Design and construction of a segmental lining for a machine-bored tunnel: Delivery tunnel North, Lesotho highlands water project

0886-7798(93)E0010-3

Design and Construction of a Segmental Lining for a Machine-bored Tunnel: Delivery Tunnel North, Lesotho Highlands Water Project

d. A. Richards, F. Remmer, d. C. Sharp

Abs t rac t - -The 21&m-long Delivery Tunnel North for the Lesotho Highlands Water Project is split into two main tunnel drives of 8 km and 11 km lengths. The tunnel is located in the sedimentary rocks of the upper Karoo series with cover of up to 350 m. The rock units comprise claystones, siltstones and sandstones. In the poorer geology, early support of the tunnel periphery is essential. The method of tunnel construction involves excavation by a TBM and the simultaneous erection of a precast concrete segmental lining that provides both rock support and a final tunnel lining. The segmental lining is designed and constructed as part of an integrated excavation / support/lining system. There are three types of segments, differentiated by the amount o f steel reinforcement. The distribution of the different segment types along the tunnel, relative to the geological conditions and depth of cover, will be optimized by monitoring of the performance of the lining during the construction period.

Rdsumd--Le canal d' dcoulement nord du Lesotho Highlands Water Project, long de 21 kin, est divisd en deux tunnels principaux de 8 et 11 kin. Ce tunnel se situe dane les roches sddirnentaires des couches supdrieures de Karoo, avec une couverture atteignant 350 m. Le terrain se compose de roches argileuses, de silt et de gr~s. Lorsque les conditions gdologiques sont difficiles, il est essentiel de mettre en place un sout~nement & la pdriphdrie du tunnel. Let mdthode de construction utilise un TBM et la mise en place simultande de voussoirs en bdton p rdfabriqu~s, qui assurent dt la fois le sout~nement de la roche et le rev~tement ddfinitif du tunnel. Le rev~tement en voussoirs est conqu et construit comme partie du syst~me intdgr~ creusement/sout~nement/revdtement. II existe trois types de voussoirs, diffdrencids par le volume d'armature mdtallique. La distribution des diffdrents types de voussoirs le long du tunnel, en fonction des conditions gdologiques et de la hauteur de couverture, sera optimisde par le contr6le des performances du revdtement au cours de la pdriode de construction.

1. Introduction

T he Delivery Tunnel forms part of the Lesotho Highlands Water Project (LHWP). It is the north-

ernmost section of the tunnel system, bringing water from Katse Dam in the Lesotho Highlands northwards to the Ash River, a tributary of the Axle River in the Orange Free State Province of South Africa. From the Axle River, the water gravitates via the Liebenbergs Vlei and Wilge rivers to the Vaal Dam, before being distributed to the Pretoria- Witwatersrand-Vaal (PWV) area of South Africa.

The intake to the Delivery Tunnel is sited at the tailpond ofthe'Muela Power Station. The tunnel crosses the border between Lesotho and South Africa some 16 km to the north at the Caledon River and continues for a further 21 km to the outfall at the Ash River.

The Delivery Tunnel North comprises the 21-kin section in South Af- rica, and is arguably located in the most complex geology of the whole tunnel system.

2. Delivery Tunnel North 2.1 Genera/

The Delivery Tunnel North at its southern end is located in the foothills of the Lesotho mountain ranges at the Caledon River. The topography along the tunnel route northwards becomes progressively less mountainous, giving way to a predominantly rolling landscape with relatively low cover.

The north-south tunnel route is in- tersected by east-west trending river valleys of the Caledon and Little Caledon rivers, which provide conve- nient access to the tunnel drives. The Little Caledon River, lying approximately halfway between the Caledon River and the Outfall, forms the divi- sion between the two main tunnel drives, referred to as the Caledon and the Ash Tunnels.

The Caledon Tunnel lies to the south, between the Caledon and Little Caledon Rivers, and is 10 km in length. The 11- km-long ASh Tunnel lies to the north, extending up to the ASh River Outfall. The tunnels have an internally lined diameter of 4.6 m.

