Second stage

16 March 2005

M M Madaan discusses the rock formations that will be encountered during tunneling works at the Parbati hydroelectric project stage II in India, and explains how the project developers are planning to achieve the completion schedule in five years

The Parbati hydroelectric project is a cascade scheme to be developed in three stages with an aggregate generating capacity of 2070MW. Parbati project stage-II is a run-of-river scheme comprising an 85m high concrete gravity dam and a 31.5km long headrace tunnel which will utilise a gross head of 862m to generate 800MW of power. The tunneling work is of special importance as the longest reach of 9km shall be excavated with the use of TBM, as well as the two 2km long inclined pressure shafts. In all, 56km of tunneling works will be needed to complete the project.

The Parbati scheme lies in very complex geological groups like the Larji, Rampur and Kullu group. The main stretches which are likely to be problematic consist of biotitic schist, carbonaceous phyllite and talc chlorite schist. The tunnel also passes under a high rock cover of more than 1000m, which may generate popping and rock bursts in competent rocks like quartzite. There is also a possibility of hot springs occuring. All types of geological formations of ages with shear zones to big thrusts shall be encountered.

Regional geology

The project area lies within the Lesser Himalayan domain of Himachal Pradesh. The area exhibits a rugged topography and is dissected by a number of perennial streams. A thick vegetal cover generally covers the area. The Lesser Himalaya is further divided into outer and inner Lesser Himalayan zones due to its thickness. The Parbati project is located within the inner Lesser Himalayan zone. This is a complex zone and comprises poorly fossiliferrous and variably metamorphosed rocks of thrust sheets, windows and semi-windows. The area has high mountains, glaciated valleys, steep slopes and escarpments. The great difference of the elevations suggests a very young and immature topography conforming to late orogenic uplift of the Himalaya. This is further supported by the occurrence of raised terraces, fossil valleys, and deep gorges with overhanging valleys etc. The Parbati valley, which includes other tributaries of Beas, exhibits a complex geology where various rock formations have undergone extensive structural deformation.

Larji Group

Larji Group comprises of a sedimentary sequence of dolomites, limestones, quartzites and slates etc. This formation occurs as a tectonic window bounded by the Banjar thrust on all sides except in the west where it is concealed under the Kullu thrust. Structurally, the rocks of Larji Group have undergone large-scale disturbance in the form of series of overturned folds and associated faults. Due to these movements the dolomites and quartzites members of Larji Group are highly sheared and crushed and have become very brittle.

Description of Parbati stage-II

The Parbati hydroelectric project (Stage-II) is a run-of-the-river scheme to harness the hydro potential of the lower reaches of the Parbati river. The scheme is an inter basin transfer type. The river near Village Pulga in Parbati valley will be diverted by an 85m high; 113m long concrete gravity dam, now under construction across the Parbati river. The reservoir will have a live storage capacity of 3.09Mm3 with gross storage capacity of 6.83Mm3. This diurnal storage will be sufficient to run the power station at full capacity for four hours in a day even during lean flow period. The spillway section consisting of three bays is designed to pass a maximum probable flood of 1850m3. Further, it is planned to divert the discharge of Jigrai Nallah, located just downstream of Pulga diversion dam, into the reservoir for augmentation. The proposed intake structure is located on the left bank of Parbati river just upstream of the Pulga diversion dam to cater to a designed discharge of 145m3. An underground desilting chamber at the end of approach tunnels is under construction to trap suspended silt particles carried by the river. The water will be carried through a 31.501km long, 6m diameter tunnel on the left bank of Parbati. The tunnel is designed to carry a discharge of 116m3. The tunnel is under construction through six adits deploying modern tunneling equipments. In order to augment the power generation, the discharge of Hurla Nala and its tributaries viz Manihar, Pancha and Hurla shall be diverted to the headrace tunnel through diversion structures and drop shafts. At the end of the headrace tunnel a 17m diameter and 116m high underground restricted orifice type surge shaft to feed two steel lined pressure shafts each of 3.5m diameter and 2626m long are under construction. The inclined pressure shafts shall be excavated with TBM. The surface power house is located on right bank of Sainj river near Suind village, 200m downstream of confluence of Jiwa Nala & Sainj river. Thus, a gross head of 862 m between Pulga and Suind will be utilised for generating 800MW power. The power house will have an installed capacity of 800MW with four 200 MW units. Short tail race channels will discharge the water from the Power House to Sainj river.

