The rock mass is typically a jointed aquifer where watermoves through the most permeable discontinuities orthrough open channels along them. In general, the rockmasses close to the surface are more jointed and thejoints are more open than deeper in the rock mass.Visual observation carried out in many ungrouted tun-nels indicates that most water leakage in general occursin the part of the tunnel which is closest to the surfaceand is confined in fractures, faults and weathered zones(Nilsen and Thidemann, 1993 and Karlsrud, 2002).
Water leakage problems in unlined or shotcretelined water tunnels are not new issues in tunnelling.In many occasions severe water inflow as well asleakage problems have been faced that not onlyreduced the stability of the rock mass surrounding thetunnel but also valuable water has been lost from thetunnel causing huge economic loss to the projects.According to Kassana and Nilsen (2003), some of the notable projects, which have suffered excessivewater leakage problems are Chivor II (Columbia),Whatshan (Canada), Askora and Bjerka (Norway)and Kihansi (Tanzania). Consequently, water leakagecontrol in the tunnels plays a vital role not only inimproving the rock mass quality, but also in savingeconomic loss caused by leakages. The innovativesolutions that exist at present in tunnelling makes iteasier to control and reduce the amount of water leak-age to a target level by pre and even post-groutingtechnique. However, the main concerns are the cost
and time, which must be accounted and optimized.Thus, the proverb “prevention is better than cure” fitsvery well when it comes to water leakage control intunnels.
In the Himalaya, due to active tectonics in thisregion, the rock masses are highly fractured, folded,sheared and deeply weathered. Tunneling throughnumerous zones of weakness, fractures and faults isthus a matter of reality. Moreover, the majority of thesezones are in general highly conductive, representatingpotential sources of ground water aquifer as well aspossible sources of water leakage from the completedunlined/shotcrete lined tunnels. Accordingly, treatingthe rock mass with injection grouting against waterleakage might be very cost effective and environmen-tally friendly solutions for tunnels in this region. Asan example on the effectiveness of grouting, Karlsrud(2002) indicates that the hydraulic conductivity of therock mass closest to the tunnel periphery can bereduced to approximately one tenth the conductivityof ungrouted rock if systematic pregrouting is carriedout during tunnel construction.
This paper is thus focused on the significant rolethat grouting may play in controlling the water leak-age. For this the headrace tunnel of the Khimti IHydropower Project in Nepal Himalaya has beentaken as a case. In this Project, the grouting con-tributed significantly in reducing permeability andmeeting the contractual requirements concerning waterleakage. The main focus of the paper is the effectiveuse of pre-injection and post-injection cement grouting.
Significance of grouting for controlling leakage in water tunnels – a casefrom Nepal
K.K. Panthi & B. NilsenDepartment of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology(NTNU), Alfred Getz vei 2, N-7491 Trondheim, Norway
ABSTRACT: The rock itself is a significant barrier against groundwater movement if it has low porosity andis unweathered. The existence of discontinuities in the rock mass however increases the permeability and it mayvary widely. In the Himalayas, active tectonic movement and shearing have made rock masses weathered andfractured and increased the permeability. Having such rock masses has direct impact on stability caused bywater inflow and leakage in tunnelling. If the tunnel is excavated for conveying water, the leakage problem isnot limited only to the tunnelling phase, since there is also high risk of water loss through unlined tunnels dur-ing operation. The paper discusses the role that preinjection and postinjection grouting played for controllingleakage through the unlined headrace tunnel of the 60 MW Khimti I hydropower project in Nepal Himalaya.
2 KHIMTI PROJECT
Khimti I Hydropower Project is located in theHimalayan region about 100 kilometers due east ofKathmandu, Nepal, see Figure 1.
The Project is owned by Himal Power Limited(HPL), Nepal and is among the first privately ownedhydropower projects in the country. The Projectstarted its commercial operation in June 2000. TheCivil Construction Consortium (CCC) (a joint ven-ture between Himal Hydro, Nepal and Statkraft AS,Norway) undertook the construction work on a turnkey basis in 1996 and the construction work was com-pleted in the summer 2001 after a one year defect lia-bility period. The Project has an installed capacity of60 MW and generates approximately 350 GWh elec-trical energy annually. To generate this energy theProject utilizes water from the very steep KhimtiRiver, which has an average gradient of about 7 per-cent. Khimti I is a high head scheme, with a designdischarge of 10.75 m3/s and a gross head of 684meters. The total waterway length of the Project isapproximately 10 kilometers (HPL, 2000).
