DIFFICULTIES OF TUNNELLING UNDER HIGH COVER IN MOUNTAINOUS REGIONS

T.G. Carter1, D. Steels2, H.S.Dhillon2 and D.Brophy21Golder Associates, 2Aecon Constructors

ABSTRACT: Deep tunnels present unique geological problems that are exceptionally challenging for the design of tunnelling machines capable of excavating in such conditions. Traversing faulted and disturbed ground at significant depth requires that tunnelling procedures be able to cope with a huge range of difficult geological conditions. Investigating, evaluating and assessing anticipated geology ahead of tunnelling, and dealing with encountered difficult ground conditions requires that better understanding be gained of the interaction between complex geology and stress conditions when mining at these significant depths. Extremes of ground conditions present major contrasts to tunnelling, so much so that they often demand use of flexible rock engineering solutions in order for the tunnel to progress. This need to adopt flexible solutions is often seen as being at variance with the constraints imposed by the rigidity of design elements incorporated into the fabrication of a typical TBM. In this paper extremes of experience are cited from tunnelling in the Himalayas at the Nathpa Jhakri Scheme (Figure 1) where a variety of difficult ground conditions had to be traversed, ranging from rock-burst-prone rhyolitic units with hydrothermal water inrushes through to coping with squeezing, very soft, soil-like fault zone infills.

RÉSUMÉ: Les tunnels profonds présentent les problèmes géologiques uniques qui sont exceptionnellement provocants pour la conception des machines de perçage d'un tunnel capables de l'excavation en de telles conditions. Traverser la terre censurée et dérangée à la profondeur significative exige que le tunne l procédures de ling puisse faire face à une gamme énorme des conditions géologiques difficiles. L'investigation, l'évaluation et l'évaluation de la géologie prévue en avant du perçage d'un tunnel, et traiter dans des conditions au sol difficiles produites exige qu'une meilleure compréhension soit gagnée de l'interaction entre la géologie et les états complexes d'effort en extrayant à ces profondeurs significatives. Les xtremes de E du commandant actuel de conditions au sol contraste au perçage d'un tunnel, tellement de sorte qu'ils exigent souvent l'utilisation des solutions flexibles de technologie de roche dans l'ord heu pour que le tunnel progresse. Ce besoin d'adopter les solutions flexibles est souvent vu en tant qu'étant en désaccord avec les contraintes imposées par la rigidité des éléments de conception incorporés à la fabrication d'un TBM typique. En cela des extrémités de papier de l'expérience sont citées du perçage d'un tunnel en Himalaya à l'arrangement de Nathpa Jhakri (Figures 1) où une variété de conditions au sol difficiles a dû être traversée, s'étendant à partir des unités rhyolitiques roche-éclater-enclines avec les inrushes hydrothermiques de l'eau à travers à faire face au serrage, très doux, sol-comme des remplissages de zone faillée.

1.0 INTRODUCTION

Driving tunnels through mountainous regions poses amongst the most difficult challenges for modern tunnelling, as unforeseen conditions are the norm rather than the exception. This is because investigation of route alignments can be done only in a cursory manner, as tunnel depths are generally too great to allow economic investigatory drilling to be feasible. In consequence, regional geological interpretation has to be the mainstay of prediction. Interpretation of satellite imagery and aerial photographs provides some insight into probable geologic structure, while data from drilling that can be undertaken at the portals and from any accessible valley sites supplemented by available outcrop mapping, constitutes the typical total range of information on which all prediction must be based. In order to forewarn of possible problems and maximize drivage speeds, reliance has to be placed on forward probing and rapid reaction to changing ground conditions in order that effective and efficient advance rates can be maintained in spite of varying and often difficult conditions. These same issues also complicate initial decision-making, as to whether or it not would be viable to drive such tunnels using a TBM.

Modern TBM's can cope with a wide variety of conditions, but some compromises are generally necessary in order to balance cuttability and thrust requirements against production rate and ground support needs. Open machines with sidewall gripper pads and large cutters provide the optimum design arrangement for achieving the most rapid progress rates, but only function at their very best in good ground conditions. Closed face, fully shielded TBM’s, with telescopic articulation. provide the most versatile and safe machines for poor and saturated ground, but compromise on cuttability and efficiency. Such machines are not a panacea for advancing through all ground conditions, particularly if stress or water pressures are severe or if squeezing conditions are encountered. Current limitations on seal design creates an upper bound to maximum water pressures that can be resisted in any conventional “closed” machine construction. Overcut arrangements can be engineered to help counter problems imposed by severe closure, but such measures can only allow relatively small

(2005). Proc. Int. AFTES Congress, Tunnelling for a Sustainable Europe, Chambery, pp. 349-358

Figure 1: Nathpa Jhakri Hydro Power Project Setting

Page 1 of 10

(decimetre-scale) convergence to be dealt with. In zones of significant closure it is still possible for such machines to become trapped. In fact, because of the length of the latest state-of-the-art telescoping double shield machines, it might be argued that the risk of problems occurring due to squeeze and/or closure may actually be compounded by the use of such machines as against more open-structured TBM’s, which have a lower rock contact area.

Because major tunnelling risks can be considerable in high mountainous terrain (such as the Himalayas – ref. Figure 1), there is reluctance to use TBM’s for fear of running into insurmountable trouble. Drill and blast methods are still the approach of choice, especially if multiple cross adits can be constructed from side-hill accesses or from adjoining valleys, so that several headings can be worked at the same time, thus improving overall project progress. For the Nathpa Jhakri project, in India, Figure 2, where almost 30km. of line drive tunnelling was required to connect the Intake Works to the Power Facilities, drill and blast was the method of choice for all of the Contractors, despite the fact that some of the segment lengths between cross adits were in excess of 6km.

