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Construction of the desilting chambers for the Nathpa Jhakri hydroelectric project, India

T.G. Carter & M.J. Telesnicki Golder Associates Ltd., Canada M.L. Kenny, D.M. Brophy, J.L. Carvalho, D.E. Steels & H.S. Dhillon Aecon Constructors Ltd., Canada

SYNOPSIS: Construction of the four, closely spaced 525m long, 16m wide, 30m high caverns forming the Desilting Chamber Complex for the Nathpa Jhakri Hydro-electric Scheme in Himachal Province in Northern India posed numerous challenges as a consequence of (i) the difficult rock conditions, (ii) the end use design requirements and (iii) the physical layout arrangements of the Chambers with respect to the multiple access tunnels and waterway conduits. This paper discusses the rock mechanics measures undertaken to safely excavate and support the complex gneissic and schistose rock mass, including the reinforcing of several large wedge failure geometries evident in the curved Chamber sidewalls. The detailed construction steps taken to develop the staged sequence for excavating the full Chamber profiles are described, and outlines are presented of the controlled excavation methods and extensive rock reinforcement undertaken to create the required curved wall excavation shapes and preserve the slender pillars between the Chambers. The application of the fibrereinforced shotcrete lining is discussed. Because of the difficult rock conditions and relatively high stress state several detailed 2-D and 3-D numerical modelling analyses were undertaken to examine the stability of the Chambers as a basis for reinforcing the Chamber sidewalls, crowns and pillars, taking into account the numerous inter-connecting access and waterway tunnels and shafts. The results of the modelling are explored in the light of observed deformation behaviour of the Chambers during excavation.

1. INTRODUCTION

Construction of the Desilting Chamber Complex for the Nathpa Jhakri scheme, which was carried out from 1994 to 2004, involved excavation of four major Chambers, each of a size similar to that typical for the main cavern of an underground power station, (Figure 1). The Chamber layouts and associated tunnel works, which were principally sized and dimensioned for silt control purposes, were designed by the Central Water Commission (CWC) for Satluj Vidyut Nigam Limited (SJVN), a Joint Venture of the Government of India and Himachal Pradesh, formerly known as the Nathpa Jhakri Power Corporation (NJPC). As such the layouts of the tunnels and chambers were optimized principally from a hydraulics viewpoint, with rock mechanics aspects only considered of secondary importance. However, in the steep Himalayan topography of the site, steeply dipping geology

(phyllites, gneisses and schists) dominated rock conditions, making excavating and reinforcing the Chambers and Intake structures challenging. From the contractors perspective the main Chamber constructability issues were wall and crown control, pillar reinforcement and excavation sequencing, while for the Chamber Intakes, rockslide stability was of most concern. 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 at a total project cost in excess of US$1.2 billion, about a third of which was 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

World Tunnel Congress 2008 - Underground Facilities for Better Environment and Safety - India

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Figure 1. Desilting Chamber #3

Figure 2. Project location & scheme layout

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civil work contracts, totalling $640M. These construction contracts included the Main Dam, the Intakes and the Desilting Chambers (Figure 3) and approximately 16km of the Headrace Tunnel. Throughout the eleven years of the joint ventures contract works, Golder Associates provided geotechnical and geological engineering advice, with much of the emphasis on the Chambers.

1.1 Desilting arrangements

As designed by SJVN, the Nathpa Jhakri scheme is basically a run-of-river power development and was 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 gate intake arrangements, by a 61.5 m high concrete gravity dam, (Figure 4). In an attempt 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, 30 m high, 16m wide 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. Completion of the Chamber complex required removal of more than 1 million m3 of rock. From the Chambers, as per the right hand diagram on Figure 2, the water runs in 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 generating units of 250 MW, each set within a 225m long, 49m high and 20m span underground powerhouse, allowing full utilization of the approximately 425m developed water pressure head.

1.2 Rock mechanics influences on construction

Numerous technical papers describing various aspects of the scheme from a geological or geotechnical perspective have already been published elsewhere, (eg., Kumar and Dhawan, 1999, Dasgupta et al, 1999, Hoek, 1999, 2000; Bagde, 2000, Mahajan, 2000, Hoek & Marinos, 2000 and Carter et al., 2005). This paper does not discuss the overall scheme in any detail, rather it concentrates on examining the rock conditions encountered during excavation of the Desilting Chambers by the Continental-Foundation Joint Venture (CFJV). It is of note that the steeply dipping foliated nature of the rockmass has been a dominating influence on almost all aspects of the underground excavation works. As shown in the before and after cable anchoring photographs of the Desilting Complex Intakes works (ref. Figures 5a and 5b respectively) the foliation in the rockmass at the Desilting Complex is pervasive, also controlling much of the surface topography. Largely because of difficult rock conditions and access, the Contract works undertaken by Aecon Constructors as part of Continental-Foundations Joint Venture (CFJV), which were envisaged to take about 57 months, ended up taking 131 months to final completion and startup of operation of all the Generators. The largest delays to progress in the Intake and Chambers areas occurred because of:

(a) pillar stability issues in the Chambers due to the more foliated nature of the rockmass than expected and the slenderness of the inter-Chamber pillars, the rock support designs indicated in the tender documents needed enhancing. This was accomplished by installation of three rows of 20m long 60T cable anchors through massive concrete beams cast on each Chamber wall, together with over 20,000 supplemental deeper patterned reinforcement elements (bolts and dowels). . and .

