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PROYECTO HIDROELECTRICO ALTO MAIPO

INGENIERIA BASICA

TUNNELLING AND ROCK SUPPORTBASIC DESIGN

0 19-12-08 PARA LICITACION

EMISION FECHA PROPOSITO DE LA EMISION PREPARA REVISOCOORDINADOR

GENERALAPROBO

D. PROYECTO/ D. ADJUNTO

APROBO

SUBGERENTEDE INGENIERIA

APROBO

GERENTE DEPROYECTO

APROBO

REVISION

0

N°: 600-TU-CDD-001 

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INDEX

ITEM CONTENTS PÁG.

1.  Executive summary ..............................................................................................1 

1.1 

Introduction...........................................................................................................1 

1.2  Las Lajas..............................................................................................................1 

1.2.1  Upstream portion to connection with Alfalfal II tailrace..........................................1 

1.2.2  From connection with Alfalfal II tailrace to Las Lajas powerhouse ........................1 

1.2.3  Las Lajas Pressure shaft ......................................................................................1 

1.2.4  Las Lajas Powerhouse .........................................................................................2 

1.2.5  Cable shaft and cable tunnel ................................................................................2 

1.2.6  Las Lajas tailrace tunnel.......................................................................................3 

1.3  Alfalfal II ...............................................................................................................4 

1.3.1  Basic considerations decisive for layout ...............................................................4 

1.3.2  From upstream end of the headrace tunnel (VA4) to top of the pressure shaft (A9).............................................................................................................................4 

1.3.3  Alfalfal II Pressure Shaft .......................................................................................5 

1.3.4  Alfalfal II Powerhouse...........................................................................................5 

1.3.5  Alfalfal II Tailrace tunnel .......................................................................................6 

1.4 

Tunnel Volcán ......................................................................................................7 

2.  Las Lajas - Description of layout..........................................................................8 

2.1  Las Lajas Access tunnel.......................................................................................8 

2.2  Las Lajas Powerhouse Area.................................................................................8 

2.3  Las Lajas Headrace tunnel.................................................................................10 

2.4  Surge system .....................................................................................................10 

2.5  Pressure shaft ....................................................................................................11 

2.6 

Transition zone and valve chamber ....................................................................11 

2.7  Las Lajas Tailrace tunnel....................................................................................11 

2.7.1  General ..............................................................................................................11 

2.7.2  Tunnel outlet.......................................................................................................12 

2.7.3  Alternative with use of TBM for part of the Las Lajas Tailrace Tunnel.................12 

2.8  Las Lajas Cable tunnel (Bio Bio).........................................................................12 

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2.8.1  Tunnel dimension...............................................................................................12 

2.8.2  Cable tunnel (Bío Bío) entrance..........................................................................13 

3.  Alfalfal II – description of layout ..........................................................................13 

3.1  Basis for alignment of tunnels.............................................................................13 

3.2 

Alfalfal Access tunnel..........................................................................................14 

3.3  Alfalfal Powerhouse area....................................................................................14 

3.4  Tailrace tunnel....................................................................................................16 

3.5  Pressure shaft ....................................................................................................16 

3.6  Transition zone and valve chamber ....................................................................17 

3.7  Surge shaft and surge chamber..........................................................................17 

3.8  Headrace Tunnel Alfalfal II. ................................................................................17 

3.8.1 

General ..............................................................................................................17 

3.8.2  Tunnel portion excavated by TBM ......................................................................18 

3.8.3  Tunnel portion excavated from the upstream end...............................................18 

4.  Tunnel EL Volcán ...............................................................................................18 

4.1  Basis for alignment.............................................................................................19 

4.2  Tunnel portion excavated by TBM ......................................................................19 

4.3  Tunnel portion excavated by D&B.......................................................................19 

5.  Electrical equipment ...........................................................................................20 

5.1  Background ........................................................................................................20 

5.2  Evacuation..........................................................................................................20 

5.3  Separate transformer hall ...................................................................................20 

5.4  Generator breaker ..............................................................................................21 

5.5  Generator and transformer .................................................................................21 

5.6  Outdoor breakers................................................................................................22 

5.7  230 kV Cable......................................................................................................22 

5.8  Auxiliary supply...................................................................................................22 

5.9  Control and Supervisory Control and Data Acquisition (SCADA) System ...........23 

5.10  Protection...........................................................................................................24 

6.  Mechanical equipment........................................................................................24 

6.1  General ..............................................................................................................24 

6.2  Alfalfal II .............................................................................................................24 

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6.2.1  Turbines .............................................................................................................24 

6.2.2  Spherical valves .................................................................................................25 

6.2.3  Cooling water system .........................................................................................25 

6.2.4  Machine hall crane .............................................................................................26 

6.2.5 

Penstock steel lining...........................................................................................27 

6.3  Las Lajas............................................................................................................27 

6.3.1  Turbines .............................................................................................................27 

6.3.2  Spherical valves .................................................................................................27 

6.3.3  Cooling water system .........................................................................................28 

6.3.4  Machine hall crane .............................................................................................29 

6.3.5  Penstock steel lining...........................................................................................29 

7. 

Engineering geology in basic design...................................................................30 

7.1  General layout....................................................................................................30 

7.2  Geological information........................................................................................30 

7.3  Design of underground openings........................................................................30 

7.3.1  General ..............................................................................................................30 

7.3.2  Rock Mass Classification....................................................................................31 

7.3.3  Plastic deformation.............................................................................................32 

7.3.4  Water Leakage (WL) ..........................................................................................33 

7.3.5  Rock support as function of classification ...........................................................33 

7.3.6  Design of pressurized waterways .......................................................................35 

7.3.7  Rock mass permeability......................................................................................35 

7.4  Tunnelling conditions for selected alignments.....................................................36 

7.4.1  General ..............................................................................................................36 

7.4.2  Las Lajas project ................................................................................................36 

7.4.3  Alfalfal II project..................................................................................................37 

7.4.4  Volcán tunnel......................................................................................................39 

7.4.5  Rock support methods........................................................................................40 

8.  Tunnel and shaft excavation methods ................................................................40 

8.1  General ..............................................................................................................40 

8.2  Drill and blast tunnelling......................................................................................42 

8.3  TBM tunnelling....................................................................................................43 

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8.3.1  General on Tunnel Boring Machine (TBM) for hard rock.....................................43 

8.3.2  Open TBM..........................................................................................................44 

8.3.3  Double Shield TBM (DS TBM)............................................................................44 

8.3.4  Assembly of TBM and Backup System...............................................................45 

8.4 

Shaft excavation.................................................................................................45 

8.4.1  Raise Boring Machines (RBM)............................................................................45 

8.4.2  Raise drill and slash ...........................................................................................46 

8.4.3  Inclined TBM shaft excavation............................................................................46 

9.  Evaluation of use of TBM for tunnel excavation at the Alto Maipo project ...........47 

9.1  General ..............................................................................................................47 

9.2  Evaluation of TBM concept.................................................................................47 

9.2.1 

Open hard rock TBM (gripper type) ....................................................................48 

9.2.2  Double shield machine .......................................................................................48 

9.3  Selection of TBM for the various tunnels at the Alto Maipo project .....................49 

9.3.1  Las Lajas tailrace tunnel.....................................................................................49 

9.3.2  The Alfalfal II headrace tunnel - downstream portion..........................................50 

9.3.3  TBM for Tunnel Volcán.......................................................................................50 

9.4  Summary of TBM requirements/ configuration....................................................51 

9.4.1  Machine requirements ........................................................................................51 

9.4.2  Backup special requirements..............................................................................51 

9.4.3  Site operation requirements................................................................................51 

LIST OF TABLES

Table 1-1: Main technical data - Las Lajas ............................................................................3

Table 1-2: Main technical data - Alfalfal II and Tunnel Volcán................................................6

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1. EXECUTIVE SUMMARY

1.1 Introduction

This report presents the Basic Design of the Alto Maipo Hydroelectric Projectunderground works.

Reference is made to the Drawings 600-TU-PLA-001, 610-TU-PLA-003, 620-TU-PLA-003 and 630-TU-PLA-003 where the alignments are shown in plan and longitudinal profile,and to Drawings 620-TU-PLA-004 and 630-TU-PLA-004 showing proposed layout andarrangement of the powerhouses.

All dimensions and quantities given in this text are intended for general referenceonly. Precise values are given in pertinent documents (drawings and general and particularspecifications).