As shown on Figure 2, the 1500 MW Nathpa Jhakri Hydroelectric Power Project is located in a remote northern area of India in the

upper reaches of the River Sutluj in the state of Himachal Pradesh, (HP) almost on the Chinese border. The project has been implemented by Satluj Vidyut Nigam Limited (SJVN), a Government of India company, formerly known as the Nathpa Jhakri Power Corporation. The total project cost, which was in excess of US$1.2 billion, was one third funded by the World Bank. Aecon Constructors, through its wholly owned subsidiary, the Foundation Company of Canada, was Managing Party of one of the joint ventures constructing the project, responsible for undertaking two of the main civil work contracts, totalling $620M. These construction contracts included the Main Dam, the Intakes, the Desilting Chambers (Figure 3) and approximately 16km of the Headrace Tunnel. Throughout the ten years of execution of the joint venture’s contract works execution, Golder Associates provided geotechnical and geological engineering advice.

Figure 2: Project Location

Figure 3: Desilting Complex

The scheme is basically a run-of-the-river power development and the largest project identified on the Satluj for harnessing its hydroelectric potential. A design discharge of 486 cumecs is diverted from the river into the four intake gate arrangements, by a 61.5 m high concrete gravity dam. To exclude silt particles of up to 0.2 mm diameter from the water before it enters the Headrace Tunnel, the Intakes feed into a complex of four Underground Desilting Chambers (the largest in the world) through independent approach tunnels. As is evident from Figure 3, the Desilting Complex required significant rock engineering input in order to safely excavate and support the four 525 m long, 27.5 m high, 16m wide chambers and associated cross tunnels and shafts and more than 1 million m3 of rock were removed to excavate the Complex. From the Chambers, which are each lined with steel fibre reinforced shotcrete, and heavily supported with a surficial rockbolt anchorage pattern and long cable and bar anchor systems, the water runs along a 10.15 m diameter circular section Headrace Tunnel for 27.3 kms. to terminate in a 21 m diameter, 225 m deep Surge Shaft. Three Pressure Shafts of 4.9 m diameter each then take the water from the Surge Shaft to feed the six Francis Turbine

Figure 4 : HRT Layout and Geology from Desilting Complex to Wadhal Adit

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generating units of 250 MW each set within a 225m long, 49m high and 20m span underground powerhouse, allowing full utilization of the approximately 425 m head created by the scheme. Numerous technical papers describing various aspects of the scheme have already been published elsewhere, (eg., Kumar and Dhawan, 1999, Dasgupta et al, 1999, Hoek, 1999, 2000; Bagde, 2000, Mahajan, 2000). This paper does not discuss the overall scheme in any detail, rather it concentrates on examining the rock conditions encountered during excavation of the section of the Headrace Tunnel driven by the Continental-Foundation Joint Venture (CFJV) along the alignment configuration shown on Figure 4. While not unique, some of the conditions encountered on this drive are exceptional and amongst the most difficult that are likely to have to be dealt with when driving long tunnels beneath high mountainous terrain anywhere. Over the 16,042 m long stretch of the HRT from Stn. 0+00 at the junction point at the Link Tunnels just downstream of the Desilting Chambers (Figure 3) to the Wadhal Adit junction, at the extreme right hand side of Figure 4, not only is the geology variable and complex, with numerable faults, some of which gave rise to major convergence issues (ref. Figure 5), but the rock cover varies from a minimum of about 90m beneath one of the heavily faulted river crossings to a maximum of about 1,500m adjacent to the zone of spectacular rock cliffs shown on Figure 1, where high stress face spalling and sidewall slabbing conditions were encountered.

Now complete, the HRT is fully concrete lined, with nominal lining thicknesses varying from 300 mm to 500 mm, dependent on rock quality and cover. It was constructed in four different segments, named after local communities, from upstream to downstream, the Nathpa, Sholding, Nugalsari and Wadhal sections, ie., extending from left to right across Figure 4. Because of concerns regarding the competence of the ground, and the possibility of encountering geothermal water sources and high stress zones, and also because of perceived poor previous TBM utilization elsewhere in mountainous terrain, all of the tunnelling contacts for the HRT were tendered for Drill and Blast drivage, with none of the joint venture contactors electing to propose an alternative TBM bid. As a consequence, each of the access and main tunnel drives were driven drill and blast, with face production drilling undertaken with, for the CFJV section, sixteen two-boom electric/hydraulic jumbo drills working from 7 headings to complete more than 3 million m3 of rock excavation for the tunnel. Most of the alignment was excavated by the heading and bench method. However because of the many difficulties encountered due to adverse geological conditions, some reaches of the CFJV section of the HRT were advanced by multiple drifting methods (Figure 5) while other sections of the alignment further along the HRT, also in difficult ground, were constructed with umbrella forepoling methods as illustrated on Figure 6 (after Hoek, 2000).

Sta

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S ta

116 1

5.00

Sta

1269

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S ta

1270

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Sta

1312

5.0 0

Sta

1313

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Sta

1342

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Sta

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8.00

Sta

1359

9 .75

Sta

136 1

5.50

Sta

1419

2.00

Sta

1420

4.00

Sta

1606

2.00

NUGALSARI D/S WADHAL U/S2871m 3504m

1925

.00

1 928

.00

3372

.00

336 0

.00

2937

.00

2929

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2637

.00

2 624

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2462

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244 6

.50

1870

.00

1858

.00

A B C D E F

NUGALSARI ADIT WADHAL ADIT

Details of EGO Work from Nugalsari to WadhalS. No Location Length No. of DaysA Nugulsari D/S 11602.00 11615.00 13.00 1-Feb-98 12-Mar-98 40B Wadhal U/S V 12702.00 12690.00 12.00 26-Jul-99 11-Nov-99 109C Wadhal U/S IV 13133.00 13125.00 8.00 26-Sep-98 5-Nov-98 41D Wadhal U/S III 13438.00 13425.00 13.00 5-Dec-97 8-Apr-98 125E Wadhal U/S II 13615.50 13599.75 15.75 20-Aug-97 30-Sep-97 42F Wadhal U/S I 14204.00 14192.00 12.00 14-Oct-96 15-Jan-97 94

