state of compression till it finds a release by way of an underground excavation. In terms of RMR, the ground of shear zone was classified as VP (V) to NIL rating. The ground behaved as a continuum medium, with behavioural category of fully plastic prior to treatment. Main concept behind the treatment was to block the ingress of water by coalescing the cohesionless loose debris to form a barrier around the tunnel so as to divert it back to its regular channel. This was evidenced by a small gush of water encountered on the RHS of CH 44+500 while excavation for lowering the bench. The two-component polyurethane was the most appropriate choice to divert the water through to the drainage holes and eventually to the natural channel above and below the tunnel. The fact that there is no recurrence of water bursts in the area and also there is stabilisation of movements recorded by 3 D Monitoring at well below threshold limits prove that the measures undertaken were not only effective but economical.
Case Study 2: Tunnel collapse between Ch.48+654 to 48+636 in Main Tunnel due to presence of shale between thinly jointed Sirbon Dolomite
In the initial 202m stretch from the north portal at Ch.48+937.5 the Main tunnel was excavated in cherty dolomite, interbedded with quartzite, shale. Primary supports comprised of rigid steel ribs and backfill concrete. In addition, 25 mm dia SN Bolts were provided much subsequently, based on the site conditions. NATM was adopted in the subsequent tunneling. From Ch.48+735 to Ch. 48+692 where thinly jointed cherty dolomite, filled with gougy material and interbedded shale bands was continued to be encountered, the strata was generally classified as poor. The strata from Ch.48+692 to 48+670 was classified in Class III to IV of “Q” system of classification i.e., fair to poor rock, whereas the strata from Ch. 48+670 to 48+654 was classified as very poor. The cross-cut relationship of the joint planes have contributed to cavity formation due to wedge failures at six locations from Ch.48+702 to 48+654.
The work of excavation of Escape Tunnel was commenced from the north portal at RD 5976 in the first week of February 2015. A location of RD 5685 which is equivalent to Ch. 48+654 was reached on 06.09.2015. In the drive direction, the strata from RD 5685 had comprised of highly jointed dolomite interbedded with shale bands. Seepage water was flowing with discharge upto 1200 LPM. As the tunnel was in a falling gradient from the north portal, the seepage water from the tunnel has to be drained out of the tunnel by pumping. In spite of the geology being the same as the Main Tunnel, the progress in the Escape tunnel was relatively better owing to mitigation of adverse rock mass behaviour due to the scale effect.
Challenges in the stretch
Tunnel excavation was carried out in strata comprising of crushed dolomite interbedded with shale. Reversal of dips in the strata was observed both along and across the drive direction from Ch. 48+650 onwards. (Figs. 15 & 16). The same is a consequence of highly folded and highly jointed nature of the rock mass. The progress of work was very slow that as many as 33 days were required to negotiate the stretch of 14 m in heading between Ch. 48+650 to Ch. 48+636.
Dripping to moderate seepage up to 30 lpm further compounded the difficulty in the excavation as the shale mass was prone to be rendered to saturated condition, thereby providing opportunity for accumulation of load directly on the primary supports. With a view to arrest seepage and thereby to prevent the exposed shale strata from getting saturated, two-component Polyurethane Grout was injected at 48+638.5 and 48+640.15 on 15.10.2015 and 17.10.2015 respectively. In all, approximately 400 kg of PU was injected. Seepage was arrested but the exposed surface remained moist.
Support measures of Class F i.e., ISHB 150 @ 500 mm c/c, M30 grade SFRS and rockbolts were provided in the stretch. (Photos 8 & 9).
