Planning and design of the transfer tunnel for the Lesotho highlands water project

0886-7798 (93) E0014-V

Planning and Design of the Transfer Tunnel for the Lesotho Highlands Water Project

A. Boniface, J.G. McKelvey and S. Nthako

Abstract--The Transfer Tunnel forms part of Phase 1A of the Lesotho Highlands Water Project. The tunnel has a length of 45 km, an excavated diameter of 5 m, and is situated at an altitude of nearly 2000 m above sea level. Directly connected to an underground power station, it is probably the longest headrace tunnel in the world. The tunnel is being excavated through basalt and runs in a northerly direction from an intake structure sited on the Malibamatso River, approximately 100 km east of Maseru. The paper describes the planning and design of the tunnel from the Project Optimization stage to the present.

P~eumb--Le Transfer Tunnel fait partie de la phase 1A du Lesotho Highlands Water Project. Ce tunnel, dont la longueur est de 45 km et le diam~tre excav~ de 5 m, se trouve ~t Une altitude de pros de 2000 m. Raccord~ directement ~t la centrale souterraine, il s'agit probablement du plus long canal d'amende du monde. Le tunnel est creas~ clans le basalte et remonte vers le nord ~ partir d'un ouvrage de prise situd sur la rivi~re Malibamatso, environ 100 km ~t l°est de Maseru. L'article d~crit la conception et le dimenesionnement du tunnel, depuis la phase d' optimisatio n du projetjusqu 'd a~ourd'hui.

1. Background

T he water potential of the High- lands of Lesotho was recognized by Sir Evelyn Baring in 1950.

At the time, as High Commissioner for Basuteland, he initiated a survey of the water resources of the territory (van Robbroeck 1986). Later the High- lands were seen as a source of water for the Orange Free State, with its rapidly developing goldfields. However, i twas the eve r - inc reas ing needs of the Pretoria-Witwatersrand-Vaal (PWV) a r e a that finally led to the Lesotho Highlands Water Project (LHWP) as it is being developed today (see Fig. 1). Tunnelling was always an important element of the project.

The major rivers flow from north to south through the Highlands. Thus, the higher up in the catchment a stor- age facility is built, the higher is the available head and the shorter the Transfer Tunnel needs to be. Con- versely, a dam built further south increases the catchment dramatically, but with consequent loss of available head and increased length of tunnel.

Present address: A. Boniface and J. G. MeKelvey, Keeve Steyn Inc. and Lesetho Highlands Consultants, South Africa, 2nd Floor Cowey Park Centre, 107 Cowey Road, Durban 4001, South Africa; S. Nthako, Lesetho Highlands Development Authority, Lesotho.

As originally envisaged, the Oxbow scheme required a 16.m tunnel with a capacity of 4.37 m3/s. It also provided for alimited amount of power generation.

By 1971, the concept of the project had expanded to require a 60-kin tunnel with a capacity of 8 m3/s. The tunnel would have run northwards

Tunnelling and Underground Space Technology, Vol. 9, No. 1, pp. 79-89, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0886-7798/94 $6.00 + .00

ABBREVIATIONS A number of abbreviations are used in the text. They are given here for ease of reference.

D DB DBM FS

FSL HAB LHC

LHDA LHPC

Do|erite Doleritic Basalt Drill, blast and muck excavation Feasibility Study (Lahmeyer Macdonald Consortium: Shand Consortium, 1986) Full supply level of Katse reservoir Highly Amygdaloidal Basalt Lesotho Highlands Consultants (a consortium consisting of Sogreah, Coyne et Bellier, Sir Alexander Gibb, Ninham Shand, Watermeyer Legge, Pidsold and Uhlmann, and Keeve Steyn) Lesotho Highlands Development Authority Lesotho Highlarids Project Contractors (a joint venture of Spie Batignolles, Balfour Beatty, Campenon Bernard, LTA and Ed Zublin) Lesotho Highlands Water Project Minimum operating level of Katse reservoir Moderately Amygdaloidal Basalt altitude in metres above mean sea level Moderately Amygdaloidal Basalt Pretoria-Witwatersrand-Vaal industrial area in South Africa Tunnel Boring Machine

~ ) Pergamon

LHWP MOL MAB m A m ~

NAB PWV TBM

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PELANENG • ~ , , ADIT Mafika Lisiu Pass 3090m ~.~

/~ 10 '~HA LEJONE "~ | 11

0 | TAKE

40 km ~q~

KA TSE D A M ~

Figure 1. Locality plan for the Lesotho Highlands Water Project.