1.2 Las Lajas

1.2.1 Upstream portion to connection with Alfalfal II tailrace

The flow from the existing Alfalfal power plant will be directed into a Cámara deCarga, that will be located 500 m downstream of the Alfalfal tailrace outlet. The flow fromCámara de Carga, including also the existing flow at Maitenes intake, will be conducted undermoderate pressure through a concrete culvert that will enter the upper end of Las Lajasheadrace tunnel approximately 2 km downstream of the Alfalfal tailrace outlet, at point VL8. Theculvert and tunnel will be dimensioned for a maximum flow of 38 m3/s.

The tunnel continues for 5.8 km to point L10, where the Alfalfal II tailrace tunnel joins the Las Lajas headrace tunnel.

A construction adit of length 230 m will be located at VL7 (adit el Trescientos),some 1.8 km downstream of the upper end of the tunnel (VL8).

1.2.2 From connection with Alfalfal II tailrace to Las Lajas powerhouse

The tunnel from the connection with Alfalfal II tailrace at point L10 to point L9 at

top of the pressure shaft will have a length of 3.7 km, and a design flow of 65 m3

/s.

1.2.3 Las Lajas Pressure shaft

A vertical pressure shaft of some 160 m length leads to the Las Lajaspowerhouse. Steel lining with diameter 3.7 m has been assumed.

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The access tunnel to the top of the pressure shaft, from the switchyard area (VL5)will connect to an inclined cable shaft, and serve as cable tunnel in the operational phase.

The final evaluation of whether extent of steel lining could be reduced or must beincreased will be made when results from in situ investigations of the underground rockconditions are available.

No spherical valve will be installed at top of the pressure shaft, but preparationswill be made to allow for such installation later, if experience from operation deems it necessary.

1.2.4 Las Lajas Powerhouse

Refer Drawings 630-TU-PLA-004 and 630-TU-PLA-005

The Las Lajas powerhouse is located south west of Estero Aucayes, on the south

side of río Colorado, (Point LPS).

The access tunnel will start at point VL4, and have a length of some 2050 m atslope of 9%.

The Las Lajas powerhouse will have gross head of 483 m and an installation of2 x 135 MW, in two 6 jet Pelton turbines with a speed of 300 rpm.

Two three-phase transformers will be installed. They will be located in niches in aseparate cavern running parallel with the main cavern, and connected to the latter with two

busbar tunnels.

1.2.5 Cable shaft and cable tunnel

A short horizontal tunnel will connect the transformer hall with the foot of aninclined cable shaft.

The cable shaft will have 3.4 m diameter and approximate length of 210 m, joiningthe access tunnel to the top of the pressure shaft. This tunnel leads to point VL5 close to thenew Alto Maipo Substation.

Drainage from the steel cone area at top of the pressure shaft will be conductedvia the cable shaft to the end of the transformer hall, and further to the tailrace tunnel via holesdrilled for this purpose.

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1.2.6 Las Lajas tailrace tunnel

The Las Lajas tailrace tunnel is a free flow tunnel of mild slope, 1.1 m per km,13 km long. It will cross under río Colorado some 200 m below the riverbed which is at elevationapprox. 1030 masl.

The entire tunnel can be excavated by D&B by constructing an intermediate aditto the tailrace tunnel, adit Las Puertas at VL2. Alternatively, the downstream portion of thetailrace tunnel, some 9000 m, can be excavated by TBM. In this case, the adit Las Puertas canbe omitted.

Table 1-1: Main technical data - Las LajasLas Lajas power plant. Main dataIntake Cámara de CargaHighest operation level, Cámara de Carga masl 1 323.0Lowest operation level, Cámara de Carga masl 1 318.0

Exceptional high level at Cámara de Carga masl 1 324.0Live volume of Cámara de Carga m3  300 000

Power conduit systemPressure concrete culvert from Cámara Carga to headracetunnel, designed for 37 m3/s m 1 160Headrace tunnel, total length to top pressure shaft. m 9 480

Tunnel excavation methodDrill &Blast

Tunnel cross section (varies - one part designed for 38m3/s, one part for 65 m3/s ) m2  21/31Elevation at top of pressure shaft masl 1000

Vertical, drilled steel lined pressure shaft m 160Drilled diameter m 4.7Internal diameter of steel penstock m 3.7Tailrace tunnel, free flow, Cross-section 36 m2  m 12950

Surge shaft diameter (vertical) m 5.0Surge chamber diameter m 10

Underground powerhouseLength x Width m 62.6x18.2Number of units (Pelton) no 2Design flow per unit m3/s 32.5

Rated capacity per unit MW 134Speed rpm 300Elevation of runner masl 840Max gross head m 468,5Generators no 2Capacity, each MVA 144Power Factor 0,92Transformers, 3-phase no 2Capacity, each MVA 158Transmission voltage kV 110

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Access tunnel, length m 1930Cable shaft, inclined m 213Cable tunnel , from top of shaft m 580

1.3 Alfalfal II

1.3.1 Basic considerations decisive for layout

Alfalfal II will have a maximum gross head of 1156 m and a headrace tunnel 14.8km long.

Location of the headrace tunnel alignment and the powerhouse at an areasomewhat to the west of Estero Aucayes has been selected taking into account the geologicalconditions and their impact on tunnelling and rock support measures. It has been an aim toreduce the magnitude of the overburden, which still is very high, and more than 1000 m atplaces.

For the present layout it has been assumed that steel lining will be needed in thepressure shaft below elevation 1950 masl, i.e. for pressures above 550 m. This assumptiondetermines the location of the top of the pressure shaft, and thereby several other key elementsof the layout, such as powerhouse location and location of adits/access tunnels in thepowerhouse/pressure shaft area.

A surge chamber will be located in the Aucayes Valley, connected to theheadrace tunnel via a shaft/tunnel.

The tailrace of Alfalfal II will empty into the headrace tunnel for Las Lajas Powerplant at L10.

1.3.2 From upstream end of the headrace tunnel (VA4) to top of the pressure shaft(A9)

(Reference is made to the drawings and to the list of coordinates, Table 1-2, forlocation and data for the key points of the tunnel system).

No intermediate adit will be available for the Alfalfal II headrace tunnel, and the

following method has been selected for excavation in Basic Design:

A Tunnel Boring Machine (TBM) will be used for excavation from the downstreamend (A9) in the upstream direction, for an estimated length of 8500 m, to point A10’.

Conventional excavation by drill and blast (D&B) will be used for excavation fromthe upstream end in the downstream direction, for an estimated length of 6250 m, to point A10’.

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The downstream access to the headrace tunnel by the TBM will be located atpoint VA2, south west of Estero Aucayes. The length of this access, from VA2 to A9, will be2000 m. It has been assumed that this access tunnel also will be excavated by TBM for itsentire length.

A surge system will be located near the downstream end of the headrace tunnel,connecting to the tunnel some 250 m upstream of the pressure shaft. The system will comprise:

An inclined shaft that will be excavated by TBM from the headrace tunnel toelevation 2400 masl. A tunnel on milder slope from elevation 2430 masl to the surge chamber,will also be excavated by the TBM.

For Basic design it has been assumed that the TBM will be transported backthrough the surge tunnel/shaft to the headrace tunnel and further through the tunnel A9-VA2, butit is for the Contractor to decide wether the TBM can alternatively be withdrawn at the Aucayespond, where a 7.5 km road would have to be constructed to access the Aucayes pond.

The TBM is the same that will excavate the pressure shaft for Alfalfal II.

1.3.3 Alfalfal II Pressure Shaft

An inclined, steel lined pressure shaft has been selected for Alfalfal II. Relative toa vertical shaft this solution will reduce the length of the access tunnel by 600 m, giving areduction in critical construction time for the project.

The final evaluation of whether extent of steel lining could be reduced or must beincreased will be made when results from in situ investigations of the underground rockconditions are available.

No spherical valve will be installed at top of the pressure shaft, but preparationswill be made to allow for such installation later, if experience from operation deems it necessary.

1.3.4 Alfalfal II Powerhouse

Refer Drawings 620-TU-PLA-003, 620-TU-PLA-004 and 620-TU-PLA-005

Alfalfal II powerhouse is located south west of Estero Aucayes, on the south sideof río Colorado, (Point APS).

Two generating units with Pelton type turbines of 500 rpm with 4 jets will beinstalled, for a maximum capacity of 2 x 132 MW.

Two three-phase transformers will be installed, located in niches in a tunnelparallel to the main cavern, connected to the main cavern via busbar tunnels.

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The access tunnel will start at point VA1, and have a length of some 2500 m atslope of 8%.

Cables will be conveyed out via the access tunnel. The topography andoverburden in the powerhouse area is such that a separate cable tunnel, or a combination ofcable shaft and cable tunnel, is not feasible.

