Repair Strategy of Stoplog Sill Beams During Operation Stage of 510 MW Teesta-V Power Station in Sikkim

Virender Salman, B.E. (Civil) General Manager (Civil), NHPC Office Complex, Faridabad,
Prof. Achintya, (Corresponding Author) B.E. (Hons.), M. Tech., Ph. D. (Engg.) Professor of Civil Engineering and Principal, Darbhanga College of Engineering, Mabbi, Darbhanga

Running Power Stations serve as a boom to the economy of the country and any generation loss due to construction faults or damages during the generation results in the colossal loss of the revenues of the company and also tells upon the image of the company as the stakeholders/residents in the vicinity of the Project area have to bear the consequences.

In the present case, the Power Station is situated in Sikkim and after commissioning of the Project in the year 2008, the damages to the glacis / gate structures had developed to such an extent that the Project began to lose the revenue due to gradual decline in the generation and the leakages were so heavy that it was impossible to go ahead with the regular maintenance works. This also proved a threat to the life of the dam as well as to the local residents. The paper reviews the repair strategy adopted for the repair of the dam glacis, sill beam of stop logs/radial gate of the Dam.

Teesta-V Hydro Power Project is the second Power Station commissioned in the Himalayan state of Sikkim by NHPC Ltd. with the start of construction in March 2001 and completed in the time period of seven years. The Project was completed with a total cost of Rs. 2812 crores with Annual generation of 2573 MU (90% dependable year). Teesta-V Power Station (3x170 MW) is a run of the river scheme with diurnal storage to harness the hydro power potential of river Teesta for peaking during the lean season and is located in the state of Sikkim.

The generation target could be achieved in the financial year (FY) 2009-10 and 2010-11 only but was reduced considerably with every financial year. The leakage from the radial gates of the dam was to the tune of around 30 cumecs due to which the annual generation of the FY 2012-13 touched to 2268 MU against annual generation target of 2573 MU. The major issue of loss of generation was leakage from surge shaft area and from the dam gates. The same were attended during the FY 2012-13 and 2013-14.

Features of the Project
The Project is located at a latitude 27° 15’ 00” N and longitude 88° 30’ 20” E in the state of Sikkim. Location Plan and Plan Layout of Teesta Stage V are shown in Fig. 1 and Fig. 2 respectively.

Location Plan of Teesta River

Fig. 3 reflects the schematic diagram showing development of Teesta River along with total Hydro Potential. The forest land and the private land acquired were 147.42 ha and 197.31 ha for the project. The catchment area is 4307 sq km. A concrete gravity dam of 95 m height was built. There are three desilting chambers of size 294(L) X 21(W) X 30(H). A detail of plan of dam complex is shown in Figures 4. Photographs of intake along with dam complex are shown Fig. 5 and Fig. 6.

Schematic Diagram showing development of Teesta River along with total Hydro Potential

Dam ComplexFigure 5: Dam Complex

Major Repairs in Dam (Teesta – V) Power Station
The generation target for the FY 2012-13 for Teesta – V power station has not been achieved since July 2012 due to constraints of damaged spillway, stop log and radial gates sill beams. The repair of the gate was undertaken from October 2012 onwards, i.e. before the completion of monsoon period and the inspection of the gates for leakage was done during the monsoon period itself. On the detailed inspection of gates after putting dam stop log gate in the bays in the month of October 2012, it was observed that the stop log sill beam along with radial gate sill beam of all the gates got damaged but major damage was in gate in bay 2, 3 and 4 as most of the water leaks through these middle gates. This is due to the major erosion of the sill beam in the middle bays. The damage has occurred to the sill beams and to the lining due to movement of heavy silt and boulder/logs during the monsoon & other type of debris in the river.

Intake & Dam ReservoirFigure 6: Intake & Dam Reservoir

As all the gates were leaking and the power station having only two set of stop log gates, so just at the commencement of lean season, the gate no.01 was already repaired and the repair of gate no. 02 & 03 were undertaken. The heavy seepage in the gates even after putting in position of stop log gate in Bay No. 02 & 03 due to damaged sill beam of stop log resulted in lots of difficulties while replacing of the sill beam of radial gate and concreting of the area.

After the detailed discussions with the corporate design office and the visit to the site by design officials, the revised and detailed drawings of stop log sill beam as well as radial gate sill beam were issued by Hydro Mechanical (HM) Design, Corporate Office. Accordingly the power station procured the sill beam by November 2013.