80 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 9, Number i, 1994

from an intake on the Malibamatso River upst ream of a 94-m-high dam at Pelaneng. However, negotiations to implement this scheme collapsed in 1972.

2. Feasibility Study Interest in the project was revived

in 1975, leading in due course to the decision to commission a Feasibility Study (FS), which was begun in 1983. At the conclusion of Stage 1 of this study, the project was seen as having three phases. At the completion of the third phase, the system would be able to deliver 50 m3/s via two transfer tunnels and a delivery tunnel. By the conclusion of the subsequent stage of studies, the project had taken on much of its present form.

The Feasibility Study (1986) envis- a g e d a four-stage project which, on completion, would make use of twin Transfer Tunnels (each 48 km long), and twin Delivery Tunnels (each 34 km long), to deliver a total of 70 mVs of water to South Africa.

The FS foresaw construction of the Phase 1 Transfer and Delivery Tun- nels each taking six years. Construc- tion of Katse Dam was expected to take several months less. The construction period for the tunnelling work was therefore critical to the timely completion of Phase 1A of the project. In subsequent planning, there was continuous pressure to see if this six-year period could be shortened.

For the Transfer Tunnel, the FS assumed the use of four TBMs, with minimum and maximum drive lengths of 3.9 km and 14.4 kin, respectively.

3. The Treaty and Project Optimization

InOctober 1986,aTreatywas signed between the Republic of South Africa and the Kingdom of Lesotho, defining the terms under which Phase 1 of the LHWP would be implemented. Opti- mization studies leading to detailed planning work on the Transfer Tunnel began in September 1987. A major programme of site investigations was started in February 1988.

Lesotho High land Consu l t an t s (LHC) were required to provide input for the overall optimization of the project being carried out by others. It was necessary to examine the cost and programme implications of selecting different intake and outlet positions for the tunnel. This process led to exAmin- ing the effects of changing the tunnel diameter, hydraulic considerations, types of lining, differing methods of construction, ways of optimizing the vertical alignment, and deciding on the design and location of the access adits. It was assumed that four TBMs would be used to excavate the waterway, and

that the access adits would be done by DBM.

LHC felt that, broadly speaking, apar t from such adjustments to the positions of the intake and outlet and adit positions, the tunnel corridor pro- posed in the FS could not be improved on. The new studies therefore assumed that the maj or part of the tunnel route-- i.e., between Pelaneng and Sentel ina-- would remain unchanged. The various possible intake and outlet positions considered are shown in Figure 2. These studies yielded considerable variations in overall tunnel length, namely from 41.0 to 57.5 km, compared with the FS length of 48.3 kin.

The suggested terminal points M and N for the Transfer Tunnel intro- duced a new concept to the project. The arrangement envisaged in the FS, and those represented by the letters L and

H in Figure 2, had the Transfer Tunnel discharging into a headpond, making it independent of the underground power station. In considering end points at either M or N, the Transfer Tunnel became a headrace tunnel directly connected to the power station.

Overall project optimization studies indicated that the preferred layout was to place the Intake at either A1 or C. Considerations of hydraulics/reservoir sedimentation, access, geology, construction programme, proximity to quarry, cover requirements, and construction site needs, led to the adoption of the Intake C site for the Tender and Con- struction Design stages (see Fig. 3).

The overall project optimization studies further indicated that the preferred layout would terminate the Transfer Tunnel at a surge shaft at 'Muela (M). This layout has probably

AL TERNA TIVE OUTLET L OCA TIONS

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B:

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MOL 1980

MOL 1965

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HA LEJONE

AL TERNA TIVE INTAKE LOCATIONS

Figure 2. Alternative intake and outlet positions.

Volume 9, Number 1, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 81

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8 2 TLrNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 9, N u m b e r I, 1 9 9 4

made the LHWP Transfer Tunnel the Table 1. Average rock strengths and estimates of their relative occurrence. longest headrace tunnel in the world.

4. Detailed Design, Tenders, and Start of Construction

Work on the Tender design began in 1988, in parallel with completing optimization and conceptual design studies.

