Lower pot subanisiri hydro electric project

Informative

1. Observation and Recommendations of the Expert Group constituted for the assessment of ground situation of Subansiri Lower HE Project of NHPC

2. Observations: The in i t i a l e v a luation of the L e ft B a nk S lope a t the S ub a nsiri L o w e r H E P roj ec t indicates that the l e ft b a nk slope sl i de might have been trigge r e d due to following f a c tors: The r o c k mass on the le f t b a nk slope h a s low str e n gth a nd tends to d e g r a de u pon c onta c t with w a t e r The lat e r a l rock cover of t he tunnels at the b a se of the slope was low resulting into i nsuf f ic i e nt lat e r a l co n fin e ment . The b a se of the slope b eca me unst a ble due to the e n l a rgement of ca vi t ies created a b ove the D T s, a nd the upw a rd p r o gr e ss i on of t h e se ca vi t ies might have triggered the slope failure.  

3. 2. It is anticipated that removal of rock toe support due to cavity and saturation of the rock mass during monsoon, disturbed the equilibrium of the hill slope above the DTs which distressed the already stabilized/treated rock mass. This resulted into a massive rock slide with crown touching the High-level Road (HLR) (Fig. 1D of Annex-II). 3. The other likely contributory factor of the slope failure is high velocity water hitting the opposite bank, the high discharge of the perennial nala and incessant rain during monsoon season.

4. 4 . Plugging of DT-1 is being carried out by consolidating the muck lying inside the tunnel. A continuous leakage is observed through DT-1. As reported by NHPC, leakage of the order of 1 cumec is observed as of now. The way plugging of DT-1 is being carried out is not as per codal requirements and will pose bigger concerns after filling the reservoir to its capacity due to increase in leakage which is highly likely.   5 . A t pr e s e nt the c r o wn of t h e sl i de is a t E L ±300m with h ea d of the sl i de d e b ris is a t E L ±180m ( F i g . 2 of A nn e x - II ) . The sl i d e d ma t e ri a l, a s obs e r v e d n e a r t h e present reservoir w a ter level ( E L ±148m), c omprises of s e mi c onsolidat e d smaller f r a g ments/ro c k c hunks o f si z e 3 - 4m to 3 - 5m. The sl i de is r e tr o gr e s sive in n a ture

5. a nd c ont i n uous d e bris a nd h u ge r oc k blocks h a ve f a l l e n in t e rmit t e nt l y du e to fo r mation of s e v e r a l ov e r h a n g s, w h ich are g e t t ing d e stabil iz e d due to p e r c olation of r a in w a ter a lo n g the jo i nts. S e v e r a l o v e r h a n g s a r e st i ll pr e s e nt on t h e major s c a r f a c e b e low the c ro w n ( F i g . 3 ) 6. The vertical /sub-vertical/overhang hill slopes over DTs inlet area as well as outlet area over the DT-5, are in critical conditions even though they are protected as of now. Working under such slopes is highly risky and unsafe for the personnel. Since 2005 onwards, occurrence of repeated slides is reports at the same area.

6. 7 . Slope stability Studies of upstream left bank slope/ dam abutment as carried out by NIRM lacks in modeling the likely cavities/weaknesses. The studies do not consider the realistic rock mass and slided rock mass parameters. Hence, could not be accepted in the present form. 8 . There is a l i m i t e d lat e r a l ro c k c o v e r to the DT1 in t he downstr ea m towards river side. This may cause a p o tential risk of it g e t t ing e x pos e d during floods. 9 . The proj ec t’s e n er g y dis s ipation is in t e nd e d to o cc ur thro u g h p r e f o rm e d plun g e pool, whi c h h a s not b ee n c onstru c ted y e t . W i t h a ll d i v e rsion tun n e ls c losed, w a t e r is c ur r e nt l y flo w i n g thro u g h spi l lw a y . This a r r a n g e m e nt ma y l e a d to e r osion in the do w nstr ea m of d a m.

7. Recommendations: 1. All the possible cavities (which are highly likely) in the left bank in and around the DTs may be identified and duly earmarked. These cavities must be filled with concrete adhering to the desired quality control to avoid the possibility of land slide in future. 2. The Diaphragm wall, as discussed during various meetings, may be considered to isolate the DTs from the reservoir so that cavities can be approached for concreting. Required approach from downstream may be created in this regard.

8. 3. All the DTs, including DT1, must be plugged at dam axis location as per design requirements. Here, it is to highlight that the DT1 is filled with flowing muck hence could not be plugged as per design requirements and codal practices till date. 4. Slope stability Studies of upstream left bank slope/ dam abutment need to be carried out by modeling all the cavities/weaknesses and considering the realistic rock mass and slided rock mass parameters. 5. Strengthening of the left bank should be taken up to ensure its long term stability. Over hangs must be removed for safety reasons. After consolidation of the slided mass at toe, slope shall be reframed and stabilized.

9. 6. O c c ur r e n c e o f r e p ea ted sl i d e s is reported at the troubled a r e a in recent past . P e rm a n e nt stabil iz a t i on of the left bank can only be ac h i e v e d with hi l l slope mode ra t i on to 60 d e g ree slope with re g ul a r b e n c h e s a t a ppro p ri a t e h e i g ht. Along with conventional slope stabilization measures, 35 m d e e p p ost str e ss ca ble tendon s may also be provided a t e v e r y b e n c h with e f f ec t i ve w ee p holes. A s y s t e matic, g e ol o g ic a l r e - invest i g a t i on/s t u d y / t e st i n g r e qui r e d to r e vi e w e nt i re a r e a b a s e d on a v a i l a ble d a ta (f rom the r e ports a nd i nstrument a t i on). 7. For long term safety of the left abutment, it is recommended to monitor the rock mass through instrumentation for slope movement, seepage, subsidence etc .

10. 8. Scouring downstream of the spillway shall be monitored regularly and addressed suitably till construction of the plunge pool to ensure safety of the dam. 9. Till required treatment of the slided rock mass and complete filling of the likely cavities is ensured, it is a dvised n ot to r a ise the w a ter lev e l of the r e s e rvoir. Efforts may be done to m a in t a in the reservoir w a ter lev e l at the lowest possib l e level till then. 10. S lope st a bi l i t y stud i e s s h a ll a lso be e x tend e d a ft er w a rds on do w nstr e a m s l op e s on the le f t bank and required measures may be taken. 11. Anti-erosion measures shall be taken to protect left bank of the river downstream of the dam.

11. F ig u r e 1. S e qu e n c e of i n i t iation of landslide a bo v e DT in l e t a r e a . A) Go o gle i m a g e of 2016 showi n g DT stru c ture with mod i fi e d a nd tr ea ted slope. B ) F o r mation of a ca vi t y a t LLR a t 35m f r om in l e t o f t h e D T - 2. C) Enl a rg e ment of the ca vi t y l ea di n g to r e moval o f ro c k toe support a nd c re a t i ons of ov e rh a n g s of c l a dding w a ll a nd jo i nt f a c e . D ) Initiation of fi r st r o c k sl i de a bo v e D T - 2 a nd D T- 1 with c r o wn a t H L R a nd E) r e tr o g r e ss i on a nd e nla r g e ment of the r o c k slide a b ove H L R.

12. Figure 2. Frontal view of the slide area from Power Intake structure on right bank.

13. Figure 3. Close view of the upper portion of the slide showing jointed rock mass and formation of overhangs.

14. THANK YOU