The headrace tunnel, which is the major focus ofthis paper, is a pressurized tunnel with maximum andminimum static water head of 4 bars and 1.1 bars atits downstream and upstream end respectively. Thetunnel is approximately 7.9 kilometers long withinverted D-shape and 14 square meters cross-section.Except first downstream end of 418 meters with fullreinforced concrete lining, the tunnel is built based onNorwegian tunnelling principles and is unlined orshotcrete supported. Modern support means such aspre and post-grouting, steel fiber shotcrete, spilingand rock-bolts have been used.
2.1 Project geology
Geologically, the project lies in the crystallineTamakoshi gneiss complex representating KunchhaGroup of the lesser Himalaya. Structurally the area isbounded or surrounded by a major fault system of theHimalaya called “the Main Central Thrust (MCT)”,see Figure 2. The rocks in the project area are mainlyaugen gneiss with inter-bedded bands of chlorite andtalcose schist. This intercalation has been observedfrequently at an interval of 5–10 meters at the down-stream section of the headrace tunnel.
In contrast, at the upstream stretch the interval islonger and the rocks are more fractured and openjointed.
The foliation planes are generally striking towardsnortheast – southwest direction and dipping towardsnorthwest. Since the project area is bounded with theMain Central Thrust (MCT) the rocks along the head-race tunnel are highly jointed, sheared, deeply weath-ered and deformed. The area is also influenced by
several minor faults and weakness zones representedby very weak sheared schist and crushed zones.
2.2 Actual rock mass conditions
For a successful tunneling a method characterized bycost effectiveness and flexibility to adapt changingground conditions is a must. In case of the Khimtiheadrace tunnel this concept is fully utilized with amaximum use of the self-supporting capacity of therock mass. This is done by adopting a good system ofwater leakage control and by deciding the rock sup-port at spot based on predefined rock mass classesand rock support procedures.
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Project Location
India
Figure 1. Location Map of Khimti I Hydropower Project.
Figure 2. Geological map of the Project (source: Departmentof Mines and Geology, 1994).
On the other hand, the planning phase investigationand predictions of the rock mass conditions along thetunnel in Khimti Project were rather poor and gavefalse impression to the contractor in planning the tun-nelling work. The Design Basis Memorandum (HPL,1995), which was the main guideline for the contrac-tor in planning and detail design of the project, statesthat most of the tunnel length will be in sound rock.Exceptions were described to be the construction adits,the initial section close to intake and at the down-stream end of the headrace tunnel, see Figure 3, wherethe tunnel was predicted to be in weathered rockwhere lining might be needed.
Accordingly, it was believed that the rock massalong the headrace tunnel would be of good qualityand no measures such as water leakage control wereconsidered at the initial phase of tunnelling. However,huge deviations have been found on the rock mass qual-ity along the tunnel during construction. Less than 5 percent of the tunnel stretch was found to passthrough sound rock, and the remaining 95 percentswent through poor, very poor to extremely poor rockclass (Bajracharya and Panthi, 2002), see Figure 4.
With respect to the discontinuity patterns, threesets of joints with occasional random joints have beenobserved along the tunnel alignment. The general
strikes of the main foliation joints (Jf) have been foundvarying from N15 to 60°E. This is not very favorablerelatively to the headrace tunnel alignment, whichalso is oriented in northeast/southwest direction, seeFigure 3 and 4. The foliation joints are mostly dippingtowards northwest with a varying dip angle of 50 to60 degrees at the southern part of the tunnel (adit 4area) and this trend changes gradually making the dipangle more flat with almost 25 degrees at its northernpart. The joint set number one (J1) is oriented withalmost the same strike direction as the foliation jointsand is very close to parallel to the tunnel alignment,but is dipping opposite to the foliation joints (dipangle 50 to 75 degrees towards southeast). Joint setnumber two (J2) is oriented in northwest southeastdirection with very steep dip angle (70 to 85 degrees)towards southwest, see Figure 5.