Whilst the tender documents were being brought together, 39 contracting consortia registered interest in the work. Tender documents for two separate contracts, Transfer Tunnel South and Transfer Tunnel North, were is- sued in October 1989. Tenders from seven consortia were received in April 1990, and a combined contract for the Transfer Tunnel was awarded to LHPC in December of the same year. At the same time, LHPC were also awarded a separate contract for Delivery Tunnel South. Construction on the Transfer Tunnel began on 1 February, 1991, with a construction period of 68 months (i.e., completion is required by 1 Octo- ber 1996).

5. Transfer Tunnel Layout The 45-km-long Transfer Tunnel is

being bored with a diameter of 5.03/ 4.99 m (new/worn cutter excavation diameters). The tunnel intake is located on the Malibamatso River, about 100 km east of Maseru in the High- lands of Lesotho (see Figs. 1 and 3).

The Intake is sited a kilometre up- stream of the impressive Malibamatso road bridge which, in turn, is some 17 kin north of Katse Dam. At the Intake the tunnelhas aninvert level of 1981.50 mamsl: The tunnel has a falling gradient (for the most part, 1 in 700) towards its northern end at the underground power station at 'Muela, some 20 km east of Butha Buthe in north- western Lesotho. The design flow of the tunnel for water transfer purposes is 32.6 m3/s.

Between the Intake and chainage 16 km, the cover fluctuates between 90 and 500 m. The cover then increases to a maximum of 1200 m over the next 8 km before dropping to 120 m in the vicinity ofHlotse (Ch. 28 kin). North- wards of Hlotse there is another, shorter, section of high cover (up to 1000 m) before the tunnel turns north- west, with cover decreasing along the 'Muela ridge to 130 m.

6. Geological and Geotechnical Considerations

The Transfer Tunnel is being driven through the basalts of the Lesotho For- mation. Four types of basalt have been distinguished by visual examination in the field. Arelatively small amount of Dolerite will also be encountered. Average rock strengths and estimates

Rock Type

D

DB

NAB

MAB

HAB

Average UCS

172 176

123

104

85

Standard Deviation (MPA)

103

58

45

35

33"

Estimated % Occurrence

2

4 36

29

29

of their relative occurrence are givenin Table 1.

The geological conditions are relatively straightforward, although a number of geological factors have a considerable bearing on the construction of the tunnel. These have been described by others (Brackley, Galliers, Dell and Nthako 1991).

The site investigations included:

• Surface mapping. • Drilling (a total of 10,000 m),

trenching and test pits. • Seismic refraction. • Laboratory testing. • Testing in boreholes. • Boreability testing. • Durability studies. • Hydrofracture tests. • Virgin rock temperature and gas

measurements.

It has been estimated that approximately 15% of the highly amygdaloidal basalts are subjected to swelling and are potentially degradable. As of July 1992, it appeared that two types of HAB are susceptible to degradation. One is readily distinguishable in the field as an HAB containing dissemi- nated amygdales. The other is not visually distinguishable from the usual HAB until degradation commences. The mechanism of degradation is not yet fully understood and investigation work continues in this field.

7. Horizontal Alignment of the Transfer Tunnel 7.1 Access

Much of the route of the Transfer Tunnel lies under the Front Range of the Maluti Mountains, which rise to an altitude of 3200 mamsl (Figs. 1 and 3).

Access into this area of sharply dis- sected topographical relief is extremely difficult and results in long travel times. Air travel, either by fixed wing aircrai~ or helicopter, is subject to the vagaries of the variable mountain weather and is not reliable on a regular basis.

Even now, with tarred access roads extending over much of the project area, it takes a minimum of just over two

hours to drive from the Transfer Tun- nel Intake to the northern end of the tunnel at'Muela. The distance by road exceeds 140 kin, more than three times the straightline distance. In the course of the journey, the Mafika Lisiu pass (3090 mamsl), with a maximum road gradient of 14%, must be traversed. Snow, hail and ice, although more prevalent in winter, can occur at al- most any time of the year, making road travel over the pass hazardous or even impossible for short periods.

7.2 Time Constraint In order to meet the project time

constraints, it was imperative to estab- lish intermediate points of access, in order to permit work to be carried out simultaneously on a number of faces. The Pelaneng and Hlotse River val- leys, on either side of the Front Range, provided the best opportunities, at approximately quarter points along the tunnel for such access. To keep the lengths of the adits from becoming ex- cessive, the route of the Transfer Tun- nel was bent towards the Pelaneng and Hlotse adits. Comparative studies were carried out to optimize the amount of tunnel route deflection and adit length.

