By Imran Sayeed
In India, the Himalaya mountain range has enormous untapped potential for hydro development. According to the Central Electricity Authority of India, about 80 percent of the 148,700 MW of hydro potential in the country comes from rivers that arise in the Himalayas. In fact, only 2 percent of the potential in northeastern India and 24 percent of the potential in northern India has been developed.
One significant challenge in developing this potential is the structurally unsound rock and other issues related to the complexity of the region’s geology. Throughout 33 years of developing hydro projects in this area, NHPC Limited (formerly known as National Hydroelectric Power Corporation Limited) has developed solutions to a number of geotechnical problems. For example, it is possible to successfully excavate underground powerhouses or make high cut slopes for spillways or surface power stations in the Himalayas by carrying out detailed investigations and using appropriate rock support systems. Other challenges for which NHPC has developed solutions include dealing with difficult foundation conditions, locating construction materials, and tunneling in uncertain rock conditions.
Experience shows that it is feasible to build large dams in the Himalayas despite constraints the geology imposes. For example, the highest concrete gravity dam (Bhakra Dam), highest rockfill dam (Tehri Dam), longest headrace tunnel (associated with the 1,500-MW Nathpa Jhakri project), and largest underground powerhouse cavern (Tehri) in the country are all in the Himalayas. In addition, construction is under way on 800-MW Parbati 2 with its 31.5-kilometer-long headrace tunnel and 3,000-MW Dibang with a 288-meter-high dam, which will be the highest concrete gravity dam in the world.
Hydro development in the Himalayas
The Himalayas are the world’s highest mountain range, with more than 100 peaks attaining a height above 7,200 meters. In India, the Himalayas run, from west to east, through the states of Jammu and Kashmir, Himachal Pradesh, Uttrakhand, Sikkim, Arunachal Pradesh, and Assam.
The Indus, Ganges, and Brahmaputra rivers arise in the Himalayas and flow toward the northern plains in India. These rivers are fed by the permanent snow line and glaciers in the summer and by heavy rainfall during the monsoon season. This arrangement of a steep fall in the Himalayan river beds, together with perennial discharge, forms an ideal setting for hydropower development.
Geotechnical challenges and solutions
In the Himalayas, the geological challenges occur in part from the fact that this mountain range evolved due to the collision of the Australasian and Eurasian plates. The rocks were thrown into several folds and fault zones, giving rise to a disturbed rock mass traversed by several discontinuities. Geotechnical challenges that must be solved include: assessing foundation conditions, ensuring stability of high cut rock slopes, securing rock slopes during construction of above-ground powerhouses, locating construction materials, and tunneling in uncertain rock conditions.
Assessing condition of the foundation
Rivers that arise in the Himalayas have a high-velocity flow of water because of the steep fall. The rivers often follow weak zones of rock, such as faults. In these areas, deep erosion in the river bed may be covered by loosely consolidated deposits. These rivers often flow through narrow gorges that may be the result of a major discontinuity. Because of this situation, there is a general tendency of an irregular deep bedrock profile in many river sections. Choices of viable dam sites often are limited, so proper site selection in the deep gorges or even in relatively wider valley sections that may have buried channels is quite challenging.
Choosing a site for a dam in the Himalayas primarily requires careful assessment of foundation conditions. For rivers with deep bedrock (30 meters or more deep), building a rockfill or concrete-faced rockfill dam (CFRD) with a positive cutoff, such as a plastic concrete diaphragm wall, is an effective measure to avoid the need to excavate down to the bedrock. For example, NHPC built a CFRD at 280-MW Dhauliganga 1, where the bedrock was 65 to 70 meters deep. NHPC also used this solution at 520-MW Parbati 3, which features a conventional rockfill dam with a clay core and a cutoff wall to a depth of 40 meters. Rockfill was chosen over CFRD for this site because it is less expensive than excavating to bedrock and avoids the construction risk involved in this practice. In addition, transporting a large quantity of cement to the remote location would be challenging.
If the bedrock is shallower than 30 meters, construction of a concrete dam may be a better option. Excavation of overburden to this depth is feasible without much difficulty. Deeper excavation involves problems of seepage, slope stability, and time required to complete the work. However, if sufficient materials are available for construction of a rockfill dam and an adequate spillway can be provided, it could be less expensive to build a rockfill dam on a site with shallow bedrock.
Other factors – such as availability of construction materials and capacity required for the spillway – also plan a major role in selecting the type of dam.
For concrete dams, proper investigation is required to determine the sub-surface bedrock profile, as well as the foundation conditions. In situations where there is a large deviation in foundation depth, problems may include increased excavation, water seepage, and large increases in concrete. This can cause problems with regard to a project’s construction schedule. In addition, provisions may be required for treatment of shear zones. For the concrete dams built to impound water for the 540-MW Chamera 1 and Parbati 2 projects, NHPC was able to use a carefully planned drilling program to predict the bedrock conditions quite accurately.