As it is proposed to take up major repairs of the Dam during the lean season, i.e. from November 2013 to February 2014, so detailed planning was required to be done in coordination with Design Division and Operation and Maintenance (O&M) Division at the Corporate office for taking up the repairs

Proposed Scope of Work to be Taken Up From Dec. 13 to Feb. 14.
Stage 1
  1. It was proposed to take the repair of stop log and radial sill beam along with concreting using high strength concrete of grade M70 in the spillway at the first stage
  2. Along with repair of the stop log/replacement of the bay 3 and 4, it was proposed to change the gate seal totally of the radial gate 3 and 4.
  3. Checking of the power pack & change of hydraulic oil.
  4. Concreting and strengthening of pier in bay 3 and 4, i.e., pier P8, P7, P6 and P5.
Targets for the completion of these works were one month for replacement of sill beam of radial and stop log bay no.3 and 4 and two months for concreting of Pier/Spillway.

Stage 2 (taken if time permits)
  1. Repair of stop log and radial sill beam in gate No.05 along with high strength concrete of grade M70 in the spillway.
  2. Repair/Replacement the seal of Radial Gate 5.
  3. Checking of the power pack and change of hydraulic oil.
  4. Concreting and strengthening of piers in bay 5, i.e., pier P9 and P10.
Targets for the completion of such works were proposed to be identical as that of Stage 1.

Stage 3 (Proposed to be taken during December 2014 to February 2015)
  1. The proposed work would be taken care of during lean season.
  2. It was proposed to take the repair of stop log and radial sill beam along with high strength concrete of grade M70 in the spillway in gate no. 01 and 02.
  3. Repair / replacement of seal radial gate 1 and 2.
  4. Checking of the power pack and change of hydraulic oil.
  5. Concreting and strengthening of piers in bay 1 and 2, i.e. pier P1, P2, P3 and P4.
Targets for the completion of such works were one month for replacement of sill beam (Radial and Stop log Bay No.1 and 2) and three months for concreting of Pier/Spillway.

Method of Execution

Execution for Stage 1 and 2

The power station was proposed to be shut down for a minimum complete period of one Month. The work was to be taken from 1 December 2013 to 31 December, 2013 and if required, could be extended in the month of January 2014 for undertaking the work in bay 5. The depletion of the Reservoir had to be started after taking shutdown as per the depletion Reservoir manual. Road was to be constructed from intake to the reservoir in the month of October 2013 and after depletion of reservoir, i.e., EL 583m to MDDL level 568m (detailed estimate was in preparation). The work would be taken up to the top of the reservoir level by the end of October 2014 and completed by the engagement of excavator / loader after depletion. The waterway for the running river water would be allowed through bay no. 1 and 2, so accordingly coffer dam will be made to isolate the bay no.3, 4 and 5 in the first stage from EL 568m to EL 548m.

Isolation of bay 5 from 3 and 4 and excavation of pit up to depth of 538m to stop log was made. Dewatering of pit, alignment, anchoring & concreting of sill beam of stop log was carried out. Concreting of stop log area, spillway area, radial sill beam and piers was done with high strength concrete grouting. Putting in position of stop log and checking up of stop log sill beam/stop log gate was accomplished precisely. Miscellaneous repairing of stop log gate as per requirement and change of seal of stop log and radial gate was also carried out.

Execution for Stage 3
The stage 3 execution was planned to be undertaken during December 2014 to January 2015. The power station was proposed to be shut down for a minimum period of one Month in the month of December, 2014 to January, 2015. Depletion of the reservoir as per the manual was made. Platform was constructed at EL 542.5m on the rock ledge protruding on R/bank with the shifting of materials and manpower by hydraulic / crane / lifting trolley from non-over flow Block R-1 & R-2 top of the dam). The excavator was shifted to the platform through 40 T crane. Construction of coffer dam with steel plates, gunny bags, river bed material and clay bags was completed. Isolation of bay 1 and 2 from the river flow with the help of dewatering pumps and river flow was maintained through Bay 3, 4 and 5. The area of sill beam with the help of excavator, pavement breaker, manual labour, etc. was cleaned. Shifting of the sill beam, concreting and other miscellaneous work from top of dam to the platform at EL 542.5m by crane, chain pulley blocks / winch arrangements, etc. was executed successfully. Erection and concreting of the stop log sill beam and the radial gate sill beam was carried out. Further, concreting of spillways and piers was done with high strength concrete grouting. Putting in position of stop log and checking up of stop log sill beam/stop log gate was performed with minute precision. Miscellaneous repairs of stop log gate and change of seal and painting of radial gates were them accomplished as per requirement.

Proposed Methodology for Repair
Proposed Methodology for repair of spillway stop log sill beams area and High Performance Concrete (HPC) in glacis of bay no. 3 and 4 are described in the subsequent section.

