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Challenges of Hydro Tunnelling in India

 

With a great deal of tunnelling work under way at hydroelectric projects in India, the lessons learned by developers of the 192 MW Allain Duhangan project can prove valuable for other companies working in this country.

By Claudio Vissa, Rakesh Mahajan, German Vera Lazo, Kent Murphy and S.P. Bansal

Accompanying the significant new hydro development activity under way in India, a great deal of tunnelling work is in process. Tunnelling to develop hydro projects in India can be challenging for several reasons. One example is the potential to encounter structurally unsound rock due to the complexity of the region's geology. This case study of tunnelling work related to development of the 192 MW Allain Duhangan project illustrates some of the challenges faced and lessons learned.

Background

The run-of-river Allain Duhangan project is on the Allain and Duhangan streams in mountainous northern Himachal Pradesh. This project features a two-unit underground powerhouse operating at a gross head of 876 m. Water is supplied via the Allain Nallah and Duhangan Nallah diversions. The project has a combined flow of 26 m3/s.

In 1993, a development concession was awarded to the Bhilwara Group. Site surveys and investigations were carried out from 1995 to 2003, and final design and site infrastructure preparations began in 2004. Project owner Allain Duhangan Hydro Power Limited hired RSW Inc. (now AECOM) and Indo Canadian Consultancy Services, a joint venture company of LNJ Bhilwara group and RSW Inc., to perform the detailed design and construction supervision of the project.

The first stage was commissioned in July 2010 using water from the Allain Nallah diversion, which represents about 70% of the combined flow. The Duhangan diversion and headrace tunnel were incorporated in February 2012.

Detailed description

The Allain barrage, at an elevation of 2,750 m, diverts a nominal discharge of 19 m3/s through a desilting basin before entering a headrace tunnel. An intermediate storage basin, used during low flow periods, regulates the headwater level before transiting flow through a 2.94 km-long headrace tunnel. The intermediate basin has live storage capacity of 220,000 m3 and provides peaking generation for a minimum of four hours per day during lean flow months.

The Duhangan intake, at an elevation of 2,780 m, transits a nominal discharge of 7 m3/s through an underground desilting basin before entering a 4.34 km-long headrace tunnel. Flow in the Duhangan tunnel is transported as open channel subcritical flow over a distance of 3.12 km at a slope of 0.24 % before reaching a gravel trap. Downstream of the trap, the headrace tunnel plunges toward the surge tank with a slope of 7.7%.

Within the steep slope section, the flow regime changes from open channel to pressure flow when coming into contact with the surge tank's approximate free surface water level, allowing formation of a hydraulic jump. Both tunnels meet at the surge tank, whose invert is at elevation 2,680 m.

Downstream of the surge tank, the total flow is discharged via a penstock shaft 1,500 m long and reducing in diameter from 3 m to 2.4 m. Near the powerhouse, the penstock bifurcates into two penstocks for the Pelton units, each 35 m long. An 800 m-long tailrace tunnel discharges the water back into Allain stream.

Hydraulic design

Hydraulic aspects of the project include the design of diversion and headworks for each stream. The Allain barrage contains a spillway with two gates, sluiceway with two gates and head regulator with four gates followed by four desanding basins. Water from the desanders is collected in a pool before entering the 1.1 km-long section of Allain headrace tunnel that interconnects the barrage and intermediate reservoir. The barrage is designed to operate at a lower pond level of 2,746.3 m during the monsoon. This is achieved by keeping both spillway gates fully open to ensure safe flood passage. During the low-flow season, the barrage water level will be controlled between 2,746.3 m and 2,752.5 m to provide peaking storage of 85,000 m3.

Tunneling work for the Allain Duhangan project was completed successfully after encountering several challenges, including unexpected rock conditions during excavation of the Allain headrace tunnel.

The intermediate reservoir provides 220,000 m3 of storage between the minimum and maximum operating levels of 2,734 m and 2,748 m. The interconnecting Allain headrace tunnel is drained daily during peaking operation in lean-flow season. The Allain headrace tunnel beyond the reservoir up to the surge shaft is lined and remains under pressure at all times.

Water from Duhangan stream is diverted by a trench weir into a bottom intake that leads to an underground desander. Gravel and sand flushing tunnels transport sediments back to the river.

