Innovations in Tunnelling Technologies in Hydro Project

Dr. Rakesh Kumar Khali, Vice President - Operations, (Tunnel, UG Structures & Hydro Power), G.R. Infraprojects Ltd., describes the construction of the Tehri Pump Storage Project (4X250 MW) and the innovative technologies in Vertical Shafts and Tail Race Tunnels

This article discusses the innovations made in excavation and support of reverse slope for slanting portion of Upstream Surge Shaft - 3 & 4 and also analyses the innovative methods adopted for concrete lining of TRTs. It further describes the steps taken to ensure the stability and effective excavation of Tehri PSP U/s Surge-Shafts Slant-portion Design, and the measures taken to safeguard excavation and support, casting of collar beam and erection of truss girder for gantry movement. Here, the focus is on how the invert lining of TRT’s done first to be followed by an overt lining. The practice is very uncommon in India and sets up a successful example for other industry partners.

The Tehri Project

The Tehri PSP is located on the left bank of the Bhagirathi river in the district of Tehri, about 1.5 km downstream of its confluence with the Bhilangana river (now forming a part of Tehri Reservoir), falling between 78°30’ and 79°00’E longitudes and corresponding 30°30’ and 33°30’N latitudes. The nearest railhead is Rishikesh (Uttarakhand), located about 82 km south of the project site.

The construction of the Tehri PSP by HCC (under progress) envisages an underground Machine Hall on the left bank of river Bhagirathi, housing 4 reversible pump turbine units, each of 250 MW capacity. The reservoir of the Tehri dam will operate as the upper reservoir and the Koteshwar reservoir as the lower reservoir for this project. Two completed Head Race Tunnels (HRTs) (3 and 4) will be augmented by respective Surge Shafts at the end of each HRT. These HRTs will bifurcate into two steel lined penstocks at the base of the Surge Shaft to feed two turbines each.

The water from the turbine units will discharge into two TRT’s which, in turn, will carry the water of all the four units into the downstream reservoir. The availability of water for the Tehri Pump Storage Project shall be governed by the mode of operation of the Tehri Power Complex. During non-peak hours, water from the lower reservoir will be pumped back to the upper reservoir by utilizing the surplus available power in the grid.

Interestingly, the underground Tehri PSP, which is in an advanced stage of construction, is located on the same hillocks where the Tehri Hydro Power Project (HPP) is. The major project components of the PSP are:

• 25.4m (W) X57.3m (H) X203m (L) size Machine Hall to accommodate 4 turbines of 250 MW each
• 22m dia. 02 nos. Upstream Surge Shafts with Surge Chambers
• 77m (L) X 24m (H) X10m (W) size BVC
• 81m (L) X20m (H) X13m (W) PAC
• 18m dia 02 downstream surge chambers
• 02 nos. of 1081m & 1176m TRTs.

The geology

Figure 1: 3-D-Layout of Tehri Pump Storage Project

The geology of the Tehri Pump Storage Project is followed by an assessment of the current and anticipated conditions for each of the main project components. The most important tectonic feature in the vicinity of the Tehri dam site is the moderate to steep dipping Srinagar thrust (N60oW-S60oE), exposed at a distance of about 5km N-NE of the dam site. This tectonic surface which, at places, is displaced by transverse faults, has a regional continuity of more than 100km. The Srinagar thrust is displaced for about 500m by a transverse fault, the Dewal Tear (N60oE - S60oW), near Dewal village in Bhilangana valley. Fig.1 shows a 3D view of Tehri PSP.

Operative use of equipment for expeditions and safe operations

Figure 2: General view Plan of Tehri Pump Storage Project

The geometry of U/s Surge Shaft and its Upper Chamber connection is unique owing to the complex Geological Formations/Low Geotechnical properties of rock mass housing the excavation. The excavated dimensions of Surge Shaft just below the chamber (at chamber invert) are 22520 x 15600mm (the dimensions are not rectangular; in the plan they comprise of two parallel lines separated by 15600mm and their ends connected by arcs of 11260mm radius). The diameter of the Shaft is 20920mm. The maximum upper chamber excavated plan dimensions are 37520 (L) x15600 (W) mm. Therefore, the chamber width is restricted to 15600mm while the Surge Shaft dia below it is 20920mm, a change that was necessitated by the non-feasibility of constructing the chamber size in commensuration with Surge-Tank diameter, considering the weak geological setup encountered. Therefore, this improvisation of tapering a circular section to a somewhat oval shape (at invert of upper chamber the plan geometry described in para 2), i.e., the 10000mm height of shaft was needed for safe operations and optimum use of the equipments. The design process also evolved in line with it.