Relating to joint filling and alteration, most of thediscontinuities at the southern section (downstreamfrom Adit 3) of the headrace tunnel are filled with clayand bands of chlorite and talcose schist, and have beencharacterized as impermeable with respect to waterleakage. In contrast, the discontinuities present at thenorthern section of the headrace tunnel are either openor filled with coarse grained, permeable silt materials.In this northern section several open joints with aper-ture of up to 10 cm have been observed. Moreover, thedegree of weathering along the tunnel alignment variesgreatly and classifies as medium to highly weatheredaccording to ISRM (1978). In some sections the degreeof weathering was so deep that decomposed organicsoil was found in the tunnel. Especially the tunnel sec-tion 500 meters downstream from Adit 2 was deeplyweathered (CCC, 2002). The valley side slope in thisstretch of the tunnel is flatter (about 25 degrees) andthe rock cover is approximately 100 meters.
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Figure 3. Plan and longitudinal profile of the Project.
Figure 4. Headrace tunnel actual rock mass class distribu-tion summarized after finish.
Figure 5. Joint rosette showing the orientation of the mainjoint sets and tunnel direction.
Figure 5 indicates that the orientation and dipangle of two cross joints sets (J1 and J2) increase thepossibility for large leakage from the headrace tunnelduring operation as the valley side slope is oriented innortheast/southwest direction with a dip towardssouth, see also Figure 3. Moreover, most of the dis-continuities upstream from Adit 3 are either open orfilled with permeable materials that further increasethe possibilities of large leakage during operation.
After tunnelling almost two and half kilometers ofthe headrace tunnel from different adits, the contrac-tor realized this problem and had to change the tunnelconstruction methodology. He also incorporated adetail plan for preinjection grouting for the remainingtunnel stretch. However, there were only two alterna-tives left for water leakage control in the section ofheadrace tunnel where the excavation was alreadycompleted; full concrete lining or post-grouting.Even though it is well known that post-grouting is notas effective as pre-grouting in water leakage control,the contractor decided to go for post-grouting due tovery tight construction schedule. Even if there weremany uncertainties and risk connected to the postin-jection grouting, success was achieved with tremen-dous amount of cost and time saving.
3 GROUTING CRITERIA AND PROCEDURE
The contractual limit for maximum water leakagefrom the 7,923 meters long headrace tunnel at theKhimti Project was defined as 150 liters per second,which gives specific leakage of 1.13 liters per minuteper meter length of tunnel and is considered to bevery strict with respect to water conveying tunnels.According to the Khimti civil works contract (KC2),failing to reach this criterion would have resulted inconsiderable economic penalties (CCC, 2002). Incontrast, the traditional approach of full concrete lin-ing in the tunnel would have been a very expensiveand time consuming solution. In addition, concretelining is not a fully watertight structure due to shrink-age cracks and there would be a need for contactgrouting to make the tunnel water tight for full hydro-static pressure.
Therefore, the decision was made to utilize the selfsupporting capability of the rock mass with modernmeans of support and to use mainly injection groutingin the headrace tunnel to reduce water leakage, seeFigure 6.
As previously discussed, the best way to control thewater leakage out of the tunnel would have been tocarry out preinjection grouting ahead of the tunnel faceduring excavation. However, since almost 2.5 kilome-tres of the headrace tunnel was already excavated whenthe need of grouting was realized, both preinjectionand post injection grouting was performed.
3.1 Preinjection grouting
Two criteria were mainly followed to decide whetherthere was a need for injection grouting or not. Thesewere; (a) if the pressure of groundwater inflow to thetunnel is less than 1.5 times hydrostatic pressure dur-ing operation, (b) if the leakage through the rockmass exceeded a certain limit (water loss after pump-ing the water through exploratory drillhole with 1.5times hydrostatic pressure). In general the criterion(b) is considered to be more reliable as criterion (a) isvalid only in sections where groundwater inflow mayoccur during tunnel excavation and therefore (b) wasused extensively for defining the preinjection grout-ing at the Khimti headrace tunnel.
First, one 21 meters long exploratory drillhole wasdrilled from the valley side wall of the tunnel face atan angle of 8 degrees relatively to the tunnel axis andslightly upwards with an angle of about 5 degrees.During the drilling regular observation was made torecord possible water inflow as well as loss of flushingwater, and the approximate depth of such incidents.Secondly, after drilling was finished, the water inflow(if any) into the tunnel was measured using packerand flow-meter. After that water leakage test was car-ried out by pumping water into the drillhole (throughthe packer) with 1,5 times the hydrostatic pressure.Finally, if the water leakage exceeded one litre perminute per meter length (lugeon value exceeding one)preinjection grouting was recommended. For prein-jection grouting 12 drillholes with 21 meters lengthwere drilled around the tunnel perimeter at an angleof 8 degrees relatively to the tunnel axis to establish agrouting cone, see Figure 7.