7.3 'Muela Ridge

The last 3 km of the Transfer Tun- nel from Sentelina follow the 'Muela r idge, which is f l anked by the Khukhune and Nqoe rivers. As the ridge nears the power station site at 'Muela it narrows, providing limited vertical and side cover. Therefore, consideration was given to the relative merits of keeping the tunnel over this length at the level initially selected ("upper" alignment), or dropping it to a level corresponding to the turbines at the power station ("lower" alignment).

Hydrofracture measurements done in boreholes to determine in-situ stress conditions along the ridge over a range of depths failed to show a clear trend of increasing field stress with depth. It did, however, indicate a fairly consis- tent level of satisfactory field stress on the original or "upper" vertical alignment (see Figs. 4a and 4b).

Volume 9, Number 1, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 8 3

Accordingly, this alignment has been retained in the Tender and Con- struction design stages.

7.4 Phase 2 Transfer Tunnel A minor constraint on the overall

horizontal routing of the Transfer Tun- nel was the need to provide for the planned future duplication of the tun-

nel in Phase 2 of the Project. The Phase 2 Transfer Tunnel will be slightly larger than its Phase I coun- terpart (5.7 m excavated diameter). It has been located parallel to and 50 m to the west, centre line to centre line, of the Phase i tunnel.

Placing of the Phase 2 Tunnel to the west of the Phase 1 (as done in the FS) was reconsidered. Preliminary lay-

outs of the tunnel , and pa r t i cu la r ly the crossover and o ther deta i l s a t each of the access adi ts , were d rawn up wi th the Phase 2 Tunnel placed to the eas t of the Phase 1 Tunnel . A construct ion programme and costing were then done to compare wi th those a l r eady exam- ined. I t was found t ha t the differing m a r g i n a l a d v a n t a g e s for e i ther ar- r a n g e m e n t were insufficient to make

2150

2100 2050

2000

1950 1900

~ j 3 I I I I TRANSFER TUNNEL I

HL 0 TSE

Position of Surge Shaft

t l

JPR 127A

"MUELA ADIT

Figure 4a. 'Muela Ridge---longitudinal section.

rnssf

2300

2200

2100

2000

1900

1800

1700

K H U K H U N E ~ ~ NQOE VALLEY

31 $ I value of minimum $2 $3 3,0 Principal Stresses as determined

from Hydrofracture tests (MPa)

0 100 ~ WOrn I I

Figure 4b. 'Muela Ridge--superimposed cross-sections.

84 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 9, Number 1, 1994

one more desirable than the other. Therefore, overall layout was left as it was, with the Phase 2 Tunnel lying to the west of the Phase 1 Tunnel.

As noted above, implementation of the LHWP at this stage is limited to Phase 1. However, certain planning and construction work has to be done now in order to permit the Phase 2 work to be executed at a later date-- for example, Phase 1 and Phase 2 pro- visions in the design of the single In- take structure; and the planning of the access adit and waterway tunnel lay- outs at Pelaneng, Hlotse and 'Muela.

8. Vertical Alignment 8.1 Hydraulic Grade Line

The primary constraint on the vertical alignment is the hydraulic grade line. At the intake, this is governed by the minimum operating level (MOL) of the Katse reservoir, which is 1989.0 mamsl. At the downstream end, it is set by a lowest drawdown in the surge shaft to 1950.0 mamsl. Invert levels of the Phase 1 and 2 Transfer Tunnels were kept 10 m below this hydraulic grade line. This arrangement resulted in the need for very flat gradients. In the FS, these ranged from a falling gradient of 1:354 to horizontal. Falling gradients (south to north) were used for southward (uphill) drives, and a horizontal alignment was planned for northward drives. Ventilation shafts were positioned at the end of the northward and southward drives, i.e., where changes of grade occurred.

The tender design made use of a continuous falling gradient of 1:700 except be tween the I n t a k e and Pelaneng, where a steeper gradient of 1:450 was adopted. This design en- sured positive drainage during construction and maintenance. It also removed constraints on the final posi- tioniug of the vent shafts. Further- more, it allowed the lengths of tunnel drives from opposite directions to be varied at any time, as required, with- out having to introduce changes of grade.