Major Activities
The following major activies were taken up and accomplished:
  1. Emptying the reservoir up to the level of EL 540m.
  2. Making fishnet gabion bund having approximate weight of 1.20 T in front of bay no. 3 and 4 with the help of tower crane to be erected on the dam top at pier no. 6 / 7. (Exact location to be given by the design division).
  3. Excavation of silt entrapped between fishnet gabion and stop log and positioning of dewatering pumps.
  4. Dismantling of old concrete and removal of segments of leftover stop log sill beam at bay no.3 and 4 with pavement breakers.
  5. Drilling and anchoring of sill beam/holdfast/alignment/reinforcement and fixing, etc. and positioning of sill beam with welding and anchoring with hilti compound.
  6. Placing of ready mix self compacting HPC on the sill beam groove and surrounding area.
  7. Curing of HPC with suitable curing membrane for 14 days.
  8. Epoxy treatment of the side walls (area between stop log sill beam and radial gate sill beam).
  9. Lowering of 1 no. of stop log and wet testing after HPC setting time.
  10. Lowering Stop logs at bay 3 and 4 for further Civil and HM works.
  11. Building up reservoir (from 540.0m to 568.5m) and switching on of power station to start generation.
Necessary Arrangements for Execution of Work
Before emptying the reservoir, the following arrangements were required to be kept ready for the execution of the work:
  1. One tower crane needed to be erected in pier no: 6 / 7 at a height of about 10 m from the dam top keeping in view of free movement of gantry crane. The minimum specification of tower crane is span of 35m, rated loading capacity 4 – 5 MT, maximum lifting height 60m to take up the above work. The suggestion of placing of tower crane instead of 40 T crane was due to load restriction of road (18 T) at Rangchang Khola Bridge. During that period, the erection work of new bridge by Teesta Stage III Hydro Electric Project too was under progress.
  2. Before the commencement of the work, sufficient number of clay filled bags in the gabions of size 1m x 1m x 1m approximately needed to be kept ready and stockpiled at intake and at road to silt flushing tunnel (SFT) area and thereafter shifting to the tower crane area by tipper (the gabions will be lifted to the tipper with help of 12 T Hydra Crane) from storage area.
  3. 2 number of modified stop log sill beam required to be made available at dam site.
  4. Complete electrification of the dam and gate area u/s and d/s for working at night. Proper arrangements of lighting and sufficient storage of diesel for DG set at Dam site were to be ensured.
  5. Materials (Table 1) and equipment in working condition in the scope of contractor required to be made available at dam site before depletion of reservoir.
  6. Placing of fishnet gabion required to be started from left side bay no 4 to 3.
  7. Stop log panels needed to be shifted/vacated out from dam top to create the working space.
  8. Stop log rubber seals required to be replaced with new one for proper sealing.
  9. Sufficient manpower of different categories to execute the work in three shifts in each bay to be ensured by the contractor. Each shift will be of 8 hours duration with minimum six and half working hours.
Execution Period
The time for completion of the job was from 1st December, 2013 to 16th February, 2014, i.e. 78 days. The time period proposed in the previous methodology by design wing is from 1st December, 2013 to 6th February, 2014, i.e., 68 days.