TOTAL 73.75 451

Station Period

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1606

2.00

NUGALSARI D/S WADHAL U/S2871m 3504m

1925

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1 928

.00

3372

.00

336 0

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2937

.00

2929

. 00

2637

.00

2 624

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244 6

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1870

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1858

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A B C D E F

NUGALSARI ADIT WADHAL ADITSt

a96

87. 0

0 Sta

116 0

2.00

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116 1

5.00

Sta

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S ta

1270

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5.0 0

Sta

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9 .75

Sta

136 1

5.50

Sta

1419

2.00

Sta

1420

4.00

Sta

1606

2.00

NUGALSARI D/S WADHAL U/S2871m 3504m

1925

.00

1 928

.00

3372

.00

336 0

.00

2937

.00

2929

. 00

2637

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2 624

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2462

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244 6

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1870

.00

1858

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A B C D E F

NUGALSARI ADIT WADHAL ADIT

Details of EGO Work from Nugalsari to WadhalS. No Location Length No. of DaysA Nugulsari D/S 11602.00 11615.00 13.00 1-Feb-98 12-Mar-98 40B Wadhal U/S V 12702.00 12690.00 12.00 26-Jul-99 11-Nov-99 109C Wadhal U/S IV 13133.00 13125.00 8.00 26-Sep-98 5-Nov-98 41D Wadhal U/S III 13438.00 13425.00 13.00 5-Dec-97 8-Apr-98 125E Wadhal U/S II 13615.50 13599.75 15.75 20-Aug-97 30-Sep-97 42F Wadhal U/S I 14204.00 14192.00 12.00 14-Oct-96 15-Jan-97 94

TOTAL 73.75 451

Station PeriodDetails of EGO Work from Nugalsari to Wadhal

S. No Location Length No. of DaysA Nugulsari D/S 11602.00 11615.00 13.00 1-Feb-98 12-Mar-98 40B Wadhal U/S V 12702.00 12690.00 12.00 26-Jul-99 11-Nov-99 109C Wadhal U/S IV 13133.00 13125.00 8.00 26-Sep-98 5-Nov-98 41D Wadhal U/S III 13438.00 13425.00 13.00 5-Dec-97 8-Apr-98 125E Wadhal U/S II 13615.50 13599.75 15.75 20-Aug-97 30-Sep-97 42F Wadhal U/S I 14204.00 14192.00 12.00 14-Oct-96 15-Jan-97 94

TOTAL 73.75 451

Station Period

Figure 7 – Delays during driving through Major Fault

Zones in the Nugalsari and Wadhal HRT Sections

Overall, more that 30% of the HRT alignment encountered faults shears and shattered zones, some with significant groundwater. Not only were ground conditions in the tunnels quite adverse, but in some places high rock temperatures of up to 45°C., and hydrothermal water inflows (at about 51°C) had to be contended with in order to advance the headings. Even with 20:20 hindsight, and knowledge of the conditions, deciding whether or not to drive these tunnels by machine would still be difficult.

2.0 ASSESSMENT OF ADVERSE CONDITIONS

In order to provide some guidance for future assessment of possible tunnelling conditions under similar cover and through similar rock masses, some back-analysis examination has been undertaken of some particularly difficult segments of the HRT alignment. This has been done principally to explore whether various rock mass characterization approaches and semi-empirical procedures for assessing rockmass behaviour have validity for realistically predicting tunnelling conditions particularly from a machine tunnelling perspective. As explained subsequently, based on the back-analysis checks it would appear that such methods if used with caution have application for rapidly assessing the likelihood of encountering problematic conditions. They therefore can be used to aid decision-making regarding whether or not a given tunnel would be amenable to excavation using machines. First however, as background, some additional details are presented of the construction works and encountered conditions.

Figure 6: Umbrella Forepole Installation in progress through another of the major Fault Zones on the HRT

(photograph, Hoek, 2000)

Figure 5: Severe Closure Associated with Multi-drift Advance through 30m wide Fault in Nathpa HRT

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Figure 7 lists the locations of the most adverse faulted zones in the Wadhal and Nugalsari sections, with Figures 8 and 9 showing some examples of the ground conditions encountered whilst traversing through these faults. As is evident from Figure 8, where a major mudflow developed into the tunnel from one of the faults, water also complicated dealing with the most adverse of these zones. In general the inflows associated with the worst faults were relatively small (typically less than 300 gpm (20 l/sec)). By contrast, in some of the more blocky and brittle, faulted areas, where extreme ground condition problems were not encountered, heavy groundwater inflows, of up to 2000 gpm (150 l/sec) occurred (including in some cases, hot >50°C water). In such zones, progress rates were significantly affected, with advance often reduced to well less than one metre per day.

3.1 Back-Analysis Insights

Figure 8 – Mudflow from RD 2630 Fault in Wadhal HRT

Two of the extremes of tunnel wall stability behaviour encountered along the HRT are briefly examined to illustrate some common, and important inter-relationships that frequently exist between rock mass characteristics and overburden cover in controlling the severity of encountered problem conditions. The first back-analysis case involves a zone of significant closure and squeezing in the Nugalsari Section of the HRT associated with mining through a completely granulated quartz-mica mylonite fault zone. The second case corresponds with a segment of the HRT where high stress conditions were encountered in a zone of brittle, but competent silicified granite gneiss. These stress and brittle rock conditions induced not only sidewall spalling and face instability during the initial tunnel driving, but also lead to yield and failure of the installed support system several weeks after the top heading excavation face had left the area, and well before benching was due to commence.