Ensuring stability of rock slopes
There are several projects in the Himalayas that may involve slope cuts more than 50 meters high. This includes excavating for building side channel spillways at rockfill dams or for removal of weathered or slumped rocks, which typically are present at many sites in the Himalayas. Removal of these rocks may be needed to provide a sound foundation for placement of the dam and a proper junction between the dam body and abutments, or for building side channel spillways at rockfill dams. Rock conditions play a pivotal role in the design of such slopes and the need for adequate rock reinforcement.
For 280-MW Dhauliganga 1, a high slope cut was to be executed on the right bank in strong biotite gneiss of Pre-Cambrian age. However, the joint patterns of this rock were such that prominent unstable wedges formed. When the excavation work began in 2000, blocks as large as 10,000 cubic meters in volume started to fall from the cut slope.
To solve this problem, NHPC engineers proposed special support measures apart from modifying the slope of the rock. Rock supports already proposed for the situation included 18-meter-long cable bolts; 9-, 12-, and 15-meter-long rock anchors; shotcrete; and wire mesh. The special measures involved reinforcing the rock mass by driving 30-meter-long tunnels, with additional cross-cuts, into the hill slope and then back-filling the tunnels and cuts with concrete and steel. This solution stabilized the slope and enabled the project to be commissioned on schedule in March 2005.
In this case, the large size of the blocks was detrimental to slope stability and necessitated modifications to the slope angle, as well the support measures. The key to success for a high slope excavation lies in proper investigations, design, and support provisions and careful execution. This arrangement is described below for surface powerhouses where high open cuts may be involved.
Building above-ground powerhouses
For hydro projects in the Himalayas that involve surface powerhouses, slope stability is a problem. Because of the narrow configuration of the valleys, space must be created for powerhouses by cutting the hill slope. For Parbati 2, a surface powerhouse with hill cutting of 100 of 125 meters was planned. This powerhouse is in meta-basics and chlorite schists/phyllites with three sets of joints. This slope has suffered three collapses due to the deep cutting. Redressing of the slope with heavy supports is under way. The support elements consist of 35-meter-long cable anchors, 6- to 12-meter-long rock anchors, shotcrete, and wire mesh. The entire slope is expected to be completed by the end of 2009.
As the above example illustrates, execution of slopes, particularly in adverse rock conditions, remains a challenge. Depending on the slope height and rock conditions, heavy supports may be required. Generally, for high slopes there is considerable provision of support measures. These supports include long rock bolts or anchors (9 to 15 meters), cable bolts, treatment by injection of grout, and/or use of reinforced concrete plugs to make small horizontal tunnels into the slope. Proper drainage arrangements also are necessary.
Accordingly, it is important to provide sufficient time in the schedule for installation of systematic supports during excavation. The amount of time needed depends on the magnitude of the work.
The Bureau of Indian Standards publishes codes of practice for geological investigation for dams and powerhouses,1 which are followed in India to perform river valley investigations. In view of the problems faced in the Himalayas and also based on the successes achieved at some projects, the following steps are recommended for deep open excavations for surface powerhouses or for high cut slopes for other structures:
- Complete geotechnical mapping, on a 1:1,000 scale, covering the entire area of the cut slope, as well as about 50 meters on the sides and above the top of the proposed cutting line;
- Excavate two to three test tunnels to probe the slumped or weathered zones and determine the nature of any discontinuities;
- Drill two to three holes beyond the periphery at an elevation 10 to 20 meters above the top of the cut line, to delineate the overburden and weathered or slumped rock that must be dealt with immediately as the cutting begins and to ascertain the quality of rock in deeper excavations and thus both establish pre-treatment before commencing actual excavation and determine the adequacy of rock supports;
- Perform laboratory tests on rock samples to determine physical and engineering properties;
- Perform in-situ rock mechanic tests for shear strength, modulus of deformation, and elasticity; separate tests may be necessary for shear strength along joint planes;
- Include sufficient provisions for rock support according to numerical analysis/design calculations and proper scheduling in the tender for the works or contract with the executing agency;
- Use controlled blasting and immediate quick rock support during execution of a high cut slope (height of unsupported areas may not be more than 2 meters for individual excavation rounds); and
- Use pre-strengthening measures like grouting in weak rocks or vulnerable areas.
Locating construction materials
The choice of a dam type greatly depends on the availability of construction materials. In the Himalayas, the rocks contain a considerable percentage of free mica, rendering them unsuitable for use as aggregate. In addition, because of the steep bed slopes in the rivers, the occurrence of suitable river shoals or terrace deposits is rare. This results in greater dependence on rock quarries.
Detailed work is required to choose safe and environmentally benign locations for quarries. This work involves studying the available rock types (which could be used for aggregate) and performing surveys, testing, and confirmation regarding exploitable quantities. The optimal choice is to locate quarries in the area that will be submerged when the reservoir is impounded because this avoids affecting new areas. If suitable deposits are not found in the reservoir area, alternative locations for the quarry are identified, along with a plan for restoration of the quarry site after construction work is complete. Such restoration plans are now part of the environmental management plan included in the environmental impact assessment for all NHPC projects.
Tunneling in uncertain rock conditions
Tunneling is an intrinsic part of hydro projects in the Himalayas. Since 1975, NHPC has completed more than 200 kilometers of tunnels in the region. Although many projects with tunnels have been completed in the Himalayas by other companies as well, some have taken a decade or more, with the delays mainly attributed to tunnel completion. Contractors cite poor rock conditions as the prime reason for cost and time overruns.