Rockmass characteristics

The headrace tunnel will broadlypass through seven lithological units of two geological formations i.e. Jutogh (Kullu) Formation and Banjar Formation. Both these formations are separated by a regional thrust known as Jutogh (Kullu) Thrust. This thrust is expected to be negotiated around RD-10km (from intake). Another important feature of the area is high angle reverse fault along which Manikaran Quartzites have been thrust over the younger Green Bed Member. This fault zone of 50 to 100m thickness may be negotiated towards the end of HRT near surge shaft. Geological formations/rock units expected along the proposed tunnel alignment are given below:

Jutogh Formation

Carbonaceous slates, phyllites, schists with bands of augen gneiss and limestone represent the rocks of Jutogh Formation. These rocks cover a major part of Kullu-Rampur Group. Jutogh formation in the project area consists of:

•Schistose quartzite with subordinate bands of Biotite schist.

• Biotite Schist with subordinate bands of schistose quartzite.

• Carbonaceous phyllite/phyllite/phyllitic schist.

Banjar Formation

The rocks of Banjar Formation comprise of low to medium grade metamorphics such as slate, phyllite and chlorite schist inter-bedded with massive quartzite. These rocks were first studied in ‘Banjar’ area from where they derive their name. In the project area, only rocks belonging to Bhallan Member and Green Bed Member of Banjar Formation are exposed. The rocks in this formation consist of;

• Quartzite (Manikaran member).

• Chloritic Schist.

• Bandal Granite /Gneissose granite.

• Metabasic with bands of chloritic schist.

In general, foliations of all the lithological units are dipping northeasterly with moderate dips. The foliation trend generally cuts oblique to the tunnel alignment; however, at places it becomes sub-parallel due to kinks in alignment. The tunnel passes through fairly competent media. Nevertheless, there are certain stretches of poor rock viz. biotite schist, carbonaceous phyllite and talc chlorite schist where severe tunnelling problems may have to be negotiated. The special feature, to be negotiated along the tunnel alignment is Jutogh Thrust. It is associated with carbonaceous phyllite of graphitic character. This unit is apprehended to be highly sheared and will be encountered in a 1km length between RD 9 to 10km. Study of rock mass classification indicates that the headrace tunnel may be housed in fairly good tunnelling medium in 60% (Class-I & Class-II), satisfactory tunnelling medium in 27% (Class-III), poor to exceptionally poor medium in 13% (Class-IV & Class-V) lof the tunnel. The high rock cover of more than 1000m over a 7km reach along the proposed tunnel may generate incidence of rock bursts/popping in competent rocks like quartzite. Whereas, softer rocks, like biotite schist, carbonaceous phyllite and chloritic schist may experience squeezing ground conditions. Moderate ground water inflow conditions may occur in quartzite and granite rocks, and in the areas where the headrace tunnel crosses major streams.