1.3.5 Alfalfal II Tailrace tunnel

The Alfalfal II tailrace tunnel will be 3.1 km long, and will end in the Las Lajasheadrace tunnel.

Decisive for design and dimensioning of the tailrace tunnel, and for the elevationof the Pelton runners at Alfalfal II, is the situation when Las Lajas power plant stops operating

while Alfalfal II continues. The Alfalfal tailrace system will then be a part of the Las Lajas surgesystem, experiencing water surface fluctuations before the surface stabilizes at a higher level,when the Alfalfal II flow is directed to the Cámara de Carga of Las Lajas.

As result of hydraulic analysis of the system, the elevation of the Pelton runner atAlfalfal II has been set to 1341 masl.

The Alfalfal II tailrace will be ventilated via a shaft excavated to surface (A1A)located 1420 m downstream of the foot pressure shaft (FPS).

Table 1-2: Main technical data - Alfalfal II and Tunnel VolcánAlfalfal II power plant. Main data. 

Tunnel Volcán

Total length, Tunnel Volcán. ( No intermediate adits) m 14105Excavation by TBM from the downstream end (V6). Estimatedmaximum length by TBM m 7000Excavation by D&B from the upstream end (V1) towardsdownstream. Estimated minimum length intermediate adits. m 7105Tunnel Inlet elevation masl 2494.1Tunnel outlet elevation masl 2 480Design flow, tunnel Volcán m3/s 12

TBM diameter, estimated minimum m 4.1Tunnel cross section for D&B m2  14

Power tunnel system Alfalfal IIHeadrace tunnel, length from VA4 – A9 m 14789Additional length from A9 to inlet cone of steel lining m 100

TBM diameter ( wheel bound), on ascending slope 7% m 4.5Estimated length of tunnel by TBM m 8540Minimum area of D&B tunnel from upstream m2  13

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Estimated length of tunnel by D&B, on variable descendingslope m 6245Elevation at top of pressure shaft masl 1950,5Shaft for penstock 45° slope, excavated by TBM m 860Drilled diameter m 3.4Internal diameter of steel penstock m 2.4

Tailrace tunnel, to Las Lajas headrace tunnel. Part free flow,part pressure tunnel m 3055Cross section of major part of tailrace tunnel m2 21Diameter of inclined surge shaft, by TBM m 3.4Length of inclined surge shaft, to reservoir m 912

Underground powerhouseLength x Width of theoretical contour line at machine floor level m 57.5 x 16.4Number of units (Pelton) no 2Design flow per unit m3/s 13.5Rated capacity per unit MW 132Speed rpm 500

Runner diameter m 2.72Elevation of runner masl 1 341Max gross head m 1 159Generators no 2Capacity, each MVA 144Power Factor 0,92Transmission voltage kV 220Transformers, 3-phase no 2Capacity, each MVA 161Machine hall crane capacity, estimated t 180/10Access tunnel, cross section m2  35Access tunnel, length m 2456

1.4 Tunnel Volcán

Tunnel Volcán will conduct the flow collected from the río Volcán catchment andtransfer it to entrance of the El Yeso siphon system (Pozo de Toma) that will take theYeso/Volcán flow into the Alfalfal II headrace tunnel. Design flow for Tunnel Volcán will be12 m3/s and the size will be determined from construction requirements. The tunnel will be14.1 km long, with no intermediate adits.

Available information on geology in the area indicates that at some distance alongthe tunnel, some 5 - 7 km upstream of V6, conditions could cease to be suitable for excavationby TBM due to the high magnitude of rock overburden, with corresponding high risk of spallingand squeezing.

For Basic design it has been assumed that tunnel Volcán will be excavated byTBM from the downstream end, at Point V6, in the upstream direction for an estimated maximumof 7 km. After removal of the TBM, the excavation will continue with D&B.

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The upstream portion of the tunnel will be excavated by D&B from point V1.

2. LAS LAJAS - DESCRIPTION OF LAYOUT

Reference is made to drawings 630-TU-PLA-001, 630-TU-PLA-002, 630-TU-PLA-003, 630-TU-PLA-004, 630-TU-PLA-005 and 630-ME-PLA-006.

2.1 Las Lajas Access tunnel

The entrance for the access tunnel will be at point VL4 near the Colorado riverand the main road along the valley, at elevation 1025 masl. Length of the access tunnel willhave some 2050 m, with slope 8.5%.

The access tunnel has been designed with an excavated cross section of 35 m2.This size has been selected with basis in the estimated space needed for transport of thetransformer. If this size is not sufficient for transport of the bifurcation for the 3.7 m dia penstock,the bifurcation must be transported in parts and assembled inside.

Las Lajas will have a separate cable shaft/cable tunnel, and the cables from thetransformers will not be led out of the access tunnel.

It has been decided to supply ventilation air via the access tunnel, and evacuateair via the tailrace tunnel and a shaft drilled to the surface at point L5A.

2.2 Las Lajas Powerhouse Area

Location of the powerhouse inside the rock massif east of Estero El Sauce isdetermined by the location of the top of pressure shaft, point L9, at elevation 1000 masl.

The direction of the longitudinal axis of the powerhouse cavern has beenpreliminarily determined from what appears the optimum with respect to the geologicalconditions, based on information from geological mapping available at the time of basic design.Final orientation of the powerhouse, and hence the final arrangement of the tunnel system in the

powerhouse area will be determined when sufficient information on the in situ rock conditionshas been obtained.

The Las Lajas powerhouse will have a gross head of 469 m and an installation of2 x 132 MW, in two Pelton type turbines with speed of 300 rpm and 6 jets.

The elevation of the Pelton runners is 840 masl.

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Elevation of the main machine hall floor will be at 852 masl.

The dimension of the powerhouse is determined chiefly by the dimensions of theturbines and generators selected.

The width between the upstream and downstream walls of the power station is15.2 m. Adding 0.3 m wall thickness and 1.2 m between wall and rock on each side it gives atotal cavern width of 18.2 m.

The generator will have a diameter of approx. 8.2 m, and inner dimensions of thegenerator room are 10 x 10 m.

The distance between units is estimated to 21.2 m.

The height of the station is determined by the height of the generator rotor withshaft, and the need to be able to lift it above the machine hall floor. The height from turbinecentre level to crane rail level is approximately 21 m.

Cooling water tanks and tanks for fire water are located in a tunnel excavated asextension of the cavern top heading.

The dimensions of the powerhouse have been determined with basis in thedimensions of the turbines and generators selected. The dimensions of the equipment actuallysupplied may deviate from this design basis, which could imply minor adjustment of powerhousedimensions.

Two three-phase transformers will be located in niches in a tunnel parallel to themain cavern, connected to the cavern via busbar tunnels.

The cables will be conveyed out via a separate cable shaft, 210 m long, inclined45°, continuing in a cable tunnel of 685m length to the point VL5, at elevation 1080 masl, fromthere it will be laid in cable trays up to new Alto Maipo substation, where the cables will beconnected to the GIS plant.

The cable tunnel continues from top of the cable shaft to top of the pressure shaft.Leakage from the cable tunnel and from downstream of the steel cone area will be conducted

via the cable shaft to the tailrace tunnel, via one ore two holes drilled from the transformer tunnelto the tailrace.

A transport tunnel to the area at the foot of the pressure shaft branches off fromthe main access near the powerhouse cavern. Dimensioning for the size, alignment and curveradius for this tunnel, and for necessary space in the area at the foot of the pressure shaft, andthe lower penstock area with bifurcation, will be the space needed for transport and assembly ofthe downstream portion of the penstock steel lining with bifurcation. If necessary due to space

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limitation in the access tunnel, the bifurcation must be transported in parts and assembledinside.

A transport tunnel connects the access tunnel with the tailrace tunnel. The tunnelwill be used for excavation of part of the tailrace tunnel and to ensure air supply to the free flowtailrace tunnel. A tunnel cross section of 36 m2 has been assumed for basic design.

The resulting layout and dimension of the transport tunnel and associated area,as well as need for additional transport tunnels and space in the powerhouse area, will bedetermined by the contractor.

2.3 Las Lajas Headrace tunnel

Flow from Alfalfal and río Colorado, maximum 37 m3/s, is conveyed into theheadrace tunnel at point VL8, some 2 km downstream of the Alfalfal I tailrace outlet, by means

of a concrete culvert from Cámara de Carga. The tunnel alignment follows the points VL8 - L15- L14-L13- L12-L11- L10 to the top of the pressure shaft at L9. The minimum tunnel crosssection from VL8 to L10 is 21 m2. The tunnel invert will have concrete pavement.

An adit to the headrace tunnel, Los Trescientos, starts at point VL7.