On 19.10.2015, in the course of excavation at Ch. 48+636.75 cracks in shotcrete began to appear on the LHS, one metre behind the tunnel face. Increase in seepage from moist to 24 lpm was noticed from the RHS, leading to dripping from crown on 20.10.2015 and dripping from the tunnel face upto Ch. 48+642.5, which was hitherto dry. Chunks of shotcrete got detached on 20.10.2015 as the face was excavated up to 48+636. Simultaneously seepage increased in a stretch between48+660 to 48+636. Suddenly on 20.10.2015, at 02:00 hrs the cracks were formed on walls and crown from Ch.48+638 to Ch.48+650. Also at 06:00 hrs, a rib began to yield due to pressure from face and crown (bent at joint) at Ch.48+642. At 08:00 hrs, deformation of ribs erected between Ch.48+655 to Ch.48+637.50 was observed. At around 14:30 hrs, stresses at the crown at Ch.48+642 got released rupturing the crown and the roof collapsed along with 76 dia SDA fore-poling umbrella. Water streamed down the cavity for around 15 minutes along with gouge material consisting of shale, clay and slate closing the face and formation of a cavity of approximately 50 cum in volume. The face at Ch. 48+636 was packed with sand bags and the cavity at Ch.48+642 was attended to. The seepage was continuous from a few of the systematically provided drainage holes between Ch.48+700 to Ch, 48+650.
Behavioral mechanism of rock mass of Main Tunnel between Ch.48+660 to 48+636
Strata encountered between from Ch.48+660 to 48+630 was highly jointed dolomite interbedded with shale and clay. In such strata, failure mechanism will be as rock matrix failure or due to deformation mechanism. Shale contains minerals which cause squeezing/swelling behaviour depending on the stress condition and size of the openings and in some cases resulting in non-uniform deformations. Squeezing is a time dependent behaviour as a result of visco-plastic behaviour of the rock mass. Theory of pressure - dependent elastic moduli or bifurcation theory indicates that the failure will be initiated within the side walls and subsequently develop into deformation failures. Saturated shale, clay and rocks containing considerable percentage of mica including muscovite and/or biotite are materials with high potential for squeezing/swelling, which manifest over a long period of time. Seepage of water through the dolomite was observed. With the driving of tunnel from Ch.48+660, the proportion of shale increased on the RHS (increasing chainage) while on the LHS, highly fractured dolomite with dripping of water was encountered. The presence of saturated shale and clay reduced the bearing capacity of the rock mass and caused non-uniform deformation of rock-mass. Further, shale and clay are impervious but when they are formed in the vicinity of brittle rocks like dolomite containing water, they behave like visco-plastic materials, and in this case, the accumulated load got applied on the supports much earlier than the time-dependent action like creep.
Mitigation measures proposed by the Detailed Design Consultants were with a view to provide support systems to maintain load bearing capacity of the support measures against the passive pressures that had built up around the tunnel and also to prevent the surrounding medium from developing squeezing.
Proposed measures for mitigation and treatment of the above mentioned zone were:
- Drilling of drainage holes
- Pre-grouting with 9 m length SDAs to strengthen the strata
- Providing Yield Support measures i.e., yield/sliding steel ribs with T-H or equivalent profiles failing which installation of ISHBs150 with 350 mm spacing from CH48+655inside the primary supports.
- Providing of SFRS with 350 mm at walls and crown from CH48+660
- Installing long length radial D-bolts or equivalent
- Fore-poling with 9 m long 76 mm dia SDAs perforated at CH48+644 to protect the crown
- Post-grouting at CH48+660-636 after completion of mitigation at the squeezing zone
- Horn OPC pre-grouting at CH48+644 to consolidate the rock fragments at crown
- PU-2C injection at sections of even dripping condition to prevent the shale from saturation.
- Drilling of drainage holes immediately after PU injection
- Providing yield shotcrete from Ch.48+636,
- Precise monitoring and regular checking of the ground pressure and the load applied to the ribs to quantify the squeezing pressure that build up against the primary support measures.
All the above measures for drainage and strata grouting were taken up. T-H profiles and D-Bolts were not available at site and hence ISHB 150 sections were provided inside the primary supports. By 06.11.2015, all the works in the back – chainages were completed. The stage was set to open the plug at Ch.48+636 and resume the duel with the strata of fractured dolomite interbedded with saturated shale in moderate condition of seepage.