Figure 10 – Disruption to Primary Rib Support associated with benching towards Major Fault

Figure 9 – Rock Conditions and Strain Analysis Approach for Wadhal RD 2630 Fault Zone

Case 1 - Wadhal U/S RD 2630 Fault Zone Figures 9 and 10 respectively show as-mapped geology and as-encountered conditions in the HRT as they developed because of stress re-adjustment due to removal of the bench as it encroached into the area of the RD 2630 fault zone. As is evident, punching in of the ribs occurred associated with heave and disruption of the invert zone, but of most concern was that this was occurring some 10-15m ahead of the bench face. As a consequence of this adverse behaviour and as part of detailed evaluation of possible support alternatives to assist in completing the remainder of the required excavation through this and similar fault zones, a series of

Figure 11 - Strain Based Ground Behaviour Classification (after Hoek and Marinos, 2000)

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back-analysis assessments were completed using both numerical modelling methods and the semi-empirical approach suggested by Hoek and Marinos (2000). As shown on Figure 9, which summarizes the intersected fault zone geology through this critical area, quite a number of poor rock areas existed in this section of the HRT where adverse conditions might occur. The effects of such zones on tunnelling behaviour can be broadly assessed using the Hoek-Marinos closure prediction method, allowing credible back-analysis of the possible magnitude of convergence, in this case with σcm/po ~ 0.06 and ε >60%, (ref. Figures 9 and 11) extreme squeezing conditions are suggested (ref. descriptive strain zones A-E, as listed on Figure 11). As is also evident from Figure 9, the RD 2630 fault is located amidst an area of moderate quality rock mass where no adverse conditions would normally be expected (and none were encountered) and where there is no significant change in overburden cover crossing the faulted areas. However, back-analysis of the fault zone area, for the assessed rock mass quality (Q ~ 0.05 with RQD = 20%, ie., equivalent RMR = 17)) indicates that severe swelling was entirely feasible even though the overburden cover was less than 650 m. As is evident from the tabulation inset in Figure 7, conditions in this Wadhal section of the HRT were amongst the most adverse of anywhere along the HRT alignment, with up to seven major fault zones encountered in less than 4km. Difficulties were experienced in driving the top heading development through each of these zones and in the particular case of the RD2630 Fault, some 600mm of convergence was back-calculated to have happened associated with benching, when severe buckling of the ribs occurred over a 20 metre stretch coinciding with the most adverse section of the fault. With such significant problems occurring in the tunnels attention was focused on attempting to do enough back-analysis of the top heading information to be able to ensure that sufficient understanding was gained of potential adverse rockmass behaviour for all key fault zone areas, so that excavation and support methods could be refined prior to benching. Case 2 - Wadhal U/S RD 2176 - 2240 In this section of the tunnel, severe slabbing of the sidewalls and breakout of part of the crown and haunches developed to such a degree several weeks subsequent to excavation of the top heading that the tunnel shape became asymmetrical with the long axis of preferred cracking and spalling likely reflecting the orientation of the prevailing major principal stress. As shown in the photograph on Figure 12, a V-shaped overbreak notch zone developed. This occurred more or less subparallel to the tunnel axis throughout a 30-50m long section of the HRT. In this zone, where the rock mass was of high strength and competent, but brittle, also failure occurred of the heads and face plates of several hundred rock bolts. Back-analysis of stress conditions in this area indicated that given the overburden cover, and the probable stress regime, maximum induced tangential stresses likely significantly exceeded rockmass strengths for much of the alignment either side of the zone that exhibited the most problems. The fact that only this and a few other zones of the tunnel in the highest cover areas showed a tendency to face, crown and/or sidewall slabbing therefore become the focus for detailed back-analysis, with the result eventually determined that because other, more fractured, zones of the rockmass likely could not sustain the full insitu stresses, only the most competent, brittle zones of the rockmass appeared prone to such destressing effects.

3.2 Implications These back-analysis observations led to the initial development of a series of parametric analyses being carried out to attempt to better pre-define likely problem zones that could be encountered in the remaining tunnelling; with the aim of determining this well ahead of actually excavating into potential problem areas. Early

formulations utilized the Kirsch solution to rapidly determine induced stress conditions along the tunnel alignment so as to help determine zones where stress : rockmass strength ratios might be adverse. This formed an initial basis for assessing whether such zones would occur in areas where the rock would be hard and competent, where stress slabbing and spalling might prevail, or where the rock would be weak and deformable, where potential squeezing could be the issue of most concern. Subsequent enhancements to these initial approaches have now incorporated quantitative algorithms and other empirical guidelines from amongst others Singh, et al, 1992, 1995, Barton, et al., 1977; Grimstad and Barton, 1995, Carter, 1990 and Kirsten, 1988. These refinements have been generally targeted towards developing a

better method for rapidly ranking tunnelling conditions at early pre-feasibility and feasibility stages. In the following paragraphs these methods, which have been refined on a variety of projects worldwide for tunnel alignment optimization purposes, are outlined as a suggested approach for better developing preliminary hazard rankings for any deep tunnelling beneath mountainous areas.

Figure 12 – Zone of Crown/Haunch Overstress and Classic “Notch” Breakout Cracking

3.0 HAZARD RANKING As a basis for utilizing the proposed approaches for ranking tunnelling hazards, reasonable data on topography, geology and rock quality needs to be pre-defined for each alignment alternative that is being considered, and estimates need also to be made of reasonable cost and excavation progress rate per rock quality type. This then allows a semi-quantitative risk-based ranking to be developed to:

(i) define major foreseeable geotechnical risks, along each alignment being considered for any type of proposed tunnelling operation;

(ii) define potential cost and schedule implications that may be associated with each of these perceived geotechnical risks for each proposed alignment, and then finally

(iii) list and rank the geotechnical advantages and disadvantages of the various alternative alignments, (particularly as they may impact the viability of each of the proposed scheme arrangements).

As both cost and schedule are impacted significantly by the choice of alignment of any tunnel with respect to fault locations, river crossings, extent of alteration effects and zones of high mountain cover it is important that early attention is given to adequately defining the geological conditions along each alignment corridor being considered. This then allows a first order ranking to be carried out to define any potential impacts that might be sufficiently adverse as to negate the choice of that alignment.

