Tunnelling through Coal final_EGCON-2023.ppt

This is a presentation of successful excavation of Head Race Tunnel of Rangit-IV HE Project, Sikkim

1. Successful excavation of HRT in Methane
bearing Coaly Strata -
A case of Rangit-IV HEP, Sikkim
1. R. N. Sahoo
Senior Manager (Geology)
2. Pankaj Rawat
Deputy Manager (Geology)
Jal Power Corporation Limited
(A wholly Owned Subsidiary of NHPC Limited)
EGCON-2023
3. Saurabh Singh
Senior Manager (Civil)
4. Satya Priya Nayan
Asstt. Manager (Civil)

2. Video clip of Tunnel fire explosion

3. Project at a glance
Sikkim
120MW Rangit-IV HEP
• 44m high Dam, L-142.4m
• 6.5km HRT (6.4mΦ, mod. horse-shoe)
• Surface Power House (3x40MW)
• Project was stalled since 2013
• Entrusted to NHPC on 31/03/2021
• HRT balance length- 2696.4m
• Achieved till Oct’23- 1712.6m

4. Geology along HRT
• Gondwana Group of Rocks-Sandstone, Carbonaceous shale and Coaly Shale/Coal bands.
• Mixed Ground condition with varying strength & competence- Low cohesive Coal.
• Structural warping, pinch-swell structures, lensoid bodies, drag folds & associated faults.
• Long persistent bands- strike parallel to tunnel drive in majority length.
• Vertical cover- 300 to 955m above tunnel grade (Face-2: 6 VZs & Face-3: 5 VZs)

5. Typical Structures in Tunneling media (Face-2)
RD2362m
Fractured
Carb. Shale
Coal coated Sandstone
(Plunging Anticline)
RD2282m
Dragging & Lensoid bodies
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f o l d i n g
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RD2542m
Fractured
Sandstone
Structurally
Disturbed
Coaly Shale
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RD2255m

6. Beautiful Features but Ugly Rendering
RD2110
A typical Face Log (RD2370.5m)
RD2387RD2135
RD2114
RD2021
RD2052

7. Distress Zone/Squeezing Ground Condition-RD2173-2193m
• A Lineament feature - Feeder channel to Reshi Nala
• Sub- Parallel to HRT alignment in this stretch
• Vertical cover-290m, Lateral distance from tunnel wall-29m
• Conducive set-up for possible encounterance of adverse Geological conditions

8. Distress Zone/Squeezing Ground Condition-RD2173-2193m
• High Squeezing Ground + Popping sound
• Max. 1.8m at R/wall (Rad. sqz- 32.8-49.3%)
• Distortion/twisting of steel Ribs/spreaders
• Aided with High seepage ~135LPM
• Led to >1D cavity at Right haunch
• Provisional H/V struts/pipes, 76/102Φ PRH
• Box-Channels: Cement-Bentonite Fill Grout
• Addl. Ribs in between existing Ribs & fusion
of LGs (Flexible NATM) full encased in SFRS

9. Geological Face logs of RD2179.4m (Distress Zone)
Geological Face logs Face Observation sheet

10. Encounterance of Inflammable Gases (Mainly Methane)
• Emanation of gas bubbles with gurgle sound
~4000-10000ppm/ proportional to Coal thickness
• First incidence of sudden expulsion led to fire
incidence in HRT face-2 (RD2252m) -31
st
Oct’22
• Stoppage of welding & ignition-fireworks/ Ribs
• Complete adoption of LGs with nut-bolt system
• V-cut holes (6-8m long) above human height
•Enhancement of Tunnel Ventilation system
• Regular monitoring of Gas at Crown & SPL
Downward inclined
V-cut holes
Booster
Exhaust
MVD

11. Methane gas concentration being coherent with disposed Coal % at tunnel face
– as depicted dashed boxes. However, commensurate proportional variations
do also exist, implying presence of Coal & its effect in the rear zone of face.
Concentration of Inflammable gases (Methane) vs Coal % at Face

12. Counteractive Methodologies being adopted for Coaly strata
Obtuse Face
Multi-segmental excavation & Support
LGs + Forepoles (32Φ bars & SDAs)
• Provisional 50mm Shotcrete on face
• Obtuse face method to act as toe support for
unstable crown, location w.r.t. vulnerability
•Multi-segmental excv. by light-charge blast-
cum-mechanical means + sequential supports
• LGs with modified positioning of Forepoles
w.r.t. Coal/Coaly Shale band disposition
• SDAs as forepoles as well as for pre-lining
consolidation Grouting

13. Steady Progress till Oct’23
• Cumulative steady progress of ~85m/month in HRT despite 70% Class-IV/V & 30% Class-III
Face-2
Monthly Progress
Face-3
Monthly Progress

14. Our USP of Successful tunnelling

15. It doesn’t matter how much you do.
What really matters is how you do.
Namaskaram
&
Thank You

16. Site instruction for Rehabilitation/treatment of Distress zone

17. HRT Adverse Geology beyond DPR speculations/stipulations
• Max. thickness of Coal bands < 1m
• Sandstones-hard, compact, Mod. Jointed
• Thickness of Shear zones not mentioned
• Full-face method of excavation was
envisaged for HRT.
• Seepage was anticipated to be very less
i.e.<10LPM contrary to Nala crossings
above HRT alignment.
• No speculations about encounterance of
methane or any other harmful gases.
• No mentioning of Tunnel behaviour i.e.
mix-ground conditions or mixed-face
conditions in HRT vis-à-vis its effect in
RMR computation. No mentioning of
large persistent Lensoid bodies.
• Low cohesive Coal bands ~1-7m thick
• Sandstones- Fractured & Friable at places
• >150mm Sheared lithocontacts (max. 1m)
• Due to very poor rockmass in Face-2,
Heading-Benching method is adopted.
• Heavy ingress of seepage of the order of
~125LPM (max. 260LPM) in HRT during
monsoon season.
• Harmful gases (Methane) of the order of
~4000-10000ppm encountered/proportional to
Coal band thickness at face/proximity.
• Strata with varying strength & competence
(Mix-ground condition), Localized warping
(Mixed-face condition), Drag fold & associated
faults & Large persistent Lensoid bodies
parallel to strike.

18. Adverse Geology Incidents beyond DPR speculations
Incidences, considered for tagging as Geological Surprises:
• Multi-stage loosefalls/Cavity formation (>0.5D, Extent 3m to Max. 12.4m b/w
RD2324-2332m) due to Low Cohesive & >1m thick Coal/Coaly Shale band
disposition–Total 24 incidences, Delay-239 days
• Distress zones/High Squeezing ground conditions >200mm (>5% @IS15026-2002)-
(6%-49%, Max. Radial Sqz.–1.8m at RD2173m)-Total 6 incidences, Delay-38 days
• Encounterance of Inflammable/Harmful gases (>5000ppm as per IS4756-2002)-
Total 120 incidences, Cumulative delay- 325 hours (14 days)
• Small event of Spalling (~1-1.5cum), But, L-T Eff. with Major Equipment Damage
• Excessive ambient temperature/heat coupled with higher humidity (85-90%) despite
moderate vertical cover ~300-820m inside the tunnel- Affected working efficiency
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• Dam R8 block- High permeable strata ~60-70 Lugeon & High Grout Intake, so delay
• Power House- Necessitated Consolidation grouting of Foundation against
Differential settlement due to soft rocks-shale & coal.
• In addition to that, total 30 no. of cavities of various sizes were encountered in HRT
by previous developer (JPCL) also - Cumulative delay of 15.5 months