Brief assessment of underground works

Diversion tunnel

The 7.5m diameter horseshoe shaped 273m long diversion tunnel has been constructed on the right bank in the rock ledge available between the river and the fossil valley. It is designed to cater for a discharge of 512m3 with inlet at el 2131m and outlet at el 2121m. The work excavation has been completed and breakthrough was achieved on 31 march 2003, two months ahead of schedule. During excavation, the diversion tunnel traversed through schistose quartzite, mica schist and granite gneisses having a disposition more or less across the diversion tunnel alignment. The rockmass encountered has been classified as 70% Class II and 30% Class III. On the inlet side the strata comprised of blocky formations with joint infillings of clayey material. Tunnelling from the inlet would also have resulted in driving against the dip with an inward gradient. Owing to the presence of fossil valley on the right bank, the diversion tunnel is aligned close to the existing river. As such, self-drainage was more preferable in case there is a connection between the river and the tunnel through some of the discontinuities. Therefore, the tunnel was driven from the outlet end. At the diversion tunnel inlet, the rock strata encountered is highly jointed with wide-open joints. The infillings with fluvioglacial material and the valley dipping foliation plane partings/joints tends to slide the joint planes. This resulted in unsafe working conditions coupled with heavy snowfall and rain. There were frequent failures of joints and falls of huge blocks from the downstream side disrupting the work. Therefore, the cut slope was executed slowly and cautiously by creating small controlled blasts and providing 6m long grouted rock bolts with mesh shotcreting. The diversion tunnel has been completely concrete lined. After the lining and completion of the invert, contact grouting and consolidation grouting in the weak zones were completed. As the tunnel is sandwiched between fossil valley and open valley, no pressure relief holes were provided fearing that the holes may puncture the fossil valley causing water leakage. The river had been diverted by November 2003 with a discharge of 25m3.

Desilting basin

The intake at el 2181.4m on the left bank has three 3.7m diameter, D-shape intake tunnels 124m, 147m and 168m in length. The three dufor type desilting chambers (DC) which are 15m wide, 16m high and 170m long each are designed to remove particle size 0.2mm and above. The underground chambers are placed on the left bank inside a flat-topped hill. Overall cover on the chambers is around 100m out of which 60-75m is rock comprising of schistose quartzite with mica/quartzitic schist and augen gneiss bands.

This assessment of rock strata was based on a few drill holes and seismic refraction survey carried out in the area. However after excavation of the adits it has been confirmed that 40% of the medium shall be class II and the balance 60% class-III. The clear rock barrier between successive chambers is around 30m. For the purpose of excavation, one bottom adit 377m long and one top adit 254m long have been provided. The adits have been completed.

After excavation of the central pilot of 7m x 8m size, widening of all desilting chambers is under construction. Also provided are 6m long tensioned rock bolts with expansion shell fully grouted, along with shotcreting. At present DC-1, DC-2 and DC-3 have been excavated and side by side overt lining in DC 3 has been completed. Intake tunnels 1, 2 and 3 have been excavated from top adit to DC. Benching will be carried out after concrete lining the crown of the DCs.

For gate operation chamber (GOC) to DC, a 248m long access adit has been provided at el 2180m. Excavation of GOC has been completed. During excavation, river borne material were encountered at the crown portion at two places in this adit. Therefore steel sets were provided as supports. Work on GOC chambers has been completed. After excavation of chambers, the gate shafts will be excavated.

An 180m long main silt flushing tunnel (SFT) of size 2.2m X 3m D-shape is being constructed at el 2157m. Three SFTs below the desilting chambers are of size 1.6m X 2m having a length of 222m, 197m and 148m. Adit to GOC to SFT is 136m long and Gate Operation Chamber of SFT is 56m long having chamber size of 6m X 8.5m bifurcating from desilting bottom adit. The excavation work of SFT adit and chamber has been completed. The excavation work was carried out with an Atlas Copco Rocket Boomer drilling jumbo.

Jigrai Nallah feeder tunnel

The waters from Jigrai Nallah, which joins Parbati River about 300 down stream of dam site, are to be tapped by a Trench weir (12m long, 1.25m wide) located at elevation 2210.0m. The structure is deigned for 160 cumec. A feeder tunnel would carry the water from the trench weir into the reservoir immediately upstream of dam. The tunnel shall be 690m long, 3.0m wide D-shaped having free-flow conditions. It is envisaged to have similar geology as the de-silting chambers. The excavation shall be started during lean period.