Page 5 of 10

A matrix tabulation, such as Table 1 allows each of the major types of geotechnical tunnelling risks to be quantified and ranked with respect to each other. Here a simple qualitative scale is presented reflecting assessed comparative tunnelling difficulty for each of the hazards; with squeezing conditions considered to create problems more than double the severity of those associated with slabbing; which, in turn, are ranked as being more of a nuisance than significant groundwater inflows, while moderate groundwater inflows, which can usually be handling by active pumping systems at the tunnel face are ranked as of somewhat more importance, than zones of thin cover (on the assumption that in such zones, other geotechnical issues will be important).

At a preliminary level such simple qualitative ranking scales will usually suffice. With more detail and greater accuracy of risk assessment being needed, a more quantitative approach may be justified. At each level though, some measure of assessment of uncertainty must also be included in the rankings in order to account for data deficiencies and for the fact that risks might be better characterized on one alignment than on another, simply because more is known about one rather than the other alignment. At a preliminary level, again it is usually sufficient to apply a simple scalar relationship to compensate for differences in availability of geological and geotechnical knowledge for the various alignments.

3.1 Geological Uncertainty

Although a lack of information about conditions along any given alignment does not of itself contribute to the actual real hazards that might be encountered along that alignment, if there is no data available or only very limited data, then a measure of increased uncertainty exists that assessments of the perceived risks are not as accurate as they should be, (ref. Figure 13). This in turn complicates reliability of schedule and/or cost estimates, as these are based on being able to reasonably characterize the geological conditions along each alignment.

For accurate assessment of tunnelling risks and reliable evaluation of risk on schedules and costs, detailed geological understanding is critical. Unfortunately, at a preliminary level very often little is known about conditions along the majority of the alignments that might be under scrutiny. If not recognized lack of understanding of what is uncertainty for the various proposed alignments can lead to unforeseen problems. Typically, in early project phases uncertainty is reflected in the magnitude of overall risk associated with the most “data deficient” alignments. Only by acquiring pertinent, critical site investigation data can this “apparent risk” be reduced.

3.2 Tunnelling Problems Associated with High Stress

Two forms of tunnelling problems can occur due to encountering high stress conditions. In brittle, competent rock masses, spalling or, in extreme cases rock bursting, can occur. By contrast significant convergence, tunnel closure and even squeezing can

develop where soft deformable rocks exist, or faults have to be traversed, particularly if adverse water inflow conditions occur.

Figure 13 – Project Uncertainty and Risk Reduction

In order therefore to assess the potential for such conditions to exist, cover depths along each alignment must first be established and then rock competence and quality through the high cover zones evaluated. Based on interpretive geological sections prepared along each alignment this can readily be achieved in terms of various stress-induced damage assessment approaches available through the Q and RMR classification schemes currently used worldwide for geotechnical rock tunnelling assessments. In order to provide base parameters so that stress-induced rock mass damage effects can be examined for any particular alignment under scrutiny, mean Hoek-Brown values for rock mass strength can be ascribed for each of, in this case, five rock class divisions, as per Table 2, below.

Parameter Ranking Geological Uncertainty 1

High Stress (max. cover thickness) Rock Burst Zones (km) 50 Slabbing Zones (km) 25

Squeezing Zones (km) 100 Water Inflows

Major Inflow Zones (km) 50 Moderate Inflow Zones (km) 10

Limited Cover Zones (km) 5

Table 1: Inferred Importace Ranking of Risks (based on assumed tunnel performance)

These values have been listed using the Hoek-Brown relationship:

sCCM σ=σ (where σCM and σC are respectively the rock mass and rock material strengths and where s, the Hoek-Brown brokenness index, can be related to RMR through the expression s=exp[(RMR-100)/9], (Hoek, 1988)

I II III IV V Q >50 Q ~ 5-50 Q ~0.5-5 Q ~ 0.01-0.5 Q <0.01

Q~100 RMR~85

Q~10 RMR~65

Q~1 RMR~45

Q~0.1 RMR~25

Q~0.01 RMR~5

σCM~100MPa s~0.5

σCM~50MPa s~0.1

σCM~20MPa s~0.03

σCM~10MPa s~0.005

σCM~5MPa s~0.0001

Table 2: Generalized Rock Mass Parameters by Rock Quality

(based on assumed mean quality and Hoek-Brown values)

Given the above tabulation and a reasonable prediction of the geology and cover along any given alignment some assessment can be readily made of the potential for rock-bursting and/or slabbing to occur in any zones of high cover where competent, high strength rock masses are expected. The potential for closure or squeezing to develop in soft deformable zones can also be assessed by reference to tabulated ranges of probable ground conditions [expressed with reference to the Q-system Stress Reduction Factor (SRF) parameter (Barton,1974,1976,1988)] with rock cover SRF values determined using relationships published by Kirsten (1988) and Singh (1992) modifying earlier work by Barton et al., (1977). Development of these cover depth relationships reflects experience gained not only from analysis of rock burst conditions in the South African mines but also reflects experience gained from tunnelling in the Himalayas.

Page 6 of 10

Taking the SRF criteria established by Kirsten and modifying them still further to incorporate key aspects of a more recent knowledgebase of information compiled by Grimstad and Barton (1995), spalling and squeezing conditions can be usefully related to the Q-system SRF parameter. In this context, as a means to apply the above general approach to delineating the extent of potentially troublesome zones along each of any alignments under study, in both competent rock zones and in association with faults etc., appropriate SRF values can be computed using Kirsten’s (1988) polynomial expressions, namely:

For fractured competent rock; SRF = 0.244k0.346(H/σC)1.322 + 0.176 (σC/H)1.413, where k is the principal field stress ratio, H the cover

depth (m) and σC is the uniaxial compressive strength of the intact rock (MPa).