A shaft drilled from the surface will connect canal Aucayes with the headracetunnel.

Flow from Alfalfal II, 27 m3/s, is introduced at L10.

From L10 to L9 the total maximum design flow is 65 m3/s. The minimum crosssection at this stretch is 30 m2. The tunnel invert will have concrete pavement.

2.4 Surge system

The surge system comprises a vertical shaft excavated from the surface by raiseboring down to the headrace tunnel close to point L9, near the top of the pressure shaft. A shaftdiameter of 5 m, has been selected, the same diameter as the pressure shaft. The diameter isenlarged to 10 m by slashing, between elevations 1275 masl and 1335 masl.

It is estimated that the heaviest pieces of the drilling equipment will have a weightof 6-8 tons, exceeding the capacity of a helicopter. Suitable access to the surface of the shaft isneeded for transport of these elements.

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2.5 Pressure shaft

A vertical pressure shaft has been selected for Las Lajas. Steel lining has beenassumed up to elevation 1000 masl, i.e. for pressures exceeding approx. 320 m. This gives thecriterion for the proposed location of the steel cone shortly upstream of the pressure shaft, near

point L9.

The final decision on location of the steel cone will be made when results areavailable from in situ observations and tests, including hydraulic fracturing tests.

The vertical shaft is approx. 160 m long, and will be excavated by drilling a pilothole from the top and reaming to the desired diameter of 4.7 m. The steel lining with diameter3.7 m will be embedded in concrete.

2.6 Transition zone and valve chamber

The transition between the headrace tunnel and the steel lined pressure shaft willcomprise:

•  An expanded section of the tunnel, with sand trap, for a length of approx.90 m.

•  The steel cone followed by a 25 m long section of steel tube embedded inconcrete.

•  A horizontal section through a potential, future valve chamber. No valve willbe installed at the time of construction.

The dimension of a valve will be such that it would be necessary to produce thevalve in parts and assemble it in the valve chamber. The dimensions of the chamber will bedesigned with this in mind. Any concrete structures necessary for valve assembly will beconstructed later when and if needed.

2.7 Las Lajas Tailrace tunnel

2.7.1 General

Total length of the Las Lajas tailrace tunnel measured from the outlet at río Maipoto the point where the tunnel meets the tailrace branches from each unit is 13.0 km.

Elevation of the tunnel invert at the outlet is governed by the criterion thatsufficient free space must be provided between the tunnel roof and the water surface when thepower plant is operating at maximum design flood level in río Maipo. This criterion, combinedwith the selected slope of 0.011% slope for the free flow tailrace tunnel, has been used when

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optimizing the tunnel size and determining the elevation of the Pelton runner in Las Lajaspowerhouse.

Minimum tunnel cross section for a D&B tunnel has been set to 36 m 2, with atheoretical width of 6.1 m. In order to reduce friction, the invert will be concreted along its entire

length and a 5 cm layer of sprayed concrete will be applied on all parts of the tunnel wall whereconcrete lining or sprayed concrete has not already been applied for reasons of rock support.

2.7.2 Tunnel outlet

The Las Lajas tailrace tunnel outlet at río Maipo, point VL1, will be locatedapproximately 1950 m downstream of Estero El Manzano.

The elevation of the invert floor at the tunnel outlet has been selected to releasethe flow to the Maipo River at the natural flow elevation of the River itself.

The tunnel will pass under the public road into the rock massif. Rock topographyis unknown in the area under the public road. One cannot exclude the possibility that the Maiporiver previously had its course in this area, implying that soil could be encountered at tunnellevel.

Crossing of this area with the tailrace tunnel and establishing a tunnel cut in rockwill be at the Contractor’s design. He will also be responsible for all necessary permits, contactwith road authorities, arrangements to ensure that traffic on the road can be maintainedaccording to authorities requirements etc.

2.7.3 Alternative with use of TBM for part of the Las Lajas Tailrace Tunnel

An alternative for excavation of the Las Lajas tailrace tunnel would be by TBMfrom the downstream end, for a length of approx. 9 km. In this case, the adit Las Puertas couldbe omitted.

If a TBM is selected, the initial 1.5 km of the downstream end of the tailrace tunnelcould be excavated by D&B during the lead time for the TBM.

2.8 Las Lajas Cable tunnel (Bio Bio)

2.8.1 Tunnel dimension

The tunnel from point VL5 at the switchyard will serve several purposes:

•  Construction adit for excavation of the headrace tunnel

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•  Transport tunnel for the elements of the 3.7 m diameter penstock

•  Cable tunnel.

Dimensioning for tunnel size has been the 3.7 m diameter penstock elements. Atunnel with minimum width 5.1 m and height minimum 6.0 m2, cross section 28.1 m2, has been

selected for the Cable tunnel. Measurements refer to the theoretical contour line.

2.8.2 Cable tunnel (Bío Bío) entrance

The location of VL5 near the plateau for the switchyard has been selectedconsidering needs for the road down to the plateau and switchyard requirements. The tunnelwill cross under the public road. There will be excavation in a certain amount of soil beforereaching tunnel cut in rock. Design of this part of the tunnel/cut and arrangements formaintaining traffic on the road according to requirements, including contact with authorities,permits etc is the responsibility of the contractor. The road is exposed to traffic by heavy trucks.

3. ALFALFAL II – DESCRIPTION OF LAYOUT

3.1 Basis for alignment of tunnels

Evaluation of the geology in the area for the Alfalfal II headrace tunnel, from itsupstream end to the powerhouse area, has been basis for the selection of the tunnel alignment.It has been decided not to include an intermediate adit for the tunnel.

In order to complete the 14.8 km long Alfalfal II headrace tunnel within areasonable time, alternatives with TBMs and combining TBM with D&B have been analysed.The following method has been selected for Basic Design:

•  Excavation by a TBM from the downstream end, A9, at an ascending slopeof 7%, for approx 8 km to point A10’. The cross section of this tunnel will bedictated from constructive reasons, and a minimum TBM diameter of 4.5 mhas been assumed.

•  Excavation by D&B, with rail bound equipment, from point VA4 in direction

of A11 with a minimum tunnel cross section of 13 m2, on descending slopefor 6250 m to point A10’.

The tunnel from VA4 to A11, a stretch of 200 m, will be expanded to a crosssection of minimum 26.2 m2  in order to give room for the 3.1 m dia steel tube transferring flowfrom the Volcán/Yeso system to the headrace tunnel and passage of vehicles for futureinspection of the tunnel. At the transition between steel tube and tunnel, a concrete plug with a

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steel bulkhead will be located. Dimensions of the gate will be W x H = 2.7 x 2.7 m, to permitpassage of vehicles for inspection of the tunnel.

The Yeso/Volcan steel tube will have a high point at the entrance to the headracetunnel, at VA4. An aeration arrangement will be connected to the steel tube at the high point.

A particular investigation and laboratory test program regarding rockcharacteristics has been done in order to determine specifications for the TBM. (Ref. 600-GE-INF-005)

3.2 Alfalfal Access tunnel

The access tunnel portal has been selected at point VA1 where the tunnel for theaccess road along Estero Aucayes starts, at elevation 1507 masl. The length of the tunnel willbe approx. 2500 m. The length is subject to adjustment following decision on exact location and

orientation of the powerhouse.

The minimum cross section of the tunnel has been determined from the spaceneeded for transport of the transformer, and has been estimated to 35 m2.

The cables from the transformers will be located on cable trays fixed to the tunnelwalls out to the entrance from where they will be installed in a cable trench to the smallswitchgear just outside the portal.

Three niches for phase revolvement of the cables shall be provided along the

access tunnel.

Ventilation air will be supplied from the access tunnel and evacuated via a verticalshaft to the surface from the tailrace tunnel.

3.3 Alfalfal Powerhouse area

Ref. Drawings 620-TU-PLA-003, 620-TU-PLA-004 and 620-TU-PLA-005 forarrangements and main dimensions of the powerhouse.

Location of the powerhouse inside the rock massif south-west of Estero Aucayesis determined by the location of top the pressure shaft, point TPS, and the decision onconstructing a 45° inclined pressure shaft.

The direction of the longitudinal axis of the powerhouse cavern has beenpreliminarily determined from what appears the optimum with respect to the geologicalconditions, based on information from geological mapping available at the time of basic design.Final orientation of the powerhouse, and hence the final arrangement of the tunnel system in the

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powerhouse area will be determined when sufficient information on the in situ rock conditionshas been obtained.