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Figure 3. Desilting complex

Figure 4. Completed Intakes and dam

Figure 5a. Original rock condition Figure 5b. Desilting area intakes after cable anchor installation

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(b) slope stability issues in the Intakes area due to the fact that the valley side slopes at the intake site were not stable enough for the required rock cuts needed to reach the structure foundations, remedial stabilization works were required. To completion of this works area, eventually 600 cable anchors of 40m length and 200T capacity were required to be added as additional support.

2. CHAMBER EXCAVATION

Excavation of the four Desilting Chambers progressed in parallel as much as possible in order to expedite construction progress and also to minimize any stress-related rock interaction effects (for example of having one Chamber excavated significantly ahead of the adjacent Chamber headings). The initial stage of excavation for each of the Chambers utilized a crown access, developed by conventional top heading and benching, with side slashes to develop the full haunch profile (Segments A and B as shown on Figure 6). Several of the main adit cross cuts were also constructed at much the same time as the initial crown drift in order to allow access to two different elevations within the overall Chamber Complex (ref layouts shown on Figures 3 and 6). The remainder of the Chamber profiles were then constructed by benching, again with the intention of maintaining reasonable sequencing in the benching operations so as to avoid inter-Chamber interaction problems due to stress readjustments. Figures 6a and 6b show in section and plan, respectively, where segments of the top headings and segments of benching were planned to be underway sequentially. The diagrams respectively illustrate the staged excavation sequence, as tendered and one typical stage (time snapshot) during the development (as modelled for rock mechanics evaluation). Comparison of the tender layout (Figure 6a) with the inset diagram on the left of Figure 6b, shows the differences in excavation benching between the tender proposal and actual (as constructed) staging.

2.1 Crown top headings

Excavation of the Central Pilot Heading of the four Chambers (i.e., Excavation Sequence 1, Modelling Segment A as shown on Figure 6a) was completed concurrently over the following periods:

Pilot Side-Slashes

Chamber 1 4-10-95 to 17-4-97 20-4-97 to 23-9-97

Chamber 2 21-10-95 to 12-12-96 25-3-97 to 17-9-97

Chamber 3 21-10-95 to 10-11-96 20-4-97 to 6-11-97

Chamber 4 21-10-95 to 15-11-96 11-1-97 to 21-7-97

Typically 3.5 - 4.0m rounds were excavated for the top heading and haunches, and at each stage the rock condition and details of each face were mapped and classified using standard rock mass classification methods (i.e., following Barton et al, 1977, Bieniawski, 1976, Grimstad & Barton, 1995, and/or Marinos & Hoek, 2000). Such classifications then formed the basis for definition of blasting charge weights and rock reinforcement. Figure 7 shows the typical blasting pattern used for the mid grade rock conditions (Class III, Q = 4-10; RMR76 = GSI = 55-65) with a typical powder factor in the 1.2 1.6 kg/m3 range. Throughout these excavations, support in accordance with the Construction Drawing layouts was installed and regular proof testing of bolts was carried out by SJVN to ensure the adequacy of the installations. Due to some secondary grouting difficulties encountered with some of the crown installations in the driving of the Pilot Headings; and, with the approval of SJVN, cement cartridges were substituted for uphole reinforcement, and these agreed procedures were then utilized throughout development of the side-slashes (Fig. 7). During the latter stages of excavation of Sequences 1 and 2, (Modelling segments A & B) SJVN and Geological Survey of India (GSI) geologists recognized that on a large scale the Chambers were being crossed by numerous adversely oriented, weak geological structures (biotite and micaceous schist units, and various shear zones). The pervasive foliation and multiple shear zones which were found dipping into the development headings, together with the two predominant orthogonal joint sets also created a number of local scale (but still significant) ground control problems during excavation. In fact, the extent and weakness of the rockmass, created by these features was deemed so adverse to stability and operational efficacy that an unprecedented six months period of shear seam treatment (including additional localized excavation, cavity filling, grouting and additional bolting) was initiated in each Chamber, starting in June 1997.

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The program of shear seam treatment which was started in Chamber #4 after the main pilot was through and the side slashes were complete, was then carried out concurrently into each of other four Chambers over the following periods:

Chamber 1 - 8-01-98 to 9-02-98

Chamber 2 - 6-10-97 to 2-01-98

Chamber 3 - 31-01-98 to 27-04-98

Chamber 4 - 28-09-97 to 21-12-97

Up until initiation of the shear seam treatment, most of the excavation works had been sprayed with a 50mm layer of plain shotcrete directly applied onto all agreed final "treated" rock surfaces. Following successful trials in February, 1998 and contractual agreement relating to application of the Contractor proposed Steel Fibre Reinforced Shotcrete (SFRS), the first production phase of "final" SFRS lining was initiated under SJVN direction in Chamber #1 in March 1998. Application was conducted under skilled control of experienced shotcreting crews with regular quality control panel and spray test cores being routinely checked and approved by SJVN's testing laboratories. SFRS lining application, which proved fundamental to maintaining the integrity of the near-surface zone of the rockmass, was completed within each of the four Chambers over the period from March to July 1998.