A secondary, but nonetheless very important constraint was the need to provide access to the Phase 2 alignment at Pelaneng and Hlotse, requir- iug adit/waterway crossovers. A vertical separation of at least one tunnel diameter was required at these crossovers. Another constraint on the vertical alignment was the preference to have adits that would be self-draining towards the adit portals. This was possible at the Hlotse and'Muela Adits.

9. Adit Location and Design 9.1 General Considerations

To permit multi-face development, adits were planned at the Intake,

Pelaneng, Hlotse, and at the northern end of the tunnel. Initially, the most northerly aditwas located at Sentelina. As noted above, it was then decided that the tunnel was to terminate at 'Muela. The length of tunnel between Sentelina and 'Muela originally was regarded as part of the hydropower project. Subsequently it was decided that i t would be constructed as part of the water transfer scheme. It then became necessary to have a Transfer Tunnel access adit to 'Muela, and the adit at Sentelina became optional.

The adits at Pelaneng, Hlotse and 'Muela were sized to accommodate TBMs for the later Phase 2 Transfer Tunnel. Each adit also has been designed to provide a means of access to the waterway tunnel for maintenance] inspection purposes via a bulkhead door. The Intake adit is only of use in constructing the Phase 1 Transfer Tun- nel, as its portal will be submerged.

9.2 Intake Adit In the intial plan, the Intake Adit

approach was from the north, as this would greatly facilitate the surface works layout during construction. Un- fortunately, the site of the portal area for such an adit was found to be in deep colluvium. Therefore, a southern approach was decided upon, and has been built. The adit has been built on a downward grade of 1:100 to suit the use of railbound equipment.

9.3 Pelaneng Adit The position adopted in the FS for

the Pelaneng Adit haf been in the Mokhoulane River Valley, adjacent to and immediately to the north of the village of Ha Lejone. Because the Katse reservoir backs up several km beyond this locality, the adit portal had to be located above FSL, and the adit itself driven on a steep downward grade.

The FS adit position was no longer acceptable after it had been decided to adopt Intake C (Fig. 2). The distance between Intake C and the FS Pelaneng adit would have been only 9.5 kin, compared with the 14.9 km in the FS layout. To obtain a better balance of the TBM drive lengths, the Pelaneng adit needed to be moved northwards.

Two alternative locations were considered. Both had their portals in the Pelaneng River Valley--one on the south bank and the other on the north bank.

From what was clearly a good adit portal site, the south bank adit would have run for 2.2 km at right angles towards the waterway tunnel. This option would have given an Intake to Pelaneng drive length of 10.4 kin.

The north bank adit lent itself to providing a junction with the waterway tunnel even further north, giving an Intake to Pelaneng drive length of

11.4 kin. However, this gain in position required a greater deflection of the waterway route, adding to its length. On the other hand, the adit length could be reduced to 1.64 kin. Thus, although the adit portal conditions were less favourable, it was shown to be a less costly solution, and was adopted for the final design. The adit is being driven on a downward grade of 1:10.

9.3 Hlotse Adit The Hlotse Valley in the vicinity of

the adit portal has an elevation very similar to that of the waterway tunnel. It was possible to locate the entrance to the adit so that it was above the "one in one hundred years" floor line of the river; and also to provide it with a self- draining gradient of 1:600 towards the portal.

In the FS layout, the waterway tunnel at Hlotse hadlimited cover where it passed under the Holomo River Valley, and therefore required a watertight membrane lining. By lengthening the adit and moving the waterway tunnel eastwards by some 300 m, the cover could be increased to obviate this re- quirement, notwithstanding the rais- ing of the Katse reservoir FSL to 2053 from the FS figure of 2040 mamsl.

9.4 Adits at the Northern End of the Transfer Tunnel

The FS arrangement had the Trans- fer Tunnel discharge into a headpond at Sentelina. When it was decided to extend the tunnel and terminate it at 'Muela, it was initially thought necessary to retain an access adit at this locality.

Two possibilities were considered. One, an access from the Nqoe Valley, was referred to as the Sentelina adit. This was considered a better proposi- tion than the alternative of gaining access from high up on the side of the Khukhune River Valley (the Khukhune adit). In due course, as planning pro- gressed, the need for an adit in this locality fell away; it has become regarded as a fallback option should cir- cumstances ever require it.