Headrace tunnel

The most important structure in the intricate Parbati project stage II is the headrace tunnel. With a length of 31.501km, this is the longest tunnel in any hydro power project in the country and one of the longest in the world. It has a finished diameter of 6m, horseshoe shaped, in the drill and blast method portions and circular in TBM portion. The tunnel is designed to carry a discharge of 110m3 at velocity 4.10m/sec. Six adits are provided at different chainages for construction of this long tunnel.

• Inlet Adit (0): 300m long, from this adit face (0) shall be 113m and face 1 shall be 3387m

• Adit No.1: 346m long, from this adit face 2 shall be 2864m and face 3 shall be 4049m

• Adit No.2: 976m long,from this adit face 4 shall be 9050m and face 5 shall be 0m

• Adit No.3: 469m long,from this adit face 6 shall be 2038m and face 7 shall be 3232m

• Adit No.4: 382m long,from this adit face 8 shall be 4144m and face 9 shall be 1216m

• Adit No.5: 472m long, from this adit face 10 shall be 1216m and face 11 shall be 192m

The 13.099km portion between adit no.1 and adit no. 2 constitutes the most critical portion of the headrace tunnel to be executed in a rather tight schedule of 54 months. The entire tunnel shall pass through seven lithological units belonging to Jutogh (Kullu) & Banjar formation. Two regional thrusts – Jutogh thrust & Manikaran thrust – shall intersect the tunnel.

To achieve fast rate of progress and to negotiate critical length between adit-1 and adit-2 hard rock TBM has been deployed. Geologically, upstream face of adit-2 is the most suitable choice for deploying TBM. Selected TBM is of open type and is required to cut hard and massive granite and quartzite effectively.

Inlet Adit (0)

Inlet Adit (0) is located on the left bank of the Parbati river and theright bank of Jigrai Nala. The 300m long adit is housed in rock consisting of biotite schist with bands of schistose quartzite. Excavation of this has been completed; small bands of shear zones were encountered during excavation. From this adit 3500m of tunnelling is being executed. So far, 113m of the headrace tunnel towards the intake has been completed and 1021m of tunnelling has been done towards face-1.

Adit 1

Adit-1 is located on right bank of Hiyani nala. Adit 1, 346m in length, was completed in April 2003. The headrace tunnel at the end of this adit takes an almost 908 bend. Tunnelling 6913m in length will be done from this adit. This section of the headrace tunnel is to be negotiated through schistose quartzites with mica schist and gneiss. Mainly class II and Class III conditions are expected along with some stretches of class IV rock. The rock cover in this reach is low to moderate due to a number of cross drainages. Copious ground water seepage is therefore anticipated in some reaches. Excavation of this portion of HRT from both faces 2 and 3 has started. About 3500m of the headrace tunnel has been excavated from both faces. Atlas Copco Rocket boomer drill jumbos are being used for drilling and rock bolting in the tunnel. Full-face excavation is being carried out. The rock encountered is class II and III supported with fully grouted rock anchors and shotcreting with occasional rib supports in shear zones.

From the adit 1 downstream face, after reaching the contact of schistose quartzite and carbonaceous phyllites, special tunnelling techniques will be required to safely excavate and support the rather long reach of carbonaceous phyllites. There is no doubt that squeezing conditions will occur in generally weak to occasionally medium strong carbonaceous phyllites. The thickness of the carbonaceous phyllites is an extrapolation from surface exposures in nearby nallas etc corresponding to the tunnel alignment. Carbonaceous phyllites are associated with qualitatively better bands of phyllites, phyllitic schists and limestone bands.

The schistose quartzite with mica schists, and quartzitic schists are predicted to be encountered up to Rd 9km with a rock classification of class III and occasionally class IV. The methodology for negotiating carbonaceous phyllites (9 to 10km) must necessarily consist of advance probe drilling, pre-grouting and multiple drifting and immediate installations of supports.