…and… For weak, disturbed and faulted zones;

SRF = 1.809 Q-0.329

In these relationships the applicable SRF values for the various rock conditions and cover depths along each proposed alignment being evaluated would be defined based on these relationships. In doing this, it should be appreciated that these expressions are all referenced to the original 1977 SRF scale, not the 1993 revised scale. This is because Kirsten’s work pre-dates the 1993 revisions made to the numerical range of the SRF parameter of the Q system by Barton and Grimstad. Kirsten’s original tabulations and correlation relationships must therefore always be examined in the light of the original 1977 SRF scale. This scale has therefore been maintained in Table 3. For completeness, however, and in order to provide a benchmark for both sets of SRF scales, Table 4 has been included opposite Table 3 to outline the relationships existing between the old and new SRF values included in the current Q-system definition tables.

While calculating SRF values for each tunnel sector helps provide a reasonable indicator for zoning the tunnels and establishing the potential lengths where problematic conditions might occur, and is thus of great value for the sort of ranking requirements needed at a preliminary level of study, it is tunnel size independent when considered solely in terms of a rock strength to overburden stress ratio (σcmass/σv). Similarly Singh’s (1992) σCMass /σ(V>350Q) approach, is also tunnel size independent, as this merely relates squeezing likelihood to the minimum overburden cover at which squeezing may occur [viz., H > 350 Q⅓ (m), for a rock mass with strength defined as σC.MASS = 7 γ Q⅓ (MPa)]. Applying these relationships, which are based on collected case records from Himalayan tunnelling (principally from the Indian subcontinent), nevertheless provides a good second backup for checking inferred conditions where the SRF method suggests squeezing potential. Some better account of tunnel size and relative degree of severity of damage can however be made if the tangential stress to wall-rock strength ratio (σθ / σcmass) is derived for each segment of any alignment under study by examining the alignment with reference to cover depth and Q/RMR values for that particular tunnel segment. This method, which was formalized by Alber, 1988 for determining Factors of Safety (defined as σCM / σθMax ) for zoning rock conditions that might present difficulties for TBM applications, simply compares rock mass strength at the tunnel periphery to the induced tangential stress, as estimated from the Kirsch solution. For the crown the maximum tangential stress is given by σθCrown = σv(3k - 1), where k, the horizontal to vertical stress ratio for the various proposed alignments would be estimated as k ~ 2.0 typically for near-surface excavations, conservatively ranging down to k ~ 0.8 potentially for the deeper, highest cover zones (ie., under the centres and margins of the main mountain ridges), where vertical stresses might dominate; suggesting that sidewall conditions might be more critical, for which the Kirsch solution can be rewritten as: σθSidewall = σv(3 - k).

Table 3: Tunnel Ground Behaviour in terms of SRF and Wallrock Stress/Strength Ratios (with Jw = 1; k ≈ 1) (modified from Kirsten,

1988 & Grimstad & Barton, 1993)

While crude, application of these various semi-empirical approaches individually or in combination is remarkably effective for quickly determining the extent of potentially troublesome high-stress influenced tunnelling conditions. For each alignment under study it becomes straightforward to develop summations of rock stress/strength damage extents for the three critical issues - burst zones, spalling zones and squeezing zones, which then allows easy ranking of the merits or problems of each of the tunnel alignments under study.

3.3 Tunnelling Problems Associated with Major Water Inflows

Table 4: Comparison of Original and Updated SRF Parameter Ranges

In addition to the problems that might have be faced in faulted zones under high cover with respect to squeezing or significant convergence, as discussed above, another problem that may have to be dealt with is groundwater inflow under significant pressure. If water bearing faults are encountered during tunnelling under high cover, then not only is the magnitude of inflow a problem, but grouting and sealing is significantly more difficult against high inflow pressures than grouting under low pressure heads. In valley faulted areas, (especially where a river exists above the alignment) significant inflows may also occur, but in these cases flow magnitudes tend to be the problem more than pressures, as rivers and other water bodies create essentially unlimited sources that can become directly connected to the tunnel through fracturing. Two forms of groundwater inflow-related tunnelling problems can thus be envisaged – (i) high pressure, high cover inflows and (ii) low pressure, low cover, high volume inflows. The magnitude of the problem in each case will depend on the hydraulic conductivity of the encountered zone and the controlling pressure gradients. From the tunnelling perspective some of the faults interpreted to cross the various alignments under study can be expected to yield more water than others. Similarly, some rock units can be expected to be of intrinsically higher permeability than others, and therefore could potentially pose a greater groundwater inflow problem than other units. Accordingly, when preparing preliminary tunnel alignment drawings, some ranking needs to be completed of

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the various faults and major structure zones so that an appreciation can be gained of the likelihood of encountering water, on perhaps a three point scale – high, moderate and low potential inflow. Length estimates can then be prepared of the perceivable extent of major and/or moderate level inflows for each alignment, with the summations then incorporated into the ranking matrix as another key component for optimization study.

3.4 Problems of Tunnelling under Limited Cover Lastly, and again as part of preparation of an overall hazard ranking matrix, the extent of zones of thin crown cover need also to be tabulated for each alignment option. Defining the extent of critically thin zones (where the potential for adverse crown behaviour might be expected due simply to the inadequacy of crown thickness to span ratios), is of major importance for alignment optimization where valley crossings need consideration. This is irrespective of the potential for encountering poor quality faulted ground conditions, associated with high groundwater inflows in such low cover zones. A crude stability assessment of each zone of marginal or potentially adverse crown geometry in zones of low rock cover where faults might exist can be simply accomplished by means of the scaled span approach (Carter, 1992, 2000) developed from hard rock mining experience. In this method an approximate Factor of Safety against crown collapse, is defined in terms of the ratio between (i) the Critical Tunnel Span Sc deemed of marginal stability for a rockmass, of quality Q, viz:

(Q)sinhxQx3.3S 0.00160.43C =

and (ii) the Scaled Span CS for the actual rock crown and excavation shape geometry, appropriately scaled through the expression:

These relationships can easily be incorporated into a spreadsheet tabulation for each of the tunnel alignments under study so that the stability state of the crown for each segment of each alignment can be assessed. Cs and Sc values can be readily estimated for the each critical zone of low cover thickness, and the results expressed in terms of an approximate Probability of Failure, ie.,

where Pf = Probability of Failure (%)

Fc ≅ an approximate Factor of Safety; = SC/CS, and erf[ ] is the standard error function, and where Cs= scaled crown span, (m); S = actual tunnel span (m) γ = specific gravity of the crown rock mass t = thickness of crown cover (pillar) over tunnel (m) SR = span ratio = S / L (tunnel span / strike length of

poor ground), and θ = dip of faulted zone where it crosses the tunnel (degrees)

This procedure, although not totally rigorous, does allow some rapid appraisal to be made of the extent of potentially problematic crown cover conditions for any given rock quality. The extents of zones of thin cover can thus be readily identified by this method and then summed for each alignment, providing another key parameter for inclusion as a line item within an overall hazard ranking matrix aimed at assessing all the tunnels under study.