Methodology
  1. The work was to be taken from 1st December, 2013 (considering average of ten daily discharges for the past 30 years with ten daily average discharge of 152 cumecs in November, 102 cumecs in December, 82 cumecs in January, 86 cumecs in February and 116 cumecs in March.
  2. Power Station was to be shutdown, once the water level goes below MDDL.
  3. Emptying the reservoir had to be started after taking shutdown as per the general flushing guidelines of reservoir manual.
  4. After water level reached upto EL 540m to 542m, the lowering of 2 / 3 pieces stop log panels need to be placed in bay no. 3, 4 and 5.
  5. Began making bund from pier no. 9 to 4 by means of lowering readily available fishnet gabion of approximately size 1m x 1m x 1m, by the tower crane. (Gabions were to be made by means of placing clay filled bags in fishnet and shall be kept ready before emptying the reservoir).
    1. Removing silt in d/s of bund by using chain mounted excavator / dozer which was required to travel through proposed approach road from d/s of the dam. This work required to take up simultaneously at d/s of dam during making bund at u/s side of dam.
    2. Positioning of the dewatering pumps of making sump for dewatering.
  6. The top of the bund should be at EL ± 544m, i.e. above the existing water level as per model study report existing water level would be lying in between 542m to 543.5m for passing a river discharge of 200 cumecs in the specified shutdown duration through two gates i.e. gate no. 1 and 2.
  7. After completion of the bund, stop log from bay no. 3 and 4 was removed and it was ensured that no stop log should be dogged on bays 3 and 4. The stop log at bay 5 was proposed not to be removed till the completion of work as the approach road for movement of equipment to bays 3 and 4 will be from d/s side. The free flow of water was allowed through gate 01 and 02 only.
  8. The u/s face of the bund should have been provided with High-density polyethylene (HDPE) sheet to ensure minimum leakage through bund. Clay bags shall be laid in the u/s of bund to support and fix the HDPE sheet, as required. The maintenance of the bund must be taken care of by the contractor during execution time, and accordingly sufficient number of clay bags / gabions was stored at site.
  9. The dewatering system at d/s of bund was arranged by providing 4 nos. of submersible pumps with a capacity of 10 HP and 2 nos. of mud pumps with a capacity of 5 HP, as required.
  10. Breaking of concrete at stop log groove and widening of groove and removing of old/damaged sill beam of stop log using pneumatic/electrical concrete breakers and jack hammers, etc. with 3 shifts (each shift consists of 2 no. breakers, 2 no. helpers ) in both the bays were to be carried out.
  11. Placing, alignment and fixing of new modified sill beam and further anchoring with bolts with hilti compound and further strength to sill beam by drilling rods (as per drawing) at sill beam groove. Simultaneously, removing of damaged concrete of spillway glacis and piers and rectification of reinforcement also have been taken up.
  12. Starting of HPC work in the bay no. 3 and 4 at stop log sill beam groove, priority should have been given for the replacement of sill beam of stop log and associated HPC work around the sill beam. After achieving setting time of HPC the reservoir got built up after lowering the stop log at bay 3 and 4.
  13. Checking the stop log bottom seal by means of wet testing after achieving of HPC setting time, and if found satisfactory, dewatering arrangement was to be removed. Lowering the stop log in bay no. 3 and 4 and start build up of reservoir.
  14. Epoxy treatment of 5m height at pier walls between sill beams of stop log and radial gate.
  15. During curing time of stop log sill beam groove, the HPC works between sill beam stop log and radial gate sill beam would be taken up and removing of radial gate sill beam also be taken up on priority.
  16. Left out work of radial gates sill beam and HPC in glacis shall continue till its completion at bay no. 3 and 4 after reservoir build up and generation.
The list of material to be required for the work is listed at Table 1 and was to be arranged by contractor of the complete package. The procurement of the sill beams of radial gate and stop log would be done by the power station and the same would be handed over to civil contractor for erection. The erection of the tower crane will be in the scope of the contractor.

Table 1: Material for execution of above methodology
Sl. No. Material Quantity Contractor
1 Dry Ready Mix High Performance concrete grade of M70/80 200cum
2 Concrete batching mixtures 3 nos.
3 Epoxy treatment 200sqm
4 Reinforcement steel of different size as per actual damages and construction drawing.  
5 Fishnet gabion of 1m x 1m x1m approx. and dia. of manila rope should be 1 inch. 800 nos.
6 clay filled bags (sufficient quantity is kept ready considering the wastage and sinking of gabion in silt deposit in the river bed). 22000 Bags
7 Welding electrodes – adequate. Quantity and of specified make and type  
8 Manila ropes of size -       1 inch
-       2 inch.
6000m
4000m
9 Steel fabricated basket i.e. Jhoola of size 2m x 2m x 1m with arrangement of lowering by means of tower crane. 1 no.
10 DA & Oxygen cylinders with accessories like gas tube, cutters goggle etc. - adequate no. LS
11 Plywood,  wooden/ steel shutter – as per requirement  
12 Sufficient curing membrane. 50 sqm
13 HDPE Tarpaulin of thickness 0.5 to 1mm approx. (6 m wide) 250 sqm
14 Steel wire rope 16mm
Rope clips
D- shackle  (10 ton capacity)
Wire rope sling  (10 ton capacity),
Wire rope 432mm  (25 ton capacity)
 100m 25 nos.
10 nos.
1 nos.
8 nos.
15 Concrete in the skin d/s of radial sill beam with HPC (RMC) 700 cum
16 New modified sill beams of stoplog and radial gates   2 nos.  
17 Labour for doing all the works such as preparation and placement of gabions with clay bags, concreting and all other associated works. Sufficient in number
Conclusion
After going through the repairs, the generation of the power station has increased more than the design energy in subsequent years. The power station has generated 2587 MU, 2710 MU, 2819 MU, 2701 MU & 2832 MU during Financial Years 2014-15, 2015-16, 2016-17, 2017-18, 2018-19, and 2019-20 respectively. Increase in generation has not only increased revenue but resulted also in more benefits to the state of Sikkim and its inhabitants.
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