The elevation of the Pelton runners has been set at elevation 1341 masl. Thecriterion for determining the elevation of the Pelton runners at Alfalfal II is that they shall belocated sufficiently high above the tailrace water surface also in the situation that Las Lajascloses and the Alfalfal II flow is conducted to Cámara de Carga, at the upper end of the LasLajas headrace tunnel.

The width between the upstream and downstream walls of the station is 13.4 m.Adding 0.3 m wall thickness and 1.2 m between wall and rock on each side it gives a totalcavern width of 16.4 m.

The inner dimensions of the generator room have been set to 8.9 m x 8.9 m inBasic Design.

The distance between units is estimated at 19.6 m.

The height of the station is determined by the height of the generator rotor withshaft, and the need to be able to lift it above the machine hall floor. The height from turbinecentre level to crane rail level is approximately 22 m.

Cooling water tanks are located in a tunnel excavated as extension of the caverntop heading.

Additional cooling water volume for emergency cases will be provided byexcavating the inner part of the tailrace tunnels/turbine pits to a lower level.

The dimensions of the powerhouse have been determined with basis in thedimensions of the turbines and generators selected. The dimensions of the equipment actuallysupplied may deviate from this design basis, which could imply adjustment of powerhousedimensions.

Transformers will be located in niches in a tunnel parallel to the powerhousecavern and connected to the cavern by busbar tunnels.

A transport tunnel to the area at the foot of the pressure shaft branches off fromthe main access tunnel some distance from the powerhouse cavern. Dimensioning for the size,alignment and curve radius for this tunnel, and for necessary space in the area at the foot of thepressure shaft, and the lower penstock area with bifurcation, will be:

•  The space needed for transport and assembling of the TBM to be used forexcavation of the pressure shaft.

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•  The space needed for transport and assembly of the downstream portion ofthe penstock steel lining with bifurcation.

The resulting layout and dimension of the transport tunnel and associated area,as well as need for additional transport tunnels in the powerhouse area, will be determined by

the contractor.

3.4 Tailrace tunnel

The tailrace tunnel from Alfalfal II connects to point L10 of the Las Lajas headracetunnel at elevation 1250 masl. The tunnel length is approx. 3055 m and its cross section 21 m2,as determined from hydraulic needs.

The upstream part of the tailrace tunnel will be a free flow tunnel at mild slope,while the lower part, connecting to L10, will have a slope of 10%.

The tunnel cross section will be expanded in the transition zone between mild andsteep slope of the tunnel.

A construction tunnel connects the tailrace tunnel with the access tunnel a shortdistance downstream of the powerhouse.

A ventilation shaft will be drilled from the surface to connect with the free flowsection of the access tunnel, some 950 m upstream of A1.

3.5 Pressure shaft

The maximum gross head for Alfalfal II will be 1160 m. From interpretation of theavailable data, and also with reference to the experience from Alfalfal, it has been assumed thatsteel lining would be needed up to elevation 1950 masl, i.e. for pressures higher than approx.550 m.

The pressure shaft could be vertical or inclined. An inclined shaft has beenselected by the Gener, as this will shorten the length of the access tunnel and tailrace tunnel by600 m and thereby reduce critical construction time for the powerhouse complex.

The shaft will be steel lined, with an internal liner diameter of 2.4 m. The shaft willbe excavated by a TBM with diameter 3.4 m, i.e. with theoretical space 0.5 m between the steellining and rock. The steel lining will be embedded in concrete.

After reaching the top of the pressure shaft, the TBM will be partly dismantled andmoved a distance upstream in the headrace tunnel where it will be reassembled and used forexcavation of the surge shaft and tunnel.

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3.6 Transition zone and valve chamber

The transition between the headrace tunnel and the steel lined pressure shaft willcomprise:

•  An expanded section of the tunnel, with sand trap, for a length of approx.90 m.

•  The steel cone followed by a 30 m long section of steel tube embedded inconcrete.

•  A horizontal section through a potential, future valve chamber. No valve willbe installed at the time of construction.

The dimension of such a valve will be such that it could be necessary to produce

the valve in parts and assemble it in the valve chamber. The dimensions of the chamber will bedesigned with this in mind. Any concrete structures necessary for valve assembly will beconstructed later when and if needed.

3.7 Surge shaft and surge chamber.

The surge system for Alfalfal II comprises a 45° inclined shaft, approx. 685 mlong, excavated by 3.4 m dia TBM used for the headrace tunnel, followed by a tunnel onminimum slope of approx. 250 m length, also excavated by the TBM, ending in an upper surgechamber on the surface. The surge chamber will be excavated in rock as an open pit in the

Aucayes valley.

For Basic design it has been assumed that the TBM will be dismantled andwithdrawn back through the surge tunnel/shaft to the headrace tunnel and further through thetunnel A9-VA2 and the spoils from the excavation of the surge chamber, some 50 000 m3, willalso have to be transported out via this tunnel system. It is for the Contractor to decide wetherthe TBM can alternatively be withdrawn through the surface at the Aucayes pond, where approx.7.5 km of road would have to be constructed in order to access the Aucayes pond.

3.8 Headrace Tunnel Alfalfal II.Reference is made to Drawings 620-TU-PLA-001, 620-TU-PLA-002 and 620-TU-

PLA-003

3.8.1 General

The length of the headrace tunnel, from point A9 shortly upstream of the top ofthe pressure shaft via A10 to VA4 at the upstream end will be 14.8 km. Access to point A9 will

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be via a 2000 m long tunnel excavated from point VA2. Basis for the selected alignment A9-A10-A11 is evaluation of the topography and indications for tunnelling conditions in the area tobe traversed. The bend at A10 is recommended in order to reduce the rock cover above thetunnel to an acceptable level.

It has been assumed for Basic Design that the tunnel will be excavated by TBMfrom the downstream end and by D&B from the upstream end.

3.8.2 Tunnel portion excavated by TBM

The TBM will start at point VA2, excavating 2 km at approx. 6.7% slope beforereaching the headrace tunnel at point A9. From this point, the TBM will continue at approx 5.6%slope towards A10 and further in the direction of A11 until it meets the D&B excavation from theupstream end. Point A10’ has been assumed as meeting point.

Due to the slope requirements, the secondary transport system must be onwheels. The minimum diameter of the TBM has been set to 4.5 m. in Basic Design.

3.8.3 Tunnel portion excavated from the upstream end

The upstream portion of the headrace tunnel will start at VA4, and will be by D&Bat ascending slope, initially 1.5%, then 0.15% until meeting the TBM excavating from thedownstream end.

The cross section will be minimum 13 m2, and rail mounted excavation equipment

is anticipated. The contractor will be permitted to suggest larger cross section.

The cross section of the initial 200 m of the tunnel from VA4 will be expanded to26.3 m2  in order to accommodate the 3.1 m diameter steel tube transferring the Volcán-Yesoflow into the Alfalfal II headrace tunnel and passage of vehicles for future tunnel inspections. Aconcrete plug with a gated steel bulkhead will be installed at the transition between the steeltube and the headrace tunnel. Gate opening will be W x H = 2.7 x 2.7 m to allow entrance ofvehicles for tunnel inspection.

4. TUNNEL EL VOLCÁN

Reference is made to the Drawings 610-TU-PLA-001, 610-TU-PLA-002 and 610-TU-PLA-003.

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4.1 Basis for alignment

Basis for the design is that the tunnel will start at point V1, which is the endingpoint of the upstream flow collecting system, and point V6, at starting point for the conduitcrossing río Yeso with the Yeso/Volcán flow. The alignment has been selected based on thegeological information as described in Chapter 7. Total tunnel length is approximately 14.1 km.

The tunnel elevation and slope is determined from the aim to obtain a pressurizedtunnel system from the upstream end of Tunnel El Volcán, and to utilize the volume of tunnelVolcán for reservoir purposes. This implies that the tunnel will be excavated on mild slope of0.1%.

For Basic Design it has been assumed that a TBM will be used for excavation inthe upstream direction from V6. D&B will be used from the upstream end.

The design flow for Tunnel El Volcan is 12,8 m3/s. The minimum cross section of

the tunnel will be determined from constructional needs.

4.2 Tunnel portion excavated by TBM

The geological conditions described in Chapter 7 indicate that the risk ofencountering spalling and squeezing rock, due to the high overburden, becomes very high at5-7 km upstream of point V6. This must be considered by the contractor when selecting type ofTBM and when estimating the length of tunnel that can be expected to be excavated by theTBM. A TBM diameter of 4.1 m has been assumed for basic design. The TBM will excavate onascending slope 0.1%.

4.3 Tunnel portion excavated by D&B

For Basic Design it has been assumed that the TBM will have to be withdrawnafter 7 km, and that excavation continues in the upstream direction with D&B.