The 4.3 km-long Duhangan headrace tunnel passes under mountains, rising to about 1 km above the tunnel level. The initial 3.1 km length of tunnel between elevations 2,748 m and 2,734 m operates under free flow conditions and contributes toward storage for peak power generation, while a downstream length of about 400 m always remains under pressure flow. During transient flow conditions, the intermediate 800 m length of tunnel fluctuates between free and pressure flow.

Design of the surge shaft to handle hydraulic transients arising from combined operation of the two differing-level headrace tunnels was an interesting problem. The surge shaft is at the downstream end of the Duhangan headrace tunnel and adjacent to the downstream end of the Allain headrace tunnel. The Allain tunnel is connected to this surge shaft through a 3 m-diameter horizontal tunnel that also acts as a restricted orifice during hydraulic transient conditions. The flow in Duhangan tunnel is transformed from free flow to pressure flow, and a hydraulic model study indicated the possibility of trapped air bubbles traveling with the flow into the pressure shaft. To prevent this, the area at the junction of the Duhangan headrace tunnel and surge shaft was enlarged by increasing tunnel height. Air bubbles are trapped and exhausted to the atmosphere via a vent pipe.

Geotechnical design

With most of the project being underground, the first technical concern was evaluating the geological conditions and planning excavation methodologies. For the zone of the underground powerhouse, core drilling and an exploratory adit showed thick bands of good-quality mica-rich gneiss and schist that dipped gently north-east into the mountainside. Rock quality designations in the drill cores were in excess of 80, and Barton's tunnelling quality index "Q" values in the adit varied from 25 (good rock) to 60 (very good rock). Similar thick bands of monoclinally dipping gneissic rock were exposed in cliffs along both Allain and Duhangan valleys.

The underground powerhouse is about 150 m east of Allain Nallah, where the vertical rock cover is also about 150 m. The geometry of the powerhouse and transformer gallery caverns was based on precedent and confirmed using a numerical model with rock stress, strength and deformability data obtained from rock mechanics testing in the adit. In-situ principal rock stresses determined using the hydraulic fracture method varied from 2.4 to 7.3 MPa, compared with a vertical stress of 4.8 MPa; unconfined compressive strengths of intact rock averaged 42 MPa, while the rock deformation modulus determined from plate jack testing and other empirical relationships varied from 26 to 38 GPa.

Rock support for the powerhouse and transformer cavern roof arches and walls consisted of grouted high-strength rockbolts placed in pre-determined patterns and reinforced shotcrete.Excavation and support work was completed without incident.

With the exception of overburden sections, all project tunnels and shafts were designed to be excavated using conventional drill and blast technology with shotcrete, rockbolts and, as needed, steel sets for temporary support. Steel sets and forepoles were planned for the overburden sections. Both the Allain and Duhangan tunnels were designed with a 4 m-diameter, recumbent "D" excavation section. This section was increased to 4.5 m to facilitate equipment movement.

In view of the high mica content of the bedrock, all hydraulic tunnels were to be concrete-lined. The exceptions were the tailrace tunnel, which was finished with a shotcrete lining and concrete invert, and the upstream free-flow section of the Duhangan headrace tunnel, where only the invert and wall sections in contact with the flow are concrete-lined. Steel liners were provided in areas where rock cover is insufficient to contain the internal pressure and at intersections with construction adits to minimize seepage losses.

The permanent linings in both headrace tunnels are circular in cross section with a minimum concrete thickness of 0.3 m. Because the internal pressure in both headrace tunnels is small relative to the rock cover, external groundwater pressure generally governs the lining thickness.

Excavation of the Allain headrace tunnel did not go as planned. Numerous zones of altered and sheared rock with heavy seepage were encountered and several cave-ins, some extending to surface, were experienced, necessitating re-mining of existing sections and some realignments. To combat the difficult ground conditions, the contractor was obliged to use closely spaced steel sets together with forepiles, pipe roofing and grouting. The site management team worked with the contractor to overcome these conditions. Steps taken by site management included approval of additional construction adits to access the tunnel alignment, supply drilling equipment for setting forepoles, chemical grouting materials and shotcrete equipment. Grouting and shotcrete specialists were engaged to assist the contractor in the use of this equipment. The Allain tunnel was completed in March 2010, almost one year later than the original schedule and at slightly higher cost.

Excavation of the Duhangan tunnel and underground desanding chamber was much less eventful. Rock conditions were generally good and support requirements nominal.