2.2 Benching

As soon as the full SFRS lining had been placed down the Chamber sidewalls as far as permitted by SJVN (2.5m above the then invert) for the Sequence 1 and 2 crown excavation, SJVN then allowed initiation of Sequence 3 benching, (Modelling Segment C). This bench excavation between RD 0 and RD 490 was carried out within each of the four Chambers over the following periods:

Chamber 1 - 8-06-98 to 17-12-98

Chamber 2 - 13-08-98 to 18-06-99

Chamber 3 - 25-06-98 to 1-03-99

Chamber 4 - 21-10-98 to 23-10-99

During execution of this sequence over the period January 1999 to May 1999, difficulties were experienced with sidewall profile control in the downstream portions of several of the Chambers, in particular Chamber #3, due in large part to the adverse orientation of the prevailing geological structure. To better control rockmass behaviour, Sequence 3 and indeed all excavation of the lower parts of the Chambers, including the hoppers, was developed by conventional benching, with in the early phases the centre zone drilled and charged vertically from the bench above, and the more critical sidewall zones excavated using parallel development rounds, drilled longitudinally. Later on, after completion of sidewall cable anchoring, when excavation actually recommenced, all further bench development was carried out only by horizontal drilling, typically as indicated in Figure 8. Even with these measures, significant wedge shaped overbreak zones commonly developed due to the unfavourably oriented 50 dipping foliation. Consequently, several methods of benching development were attempted (including central pilot and slash, with parallel, longitudinal drilling). Further measures to control wedge and block slide releases from the sidewalls were also implemented, including installation of downward inclined reinforcement into the lower part of the Sequence 2 sidewalls, thereby adding some support to the walls alongside the next bench zone to be blasted. However, as these additional reinforcement members could only be installed into the corner zones of the walls and approval from SJVN was not given for placement of these elements as countersunk installations through the next bench blast invert, these measures

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Figure 6a. Chamber excavation sequence (as tendered)

Figure 6b. Stage 3 of planned development sequence (as modelled in examine 3D to check stress-interactions)

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Figure 7. Typical pilot blasting pattern

Figure 8. Hopper portion Drilling pattern & initiation sequence

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only achieved marginal wall profile improvements. As the problems of sidewall control were not completely resolved by the above methods employed during the benching of Sequence 3 (height 5.0 m), in the next bench (Sequence 4) in addition to maintaining the central pilot and slash excavation sequence, bench heights were further reduced to 3.8 m. For both Sequence 4 and Sequence 3, SFRS placement followed sequentially with the bench blasting and the regular program of shear seam treatment, as per the procedures developed and agreed with SJVN during the previous treatment program. It is of note in this regard that throughout benching, approval for final application of SFRS was given by SJVN only after completion of geological mapping, and shear seam identification and treatment. Final SFRS placement was therefore delayed somewhat, allowing some further wall loosening before the remainder of the Sequence 2 and 3 sidewalls were permitted to be sprayed. Sidewall overbreak issues continued to dominate wall profile problems, until some improvement could be achieved when the lower limit of SFRS was revised by SJVN to allow placement to within 1 m of the bench horizon. SFRS was thereafter applied to all the walls of Sequence 3 excavation in phase with the development progress, but still with some lag for shear seam treatment behind the excavation face, during the following periods.

Chamber 1 - 3rd Aug., 1998 - 8th Mar., 1999

Chamber 2 - 29th Sept., 1998 - 13th July, 1999

Chamber 3 - 16th July, 1998 - 7th June 1999

Chamber 4 - 18th Dec., 1998 - 31st Oct. 1999

Although all of the walls were sprayed in each sequence with SFRS, all as part of the lining operation, any zones of unstable rock, identified by CFJV or SJVN during initial development were immediately then sprayed with a fresh layer of 50mm SFRS. Excavation and support, including application of SFRS, of Bench Sequences 4 and 5 then continued over the period from June, 1999 to May, 2000 maintaining the lesser bench height and central gullet and side slash sequence with regular approval checks from SJVN. However cracking problems through the lining continued and at this time, all bench excavation was halted following a

third major wedge failure close to the intersection of Adit 2 and Chamber #4, (ref. Figure 3 and 6b for location). Up until this halt on excavation, sidewall bolting was being placed in sequence behind the excavation, with secondary grouting of bolts being accomplished using pumped in cement in sub-horizontal and downward inclined holes, with cement cartridges being used in any required up-holes.