2.2 Tunnel Alignment Present address: J. A. Richards, Consult 4, Only those aspects of the alignment P.O. Box 1859, Randburg2125, SouthAfrica. that have potential impact on the de- Tunnelhng and Underground Space l'ectmfdogy, Vol. 9, No. l , pp. 91 -99 , 1994 Copyright © 1994 Flsevier Science Ltd Printed in (h eat Britain. All rights reserxed Pergamon 0886-7798/94 $6.00 + .00

sign and construction of the segmental lining are considered in this paper.

The tunnel is driven through relatively horizontally bedded sedimentary rocks, as described in Section 2.3. Therefore, the geological conditions are not significantly influenced by the horizontal alignment of the tunnel.

Ideally, the vertical alignment of the tunnels should be optimized to locate the tunnel predominantly in the more competent rock strata, especially under high cover. However, a basic design criteria for the Delivery Tunnel System is that it must remain full under flow and static conditions to prevent the de- velopment of air pockets and unaccept- able surge pressures. The location and level of the Ash River Outfall therefore forms a major control for the vertical alignment of the tunnel.

Considerations of dealing with water during tunnel construction require that the tunnel be driven upgrade. This requirement led to the TBM establishment for the Ash Tunnel being located at the Little Caledon River. From here the tunnel could be driven upgrade towards the hydraulic control level at the Ash Outfall.

For a segmentally lined tunnel, the tunnel construction establishment would include a precast concrete seg-

9 1

ment manufacturing facility and stor- age area. Therefore, it was logical to consider that the location of the establishment for the Caledon Tunnel would also be located in the same area. This

consideration and the preference to drive the tunnel upgrade were the main criteria for the vertical alignment of the Caledon Tunnel.

Overall optimization of the tunnel

with respect to geological conditions has been limited. The vertical alignment and the principal lithology of the Delivery Tunnel North are illustrated in Figures la and lb.

2 2 0 0 -

2100-

2000- z

~1900- > w

~1800-

1700-

1600-

ENI) OF CALEDON TUNNEL START OF CALEDON TUNNEL I

CLARENS

TRANSITION ZONE

ELLIOT FORMATION - - MOLTENO FORMATION

-- TARKASTAD SUBGROUP

f , ,

r r T 1 ] ] 1 "I T 1

GRADIENT

EXCAVA~0N

~E'TH~

O O O O O O O O O - - 04 rO

O O O O O O O O O O O O O O O O O O O O O

CHAINAGE (METRES]

..~_L-IO I:I00o

DEIB TBM WITH SEGMENTAL LINING

=ENGTH 1941 8071

Figure la. Caledon Tunnel--longitudinal section.

-= CALEIX)N=, TUNNEL

2200-

2100-

z2OOO- o

~ I 900- .J bJ 1800-

START OF ASH TUNNEL

OE bJ >

CLARENS I TRANSITION ZONE

^I I I ELLIOT FORMATION

o / ' ~ I I I MOLTENO FORMATION

- 7 ; 2 - - -

I I I I I I I I I I I I 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

CHAINAGE IMETRES}

,1:1053 3RADIENT fl ~ cuTVARIESB

"XCAVATION COVER LDS=B VETHOD SIPHON I I

_ENGTH 622 ~1~96

TBM WITH SEGMENTAL LINING

1098~

Figure lb. Ash Tunnel--longitudinal section.

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

2.3 Geology The Delivery Tunnel North passes

through the upper Karoo Sequence For- mations of the Triassic period. These comprise a nearly horizontal layered sequence of sedimentary rocks of conti- nental origin. The tunnel will be excavated mainly through the rocks of the Elliot and Molteno Formations and the Tarkastad Subgroup. The basal transition zone of the Clarens Formation will be encountered at the southern end of the Caledon Tunnel.

It is generally accepted that the Drakensberg lavas of the Jurassic period which overly the Clarens Forma- tion were intruded along fissures in the sedimentary rocks. The basic magma solidified within the fissures to form dykes, sills and plugs of various

sizes, shapes, and inclinations. These intrusions will be encountered by the tunnel at various intervals.