Figure 14: Example Hazard Matrix

4.0 HAZARD MATRIX For each geological rock quality sector along each alignment of interest, spreadsheet calculations can then be undertaken to determine each of the key hazard parameters. This allows some estimate to be made of the possible extent of each type of hazard per alignment length, thereby allowing comparative ranking to be undertaken between each of the proposed tunnel alternatives.

( )( )

5.0

RS )(cos4.01S1t

SC⎭⎬⎫

⎩⎨⎧

θ−+γ

=

Figure 14 shows a typical hazard ranking matrix while Figure 15 shows some excerpts from spreadsheet tabulations illustrating the basic calculation output used for defining the various diagnostic indices for each hazard type for each alignment.

⎥⎦⎤

⎢⎣⎡ −

−=4

1F9.2erf1P cf In the example shown on Figure 14, two each of the alignments

have been ranked with the same degree of geological uncertainty reflecting the perceived equivalence of data uncertainty on a scale of 1 = low risk of surprises to 10 = high risk of unknowns. In the example spreadsheet tabulations shown in Figure 15 none of the tunnels are deep enough that rock bursting is seen to be a problem. There are indications though, that spalling is possible in some of the better quality rock units under the anticipated cover depths. There is also some suggestion that mild and severe closure could occur in some of the poorest ground conditions under high stress, particularly in the presence of water. In ranking the

Figure 15 – Typical Tabulation for Stress State, Spalling or Squeezing Conditions, Water Inflows and Low Cover Zones

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extent of the potential problems for each alignment in this example tabulation the total summation of “at risk” lengths for each category have been summed to give a length value in kilometres. On Figure 14, these have then been ranked and colour coded on the following scale … 0 – 0.1km = low risk, yellow; 0.1 - 0.5km = moderate risk, orange; and >0.5km = high risk, red.

Figure 17 Broad Outline Ranges of Current Machine Type Applicability (data from Robbins)

The same approach and ranking procedure is illustrated on Figure 14 for the prescribed extent of limited crown cover and the extent of zones that could give rise to water inflow problems. For plotting water inflows on the matrix table, two magnitude levels have been considered based on the presumption that the bigger, wider faults and those under river crossings will generate more water than those under high cover.

Prediction of the scale of problems that might be encountered in each of the defined high and moderate risk zones is obviously of significant value in attempting to decide on what sort of support procedures and potential advance rate reduction might be incurred in traversing through such zones. These empirical ranking procedures also help to focus attention on the key issues. IN particular they help in defining length estimates of zones where high magnitudes of convergence might be encountered, where significant reductions in advance rates might occur and heavy support costs might be anticipated.

5.0 CONCLUSIONS

For the Nathpa Jhakri Scheme, empirical index analysis and hazard ranking of the type suggested in Figures 14 and 15, when carried out retroactively, still tends to confirm the original field inferences that many zones of extremely heavy support pressure requirements would be anticipated in the worst area fault zones, with magnitudes of convergence, and potential pressure far in excess of the typical maximums that could be tolerated by even the latest state-of-the-art machines, thus justifying the decision to drive the HRT conventionally, with drill and blast methods.

Whole sections of the tunnel however would be predicted to require lower support levels, that would be well within the pressure range feasible to be borne by a modern TBM (ref. Figure 16). This highlights the fact that there is a need for still further flexibility in TBM designs to be able to cope with greater magnitudes of closure as anticipated in such variable ground conditions.

Hybrid machine design may provide the best way forward, with the next generation of machines being built to have the capability to

undertake umbrella arch forepoling with jet grouting, thereby giving the machines greater ability to cope with much wider ranges of ground conditions than current machines (Figure 17). Adopting the best aspects of mechanized umbrella arch tunnel advance drilling configurations into a conventional head design for a full face TBM, while it will present a significant mechanical challenge for machine manufacturers, appears to hold the most promise for advancing the state-of-the-art in hard rock machines to a position where they would have the capability to be able to mine through the worst faulted ground that Himalayan (and other young mountain fold belt – Alps, Andes etc) conditions could present them with. For the Nathpa Jhakri situation all but four of the faults on the entire alignment could have been machine mined by a current, state-of-the-art machine. How easily these four very adverse faults could have been traversed if umbrella forepoling on a TBM could have been employed is a matter of conjecture, but some indications can be inferred based on the more than six fold increase in progress rates achieved in the umbrella forepoled faulted section compared to the typical advance rates achieved with the conventional multiple drift approach.

Figure 16: Support Pressure Estimates compared to typical pressure handled by modern TBM (plot modified from Barton et al., 1977; Grimstad and

Barton, 1993, and Singh, Jethwa and Dube, 1995)

While comparison of actual excavation progress rates suggests that major reductions in excavation and support time could have been realized by adopting such methods, only when there is flexibility in contractual arrangements can such measures be implemented “on the fly”. Contract rigidity and inflexibility is oftentimes a major constraint to the introduction of novel and/or innovative measures (such as new advances in mechanized tunnelling). This points to a worldwide need to develop standardized tunnelling contracts which build in greater flexibility in tunnelling contractual language so that contractors can be encouraged to negotiate reasonable payment terms for use of innovative approaches (not perhaps already foreseen by the Designer). Flexible contractual language is needed that, for instance, would allow contractors to bid a project scoped as conventional drill and blast and not unduly penalize them for adopting some other advanced, perhaps prototype, mechanized ground improvement procedure or even changing to using a TBM.