Keys to successful tunneling include:
- Investigation and rock mechanics testing, before construction begins, to develop a suitable tunneling method, select support elements for different types of rock, estimate the quantity of work to be performed, and identify geohazards that require treatment (such as fractured and crushed rock zones, fault crossings, water ingress under high pressure, and rock burst areas); and
- Provision of immediate primary support in the heading portions, consisting of shotcrete or fiber-reinforced shotcrete, together with rock bolts. When this support is not in place, there are likely to be collapses and subsequent disruptions to the work.
The tailrace tunnel for 480-MW Uri 1 provides an example of efficient support methods in poor and highly stressed rock conditions. Along the course of the 2-kilometer-long tunnel, the contractor encountered a folded thrust zone between the sedimentary formation and the meta-volcanics. According to the geomechanical classification developed by Z.T. Bieniawski, the rock encountered in the tunnel included 1 percent class II, 49 percent class III, 28 percent of class IV, and 22 percent class V. There are five rock classes in the geomechanical classification, from I (very good) to V (very poor).
Despite the poor rock quality at this site, no steel arches were needed for support, nor was there a single instance of cavities or heavy overbreak. (Overbreak is rock excavated in excess of that needed to install the tunnel. In this case, overbreak would result from weak rock or close fractures or shear zones that could not be controlled.) Support elements used at this site consisted of pre-grouting of the rock mass, division of the tunnel section into several parts for easier excavation in poor rock, application of 200- to 250-millimeter-thick shotcrete with a double layer of wire mesh and water-expanding bolts, and grouted dowels. Successful completion of Uri 1 brought about changes in tunneling techniques in the Himalayas, particularly with respect to the use of flexible supports and pre-grouting as a stabilization measure. The tunneling method used at this site, designed by Sweco of Sweden, is based on the New Australian Tunneling Method. The civil contractor was Uri Civil Contractor AB, a Swedish-British consortium led by Skanska.
The use of tunnel boring machines in the Himalayas has met with limited success. The Parbati 2 project is a good example. This project involves trans-basin water transfer between two rivers via a 31.5-kilometer-long headrace tunnel. This is one of the longest water-conducting tunnels in the world. Total tunneling for the project, which includes feeder tunnels and access adits, is 57.2 kilometers. Construction of this project began in 2002 and is scheduled to be complete in 2010, according to the revised program.
One element of success at this project involved use of a double shield inclined tunnel boring machine to excavate two inclined pressure shafts. These shafts are 1,546 meters in length and 3.5 meters in diameter, at a difficult angle of 30 degrees, and run through meta-basics and chlorite schist bands. Progress in the second pressure shaft was so fast that the tunnel was completed in 136 days in 2006. The main reason for the success of this tunnel boring machine is the moderate strength of the meta-basics, which is amenable to boring without difficulty.
Use of an open shield tunnel boring machine to complete the portion of the headrace tunnel that passes below a high ridge met with less success. Initially, the machine worked fairly well in granite gneiss, schistose gneiss, and schist bands. However, progress slowed as the tunnel entered quartzites, and heavy-duty cutters were required. Simultaneously, as the rock cover on the tunnel increased to 800 to 1,000 meters, the jointed quartzite gave rise to wedge failures near the cutter head.
Because there were site limitations to using shotcrete, the contractor used wire mesh with channels in the crown portion, together with rock bolts, as support measures in class III conditions. In class IV and some area of class III, ring beams were used.
From 4,000 meters onwards, the tunnel encountered closely jointed zones and silt-filled discontinuities in the tunnel. At 4,056 meters, water and silt emerged from a probe hole under high pressure, with a discharge of 5,000 to 6,000 liters per minute. This caused inundation of the tunnel for nearly 2 kilometers, and the tunnel boring machine was virtually buried under silt. The discharge slowly subsided to 2,000 liters per minute and continues more than two years after the leak began. As of February 2009, discharge had reduced to 1,350 liters per minute. An expert group consisting of experts in contracts, civil design, cost engineering, geology, and financing has recommended options for treating the difficult zone so that the balance of the tunneling can be completed. These options include building a bypass tunnel and treating the weak zone by grouting.
Development of the nearly 119,000 MW of hydro potential from rivers in India that arise in the Himalayas relies on finding solutions to challenges posed by the region’s complex geology. Through 33 years of experience developing hydro projects in this area, NHPC Limited has developed solutions to a number of geotechnical problems. These solutions have allowed construction of some of the largest dams and hydraulic structures in the world.
The views expressed in this article are those of the author and not of NHPC Limited management.
1Code of Practice for Sub-Surface Investigation for Power House Sites, IS 10060: 1981, Bureau of Indian Standards (BIS), New Delhi, India, 1981 and 2004. A number of other standards for geotechnical investigation and subsurface exploration are available under Division 14 Water Resources, Section WRD-5 of the BIS.
Imran Sayeed is chief (geology) for NHPC Limited, which develops and operates hydroelectric projects in India.