9.5 'Muela Adit The 'Muela adit is 625 m long and

has been built on an upward gradient of 1:200 from the portal. Its alignment is virtually along an extension of the line of Transfer Tunnel, and joins the waterway tunnel at a point very close to the hydro-power station surge shaft.

10. Ventilation Shafts Five ventilation shafts have been

provided for along the route of the Transfer Tunnel. The shafts, which are required for construction and op- erational purposes, are 1.5 m in diam-

Volume 9, Number i, 1994 TUNNELLINO AND UNDERGROUND SPACE TECHNOLOGY 85

eter and are offset 10 m from the line of the tunnel . They va ry in dep th between 90 and 315 m.

11. Construction Programmes 11.1 Optimization Studies

I t should be evident from the above discussion tha t i m p o r t a n t in te r re la - t ionships exis t be tween tunne l al ign- men t (horizontal and vertical) , ad i t po- s i t ioning and design, and the construc- t ion programme.

The overiding cons t ra in t of t ime requi red commissioning of the Trans- fer Tunnel to commence in Project month no. 66.

I n the ini t ia l project opt imiza t ion studies, the FS a s sumpt ion of a fully l ined Transfer Tunnel was re ta ined. Also re t a ined were the a s sumed ra tes of advance for TBM excavat ion and continuous l ining, a t 500 m and 1000 m per month, respect ively. These ra tes were held to be appl icable for a l ined d iamete r of not more t han 4.2 m. The l ined tunnel d i ame te r a s sumed in the FS was 4.05 m.

In subsequent , more ref ined project o p t i m i z a t i o n s t u d i e s , e s t i m a t e d costings were required for a l ined Trans- fer Tunnel , wi th d iamete r s rang ing between 4.30 and 4.80 m. The opti- mum, programmed for the 4.80-m lined- d iamete r tunnel , was found to requi re the use of four TBMs and adi ts a t Pelaneng, Hlotse, and Sente l ina . One of the TBMs would have had to be used on two sepa ra t e drives---one on each side of the centra l h igh cover section. This scheme could have in t roduced complications, as the two dr ives fell into different contracts . Fu r the rmore , following del ivery to site, the four TBMs were requi red to be commissioned over a f ive-month period. This schedul ing was felt to be overly ambi t ious .

11.2 DBM Excavation Cons ide ra t ion was also given to

having the tunne l excavated ent i re ly by DBM at an overal l advance r a t e of 200 m/month. In order to meet the overall t ime constra ints , th is scheme would have requi red s imul taneous de- ve lopment of nine head ings from six access ad i t s using e ight crews.

I t then became a p p a r e n t t h a t the cost of a l ined TBM tunne l was very s imi la r to t ha t of a l ined tunne l exca- va ted by DBM. On the o ther hand, a fully DBM solut ion would involve ex- t r a costs for access roads and adi ts .

These considerat ions led to the de- ve lopment of a t leas t five a l t e rna t ive programmes , us ing a combinat ion of TBM and DBM excavat ion drives. The prefer red solution envisaged two con- t racts , which would make use of three TBM and th ree DBM dr ives be tween them (see Fig. 5a). Such a solut ion provided a g rea t deal of flexibili ty, par-

t i cu la r ly i funforeseen difficulties were encountered.

11.3 TBM Tender Arrangement Meanwhi le , there was increas ing

opinion t ha t much of the Transfer Tun- nel could be left unl ined i f i t were excavated by TBMs. Considera t ions favouring this concept a re dea l t wi th e lsewhere in the paper . In response to these concerns, the Engineer ' s Tender construct ion p rogramme was based on the use of five TBMs, wi th drive lengths be tween 7 and 11 km. The p rogramme a s s u m e d acces s a d i t s a t I n t a k e , Pelaneng, Hlotse, a n d ' M u e l a (see Figs. 3 and 5b).

11.4 DBM Tender Arrangement To cater to Tenderers who might

have wished to bid on a fiflly l ined DBM solut ion, a p p r o p r i a t e b i l l s of quant i t i es were made avai lable as an a l t e rna t ive to the TBM configuration. These were based on the use of nine head ings us ing six access adi ts . As i t t u rned out, no bids were received for th is solution.

11.5 LHPC Arrangement The bid of the successful Tendere r

(LHPC) envisaged the use of three TBMs for the four adi ts provided for in the Engineer ' s layout. As shown in F igure 5c, the work is being carr ied out in th is way.