The tunnel starting in the upstream end near V1 will excavate on descendingslope of 0.1%, until meeting the TBM excavating from downstream.

A minimum cross section of 14 m2 has been assumed for the D&B tunnel in Basic

Design.

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5. ELECTRICAL EQUIPMENT

5.1 Background

The basic design for the electrical works in the two new power stations is definedin the following documents:

•  Proposed single line diagrams for Alfalfal II and Las Lajas Power Station.

•  Proposed topological diagram for the control of the Alto Maipo Complex.

•  Proposed arrangement drawings for Alfalfal II and Las Lajas Power Station.

•  Proposed general specifications.

•  Proposed general technical specifications.

•  Proposed project specific data.

This section supplements the documents mentioned above and addresses somebasic design considerations not immediately evident in the design documents.

The overruling design basis is that the stations should be designed for reliableunmanned operation, long lifetime as well as low and easy maintenance. Special attention isgiven to the matter of safety for personnel and equipment.

The detailed engineering shall be based on Chilean and international acceptedstandards (in particular IEC standards) and well proven solutions adapted to local conditions.

Special attention will be paid to the seismic requirements in the region.

5.2 Evacuation

The powerhouses, and Alfalfal II in particular, are located far underground withonly one fully functional evacuation route. Should this route be blocked by smoke, escape wouldbe difficult even with smoke diving equipment. In line with modern health and safety practice,the station is, therefore, designed with a centrally located safe room with air storage, storage ofsmoke diver equipment, first aid equipment and direct voice communication to the outside.

5.3 Separate transformer hall

Each of the two power stations in the Alto Maipo project will have units in therange of 150 MVA which is quite sizable. The rated current will be in the range of 7 kA and thesub-transient short circuit current in the range of 100 kA.

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This means that the amount of energy stored in the system is large. If a faultoccurs, the destruction can therefore be considerable provided the fault is not confined to alimited space and time.

Transformer faults are very rare but the consequences are often dramatic. A fire

in the transformer normally results in development of heavy smoke that can damage nearbyequipment and be quite hazardous for personnel. Hence, recent trends have been to erect thetransformers in a separate cavern which can be sealed off from the main machine hall. A firemay then be extinguished faster due to lack of oxygen, smoke will not penetrate to the rest of thestation, personnel will be able to evacuate to smoke-free locations and possible oil mistexplosions will be relieved into empty space. Such location also offers advantages for operationas the transformer is more accessible for inspection and maintenance.

An arrangement with transformers in niches in a separate cavern has beenassumed.

5.4 Generator breaker

The rotational energy stored in large generator units will continue to be pumpedinto a fault until the generator is disconnected or de-energised. A generator breaker will removethe energy supply from a transformer or cable fault within milliseconds, thus reducing theconsequences of a fault. Without a generator breaker it will take seconds before the faultcurrent stops flowing. A generator breaker is indicated as an option on the drawings. TheGener will make the decision on whether to include this item, based on considerations of cost,risks and insurance policy.

5.5 Generator and transformer

The rating of the generator is governed by the rating of the turbine. For Alfalfal II,the 600 r.p.m turbine alternative was discarded due to the high runaway speed given by a Peltonturbine. No references have been found for generators with this combination of speed andpower. A generator meeting the pertaining runaway requirements could most likely be designedby more reputable manufacturers but will be relatively costly. A more conservative approachwith 500 r.p.m. has been chosen. Bidders could be invited to present costs for 600 r.p.m as aseparate option.

The design is based on “erection in the pit”. Extra room for “floor erection” or forunloading will be considered in the detailed design. The waterways are relatively stable and noextra moment of inertia above the natural design values are considered necessary.

Considering that a certain degree of reactive power capacity gives some room foroverproduction and some temperature margins, a power factor of 0.9 has been selected. Thegenerator will be designed to run at full MVA rating at power factor 1.0.

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The transformer rating follows the generator rating. Three phase transformers areselected, as the transport roads can reportedly can be prepared for such loads. Approximatetransport weight of one transformer is 120 tons.

5.6 Outdoor breakers

The portal of the Las Lajas cable tunnel is located close to the new Alto MaipoSubstation and the cables will be terminated in the proposed GIS switchgear here. This meansthat each of the Las Lajas power transformers will have a normal HV transformer breaker. Caremust be taken in coordinating the protection and control as Las Lajas and Alto Maipo belong toseparate contracts.

For Alfalfal II the nearest substation is far away and the connection has to be on adouble overhead line passing rather difficult terrain. This means that the chances for short

circuits (temporary and permanent) are not to be neglected. If no breaker is accommodatedbetween the cable and the overhead line the trip signal has to be transmitted up to the nearestsubstation which is a substantial distance for conveying trip signals. Anyhow, fault currents willbe fed from the generator and stress the cable and transformer until the generator breaker tripsor the generator is de-energized. If no generator breaker is installed this can take seconds.

It is possible to have a simplified substation just outside the portal. It is thereforeproposed to have a simplified possibility to disconnect the line from the cable. This is in order tohave a backup in the event of faults and to have a safe isolation point for work on the line. Suchdisconnection point could easily be arranged by using the modern combined circuit

breakers/isolators/ earth switches.

5.7 230 kV Cable

The main 230 kV cable will be of solid PEX design. In Alfalfal II the cables areproposed to be installed in cable trays in the access tunnel. In Las Lajas a separate cabletunnel is proposed. In the inclined part of this tunnel the supplier will be requested to present areinforcement, fastening and monitoring system assuring that no creep will develop over thelifetime of the cable.

5.8 Auxiliary supply

The auxiliary supply is proposed to be based on the unit transformer systemwhere each running unit is supplied by its own auxiliary transformer. If this supply is unavailable,the supply will be switched to the other unit. If both auxiliary transformers are unavailable, thesupply is taken from the outside 12 kV line. If that line is down, the emergency generator in theportal building will start.

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The distance from the portal building to the machine hall is so long that thetransmission must be based on medium voltage level. It is therefore proposed to combine thecable from the outside 12 kV connection and for the emergency generator by a switch overarrangement in the portal building. The cable must be designed for emergency operation (fireproof) and disconnection of non priority loads will be done by contactor operation in the motorcontrol centres.

Low voltage switchgear can be built for up to 50 kA short circuit current but amore modest requirement of 32 kA gives a smaller and less expensive board. Provisionally, ashort circuit limiting reactor is planned but the final short circuit calculations will determinewhether such reactor will be necessary.

5.9 Control and Supervisory Control and Data Acquisition (SCADA) System

The local control system will be based on distributed control with intelligent

input/output units close to the main components. The basic intention should be to interact andcontrol the equipment as close to the process as possible. Internal communication will be by buscommunication, most likely with fibre optic cables.

Each independent system (Unit 1, Unit 2, Auxiliary Systems, Water Managementetc. will have an independent control computer (object computer). The object computer must beautonomous, assuring safe operation of the unit if communication with overlaying system fails orthe control system for another system fails. The principle of the system is shown in thetopological control diagram.

If the Gener would wish a conventional back up control cubicle for each unit, suchwill work through the object computer.

The three stations (Alfalfal II, Las Lajas and Alto Maipo Substations) will workindependently with full control functionality from the respective control rooms. Local data will bestored and dealt with as close to the process as possible.

For normal operation these control rooms will be unmanned and all three stationstogether with the existing Alfallfal will be controlled from a SCADA system in the central controlroom located in the Alfalfal administrative building. Communication to the grid company’sdispatch centre is foreseen to be performed from this main control centre and is outside thescope for this contract but necessary data will be available in the SCADA system.

The main connection with the power stations will be on a fibre optic earth wire onthe transmission line (OPGW). As OPGW is lacking on the old transmission lines, a ring cannotbe closed. A separate backup should be considered (Power Line Carrier or microwave orsimilar).

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An emergency stop system acting directly on the units will be included, allowingemergency stop and disconnection from the mains by activating centrally located pushbuttons inthe machine hall and the portal building.

Company proprietary communication protocols have up to recent years been agreat hindrance for mixing of makes within a system. The development of internationalstandards (in particular the IEC 60817 standards and newer IEC standards) has solved thisproblem and use of such protocols should therefore be demanded.

Communication to the critical intake reservoirs (Las Lajas intake and Alfalfal IIreservoir) should be given special attention in the detailed design phase. The measurementsare simple and communication capacity requirements small but the importance of themeasurements is high.

5.10 Protection

Protection is achieved with multi-functional numeric protection units. The detailedprotection scheme will be worked out as part of the technical specifications. The basis will be tohave selectivity avoiding total blackout, and safety in the form of at least two independentsystems for detection of short circuit and earth fault.