3. GEOLOGICAL RE-EVALUATION

The onset of significant cracking in the SFRS lining in several of the Chambers and the failure of three almost identically shaped large rock wedges (ref. Figure 9b for geometry of one of the failures) highlighted the need for re-examining prevailing geological conditions within the Desilting Complex. Detailed geological mapping by SJVN staff and staff of the Contractor showed that the Chamber area, like the Intake cuts (Figure 5a), was dominated by north-east striking, north-west dipping foliation, interacting with several other cross-cutting steeply dipping joint fabrics, and that these discontinuity fabrics had played a major role in the wedge slides.

3.1 Foliation

The intensity of foliation in the rockmass forming the sidewalls of the various Chambers varied markedly depending on rock type, with zones of the most intense fabric exhibited in the more schistose sections of the rockmass, as compared with a much more gneissose texture in the more competent rock units of the Chamber complex. In the vicinity of the large sidewall wedge failures foliation was well developed, but not especially schistose. It however had a controlling influence on wedge geometry, as can be inferred from the photograph and isometric diagrams in Figure 9b, (note, for scale, the people standing in the Chamber to the right of the wedge). Detailed examination of the individual wedges and plotting of stereonets of the controlling jointing showed that the configuration of bounding jointing was quite complex. The failure block in all three cases was found to have been created by the intersection of no less than 5 joint sets with the Chamber sidewalls, as follows:

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Joint Set Dip Dip Direction

J1 (Foliation) 57 352

J2 60 173

J3 82 098

J4

Main Cross Joints

60 253

J5 (Top release) 30 229

As indicated in the above table these five joint planes, which formed the block margins as illustrated in the isometric diagrams on the right hand side of Figure 9b, can be subdivided into three groups:

the foliation, on which the wedge slid,

the three steeply dipping major cross-joints that bound the subvertical sides of the block, and

the 30 low angle joint fabric that formed the top release plane of the wedge.

In fact, several families of similar structures were mapped (as shown on Figure 10), extending across the entire Chamber complex, potentially defining a whole suite of wedges and combinations of wedges transecting the inter-Chamber Pillars.

3.2 Pillar conditions

Figure 10 shows in section and plan the typical geological characteristics of the rockmass surrounding the Chambers and constituting the core of the pillars separating the Chambers. As is evident from Figure 10a, the cross-sectional dimensions of the inter-Chamber pillars varies markedly because of the curved profile layouts of the Chambers as required for hydraulic reasons. The pillars are nearly 60m wide at the Chamber crowns and Hopper chutes but reach minimum widths of only some 30m at mid-height. Because of the size of the wedge failures that had occurred in the Chambers, and as it was considered that any further, possibly progressive, wedge release could compromise the stability of the pillars between the Chambers, a detailed phase of geological structure mapping was initiated by GSI geologists on behalf of SJVN and a major program of numerical modelling and analysis

was undertaken both by the contractor and the designer. Based on the mapping, the zones highlighted as being of most concern were where adverse combinations existed of steep cross-jointing cross-cutting the NE-SW striking foliation. As the foliation, although typically dipping NW at about 55 was found to swing up to 30 in strike and vary in dip by up to 15 or so degrees, a variety of different wedge geometries were possible in different segments of the Chamber sidewalls.

4. NUMERICAL MODELLING

In order to examine these possibilities in some more detail, several phases of numerical modelling were conducted by Golder on behalf of the Contractor and by NIRM on behalf of SJVN. Modelling was directed at answering several different questions at various stages of the project, namely excavation sequencing, stress-interaction effects and pillar stability issues. Although the purposes of the modelling at the various stages was different, results from all of the modelling phases were found germane to (a) furthering understanding of the behaviour of the rockmass, and (b) developing appropriate support layouts to eliminate further wedge instability issues. For the Chamber sidewalls and pillar zones initial reinforcement layouts (as per the Tender drawings) required 5m and 6m bolts as the standard pattern of routine surface zone support, but not specifically providing deep reinforcement into the pillar cores. With the pervasive nature of the foliation, the degree of geologically controlled overbreak of the sidewalls due to the 50 foliation dip and other cross-cutting pervasive jointing, pillar thickness concerns and sidewall stability issues suggested that much deeper, heavier reinforcement patterns would be needed to completely stabilize the pillar core zones. The fact that three major wedge failures, each up to 15m high and 8m deep had occurred was further impetus to re-evaluate the need for deeper, through-pillar reinforcement and more comprehensive integral sidewall support. The fact that these failures developed deeper than the surface support, and involved release on foliation and major cross-joints prompted a further phase of very detailed numerical modelling to examine the influence of potential wedges on overall pillar stability. The modelling work undertaken by Golder

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on behalf of CFJV (which used the Examine2D or 3D, UDEC, Flac and/or Phase2 codes) was directed towards checking the magnitude of deeper, heavier reinforcement necessary to ensure stability for excavation, while the work undertaken by NIRM on behalf of SJVNs design team (principally using 3DEC) concentrated on Chamber stability under operating conditions.