The preinjection grouting was performed usingmicro cement (Reocom) with some accelerator addedto it. Three different mixes were used, starting with mixone having water/cement ratio 2. If mix one reached1.5 times the operational hydrostatic pressure in the
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Figure 6. Headrace tunnel profile showing hydro-staticline and areas with preinjection and postinjection grouting.
tunnel, or the grout volume exceeded 200 litres ineach hole, the grouting was stopped. However, if thegrout pressure after 200 litres was still below 1.5times the hydrostatic pressure, mix two with watercement ratio 1.5 was used with criteria as for mix one.In case of failing to reach the criteria, mix three withwater cement ratio one was used. Finally, sawdust wasadded in case the desired pressure was not achieved.
3.2 Postinjection grouting
The orientation of joints shown in Figure 5 and theopen character of the joints gave clear indication thatthere was a strong need for postinjection grouting at thesection of tunnel where no preinjection grouting wasperformed during early stage of tunnel excavation, seeFigure 6. Nevertheless, to be able to concentrate thepostinjection grouting operation only to the requiredsections of the tunnel, it was very important to identifythe areas from where maximum leakage could beexpected. To verify that, early test water filling was per-formed in the headrace tunnel. A temporary plug wasconstructed at the downstream end of the headrace tun-nel at chainage 7,505 meters, since the constructionwork at the pressure shaft and powerhouse were still notcompleted. The test water filling carried out in January2000 indicated considerable leakage from the tunnel(approximately 700 liters per second), see Table 1.
As expected, the leakage was insignificant fromAdit 3 and Adit 4, giving indication that the preinjec-tion grouting was effective. On the other hand, theleakage from Adit 2 reached approximately 200 litersper second, giving indication that unacceptable leak-age was occurring from the headrace tunnel near theAdit 2 area, where no preinjection grouting had beenperformed.
Based on the test water filling results and review ofthe geological tunnel log, a plan was made for theimplementation of comprehensive postinjection
grouting. Main focus during grouting was on theungrouted 910 meters upstream and 330 meters down-stream stretch of the headrace tunnel from Adit 2(chainage 2385 to 2965) where the joints were veryopen and the rock mass highly weathered. In addition,some length of the headrace tunnel near Adit 1 areawhere it was suspected that water was leaking, was alsogrouted, see Figure 6. Since the joints in these sectionsof the tunnel were more open, ordinary cement wasused for the grouting. In addition, five percent ben-tonite clay was added in the grout mix to increasegrouting effect. Four bars maximum pressure was usedfor postinjection grouting with a grouting pattern asshown in Figure 7-b. As indicated in the Figure, a 45degrees angle relatively to the tunnel axis was used fordrilling of the grout holes with alternating length of 12and 6 meters.
4 GROUTING RESULTS
Both pre- and postinjection grouting were found to bevery effective and economical solutions for control-ling water leakage from the pressurized headrace tun-nel. Compared to preinjection, the postinjectiongrouting was found somewhat difficult and challeng-ing as there was a tendency of grout coming out in thetunnel wall, and due to very open joints it was oftendifficult to achieve the desired pressure. Consequently,the grout consumption was considerably higher thanfor preinjection grouting, and the grouting operationsmore costly. Nevertheless, the grouting costs werefound much lower than those of concrete lining. Thetotal consumption of grout material in the headracetunnel is presented in Table 2.
The final water filling of the tunnel in March 2000,and leakage measured according to the performanceguarantee in the civil works contract (KC2), gave aleakage of 120 liters per second in the 7,923 meterslong headrace tunnel. This water leakage gives spe-cific leakage (q) of 0.91 liters per minute per metertunnel length, see Table 2, which is far below than thecontractual leakage limit.
An analysis has been done to find out whetherthere exists correlation between the specific leakage
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(a) Preinjection grouting pattern
4 m
Grout holes 21 mlong with an angleof 8 degrees
Groutcone
4 m
(b) Postinjection grouting pattern
12 and 6 metersalternating grout holeswith an angle of 45degrees
Figure 7. Used preinjection and postinjection groutingpattern.
Table 1. Water leakage measured after test water filling.