Development of design process and equipment to achieve improvised functions

The most critical portion of the Surge-Shaft is its slant, 10m height below chamber invert.

When the design of the Shaft was conceived, the following steps were taken to achieve stability for this portion of the shaft:

Figure 3: 10m slant height; overhung beyond chamber invert

• The chamber was excavated to invert EL. 859.50m.
• Complete levelling concrete at the chamber invert was laid.
• The excavation of slant portion of the Surge Shaft was done in two semicircular parts. First part will be excavated to EL. 854.50m; this will allow working space (depth) of 2.4m below bottom chord of truss girder (gross depth 2.6m). During downward excavation, the excavation of collar beam area around the periphery atop the Surge Shaft was undertaken and stabilized, depending on the type of the rock as per approved GFC drawings. The excavation of the second half was taken up and stabilized subsequently.

Step 1: The initial excavation below chamber area up to EL. 856.50m, i.e., 3m was done in steps of 1.0m with hydraulic breaker mounted on excavator, and thereafter up to EL. 854.50 excavation carried out with control blasting. The slant portion was worked out using chiseling method.

Step 2: The step 1 was followed with first layer of 50mm thick shotcrete and fixed with the help of a high strength cable -net properly securing all the edges (full rock support as per approved drawings has to be provided after every round (1 m) of excavation.

Step 3: The step 2 was followed by the following second and final layer of supports:

• Lattice girders at 0.5 clc slant length on the first layer of wire mesh.
• Complete 2nd layer of shotcrete.
• Fix 2nd layer of high strength cable net properly securing all the edges.
• Applied final layer of shotcrete.
• Installed rock bolts as per approved GFC drawing on the final shotcrete surface.

Steps 2 & 3 were followed until the excavation & support reaches EL. 854.50m. Care shall be taken to finish the balance supports in the previous bench prior to the next bench round is excavated.

After completion of excavation & support upto EL. 854.50m, the collar beam were cast with reinforcement and concrete around the defined periphery atop the Surge Shaft with provision of base plate & bolts arrangement for support of truss girder as per GFC drawing of collar beam and truss girder. Allowed sufficient time for the collar beam to gain its strength.

Both the truss girders were installed one by one at the defined positions. All the proper precautions to fix truss girders were taken. The struts and 20mm dia. 1.5m long rock bolt were executed as per approved GFC drawing.

The excavator was positioned on the chamber invert. Any ramp to enter in the Surge Shafts was strictly avoided. ln order to maintain the conical shape and to avoid over-break, careful excavation at the initial stages of slant portion shall be done. Blasting was not to be carried out in the initial stage of excavation at least for an initial depth of 3m.

For the remaining slant area, a continued bench excavation was conducted from depth 5m to 10m with each bench round of 1 m dept, using soft blasting with line drilling. Chiseling method was followed in the slant portion in order to avoid over- breaks and to maintain the shape. Soft blasting followed below EL. 854.50m, in order to give negligible disturbance to truss girder arrangement. At each stage of working in the slant portion excavation, proper platform arrangements to cover the pilot hole were ensured. Any rock fall, cracks, wedge failure etc. were recorded and informed immediately.

Savings on materials & energy instruments

Both Standard Monitoring Systems (SMS) and Main Monitoring Systems (MMS) have been installed at various levels so as to keep a check on design deflections envisaged. Proper instructions regarding the steps to be taken in case the deformation levels exceeded, the “designed values” have been issued to the site.

The instruments are to be installed simultaneously with the excavation process as soon as the round is completed (as per the approved GFC drawings). It also ensured adequate checks on design assumptions, as this data has to be rigorously monitored and recorded and the readings thereof immediately to be forwarded to the designer. Frequency of Measurements has to be followed is as per approved GFC drawings. This ensures standard stability as the support system can be further optimized based on reliable instrumentation data.

Major equipment to be deployed after deliberations on safety and economy:

• Two boom hydraulic drilling jumbos with basket
• Hydraulic breaker
• Wheel Loader I Excavator of suitable make/model available at site
• Wet Shotcrete Machine CIFA or any equivalent make
• Dumpers
• Gantry Crane

Savings of materials or energy

Enough thought has been given on cost effective design, excavation and support measures and excavation methodology. However, safety and serviceability of a structure for its intended life is of paramount importance. Various design assumptions based on the sound engineering judgement have been made, so as to add to material saving. Rigorous instrumentation scheme, followed by design checks, also adds to the overall confidence. However, it has to be emphasized that we are excavating in a complex underground strata and at the same time have to deal with a complex geometry. Therefore, safety and stability are of immense importance.