Adit 2

Adit 2 is located on left bank of Hurla nala where the valley is narrow V-shaped. The right bank is rocky with sub vertical escarpment whereas the left bank is relatively gentle. Outcrops of granite/gneissose granite are seen on either bank. Adit 2 is the longest adit having a length of 976m. The adit is aligned sub parallel to oblique of the rock formations and will be housed in granite/gneissose granite, which has been assessed to be good tunnelling media. The length of the tunnel to be excavated from adit 2 is 9050m. The rock cover varies from 300m to 1500m. In the upstream part up to Rd 9km, schistose quartzites and mica schist are expected. Thereafter approximately 1km of stretch is expected to be in carbonaceous phyllites.

As such, the TBM to be deployed from Adit 2 upstream face will have to negotiate 3.66km of granite gneiss and 5.75km of Manikaran quartzite. As it is not desirable to bore through the carbonaceous phyllites under a rock cover of 1000m by TBM, the reach between 9 and 10km is to be tackled by DBM from adit no.1 downstream face only.

The Manikaran quartzites to be encountered from 10km to 15.69km shall generally occur as heterogeneous medium rather than a homogeneous formation. Pockets of class I and IV are likely to occur along with a majority of class II & III rock conditions. The quartzites are notoriously known for perched/confined aquifers; therefore a lot of seepage is expected during tunnelling through Manikaran quartzite.

In the field, two varieties of quartzites can be clearly identified. There is the dirty white, cream to pink hard compact variety and the grey coloured one, which is more sericitic. Talc schistose bands are developed in quartzites whereas some varieties are massive with feeble foliation. This intermingling talc chlorite schist bands within the quartzite adds to the heterogeneity of the formation. At some points, the rock cover exceeds 1000m, which may lead to mild rock bursts. With four sets of joints in the quartzite a lot of wedges are probable in the crown arch, which shall need support, essentially, by rock bolts.

From Adit No-2 an upstream TBM has been deployed, initial 3.6km from 19.463 to 15.863km is manifested by granite gneiss. These granites are in general hard, compact, and contain phenocrysts of feldspars. However, at places it is metamorphosed to gneissose granite particularly in the Hurla valley. The rock types are also associated with concordant metabasic hands and chlorite schist bands.

At the contact of quartzites and granites 10-20m thick weak zones due to presence of talc chlorite schist should be expected. There is also a possibility of encountering hot water springs at the contact, as well as in Manikaran quartzites. The Bandal granites constitute by far the best tunnelling media in the headrace tunnel. They can be classified as predominantly good with patches of very good and fair according to geomechanical classification. The initial 3.6km length shall be bored with TBM in minimum possible time frame.

As per RMR classification, almost 79% of the HRT rock strata is expected to fall in class II and III, about 8% in Class-I and Class-III and Class-IV constitute the remaining 13%. In the TBM portion Manikaran Quartzite is expected to cover 60% of the reach and rest 40% granite gneiss.

Manikaran Quartzite can be described as strong to very strong massive rock. It is very fine grained and Manikaran Quartzite can be described as cream coloured very fine-grained sericite quartzite and relatively coarse-grained bluish gray colored quartzite, which is extremely strong. 72% of Manikaran Quartzite shall fall in class I & II category.

Excavation of Adit -2 has been completed and excavation of tunnel with TBM has begun.

Adit 3

Adit 3 is placed on the right bank of Pulia nala where the valley is U-shaped under a cover of slope wash material. Bedrock in the area is granite/gneissose granite with bands of metabasic. The 469m long adit is aligned oblique to the trend of foliation. The 2.038km stretch of headrace tunnel between adit-2 and adit-3 shall pass through Granite Gneisses. Excavation of adit 3 has been completed. Maximum length of adit passed through class 4 rock requiring steel rib supports. A total length of 5270m of tunnel shall be excavated from this adit. At present 2800m tunnel excavation has been completed from this adit.