Selective protection of short lines has up to now been difficult. New technologyhas expanded the differential protection technology by using the fibre optic cable forcommunication of the differential currents. Such system is strongly recommended in this case.

6. MECHANICAL EQUIPMENT

6.1 General

Data for the main mechanical equipment and the steel penstock are describedbelow, and will be further detailed in the general and particular specifications to be elaborated.

The following is supplementary information on basis for the decisions made.

6.2 Alfalfal II

6.2.1 Turbines

Two vertical Pelton turbines will be installed, with the following main data;

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Item Data

Gross head 1156 m

Net head 1118

Nominal capacity 132 MW

Number of jets 4

Speed 500

Runner diameter 2.72 m

An alternative with 5 jets and a speed of 600 rpm is possible from the point ofview of the turbine, and has been analyzed. It has the advantage of giving a smaller and lessexpensive turbine, but the combination of output and speed is problematic for the generator dueto the high runaway speed. Although such a generator could be technically feasible, generators

with this combination of output and speed have not yet been constructed. This alternative has,therefore been discarded for basic design.

6.2.2 Spherical valves

Each turbine will be equipped with a spherical valve.

Opening will be by oil pressure. Closing by counterweight is proposed. Usedoriginally mostly for smaller units, this method is increasingly used also for large sphericalvalves. A recent reference is the 200 MW Pelton turbines with 1 100 m head at Tyin powerstation in Norway.

Alternatively, closing by water pressure (as for Alfalfal I) is possible, but acounterweight solution will eliminate problems related to the quality of the water.

6.2.3 Cooling water system

An open circuit cooling system has been selected. Pumps will take water from acooling water sump in the tailrace, pumping it up to a cooling water tank. The cooling water isthen distributed to the different consumers:

•  Generator air coolers

•  Turbine and Generator bearings

•  Bearing coolers and governor cooler

•  Turbine Governor

•  Transformer coolers

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The pumps will be either long shafted units with the motor located at the turbinecellar level, or submersible pumps. The latter solution is increasingly used. Automaticback-flushing strainers will be provided, located in the pump room.

The tank is double to permit cleaning of one section while the other is inoperation.

Preliminary data for the cooling water system:

Total cooling water consumption (units andtransformers

210 l/s

Number and capacity of pumps: 4 pumps - 70 l/s

Total volume of tank: 300 m3 

One pump is spare, 3 pumps are normally on duty. One spare plus two pumps onduty is an option, but a total of 4 pumps is preferred as it reduces the size and power of theunits, and reduces the frequency of start and stop of the pumps. Start and stop of the pumpswill be by level switches between defined levels in the tank.

6.2.4 Machine hall crane

One machine hall crane will be installed, with sufficient capacity to lift thegenerator rotor. Estimated weight of the rotor is 180 ton. The crane will also have an auxiliary

hoist with a capacity in the range of 20 ton.

The spherical valve will be installed through hatches in the machine hall floor andthe generator floor.

The turbine runner will be installed and removed by means of a platform with railsto a position below hatches on the downstream side of the station, and lifted by the machine hallcrane.

The inlet tube to the valve has an angle of 60° relative to the longitudinal axis ofthe powerhouse. This arrangement allows bringing the valve farther in, thus reducing the width

requirement of the powerhouse on the upstream side.

Cooling water piping to the units will be routed in the space between the upstreamwall and the rock, from where it will be distributed to the respective units. A common outlet pipefrom the units will be routed to the outlet channel of the unit on the downstream side of thestation.

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6.2.5 Penstock steel lining

A steel quality with denomination “460” (Steel grade: P460NL1 acc. to EN10 028-3) is now frequently used for this kind of application, and has been assumed for basicdesign.

The figure 460 is the yield strength in MPa for smaller thicknesses. The yieldstrength is slightly reduced as the thickness increases. For thickness above 40 mm, the yieldstrength is 440 MPa. For simplicity at the present stage, 440 MPa yield strength has beenassumed for all the elements of the lining.

Calculation of thickness with empty pipe submitted to outer pressure has beenperformed according to Amstutz, assuming generally accepted material and load factors.

Design of the penstocks has been based on a maximum velocity of 6.0 m/s.

6.3 Las Lajas

6.3.1 Turbines

Two vertical Pelton turbines will be installed, with the following main data;

Item Data

Gross head 483 m

Net head 468 m

Nominal capacity 134 mw

Number of jets 6Speed 300 rpm

Runner diameter 2.91 m

Opinions vary between suppliers concerning the question of whether a six jetturbine would be more sensitive to sand erosion than a five jet turbine, and whether there is areduction of the efficiency at full load. The natural choice of speed for a five jet turbine would be250 rpm. The main conclusion is that the influence of these two aspects will be marginal and willnot justify the significant increase in total equipment cost that would result from reducing the

speed to 250 rpm.

6.3.2 Spherical valves

Each turbine will be equipped with a spherical valve.

Opening will be by oil pressure. Closing by counterweight is proposed. Usedoriginally mostly for smaller units, this method is increasingly used for large spherical valves. A

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recent reference is the 200 MW Pelton turbines with 1 100 m head at Tyin power station inNorway.

Alternatively, closing by water pressure (as for Alfalfal) is possible, but acounterweight solution will eliminate any problems related to the quality of the water.

6.3.3 Cooling water system

An open circuit cooling system has been selected. Pumps will take water from acooling water sump in the tailrace, pumping it up to a cooling water tank. The cooling water isthen distributed to the different consumers:

•  Generator air coolers.

•  Turbine and Generator bearings

•  Bearing coolers and governor cooler.

•  Turbine Governor

•  Transformer coolers.

The pumps will be either long shafted units with the motor located at the turbinecellar level, or submersible pumps. The latter solution is increasingly used. Automatic back-flushing strainers will be provided, located in the pump room.

The tank is double to permit cleaning of one section while the other is in

operation.

Preliminary data for the cooling water system:

Total cooling water consumption (units andtransformers

210 l/s

Number and capacity of pumps: 4 pumps - 70 l/s

Total volume of tank: 300 m3 

One pump is spare, 3 pumps are normally on duty. One spare plus two pumps onduty is an option, but a total of 4 pumps is preferred as it reduces the size and power of theunits, and reduces the frequency of start and stop of the pumps. Start and stop of the pumpswill be by level switches between defined levels in the tank.

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6.3.4 Machine hall crane

One machine hall crane will be installed, with sufficient capacity to lift thegenerator rotor. Estimated weight of the rotor is 200 tons. The crane will also have an auxiliaryhoist with a capacity in the range of 20 tons.

The spherical valve will be installed through hatches in the machine hall floor andthe generator floor.

The turbine runner will be installed and removed by means of a platform with railsto a position below hatches on the downstream side of the station, and lifted by the machine hallcrane.

The inlet tube to the valve has an angle of 60° relative to the longitudinal axis ofthe powerhouse. This arrangement allows bringing the valve farther in, thus reducing the widthrequirement of the powerhouse on the upstream side.

Cooling water piping to the units will be routed in the space between the upstreamwall and the rock, from where it will be distributed to the respective units. A common outlet pipefrom the units will be routed to the outlet channel of the unit on the downstream side of thestation.

6.3.5 Penstock steel lining

A steel quality with denomination “460” is now frequently used for this kind ofapplication, and has been assumed for preliminary dimensioning and cost estimates.

The figure 460 is the yield strength in MPa for smaller thicknesses. The yieldstrength is slightly reduced as the thickness increases. For thickness above 40 mm, the yieldstrength is 440 MPa. For simplicity at the present stage, 440 MPa yield strength has beenassumed for all the elements of the lining.

Calculation of thickness with empty pipe submitted to outer pressure has beenperformed according to Amstutz, assuming generally accepted material and load factors.

Economic optimisation of the penstock results in velocities that are consideredunfavourably high in practice. Design of the penstocks has been based on a maximum velocityof 6.0 m/s.

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7. ENGINEERING GEOLOGY IN BASIC DESIGN

7.1 General layout

Drawing 600-TU-PLA-001 shows the general arrangement of the Alto MaipoHydroelectric Project. For the Alfalfal II power station the ca 14 km long Volcán transfer tunnel

combines with flow from the Yeso reservoir into the ca 15 km long headrace tunnel to the top ofthe pressure shaft. The inclined pressure shaft, 860 m long, is planned as steel lined shaft. Theremaining 530 m of head is accommodated in an unlined pressure tunnel.