4.1 Initial 2-D modelling assessments

Early phases of Golder Associates modelling studies for CFJV were targeted at examining optimum construction sequencing that would minimize excavation difficulties and assist in maximizing excavation progress throughout the benching sequencing (ref. Figures 6a and 6b). On the basis of these studies, recommendations were made regarding preferred sequencing. These modelling studies, which were carried out either in Examine2D or PHASES concluded that:

sliding failure problems would be more prevalent in the west (right) walls of each Chamber, with buckling and delamination problems more prevalent in the east (left) wall,

zones of the most adverse potential distress would occur in the west (right) haunch, and in the vicinity of either side of the desilting drift (Hopper area),

optimum wall control of benches required pre-support of the sidewalls, as foliation dips were steep enough to allow wedge sliding and/or fall-outs immediately on excavation, prior to being able to place sidewall reinforcement, and

excavating Chambers 1 and 4 ahead of Chambers 2 and 3, stress shielded the inner chambers, thereby minimizing differential displacements in the pillars between all the interior chambers.

4.2 Initial 3D sequence modelling

The initial 2D modelling analyses concentrated on overall excavation sequencing and stress-interaction effects, but did not specifically examine behaviour of the individual Chambers to benching. These analyses were therefore supplemented by specific 3D evaluations aimed at checking intersections and benching in order to optimize pilot bench drift and sidewall slash approaches. To achieve this, two major series of 3D modelling were undertaken using the Examine3D code, one looking at intersections, the other examining sequencing within a single Chamber. Example outputs from the two series of models are shown in Figure 11. These modelling studies, which were mainly undertaken using a multi-chamber configuration for the 1995 analyses and a single chamber, multi-staged sequence for the 1996 analyses, concluded that:

major interference problems could occur for many of the intersection areas of the construction access drifts (Adits 1, 2 and 3) with the Chambers, with the Adit 2 configuration, potentially being the worst,

minimal adjacent Chamber influence during benching was achieved by keeping benching ahead in Chambers 1 and 4, over those in Chambers 2 and 3, with most adverse interaction occurring when bench faces corresponded with the foliation trend, ie, Chamber 4 leading and Chamber 1 trailing,

optimum bench sequencing was achieved by excavating from the upstream end to the downstream end in order to allow early attention to reinforcement installation in heavily foliated areas, (ie, ensuring that, in general, the foliation always would dip out of the bench face being excavated),

pilot drift development during benching could be taken up to 25m ahead of the sideslash excavations for a pilot and slash approach, without adversely affecting sidewall behaviour, and

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Figure 9a. Normal sidewall chamber conditions in vicinity of Adit 2

Figure 9b. Wedge Geometry in Chamber #2 Sidewall Zone created by foliation & major cross-jointing (evident downstream of Adit 2)

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Figure 10a. Typical Cross-Section of Chambers 3 and 4 showing complex pattern of cross-jointing and shears in vicinity of Adit 2

Figure 10b. Typical plan detail of geology of chambers & pillars

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Figure 11a. Example of detailed chamber intersection zone 3D modeling

Figure 11b. Example output from Initial 3D modelling for optimizing chamber bench sequencing

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potential damage zone depths in weak foliated rock zones (schists) were double those predicted to occur within the more competent gneissic materials.

4.3 Supplementary 2D & 3D modelling With ongoing movement continuing to cause cracking and displacement of the SFRS lining and with the three major sidewall wedge failures having occurred to depths greater than the, by then, installed bolting patterns, concerns regarding wall stability prompted action by all parties to better understand rock mass behaviour. This was achieved by (a) increasing the number of installed Chamber convergence arrays to improve displacement monitoring coverage, (b) installing additional instrumentation principally sidewall extensometers and load cells) and (c) initiating a further phase of numerical modelling based on re-analysis of conditions highlighted by improved geological understanding of the rockmass. As there appeared to be significant differences in excavation-related rockmass behaviour along the Chambers, it was suspected that because of the steep valley topography (Figure 12a), some of these differences might relate less to changes in excavation sequencing and Chamber geometry and more to changes in insitu stress state with distance into the mountainside. Accordingly the available insitu stress measurement data for the Chamber area (eg., Kumar et al., 2004) was re-evaluated in terms of ground surface topography, as summarized on Figure 12b. Based on topographic surveying of the steep slopes rising up from the river towards the mountains, the rock crown cover over the Chambers was found to vary by more than 400m along their alignment, as per the following Table:

Chamber u/s end (Adit 1) d/s end (Adit 3)

1 West 109m 501m

2 124m 502m

3 141m 506m

4 East 158m 478m

In consequence of these elevation differences, quite significant changes in insitu stress state across the geometry of the Chamber complex could be envisaged. Based on Figure 12b, which presents best estimate fits to available insitu stress