6.0 ACKNOWLEDGEMENTS

The views expressed in this paper reflect the opinions of the authors and may not reflect those corporately held by the various organizations involved in the construction of the Nathpa Jhakri scheme. Acknowledgements are due to many individuals in the various organizations involved in the project whose views and insight have helped formulate the thoughts expressed in this paper.

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7.0 REFERENCES Alber, M. (1988) Design of High Speed TBM Drives. Proc. Canadian Tunnelling; pp.181 - 187. (From selected papers from the 15th Can. Tunnelling Conference, Vancouver) Bagde, M.N., (2000) Finite element analysis of underground caverns of Nathpa Jhakri Hydel Project. Proc. Int.Conf. Tunnelling Asia ‘2000 544 pp Barton, N., Lien, R. and Lunde, J. (1974). Engineering Classification of Rock Masses for the Design of Tunnel Support, Rock Mechanics. Vol.6, pp.183-236. Barton, N. (1976). Recent Experiences with the Q-system of Tunnel Support Design. Proc. Symp. on Exploration for Rock Engineering, Johannesburg, pp.lO7-l17. Barton, N.R. (1976). The Shear Strength of Rock and Rock Joints. Intnl. J. Rock Mech. Min. Sci., Vol. 13, No. 10, pp. 1-24. Barton, N., Lien, R. and Lunde, J. (1977) Estimating Support Requirements for Underground Excavations, in Design methods in Rock Mechanics. Proc. 16th US Sump. On Rock Mechanics, Minneapolis, USA, pp.163-177. Carter, T.G., (1992) A New Approach to Surface Crown Pillar Design. Proc. 16th Can. Rock Mechanics Symposium, Sudbury, pp. 75-83

Carter, T.G., (2000) An Update on the Scaled Span Concept for Dimensioning Surface Crown Pillars for New or Abandoned Mine Workings. Proc. 4th North American Rock Mechanics Conf., Seattle, pp.465-472 Castro, L.A.M.; McCreath, D.R. & Kaiser, P.K. (1995). Rock Mass Strength Determination from Breakouts in Tunnels and Boreholes. 8th ISRM Congress, Tokyo: 531-536. Dasgupta, B., R. Singh and V. M. Sharma. (1999) "Numerical Modelling of Desilting Chambers for Nathpa Jhakri Hydroelectric Project," in Proceedings of the 9th ISRM Congress on Rock Mechanics. Paris, 1999, Vol. 1, pp. 359-360. Rotterdam: Balkema. Duncan Fama, M. E. (1993) Numerical modeling of Yield Zones in Weak Rocks. In Comprehensive Rock Engineering. (ed. J.A.Hudson) Vol. 2 pp. 49-75, Pergamon, Oxford. Eberhardt, E. (2001) Numerical Modelling of Three-Dimension Stress Rotation ahead of an Advancing Tunnel Face. Int. J. Rock mech. Min. Sci. Vol 38, pp499-518 Fasching A. and Bofer, M. (1997) A Tunnel Under the Andes. Tunnels and Tunnelling International. April Edition pp.34-37 2005. Proc. Int. AFTES Congress, Tunnelling for a Sustainable Europe, Chambery, pp. 349-358

Grimstad, E and Barton, N. (1993) Updating the Q-System for NMT. Proc. Int. Symp. On Sprayed Concrete, Fragernes. Grimstad, E and Barton, N. (1995) Rock Mass Classification and the Use of NMT in India. Proc. Int. Conf. On Design and Construction of Underground Structures, New Delhi, India. Hoek, E (1999) Putting numbers to geology—an engineer's viewpoint. Q. Jnl Eng. Geol. & Hydrogeol. vol. 32, no. 1, pp. 1-19(19) Hoek, E., (2000) Big Tunnels in Bad Rock, The Terzaghi lecture – presented at the ASCE Civil Engineering Conf., Seattle, Oct 18-21, 2000 Hoek E and Brown E.T. (1988) The Hoek-Brown failure criterion - a 1988 update. Proc.15th Canadian Rock Mech. Symp. (ed. J.H. Curran), Toronto: Civil EngineeringDept., University of Toronto, pp. 31-38. Hoek E. and Brown E.T. 1980. Underground Excavations in Rock. London: Institutionof Mining and Metallurgy Hoek, E. and Marinos, P. (2000). Predicting Tunnel Squeezing. Tunnels and Tunnelling International, Part 1 – November 2000, Part 2 – December. Kirsten, H.A.D., (1988) Discussion Contribution relating to the Norwegian Geotechnical Institute Q System. In Rock Classification Systems for Engineering Purposes. ASTM STP 984, ed. Kirkaldie, L., Philadelphia, pp.86-88 Kumar, R and Dhawan, A. K., (1999). Geotechnical Investigations of Nathpa Jhakri Hydro Electric Project Proc. Workshop on Rock Mechanics & Tunnelling Techniques, Shimla Kumar, N., Varughese, A., Kapoor, V.K., Dhawan A.K. (2004) In Situ Stress Measurement and its Application for Hydro-Electric Projects—An Indian Experience in The Himalayas , Paper 1b 02 — Sinorock-2004 Symposium, Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3 Mahajan, S., (2000) Practical application of steel fibre reinforced shotcrete in desilting chambers of Nathpa Jhakri Hydroelectric Project Proc. Int.Conf. Tunnelling Asia ‘2000 544 pp Singh, B., Jethwa, J.L., Dube, A.K and Singh, B (1992) Correlation between Observed Support Pressure and Rock Mass Quality. J. Tunnelling and Underground Space Technology, Vol 7, #1, pp 59-74

Singh, B., Jethwa, L.L and Dube, A.K (1995) A Classification System for Support Pressure in Tunnels and Caverns. J. Rock Mech and Tunnelling Technology, Vol 1, #1, pp.13-24

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