Adit 4

Adit-4 is identified on the right bank branch of Kothiari nala at el. 2108m. The adit is located in Manikaran Quartzite that is moderately to closely jointed in the area and expected to pose difficulties during adit excavation. The potion between adit-3 and adit-4 is the second longest at 7.378km and single longest face with DBM is 4144m long. The geological formations to be encountered are granite gneisses and Manikaran quartzites intervened by talc chlorite schist. The cover at 24.8 and 27km increases to over 1000m, which may result in mild rock bursts in Granite Gneisses. About 400m of tunnel has been excavated from this adit.

Adit 5

A 426m long Adit-5 is placed on the right bank of Railah nala near Railah village. Adit portal is located on sub vertical rocky escarpment of metabasic. The adit is aligned right angle to the foliation trend of metabasic. Metabasic in the area are associated with thinly foliated bands of chlorite schist, which may pose difficulties during excavation. From adit-5 a reach of 2.433km shall have to be excavated between adit-4 and adit-5. From 28.474 to 30.1km Manikaran quartzites are expected whereas at 30.1km the thrusted contact between Manikaran quartzite and meta volcanic is anticipated. The crushed zone owing to the above mentioned Manikaran thrust is about 100m wide after which meta basics near Manikaran thrust are completely choritized and thus occur as very thinly layered weak formation. Otherwise more competent banks of Meta volcanic shall constitute good tunnelling medium. The short length between adit 5 and the Surge shaft lies in Meta basics. At the contact of quartzite and meta basics there is a possibility of encountering water due to shattered rocks. Excavation is underway at Adit-5 along with bottom adit to surge shaft and top adit to inclined pressure shaft and valve house.

Surge Shaft

The surge shaft at the end of the tunnel is 130m high with 17m diameter. One limb having a length of 250m from adit 5 reaches the bottom of the surge shaft at el. 2106m. The crown arch of the surge shaft is at el 2244.5m. A 393m long, 6m diameter approach adit shall reach the top of the surge shaft (gate hoisting level) at el. 2236m. The surge shaft is located within meta-basics. These rocks occur as banded formation having rather massive or feebly foliated bands intervened by thinly layered and foliated chlorite schists and phyllites. The Metabasics are also intervened by bands of quartzites particularly near the contact with Manikaran quartzites. The quartz veins on the other hand which are traversing the basic rocks exhibit pinching and swelling structure. At places talc pockets are present with chlorite schists. A drift was excavated at el 2230m to ascertain the conditions around the surge shaft. It has intercepted metabasics with chloritic phyllites and schists having 9% class II, 83% class III and 8% class IV rock.

Pressure shaft

The area encompassing the two pressure shafts, which are circular and steel lined having 3.5m diameter and 1914m lengths with c/c spacing 48m and an inclination of 30º has been a subject of intense exploration. Overall five drill holes have been completed in the pressure shaft area. In general thick overburden is present below el 2000m. Around el 2500-1500m rock exposures are seen on the power house slopes. Huge dislodged blocks of Meta basics are present in the intervening area. A considerable portion is occupied by cultivated land in the overburden reaches. The bottom adit to pressure shaft has been provided at el 1332m near the confluence of Jiwa and Sainj rivers. Inclined portions of pressure shafts shall be excavated with a double shield TBM.

There is a valve house of 8mx12x63m size housing two butterfly valves intersecting the top horizontal portion of pressure shafts. A 406m long approach adit to the valve house is under construction at el 2105m.

The pressure shafts run in Meta basics, which are traversed intermittently by weak chlorite schist. Whereas the meta basics have feebly foliated massive bands with some jointed portions, the chlorite schists are thinly layered with a low compressive strength. However the foliation direction is oblique to the pressure shaft, which makes tunnelling through them slightly less problematic.