measurement data, more than a doubling of the vertical stress to the crown of the Chambers occurs at the downstream end of each Chamber relative to the upstream end, with an almost quadrupling of the horizontal stress as one moves downstream along the chamber axes. In addition to these topographically controlled stress differences across the Chamber complex area, it was also felt that changes in rockmass stiffness might also exert some influence, due to some quite significant changes in fracture density and orientations along the cavern axes, as identified by the geological mapping (Figure 10). Modelling for this phase of evaluation was therefore focussed more towards use of discrete element codes that could replicate the actual mapped fracture patterns rather than just accounting for changes in fracture intensity by making global modulus alterations in rockmass properties, as a way to reflect the mapping information. In order to model the behaviour of the Chambers to the date of the onset of cracking of the SFRS and the fall-out of the three major wedge failures, a series of 2D section and plan models were set up in UDEC by Golder and in parallel a 3D model of the Chamber Complex was built by NIRM using the 3DEC program code. As can be appreciated from examination of the diagrams in Figure 13, these UDEC models were very complex and time consuming to run. Because of this complexity they also needed very careful calibration to actual conditions in order that any forward predictive modelling could be considered realistic. Such calibration was however also not simple. A two stage approach was therefore taken for calibrating the discrete element models. First some overview modelling (in plan and in section) was carried out using the Phase 2 FEM program code, specifically so that the data from the convergence arrays and extensometer installations could be rapidly tracked backwards over the known excavation sequence for which instrumented response data was available. Then the UDEC sections were time-stepped through the same stress change sequence as found from the Phase2 responses and Barton-Bandis shear strength parameters (Barton & Bandis, 1982) for discontinuity fabrics in the UDEC models adjusted within realistic ranges until accurate replication of as-measured convergences was achieved (as shown in Figures 13 and 14).

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Typically it was found that the best data source for reliable calibration was the detailed extensometer records (an example of which is shown in Figure 14). On this diagram the plot shows the response for the three anchors referenced to the deepest as datum. A suite of vertical sections were specifically set up in Phase 2 to model rockmass behaviour at each extensometer site and input parameters (stress state, and fracture and rockmass properties) adjusted in the modelled sections until good replication was achieved for the complete benching sequence for which instrumentation data was available. The parameters so defined were then transferred to the horizontal section models and further refinement of parameters completed until predicted convergences matched observed readings. Figure 14 shows one of the extensometers in the Chamber 3-4 pillar, on the Chamber 4 side in response to excavation in Chamber 4 of Sequences 5 and 6, and then of the hopper area (Sequence 7). The multi-point borehole extensometer anchor responses are colour coded, with the deep zone showing about 2mm, the intermediate zone about 12mm and the surface zone about another 10mm response, which then rises to almost 25mm with excavation of the Hopper zone. Replication of these responses by Golder in UDEC and calibration of the overall trends by NIRM into 3DEC and then carrying out predictive modelling of the excavation progression to completion of all of the Chambers, including considering watering up the Chambers for operational conditions, indicated that quite significant further displacements could develop, that without additional reinforcement, would lead to significant distress developing in the pillars. The displacement trends shown in the upper diagram on Figure 13 give some indication of the type of fracture-controlled differential displacements that were seen in all of the UDEC runs. The pattern of displacements shown indicates that with full Chamber excavation general relaxation occurs, that if unrestrained would pose a risk for further

potential unravelling of the blocky rockmass constituting the pillars, allowing further wedge fall-outs. With the information generated from this modelling and with continued confirmation from the convergence arrays and installed extensometers that movements were still ongoing in the pillars, and that the trends were closely matching the model predicted behaviour, SJVNs design team began to formulate detailed construction working drawings to add significant support to the pillars to enhance their stability.

5. CONSTRUCTION SOLUTIONS

The remedial support arrangements for the inter-Chamber pillars, which were finally adopted with input from the Panel of Experts and from various members of the contractors staff, who looked specifically at constructability issues, are shown in Figure 15. This diagram shows the various components incorporated into the final configuration, together with the constructed measures necessary for their installation. The SJVN design arrangement basically comprised three rows of overlapping 60T cable anchors into each pillar, installed through cast-in-place anchor beams, and supplemented with additional rebar surface support and additional SFRS to stich together the blocky, loosened near-surface zone of the rockmass. As excavation had already proceeded down to Sequence 4 (el. 1450m) in most of the Chambers and to Sequence 6 (el. 1445m) in one part of Chamber 4, and the upper row of anchors was needed at elevation 1462m, (ie., some 12 to 18m above the then current Chamber floor levels) a considerable effort was required to get back up to the elevation required. In order to overcome the challenge of casting the beams and installing the anchors, CFJV came up with the hybrid scheme of backfill and scaffolding arrangements as shown on Figure 15.

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Figure 12a. Topography at intakes to chambers

Figure 12b. Insitu Stress relationships for Desilting Chamber area with respect to depth below rock surface

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Figure 13. UDEC representation of discrete fracture mapping showing predicted and measured displacements

Figure 14. Typical sidewall MPBX behaviour

Figure 15. Benching & final support

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5.1 Pillar remedial support

Once SJVNs design staff authorized re-initiation of excavation and issued formal instructions to start work on the beams and anchors, forming the bulk of the remedial construction works required for the pillars, plans were put in place to sequentially cast the required concrete beams, necessitating, in some of the Chambers, bringing in backfill and erecting scaffolding according to the schematic shown on Figure 15. This arrangement was developed by the contractor as one of many different construction sequences and execution plans that were explored with the designer, all aimed at minimizing further schedule delays and maximizing constructability. The optimized solution required leap-frogging the fill in the various Chambers in sequence with scaffold erection, concrete placement and anchor drilling and stressing. By the time that all works were complete to the state shown in the photograph in Figure 1, the following measures had been completed:

Works Item Quantity

Refilling of Chambers with backfill

92,000m3 (Fill)

Installation of 60 T Cable Anchors

1,624 each

Placement of 24 @ 525 metre long concrete beams

12,400 metres long

Installation of Additional Rock Bolts (6 m to 7.5 m long)

20,000 each

Installation of Additional grouted anchor bars

6,000 each

Additional Rock Bolts 12.0 m long

4,480 each

Installation of three concrete beams along the entire length of each side of each Chamber (as pictured in Figure 1) was seen as the most expedient means for a) providing sufficient bearing area for anchor stressing for each anchor, whilst also providing a good measure of surface restraint and integral fixing along the Chamber axes.

5.2 Anchor arrangements

The anchors for the Chamber pillars, which were generally planned to be spaced at 7.5m c/c along the beams were selected as four strand 20m long cable packages with an allowable capacity of 60 tonnes and an ultimate capacity of 105 tonnes. Each had a fixed length (anchor zone) of 5m, and a debonded free-length of 15m. For the installations after drilling to full depth water-testing was required prior to grouting in the fixed length, with on-site agreed procedures adopted for regrouting based on site conditions and water test results. Each anchor was then proof tested, and locked off with a pre-stress of 36 tonnes.

6. CONCLUSIONS

The scheme, the largest in India at 1,500 MW, has been generating power now for almost five years with the Desilting Complex Chamber excavations and ancillary tunnels providing their designed role. In spite of so many rock-related problems requiring significant engineering input, the Desilting Complex, comprising four Chambers over 500m long, 16m wide, approximately 30m high and only at 45m centre to centre has been completed to the original hydraulically designed shapes. Execution of the final works has not been without challenges however, and the final arrangements have really only been accomplished through the cooperation and integration of important ideas put forward by many parties, and then engineered to detailed design level only as part of the construction works. The overall final arrangements now comprise several support elements not foreseen in the original tender design drawings. Whether layout re-arrangements (spacing, orientation etc) of the Chambers or different surface and deep rock support arrangements incorporated into the tender layouts could have eliminated or reduced the problems encountered in properly stabilizing the weak foliated rockmass is a moot point. The design layout as tendered has been completed, but only by placing three pairs of concrete beams and hundreds of 60tonne cable anchors and thousands of square metres of steel fibre reinforcement over the pillar zone rock mass in each Chamber, suggesting to future generations of designers that hydraulics should not necessarily dominate early design decisions, especially in a Himalayan setting.

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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. REFERENCES

1. Bagde, M.N., (2000) Finite element analysis of underground caverns of Nathpa Jhakri Hydel Project. Proc. Int.Conf. Tunnelling Asia 2000 544 pp

2. Barton, N.R. & Bandis S., (1982). The Shear Behaviour of Jointed Rock. Issues in Rock Mechanics, 23rd US Symposium on Rock Mechanics, pp.739-759.

3. 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.

4. Bieniawski, Z.T. (1976). Rock mass classification in rock engineering. In Bieniawski (ed.), Proc. of the Symp. Explo-ration for Rock Engineering, Vol. 1: pp.97-106. Cape Town. Balkema.

5. Carter, T.G., Steels, D., Dhillon, H.S. and Brophy, D., (2005). Difficulties of Tunnelling under High Cover in Mountainous Regions. Proc. Int. AFTES Congress, Tunnelling for a Sustainable Europe, Chambery, pp.349-358

6. 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.

7. 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.

8. Hoek, E (1999) Putting numbers to geologyan engineer's viewpoint. Q. Jnl Eng. Geol. & Hydrogeol. vol. 32, no. 1, pp. 1-19(19)

9. Hoek, E., (2000) Big Tunnels in Bad Rock, The Terzaghi lecture presented at the ASCE Civil Engineering Conf., Seattle, Oct 18-21, 2000

10. Hoek, E. and Marinos, P. (2000). Predicting Tunnel Squeezing. Tunnels and Tunnelling International, Part 1 November 2000, Part 2 December.

11. Kumar, R and Dhawan, A. K., (1999). Geotechnical Investigations of Nathpa Jhakri Hydro Electric Project. Proc. Workshop on Rock Mechanics & Tunnelling Techniques, Shimla

12. Kumar, N., Varughese, A., Kapoor, V.K., Dhawan A.K. (2004) In Situ Stress Measurement and its Application for Hydro-Electric ProjectsAn Indian Experience in The Himalayas , Paper 1b 02 Sinorock-2004 Symposium, Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3

13. 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

14. Marinos, P. & Hoek, E. (2000). GSI A geologically friendly tool for rock mass strength estimation. Proc. GeoEng2000 Conference, Melbourne: 1422-1442

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BIOGRAPHICAL DETAILS OF THE AUTHORS