Innovation in construction sequence of concrete lining at TRT 3 & 4 at Tehri PSP considering multilayer reinforcement in invert and overt

Construction difficulties, options, and preferred construction sequence

Option A: Practical difficulty with construction sequence of Kerb, Overt and finally Invert

Figure 4: Cross section of Tail Race Tunnel in different rock classes

• The normal construction practice of casting kerb concrete, overt concrete and invert concrete is not practically possible to adopt in the present case. This sequence of construction is possible where there is maximum two layers of reinforcement (one layer at rock face and the other at the water face).
• In the present case, number of reinforcement layers to be placed in rock class IV and V are 05 and 04, respectively.
• It is practically possible to bend maximum two layers of reinforcement during casting of kerb concrete.
• Please refer to fig. 5 (from another hydropower project) which shows one layer of reinforcement bend during kerb concrete at water face and one layer of reinforcement bend at rock face. The construction sequence adopted here is Kerb concrete, overt concrete and finally invert concrete (as only two layers of reinforcement have to be placed in position).
• In case of Tehri PSP TRT # 3 and TRT # 4 concrete lining, number of reinforcement layers are more than two, it is not possible to bend the reinforcement during kerb concrete and again to straighten them before invert concreting.
• The bottom excavation width is 5.80m. If the kerb is cast, there will not be sufficient space to travel overt lining gantry and none for the movement of transit mixers.

Hence, the above construction sequence of Kerb concreting – Overt concreting – finally invert concreting is practically difficult.

Option B: Practical difficulty with full circle reinforcement binding & concreting at a time (invert and overt together)

• In this case, it is possible to bind only one full layer of reinforcement. It’s practically difficult to bind more than one layer of reinforcement for full circle and then to follow concreting. The plate below shows reinforcement binding in one layer at full circle (this picture is for reference only).

Option C: Preferred construction sequence – 1st Invert Concrete and then Overt Concrete

Figure 5: View of reinforcement of overt and invert concrete in one go

Considering the practical difficulties (as stated above), Option-C has been adopted for Tehri PSP TRT # 3 and TRT # 4 concrete lining. The details of construction sequence are listed below:

• Cleaning of loose muck at invert
• Reinforcement binding at invert portion
• Concreting of invert portion using invert shutters.
• During the invert concreting, necessary embedment will be fixed atop the invert surface which will remain a support for the rails to travel the gantry. The typical arrangement is shown in the following photograph. To hold the gantry shutters with invert concrete, if necessary, some sort of bracing will be provided.
• Erection of Overt lining gantry formwork
• Binding of reinforcement at the overt ahead of overt lining gantry formwork erection.
• Concreting of overt lining
• grouting at overt lining (as required)
• Removal of embedment at invert top surface.

In the above sequence, invert and overt concreting will follow each other with a lag not more than pour length of two gantries (say 24m). This will ensure proper placement of concrete pipelines and concreting equipments (concrete pump, transit mixer) to be placed close to the pouring gantry.

Figure 6: Invert – Rebar and shuttering @ TRT-4 U/S

For the above sequence of concreting, discussions were held and approval received from the client as:

• Overt lining gantry design is based on the above sequence.
• The embedment fixed atop the invert surface will be removed after completion of overt lining
• There will be movement of tyre mounted vehicles on the invert (except crawler type), if necessary.

Figure 7: Overt- Rebar and Gantry Shutters @ TRT-3 U/S

Proposal of concrete barrier wall below the downstream Surge Shafts for parallel working at Draft tube excavation, Machine Hall excavation up to the bottom parallel with the Shaft slashing from the top.

Figure 8: Completed view of overt and invert

The bench excavation of the Powerhouse from EL 562.30 up to EL 552.55 (unit pit bottom) is to be carried out through downstream penstocks. The Draft tube has to reach Powerhouse at EL 554.00.

As per the initial stage construction planning, the excavation of draft tubes was planned to be completed before completion of excavation of the downstream Surge Shafts. However, in the present scenario, the slashing excavation of the downstream Surge Shafts 3 and 4 has also to be carried out parallel with the excavation of draft tubes.

After completion of Powerhouse bench excavation till EL 552.55 (unit pit bottom), the concreting of the draft tube pedestals and further installation of the draft tube segments will be started. The concrete lining of unit-wise draft tubes will start after installation of the draft tube segments at unit pits.