12. Hydraulic and Tunnel Lining 12.1 Friction Losses and Transients

Depend ing on age, the T rans fe r Tunnel , i f fully l ined, pass ing a flow of 35 m3/s wi th a l ined d i ame te r of 4.35 m, can be expectrd to have a friction loss of be tween 0.61 and 1.03 m/kin (Manning "n" r ang ing between 0.0110 and 0.143 ). This f igure is comparable to t h a t of an un l ined TBM tunne l approx ima te ly 5 m d iamete r , where the corresponding f igures have been as sumed to be 0.70 to 1.14 m/kin.

This for tu i tous s im i l a r i t y al lows quite large var ia t ions to be made in the l ined lengths of the tunne l wi thout s ignif icant ly changing the overal l fric- t ion loss. Allowing for contract ion and expans ion losses resu l t ing from l ining approx imate ly 90 discrete lengths of the TBM tunnel ( total l ing 6.6 kin), the to ta l friction loss over the full l ength of the newly commissioned Transfer Tun- nel is expected to be 32.3 m. If 119 lengths total l ing 10 km are constructed, the corresponding friction loss is esti- m a t e d to be 33.3 m - - a b o u t 12% of the gross avai lable head wi th Katse reservoir a t FSL.

The t r ans i en t effects of full load acceptance and fifll load reject ion for a

ful ly l ined or a ful ly un l ined Transfer Tunnel were checked and found to be sat isfactory.

12.2 Concrete Lining Although the tunnel may be pre-

dominant ly unlined, there are sections where an in-situ concrete l ining will be required. Based on pract ical considerations, the min imum concrete l ining thickness was set a t 200 mm. To allow for var ious work ing to lerances , the payl ine was set to provide a nominal l ining thickness of 300 mm. The payl ine d iameter is therefore 4950 mm. LHPC have elected to size thei r TBMs to provide an excavated d iamete r of 5030 mm when the gauge cut ters are new, reduc- ing to 4990 m m with worn cutters.

Some types of HAB d i sp lay swell ing character is t ics . Swell p ressu re and free swell p ressure tes t s were car r ied out on a r ange of samples of this mate- r ial . F rom the resu l t s of these tes ts , a r e l a t ionsh ip be tween swell and re- s t ra in ing pressure has been pos tu la ted as follows:

Swell ing s t r a in % = 1 - 0.25 loglo (Swell p ressure kPa)

Assuming tha t swell ing is res t r ic ted to an a nnu l a r th ickness of not more t han 0.5 m, the design s t r eng th of the 3 0 0 - m m - t h i c k l i n i n g wi l l no t be exceeded.

Where a wa te rp roof l ining is required, a nominal ly reinforced concrete l in ing is to be constructed, wi th a plas- tic membrane placed aga ins t the rock. Where the su r round ing rock cannot provide the necessa ry con ta inment , steel l iners a re planned. The to ta l length of these special l inings al lowed for in the Bill of Quant i t i es is 1.3 km.

13. Planned Omission of Concrete Lining

Unlined hydraul ic tunne l s in basa l t h a v e been b u i l t a n d s u c c e s s f u l l y opera ted in the F~eroe I s lands (North Atlantic). Two mini-hydro projects wi th unl ined tunne ls have also been in use a t Mantsonyane and Tlokoeng in the h igh lands of Lesotho. Sa t i s fac to ry performance of these has been noted over periods of four and five years , respectively.

An examina t ion was also made of an adi t in basa l t a t the abandoned Le t s eng - l a -Te rae d i a m o n d mine in no r theas t Lesotho. The ad i t was exca- va ted in 1969 and has been subjected to flowing wa te r since the closure of the mine in 1982. Very l i t t le degrada t ion of the basa l t appea r s to have t aken place in the tunne l walls .

These experiences led to the decision to provide an in-situ concrete lining only to those sect ions of the Trans- fer Tunnel which migh t requi re i t for s t ruc tu ra l reasons or where s teel or plas t ic l iners a re employed. Accord-

86 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 9, N u m b e r 1, 1994

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Volume 9, Number I, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 87

TUNNEL

Rock bolts 1 000mm long 25mm (~ bolts to be end anchored only (no column grouting), Between 4 and 8 No. bolts on opposite sides of periphery at 750mm centres. Fan of bolts to be rotated as required to support area of spalling rock. Bolt fan to cover full circle where required. Triangular face plates as shown

300

- -] f

i Ioo I"

1 O00mm long 25mm (~ bolt end anchored, tension to 5 tonne

face plate 8ram thick

Figure 6. Class BM VI support details.

ingly, provision was made for lining 6.7 km of the Transfer Tunnel.