Total leakage from headrace tunnel 700From Adit 1 tunnel near concrete plug 2.6From Adit 2 tunnel near concrete plug 200From Adit 3 tunnel near concrete plug 0.4From temporary plug at Adit 4 1.8From remaining headrace tunnel 495.2
(q), the grouting pressure (P) (1.5 times static pres-sure during operation) and the specific grout con-sumption (c) for preinjection. As shown in Figure 8, acorrelation has been found that might be used to esti-mate preinjection grout consumption for future tun-nels in similar ground conditions.
Analysis has also been carried out of the specificgrout consumption with respect to the rock massquality of the tunnel section were both preinjectionand postinjection grouting was performed. Theresults of this analysis are presented in Figure 9.
Figure 9 clearly indicates that the grout consump-tion for both preinjection and postinjection groutingis highest for rock class II and III. This is because therock mass in these categories of rock class are typi-cally open jointed. Poorer rock mass quality typicallyhas clay filled joints, deformed rock mass and fold-ing, which reduce the permeability. Figure 9 alsoindicates that the postinjection grouting has a consid-erably higher consumption than pregrouting.
5 CONCLUDING REMARKS
By applying systematic preinjection and postinjectiongrouting at the headrace tunnel of Khimti I HydropowerProject it was not only possible to control the waterleakage to the contractual limit, but also to reduce thecost and time. The grouting carried out at this projectclearly demonstrates that the use of systematic prein-jection as well as postinjection grouting played a verysignificant role in the reduction of water leakage fromthe pressurized unlined / shotcrete lined water convey-ing headrace tunnel. Moreover, the grouting has playeda major role in improving the rock mass quality bycementing the cracks and joints with grout materialand reducing the need of excessive rock support.
The factual based analysis carried out has con-firmed that there exists good correlation between thespecific leakage (q), grout pressure (P) and specificgrout consumption (c) that could be used for estimat-ing grout consumption for future water conveyingtunnels in similar rock conditions. The Khimti casealso illustrates that the grouting operations are noteasy tasks especially when it comes to postinjectiongrouting, which may reach a cost of many times thatof preinjection grouting. Therefore, systematic prein-jection grouting should be used as much as possible.Finally, to achieve a good grouting result at lowestpossible cost, a good understanding of the groundconditions is of great importance.
REFERENCES
Bajracharya B. B. and Panthi K. K. 2002. Rock support opti-mization in Himalayan tunnels. Proceedings. 28th ITAGeneral Assembly & World Tunnel Congress: 461–467.Sydney: Australia.
Civil Construction Consortium. 2002. Construction report.Khimti I Hydropower Project. Volume 1-main text: Nepal.
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Table 2. Grouting results and final water leakage measured.
Activity descriptions Units Quantity
A. Preinjection groutingTotal grouted length m 4612Grout pressure used bars 2.8–6Total grout consumptions kg 754,975Specific grout consumption kg/m tunnel 164
B. Postinjection groutingTotal grouted length m 1359Grout pressure used bars 4Total grout consumptions kg 941,260Specific grout consumption kg/m tunnel 693
C. Water leakage after final water fillingHeadrace tunnel length m 7923Total water leakage liters/second 120Specific water leakage lt/min/m tunnel 0.91
y = 0.0046x + 0.0996R2 = 0.8574
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Lower line representsfracture rocks withinflow conditions
Upper line representsopen jointed rock massin dry conditions
Figure 8. Correlation between specific leakage, groutpressure and specific grout consumption.
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Figure 9. Relation between specific grout consumptionand rock mass class.
Himal Power Limited. 1995. Project definition report.Khimti I Hydropowe Project: Nepal.
Himal Power Limited. 2000. Final design report. Khimti IHydropower Project. Volume 1 – main text: Nepal
ISRM. 1978. Suggested methods for the quantitive descriptionof discontinuities in rock mass. International Journal ofRock Mechanics, Mining Sciences & GeomechanicsAbstracts. Volume 15: 319–368.
Karlsrud K. 2002. Control of water leakage when tunnellingunder urban areas in the Oslo region. NorwegianTunnelling Society (NFF). Publication 12. 27–33.
Kassana L. B. and Nilsen B. 2003. Analysis of water leakageat Lower Kihansi Hydropower Plant System in Tanzania,East Africa. Waterpower XII – HCI Publications.
Nilsen B. and Thidemann A. 1993. Rock Engineering.Hydropower Development Series. Volume 9: 156. Nor-wegian Institute of Technology. Division of HydraulicEngineering. Trondheim: Norway.