In order to facilitate parallel activities of slashing excavation of the downstream Surge Shafts (muck dozing through excavated pilot hole) and draft tubes, the proposal of the concrete barrier wall below downstream surge shafts have been given below.

Figure 9: Cross section of Machine Hall-Draft tube-D/S Surge Shaft bottom

The concrete retaining wall is proposed to have an independent access for draft tubes leading to the Machine Hall bottom. The proposed construction sequence is given below.

Excavation below the downstream Surge Shafts 3 and adjacent to already excavated pilot hole, has to create access up to start of Draft tube. Construction of the concrete retaining wall is done thereafter.

During the excavation of the Draft tube and parallel excavation of the downstream Surge Shafts, slashing (muck will be dozed through pilot hole). Powerhouse benching (below EL 562.30 till EL 552.55), movement of excavation, mucking and rock support equipments has to be done through Draft tubes. Thereafter, pedestal concreting of the unit wise Draft Tubes to be done through the same access.

Figure 10: (Plan of Concrete retaining wall along with Surge Shaft bottom and draft tube & Figure 11: Cross sections Surge Shaft bottom, draft tube access and concrete retaining wall

Advantages of this proposal

• The muck removal from bottom of downstream Surge Shafts will be carried out independently.
• We have to facilitate as an independent access for excavation of Draft tube and furthermore for Powerhouse benching below EL 562.30 till EL 552.55.
• We have to provide independent access for Pedestal concreting of the unit Draft Tubes.
• Powerhouse concreting works at Unit Pits shall not be stopped till completion of the excavation of the downstream Surge Shafts. As soon as the concreting at unit Pit is completed, transition portion and draft tube concreting will be independently started.

Figure 12: Concrete barrier wall in construction stage

Health and safety implementation

Being the prime civil contractor, HCC has established a health and safety department at the site with a team of safety experts and medical staff. Regular awareness programmes regarding work safety, importance of PPE’s, project activities information, hazards and mitigation measures of hazards, safe working procedures of every activity, basic safety rules, fire and fire protection system, accident prevention and control, emergency systems and preparedness, motivational training and safety movie presentation have been conducted on a regular basis for the engineers, workers and laborers. The safety department conducts a routine induction programme for visitors coming to the site before they enter the project area.

Figure13: Access to draft tube after construction of concrete barrier wall on right

Usually, underground operations are quite hazardous and the chances of minor and major accidents are high. Maximum precautions were taken during the construction of the powerhouse caverns, the upstream and downstream chamber 3 and 4 and shafts, the TRT 3 and 4 upstream and downstream excavation, upstream and downstream Bus bars and TRT outlet structures. Utmost care was taken for the safety of our people and equipment while erecting the ribs, gantries, and other structures. Only then the boring of shafts and erection of steel liners was carried out.

During the construction stage, the existing medical facilities of the District Hospital of New Tehri and the existing medical facilities of the nearby THDC Hospital of Tehri PSP Project were utilized and arrangements were made with the nearest PGI - The Himalayan Hospital - by signing a contract for the workers of HCC. A first-aid post has been provided at each of the major construction sites, so that workers are immediately attended to in case of an injury or accident. The first-aid post has a qualified doctor, male nurses, first aid box with essential medicines including ORS packets, first aid appliances like splints, dressing materials, stretchers, wheel chairs, ambulance vans etc.

Conclusion

The construction of the Tehri PSP (4x250MW) is unique, both because of the precise and meticulous planning that was done in advance and because of the way the innovations done in various structures for getting the desired results. The selection of proper equipment and bringing international design consultants for the project, helped us make progress successfully, irrespective of the adverse geology and other unforeseen contingencies. With this experience the following findings were established:

Excavation of underground structures in highly adverse geology where surprises are common, can be done successfully with meticulous planning, proper investigation and innovations in technologies.

A good approach road to the project location is a must for the success of any project in a hilly area. There must be proper planning of highly technical works, which are usually difficult and hazardous in nature, and where the workplace is very restricted. Various innovations in vertical shafts excavations and in TRT’s, especially in concrete lining, were keys to the success of this project.

The selection of proper equipment for each structure and on-the-spot decisions at site with the consent of all stakeholders were also the two most important factors for the success of the project.

Acknowledgements

The author (Rakesh Kumar Khali, Project Director-Hydro, Hindustan Construction) is thankful to the THDCIL management for their continuous support and without whose help construction of the TEHRI PSP was not possible.