On the other hand the dip of foliations varies from 50o-70o. With the inclination of 30º of the pressure shafts, failures in crown are likely to occur in case of gently dipping formations. Tunnelling through chlorite schists and phyllites has to be cautious with the excavation requiring immediate supports. Therefore for excavation of inclined pressure shafts a double shield TBM has been planned. Immediate supports shall be carried out with precast concrete segments.

Overall, the pressure shaft is expected to be in predominantly class III rock with intermediate reaches of class IV and class II rock. The pressure shaft is expected to be in 60% Class III, 20% in class II and 20% in class IV & V rock conditions. It would be very helpful to have advance probe drilling and pre grouting in probable weak zones, particularly in view of proposed TBM deployment.

Due to non-availability of rock at a reasonable depth to form the portal and due to higher bed level of Jiwa Nallah, a combined adit to the bottom of the pressure shaft and penstocks having a length of 630m at el.1332m has been excavated. The adit bifurcates for penstocks and later bifurcates for the second pressure shaft. Work is currently underway on the steel lining in the bottom penstocks. TBM for inclined pressure shaft is also under erection.

Jiwa Nallah feeder tunnel

For diversion of Jiwa Nallah flow, which is a tributary of the Sainj river, a trench weir is proposed at el 2220m across Jiwa Nallah. Jiwa Trench weir contemplates a trench weir 14m long and 4.9m wide to collect water from the Nallah. Diverted flow of Jiwa Nallah is led to underground desilting basins through a 3m D-shaped approach tunnel. The approach tunnel is 580m long. To minimise entry of silt into the feeder tunnel, an underground desilting chamber has been provided downstream of the approach tunnel. The size of desilting chamber is 10m x 14 m x 100m. The water from the desilting chamber is carried to the main headrace tunnel through a 4250m long feeder tunnel. The size of tunnel is 3m D-shaped.

To facilitate construction of a 4.25 km long feeder tunnel from Jiwa Nallah, a construction Adit 3A, 252m long near its tail end has been provided. This will also be useful to complete the vertical drop shaft between feeder tunnels at el +2202m and main headrace tunnel at el +2124m. At the end of feeder tunnel at RD 4250m a 4m diameter 85m deep drop shaft is provided. Water from the feeder tunnel will be joining the main headrace tunnel at RD 21280m through the drop shaft. In the vicinity of the proposed weir site the Nala flows through a narrow gorge with steep gradient. The right bank is nearly vertical whereas the left bank is relatively gentle and under thin vegetal cover. A shallow overburden cover of 3-5m is expected in the nala bed. The feeder tunnel and desilting chamber is aligned sub-parallel to the strike of the formations and will be housed in gneissose granite/ granitic gneiss. The medium of tunnelling is expected to be good. Excavation of the feeder tunnel has been started from downstream end, while excavation of the escape tunnel has been completed.

Power house

A srface power house to accommodate four 200 MW units is proposed on the right bank of the Sainj river about 200m downstream of its confluence with Jiwa Nallah. The deepest foundation level is 1324.25m and the service bay level is 1347.6m. The tail water level 1331m is coinciding with MWL of stage III whereas the FRL of stage III is 1330m.

On the power house back hill slope, outcrops of meta basics are present between el. 1346 & 1500m. The design of cut slopes and rock supports are very important for overall stability of the slopes. The back slope of the power house has been provided with 1:8 cut slopes and 3m wide berms at every 15m interval. The top of the cut reaches up to a maximum elevation of 1417m. Considerable rock supports consisting of 9m and 6m long rock bolts and shotcrete with or without wire mesh have been provided.

The 60m long tail race channel is proposed which runs in overburden.