14. R o c k S u p p o r t

14. I General Considerations

Initially, Tender Designs for both the Transfer and Delivery Tunnels were based on short-term support systems, complemented by the subsequent provision of an in-situ concrete lining. The initial support systems made use of full column grouted rockbolts, mesh, shotcrete and steel arches.

The decision that the tunnels should remain unlined as far as possible meant that ways of ensuring long life of the initial support systems became necessary. This was done by requiring rock bolts and accessories to be hot-dip- galvanized to a thickness of 85 mi- crons. This process has been more fully described elsewhere (Richards and McKelvey 1990).

For the Transfer Tunnel, six support class designs were prepared. Five of these ranged from Class BM I (requiring minimal to no support) to BM V (providing for a combination of steel arches, weldmesh and shotcrete). Con- ditions corresponding to Class BMI are expected to apply to about 41 km of the 45 km tunnel. A separate class, Class

BMVI, was developed to cater to spaUing conditions (see Fig. 6). This design, which is based on Norwegian experi- ence, makes use of relatively short (1000 mm) end-anchored bolts fitted with large triangular face plates.

14.2 High Cover Under the maximum cover of 1200

m, the virgin field stresses are likely to be of the order of 30 MPa. Tangential stresses in excess of 70 MPa around the circular tunnel will therefore al- most certainly exceed the strength of the weaker basalts. Indications are that under deep cover, the ratio of in- situ horizontal to vertical virgin stress (k) will be less than unity. Spalling of the tunnel sidewall can be expected under these conditions.

The following four prediction models were used in an attempt to quantify the likelihood of spalling failures:

• An elastic model (massive yield- ing rock; Stacey and Page 1986).

• A non-elastic analysis using the FLAC programme.

• A brittle rock model, with frac- turing taking place when the extensional strain exceeds 300 microstrains (critical extensional strain; Stacy and Page 1986).

• A critical extensional s train model based on classical plane strain calculations, accepting 300 microstrain as the critical level.

Predictably, these models yielded considerably differing results. From an examination of a range of rock mass strengths, k ratios and tunnel depths up to 1200 m, it was possible to assess the maximum radial extent ofspalling rock. The periperhal extent of this phenomenon may, of course, vary from nothing, where no spalling occurs, to a full-circle phenomenon under hydro- static conditions, in a weak rock mass and as predicted by the more conserva- tive analytical models.

The conclusions drawn from the results of these analyses led to the adoption of the Class BM VI support design described in the previous section.

15. Conclusions This paper has attempted to sum-

marize a large volume of planning and design work done by a small team over a period of several years. As is the case with most tunnels, design continues concurrently with construction.

Much still remains to learned of the behavior of the Lesotho basalts under tunnelling conditions. It is essential

88 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 9, Number 1, 1994

t ha t no effort be spa red in doing this , as the resu l t s will not only benef i t the p resen t work, bu t m a y well have a s ignif icant bear ing on l a t e r phases of the project. [ ]

Acknowledgment The au thors wish to t h a n k the fol-

lowing organizat ions for permiss ion to publ i sh th is paper : Lesotho High lands Development Authori ty; and Lesotho High lands Consul tants .

References Brackley, I. J. A.; Galliers, 1~ M.; Dell, A. G.;

and Nthanko, S. 1991. Geotechnical investigation for a water transfer tunnel in Lesotho. Paper presented at SANCOT Seminar, November 1991.

Lahmeyer Macdonald Consortium: Olivier Shand Consortium. 1986. LHWP Feasi- bility Study.

Richards, J. A. and McKelvey, J. G. 1990. The designed ommission of linings in

the tunnels for the LHWP. Paper p r e s e n t e d a t SANCOT Seminar , November, 1990.

Stacey, T. R. and Page, C. H. 1986. Practical Handbook for Underground Rock Mechanics. Trans Tech Publications.

van Robbroeck, T. P. C. 1986. The Lesotho Highlands Water Project--an alternative. Overview of the LHWP presented at a one-day seminar, 28 November 1986.

Volume 9, N u m b e r 1, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 8 9

Documents

Planning and design of the transfer tunnel for the Lesotho highlands water project