In the same period, the Escape Tunnel work was progressing relatively well. As on 31.10.2015 the face of Escape Tunnel was at RD 5594 i.e. equivalent Ch. 48+554.5. The shale bands had disappeared from RD 5633 (MT Ch 48+588) onwards. On 06.11.2015, the face of Escape Tunnel was at RD 5631. A ray of hope was seen with the disappearance of shale and clay in the limited stretch of 2 metres. Geological Reports had also indicated that shale would be terminated at the disconformity with the quartzite band expected to be encountered at Ch.48+200. However, as the lenses of shale and clay had appeared much in advance it was felt that the zone could also disappear much earlier than at the previously estimated location. Probe hole investigation was ordered from Escape Tunnel RD 5633 towards Main Tunnel. Probe holes revealed cherty dolomite and more importantly, no trace of shale or clay. The crew were ordered to go for an additional Cross Passage from RD 5633 to emerge at Ch.48+588 in the Main Tunnel and thereby by-pass the cavity. It was a leap of faith moment considering the fact that data available was too small and shale could easily re-emerge. The construction crew reposed full faith in the decision and embarked upon the task from 07.11.2015. The rest is history.
The 25.7 m long cross passage CP 15 was completed on 21.11.2015 and thus the by-pass maneuver was completed. Thereafter, excavation of the Main Tunnel in the direction of south Portal was commenced from 25.11.2015 after making all necessary arrangements. A progress of 8.5 m in heading was achieved in the remaining six days of Nov. 2015. Progress in heading was 55 m, 40 m, 56 m, in the months of December 2015, January 2016 and February 2016 respectively. In March’16, a progress of 102m in heading and 33 m in benching was registered. It was followed by a progress of 107 m in full face in April’16. ( Figure 18). As a result of the by-pass maneuver, at least one year was saved.
With the advancement of face, seepage had disappeared in zone from Ch.48+650 to Ch.48+588 as all seepage water was intercepted in the advancing face. As a consequence, Yield Supports viz, T-H Sections, Lining Stress Controllers and D-Bolts were not required to be provided in the hazard zone. Similarly, there was no cause to inject any further quantity of the two-component Polyurethane grout to prevent saturation of shales. Hence, trapping of seepage water by natural means at the face of the tunnel was a significant contributing factor in tackling of the stretch in terms of time and expenditure. The section between Ch.48+650 to Ch.48+588 was made through on 16.02.2016 adopting “F” Class supports.
- Problems due to occurrence of shales are largely unsolved since the in-situ behaviour cannot be replicated in the laboratories.
- In strata comprising of materials with potential for squeezing, yield support measures must be utilized to allow the surrounding rock mass of the tunnel to release the stresses gradually, to prevent the medium from accumulating and building up stresses against the tunnel’s support measures. Built up stresses against the support measures can cause sudden stress release, which may develop in failure.
- However, when isolated bands of squeezing material leading to mild squeezing is encountered, it is prudent to increase the deformation tolerance, erect suitably designed lattice girders, provide a sealing SFRS and allow for deformation. Further support measures can be provided after deformation stabilizes.
- In squeezing strata accompanied by ingress of seepage water, it is imperative to prevent saturation of material with potential for squeezing.
- It is essential to monitor the ground pressure and the load applied to the primary supports to quantify the squeezing pressure that get built up.
- No stone to be left unturned in looking for a solution. Out-of-the box thinking is essential in overcoming tough challenges.
- GDE Report No.U03040/XR0135/0313 /14.09.2015 issued by Geodata Engineering.
- GDE Report No.U03040/XR0136/0314 /14.09.2015 issued by Geodata Engineering.
- GDE Report No.U03040/XR0140/0318 /01.10.2015 issued by Geodata Engineering.
- GDE Report No.U03040/XR0140/0325 /19.11.2015 issued by Geodata Engineering.
- T5P1 - Technical Advice and Summary GDE: U03040/XR0149/0327/20.11.2015
- Comprehensive Rock Engineering- Principles, Projects & Projects, Vol,-4: John.A.Hudson