Pressure shaft

Excavation and steel lining of the 3 m-diameter, 1,580 m-long inclined pressure shaft (penstock) between the surge shaft and powerhouse was one of the most challenging elements of the project. The decision to locate the pressure conduit underground and line it with steel was made to minimize consequences to villages and infrastructure along the valley bottom if there was a failure of a surface conduit.

The geometry of the pressure conduit, with three sections inclined at 52 degrees linked by horizontal sections, was determined on the basis of the height limitations of Alimak raise climbing equipment and the availability of intermediate access points. It is noteworthy that the 567 m-long middle section of inclined shaft is among the longest ever attempted using Alimak equipment.

During excavation of the upper limb of the pressure shaft, weathered rock condition was encountered about 100 m below the valve house level. Excavation using a raise climber had to be abandoned, and the balance of the inclined shaft excavation was done from the top using a winch and by providing steel rib supports with backfill concrete.

The penstock design is based on the steel liner carrying all the external hydrostatic load, assuming the encasing rock to be saturated to the ground surface. Based on available rock parameters, it is estimated that 85% of the internal hoop stresses are transferred to the steel liner, with the remainder transferred to the rock. This load distribution is conservative. No contact or consolidation grouting was done along the inclined sections of the liner because the concrete backfill was certain to fill all the space. Also, designers wished to avoid any possibility of leakage through failure of grout hole plugs, as experienced on some other Indian projects.

Another element of penstock optimization was the decision to reduce the excavated shaft diameter by making full-penetration welds of all circumferential joints from the inside only, using a backing plate. To facilitate preparation of the steel plates and welding procedures, a high-strength steel was used, Sumiten 610 manufactured by Sumitomo Corporation of Japan. Steel plate thickness varies from 16 mm at the top of the shaft to 65 mm at the bottom.

The bifurcation piece is designed to resist an internal pressure of 100 MPa and, after erection, the piece is also subjected to hydrostatic and waterhammer stress testing on site. Inspection manholes are provided at each of the two horizontal steps in the pressure shaft.

Penstock steel erection, with all welding of circumferential joints done from within the ferrules using a backing plate, was followed with concrete backfill placed a short distance behind the welding operations. No leaks were detected during initial filling, and the penstock has operated satisfactorily.

Plant design

The design of the underground complex is largely conventional but does present some interesting features.

Although the cavern excavations were essentially dry, a lightweight modular steel ceiling is provided in the powerhouse to protect equipment and personnel from groundwater seepage or loose rock in the arch. The ceiling is suspended from rock bolts, a concept originating in Canadian powerhouse design. Powerhouse lighting is integrated in the ceiling design, and the ceiling can be accessed for maintenance of the lighting system and inspection and maintenance of the roof arch.

The main gantry crane beams in the powerhouse are supported on steel columns. The design allowed erection of the support beams and rails immediately after excavation of the powerhouse substructure. Thus, the crane could be put in service for substructure concreting and erection of the ceiling and lighting system.

Lessons learned

Despite a lack of sophisticated support tools (helicopters, satellite imagery) during the feasibility stage of site investigations, the predictions of geological conditions were generally accurate for the underground power complex, pressure shaft and Duhangan headrace tunnel. With regard to the poor conditions encountered along much of the Allain tunnel alignment, more investigation of this area might have revealed the real state of affairs and helped in construction planning.

While many aspects of the project may be considered conventional practice, the design contains innovative features that saved costs and will ensure long-term operating efficiency. These include:

— Hydraulic design of the Duhangan tunnel using free flow as well as pressure flow condition;

— Locating the powerhouse and pressure shaft underground;

— Steel lining the entire pressure shaft;

— Establishing a storage reservoir on the Allain circuit rather than near the surge shaft, which added to project safety;

— Design of intermediate reservoir walls (cantilever rather than gravity wall);

— Hydraulic model testing of the surge shaft at the junction of the Allain and Duhangan tunnels; and

— Using the Allain and Duhangan tunnels to provide water storage for peaking generation.

Claudio Vissa is vice president of AECOM. Rakesh Mahajan is director at Indo Canadian Consulting Services. German Vera Lazo is project manager and Kent Murphy is chief geologist for AECOM. S.P. Bansal is heading civil works for construction of the 192 MW Allain Duhangan project.

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