Tunnel boring machines

The TBM to be used for the headrace tunnel is a refurbished Robbins TBM MK 27 of 6.8m diameter. The machine is equipped with ring-mounted probe drilling equipment, which can cover 360° of tunnel. This machine is open type-high performance with six 525kW main drive motors. The conveyor has a straight alignment without the down dip in the cutter head location. The machine is equipped with 49 432mm diameter cutters. Maximum operating load per cutter is recommended as 267kN. The cutters are back loading type. TBM conveyor of 1000mm width has normal capacity of 875m3/hr. Normal estimated power consumption is 2525kVA. The TBM has arrangement of rock bolting, wet and dry shotcreting and ring beam erector for erection of heavy steel arches. The machine is also equipped with high performance injection grouting plant. The machine has reached the site. Erection outside the adit –2 started in December 2003. The TBM, after being assembled outside the adit portal along with its back up system, shall walk up to the launch chamber. Based on experience of Himalayan geology, an average progress of 300m per month has been estimated in the type of geology to be encountered.

A refurbished Mitsubishi TBM MHI-NFM-BORETEC shall be used for boring two inclined pressure shafts. This is a double shield TBM 4.88m in diameter. The main drive motors consists of six units of 250kW each. The machine is equipped with 33 432mm cutters. Maximum operating load per cutter is 267kN. Normal operating power consumption is estimated as 1500kVA. The machine will be erected inside the tunnel. The TBM for the inclined pressure shaft shall be erected inside for which two chambers 35m long and 12m high at the end of the adit shall be constructed. The TBM after boring one pressure shaft shall be dismantled at the top and taken out again to be assembled in the second pressure shaft at the bottom. Two chambers for dismantling at the top shall also be constructed. Precast segmental lining shall be used for supporting the excavated area. Thereafter the pressure shaft shall be steel lined. The segmental linning is hexagonal ends type. The back thrust of the TBM shall be transferred to the segmental lining. In addition the TBM has gripper pads and an anti skidding device. The excavated muck shall be transported through a steel trough and flushed with a jet of water from the working face down to the adit bottom in a hopper where water and muck shall be separated out. The dry muck will be transported from hopper out of the tunnel through conveyor belt. The machine is equipped with a probe drilling machine and segment erector.

Challenges and construction methodology

As described above the tunneling in this project is the most challenging work. The faces are quite long due to the topography which puts the tunnel on a critical path. Modern equipment like electro hydraulic drilling rigs and wet shotcreting machines with robo arms are being used. In two reaches, a TBM is being used. NATM is being used for exacavation. Five r trench weirs are situated in the remote area with steep slopes of streams, making the task more difficult. Small size feeder tunnels are planned to be excavated with special sizes of equipment manufactured by Technicore Underground Mfg.

The project area lies in a high mountainous region in the remote part of Himachal Pradesh. This necessitated construction of 120km of roads and 16 new bridges. The area is prone to land slides and cloud bursts.

Tunnelling media so far encountered has been from class II to IV. In face 3 a lot of water seepage is being encountered which slows down the progress. Bands of shear zones have been encountered frequently, therefore steel rib supports are being provided regularly. Similarly in face 1 and adit 3 frequent changing of rock strata and shear zones requires steel rib supports. In face 2 shear zone was encountered all along the tunnel in the crown at Rd 650-680m. In the bottom adit to the pressure shaft the rock from Rd 300 onwards is required to be supported with wire mesh and shotcreting.

Due to the proximity of the Great Himalayan National Park, the work remains suspended in Adit-4 and Jiwa nalla area during 15 Nov to 15 Mar every year as this is the mating season of wild animals.

Most of area is snow bound and winter and monsoon rains make the task more challenging. The area is also prone to cloud bursts. On 16 July 2003 a cloud burst in Pulia Nalla and again on 8 August 2004 in Jigrai Nalla caused problems. A big portion of road was damaged and a 20m span concrete bridge was washed away. Many lives were also lost, and all major equipment was washed away.

Parbati river was diverted on 7 November 2003, with the project scheduled for commissioning in September 2009. Despite problems such as cloud bursts, and uncertain Himalayan geology, efforts are being made to compress the schedule under the accelerated hydro power development programme.

Author Info:

Mr. M. M. Madaan is General Manager in India’s National Hydroelectric Power Corporation.


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