Dr. Trevor G. Carter is a Principal with Golder Associates in Canada. He is also a specialist geological engineer with world-wide experience in unravelling difficult and complex rock conditions for design of heavy civil or mining projects. He has over 30 years of experience relating to engineering

geological and rock mechanics aspects of tunnelling and civil and mining underground design and construction. Dr. Carter has worked with Golder Associates since 1976 and after spending 4 years on the construction and design aspects of the 1000MW Drakensberg Pumped Storage Scheme in South Africa, has mainly been involved with solving geomechanics problems of large scale surface or underground excavations for mining or civil engineering applications. Throughout his career he has remained involved with hydropower projects worldwide, most recently in India as a specialist review consultant on rock mechanics aspects of the underground chambers and tunnel construction works for the 1500MW Nathpa Jhakri Scheme in India. He is currently serving on a review board for two similar power schemes in Chile.

Mike Kenny graduated from the University of Manitoba Canada with a B.Sc. in Civil Engineering in 1968 and is a licensed professional engineer in the Canadian provinces of Ontario and British Columbia. He has been employed by the Foundation Company of Canada Division of Aecon

Constructors for 25 years and is currently Vice President. Mr. Kenny was involved for 7 years at the jobsite for the companys Nathpa Jhakri projects with the last 4 years as Project Manager, where he was responsible for overseeing all aspects of the construction works for the Dam, the Desilting Chambers and several km of associated tunnels.

Don Brophy joined Aecon in 1977 after graduating from the University of Ottawa with a Bachelor of Applied Science degree in Civil Engineering. He has held the positions of Project Engineer, Project Manager, Contracts Manager and Estimating Manager. Don has extensive experience as

Senior Estimator, bidding heavy civil construction projects in the domestic market as well as internationally. He has worked on major projects such as the Highway 407 Express toll Route (407ETR) and the Toronto Airport Terminal Development projects in Canada, and the Nathpa Jhakri Hydroelectric Power project in India. Don is

currently responsible for overseeing the execution of all heavy civil projects under Aecon Constructors.

Dr. Jos L. Carvalho Joe Carvalho graduated in Civil Engineering from the University of Toronto in 1982 and currently is a Principal with Golder Associates Ltd. in Mississauga, Ontario. He has over 20 years experience in heavy civil engineering and mining projects with extensive

capability in detailed rock mechanics analysis and numerical modelling; with particular experience also in coding instrumentation and monitoring databases. Dr. Carvalho has been involved in application of advanced numerical methods for numerous applications related to open pits, underground mine excavations, underground hydroelectric caverns, utility and transportation tunnels and foundation projects. Dr. Carvalho was responsible for the numerical analyses of several aspects of the Nathpa Jhakri Hydro-electric project in India.

Doug Steels is President of Aecon Constructors and also manages Aecon's interests on Joint Venture projects as Board member of the Executive Management Committee. Mr. Steels' 35-year construction and engineering career with Aecon started as a Field Engineer on the massive St. Lawrence Seaway, Welland Canal

Relocation project. His unique combination of practical skills and management capabilities were developed on Aecon heavy civil construction projects as diverse as British Columbia's Revelstoke Dam and Mount MacDonald Railway Tunnel, India's Nathpa Jhakri Hydroelectric project and the Cross Israel Toll Highway project. Prior to his appointment as President of Aecon Constructors, Mr. Steels held roles of increasing management responsibility in Aecon's corporate office, including the positions of Chief Estimator and most recently as Senior Vice President. Mr. Steels holds a Bachelor of Science degree in Civil Engineering from the University of Windsor.

Harjit Dhillon began his heavy construction career in North America working for the Perini Corporation out of Framingham, Massachusetts. He joined Aecon Construction Group Inc in 1963 as Chief Engineer for The Foundation Company of Canada, Aecons wholly owned subsidiary. Throughout his years with Foundation,

and eventually in the position of President of The

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Foundation Company, Harjit was in charge of many of Aecons largest and most complex joint venture projects, with special emphasis on hydroelectric developments and their related underground civil construction works. Some of the more noteworthy projects included the Revelstoke Dam and Powerhouse for British Columbia Hydro and the Jenpeg, Longspruce, and Kettle Rapids power projects for Manitoba Hydro. Most recently, Harjit completed the Nathpa Jhakri Hydroelectric project in Northern India, a $640 million Aecon sponsored Joint Venture.

Mark Telesnicki graduated in Geological Engineering from the University of Waterloo in 1987.and has over 20 years experience in heavy civil engineering and mining projects, particularly focusing on geotechnical investigations; rock mechanics analysis and design; instrumentation

and monitoring; and preparation of construction specifications and drawings. He is currently a Principal with Golder Associates Ltd. and Manager of the Rock Engineering Group in Mississauga, Ontario. Throughout his career, he has been involved with underground hydroelectric schemes, several utility and transportation tunnels, and numerous rock slope stabilization and foundation projects. Mr. Telesnicki spent just over a year on site at the Nathpa Jhakri Hydro-electric project in India where he was heavily involved in all the early aspects of the construction works for the Desilting Chambers and appurtenant tunnels.