From Alfalfal II the ca 3 km long tailrace tunnel meets the ca 6 km long tunneltaking the flow from Alfalfal and the combined flow is taken by a ca 3 km long pressure tunnel tothe top of the vertical, 160 m long pressure shaft of the Las Lajas power station. The remainingabout 280 m of head is accommodated in the pressure tunnel. From the Las Lajas powerstation, a ca 13 km free flow tailrace tunnel leads to the outlet in río Maipo downstream of ElManzano.

The layout outlined above is a result of a process where technical requirementsfrom the hydropower engineering have been mated with geological constraints and constructionaspects.

7.2 Geological information

Available geological information for the basic design has been the investigationsperformed for the concept studies, feasibility study and the geological mapping done for thebasic design. The interpretation of this information is documented in the reports N° 601-GE-INF-

001, 610-GE-INF-001, 620-GE-INF-001 y 630-GE-INF-001, with geological maps and profiles.

7.3 Design of underground openings

7.3.1 General

Design criteria for underground openings are linked to functionality. One basicrequirement is that the design shall ensure the integrity of the opening during the constructionphase and during the operational phase, limited to the design life expectancy of the opening.The integrity of the opening relates to the rock mass quality and thereby the amount of rock

support required in order to provide this integrity.

For pressurized waterways without impermeable lining, the requirement to globalstability is that the minimum principal stress in the rock mass shall exceed the internal pressure.The consequence of deficiency is hydraulic jacking and uncontrollable leakage from the system.

For pressurized waterways permeability is also a case to consider. Acceptableleakage from the waterways during operation is negligible. The significance of this is that areas

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with unacceptable permeability must be identified and appropriate rectifying measuresundertaken.

7.3.2 Rock Mass Classification

The classification system used for this project is an empirical system based onpractical experience from underground construction during the last 25 years. Several projects inthe Andes Region in South America form part of this practical experience. Factors as lithology,discontinuities, faults, hydrothermal alteration, rock stress and groundwater and leakage areconsidered in the process of assigning a quality class to the rock mass. Seismicity is not directlyincluded as a factor in the classification because general experience is that only surfacestructures as portals are at risk during earthquakes. A possible exemption from this would beactive faults or reactivation of secondary faults. The classification system divides rock massquality into 5 classes. A description of the geological conditions related to each class is givenbelow.

q1 Very Good

Massive to low joint frequency, Jv < 5/m3. Tight joints, unaltered strong rock andinsignificant stress slabbing.

q2 Good

Low to moderate joint frequency, 5< Jv < 10. Strong rock with no or insignificantalteration and some joints with coating. Low to moderate intensity stressslabbing.

q3 Fair

Moderate to high joint frequency, 10< Jv < 20. Moderately strong to strong rockgenerally with coated joints and with some seams and some minor weaknesszones. The rock mass may be slightly weathered. Also applies to q1 and q2class with moderate to high intensity stress slabbing; and to medium to lowstrength rock subjected to low to medium stresses causing plastic deformations.

q4 Poor

High joint frequency, Jv > 20, clay seams (fault zones, swelling clays) inmoderately strong rock. Also applies to moderately weathered strong rock and tohigh to very high intensity stress slabbing in q1 and q2 class rock mass; and tomedium to low strength rock subjected to swelling and/or medium to high stressescausing plastic deformation.

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q5 Extremely Poor

Completely crushed rock containing a significant amount of secondary clayminerals as in major fault zones. Smectite clays may lead to significant swellingand squeezing. Also applies to highly weathered rock and to low strength rockmass subjected to swelling and/or high stresses causing plastic deformation.

7.3.3 Plastic deformation

Plastic deformation of the rock mass around an opening is a function of strengthof the material and the stress. The strength of the rock mass depends on the strength of thematrix and joint frequency. This means that the strength of a jointed rock will be less than for amassive rock given identical matrix strength. Occurrence of secondary minerals in the rockmass will further reduce strength. The rock mass will respond elastically up to a stress levelwhere new fractures start to form.

There are failure criteria that based on assumptions regarding strength and jointfrequency for rock mass makes it possible to calculate critical stress at failure. One suchcriterion is the Hoek-Brown criterion. The problem is that failure will affect material strength thatin turn will affect stress distribution, thus creating a loop. The criterion will, however, indicate ina qualitative way the onset of failure.

Not surprisingly, the criterion indicates that strong matrix and moderate jointfrequency will result in failure at opening surface for overburden in the 500 – 1000 m range, butthat moderate support pressure will contain the problem. For weak rock, massive failure willtake place at 500 m overburden. The criterion gives no clue to time scale. A Lauffer diagram for

prediction of stand up time is used in the RMR classification system, but rock stress is notconsidered in the RMR system.

In the Q-system the ratio of tangential stress over compressive strength of rockmass is used as indicator. Ratio in the 1 – 5 range is supposed to result in “mild” plasticdeformation (squeezing), more than 5 to heavy squeezing. A medium strong rock (25 – 50 MPa)with a volumetric joint number of 10 will have Rmi (rock mass index) rating of about 5 MPa.Overburden of 1000 m results in a theoretical vertical stress of about 25 MPa and tangentialstress at least twice this figure.

The conclusion that may be drawn is that plastic deformation will occur in thetunnels on the project where the weaker rock types are encountered at depth. On the Volcantunnel where effective overburden may reach as much as 2000 m, plastic deformation isexpected to be a significant factor in design.

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7.3.4 Water Leakage (WL)

Water leakage is treated separately from rock mass quality classification. It ispresupposed that minor seeps or drips will have no practical significance on rock mass quality.It is, however, recognized that the application of sprayed concrete is hampered by wetconditions. Below are listed two classes of leakage that will require grouting and sealing.

WL 1 Moderate

Water leakage of less than 2000 l/min. at moderate pressure occurring in mediumstrong to strong rock. Applies also to low water flow in medium to low strengthrock, and to relatively dry (drips and seeps) but poor to extremely poor rockmasses.

WL 2 Major

Water leakage larger than 2000 l/min. at moderate to high pressure, encounteredin medium strong to strong rock. Applies also to medium to high water flow inmedium to low strength rock and to low water flow in poor to extremely poor rockmasses.

It is recognized that neither the presence of erodible materials nor the full effectsof high pressures are covered by the above classes, but they do still form an acceptable basisfor scheduling and costing.

7.3.5 Rock support as function of classification

The required rock support for an opening is a function of the rock mass qualityand the size of the opening. In order to calculate support quantities, construction time andconstruction costs, the rock mass quality is combined with the appropriate rock support for therelevant category and opening size. The basic support resources (RS) used for each rock masscategory (q) are listed as follows:

q1 => RS 1: Scaling and spot bolts.

TBM: Scaling and spot bolts.

q2 => RS 2: Scaling. Spot bolting for smaller cross-sections. Pattern boltingand spot applied fibre reinforced sprayed concrete for larger cross-sections.

TBM: Scaling and spot bolts.

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q3 => RS 3: Scaling. Pattern bolting and minimum one layer of fibre reinforcedsprayed concrete in crown and walls for smaller cross-section.Number/length of bolts and thickness of sprayed concrete dependon the cross-section.

TBM: Scaling, spot bolts and use of fibre reinforced sprayed

concrete.

q4 => RS 4: Pattern bolting and a minimum of two layers of fibre reinforcedsprayed concrete in crown and walls for smaller cross-section.Occasionally (i.e. on 10 - 20% of the length) reinforced ribs oflattice girders and sprayed concrete or concrete lining.Occasionally concreting of the invert at face. Number/length ofbolts and thickness of sprayed concrete depend on the cross-section. Also applies to short blasting rounds and subdivision ofrounds depending on cross-section.

TBM: No use of concrete lining at the face, segmental lining maycome into use.

q5 => RS 5: Pattern bolting and spiling bolts. When stand up time is shortreinforced ribs of lattice girders and sprayed concrete is applied atthe face. Additional fibre reinforced sprayed concrete or concretelining is applied behind the face. Reinforced ribs may be deleted ifconcrete lining is cast at the face. Concreting of the invert at face.Number/length of bolts and the thickness of sprayed concrete as

well as the distance between the lattice girders depend on thecross-section. Also applies to systematic short rounds for smallcross-sections and subdivided rounds or multiple drifts for largercross-sections.

TBM: No use of concrete lining at the face, segmental lining maycome into use.

The basic treatment of rock mass with water ingress by exploration drilling andgrouting (EG) for each water leakage category (WL) is listed as follows:

WL1 => EG 1: Exploratory drilling, 2 - 4 holes of 12 - 30 m length ahead of theface. Minimum overlap between exploratory drilling is two blastrounds. Also applies to grouting, 5 - 10 fan shaped drillholes of12 - 20 m length ahead of the face. Also applies to 3 - 6 drillholesfor control of the groutin