A summary stratigraphic column for Delivery Tunnel North is presented in Table 1.

The geological layers have a shal- low southerly dip. The tunnel progressively passes down through the formations, from the Clarens Formation at the southern end of the Caledon Tun- nel to the Tarkas tad Sub group at the northern end of the Ash Tunnel, at the ouffall.

2.3.1 Tarkastad Subgroup The Tarkas tad Subgroup as en-

countered comprises an interbedded sequence of rock types ranging from c lays tones to coarse sands tones . However, fine and medium sandstones, silty claystones and clayey siltstones predominate.

Below the Molteno Formation and 20 to 40 m of mixed sediments, a sandstone-rich zone occurs. Towards the outfall, this sandstone zone occurs closer to the Molteno Formation and is more thickly developed.

2.3.2 Molteno Formation The T a r k a s t a d S u b g r o u p is

unconformably overlain by the Molteno Formation of the Stormberg Group sediments. This sandstone horizon as encountered by the tunnel is generally massive and medium to coarse sand, with silty claystone and clayey silt- stone interbeds present.

2.3.3 Elliot Formation The Elliot Formation lies conform-

ably above the Molteno Formation rocks and comprises mainly siltstones, clayey s i l t s t ones and s i l ty c l ays tones interbedded with fine- and medium- grained sandstones.

The occurrence of disturbed bedding within rock types and the alter- nating and interlaminated, inter-bed-

ded nature of the horizons within the Elliot Formation can be expected to produce significant bedding discontinuities and mixed-face conditions.

2.3.4 Clarens Formation The Clarens Formation conformably

overlies the Elliot Formation. How- ever, a boundary between the Forma- tions is often obscure because ofa gra- dational change of rock types. This transition zone is characterized in some cases by potentially weak planes in the form ofweU-defined clayeylAminations or narrow, laminated zones.

3. Alternative Excavation, Rock Support and Lining Systems

The method of construction is influenced by a number of factors, such as the length of the tunnel drive, the cross- sectional area of the tunnel, and the vertical alignment.

Sections of the tunnel at the river crossings have relatively steep grades (approximately 1 in 10); these sections are more feasibly excavated by drill- and-blast techniques. Another factor that influences the construction tech- nique in these areas is the requirement to provide a watertight lining because of therelativelylow cover. The watertight linings take the form of steel linings or reinforced concrete linings up to 800 mm thick. Because of the limited extent

Table 1. Summary stratigraphic column for Delivery Tunnel North.

Geological Approx. Max. Period Stratigraphic Nomenclature* Lithology Thickness **

Basalt lavas 1400 m

Lower Jurassic to Triassic

Karoo Sequence/ Super Group

Drakensberg Lesotho Group Formation

Clarens Formation

Stormberg Group

Elliot Formation

Molteno Formation

Beaufort Tarkastad Group*** Subgroup

Aeolian 180 m sandstones

Siltstones and claystones with subordinate sandstones

Sandstones with subordinate siltstones and claystones

Siltstones and claystones with subordinate sandstones

120 m

50 m

150 m

* The Nomenclature adopted is that generally used on LHWP, and differs slightly from that recommended by the South African Committee for Stratigraphy (1980). ** Approximate maximum thickness in the Delivery Tunnel area. *** The Ecca Group and Dwyka Formation underlay the Beaufort Group completing the Karoo Sequence, but do not crop out in the Delivery Tunnel area.

Volume 9, Number 1, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 93

of these special sections, drill-and-blast excavation methods were adopted, in view of their inherent flexibility.

With the exception of these sections, the lengths of the Delivery Tunnel North main tunnel drives are 8 kin (Caledon Tunnel) and 11 km (Ash Tunnel), which tend to favor mechanical excavation by TBM. However, drill-and-blast techniques are also feasible because of the opportunity to develop a number of shorter headings simultaneously.

The tender documents were pre- pared on the basis of the following basic construction methods:

= Excavation of the tunnel drives by TBM and/or drill and blast.

• Temporary support of the rock required, using rockbolts, shot- crete, and/or arches.

• Constructionofacast-in-situ concrete lining on completion of the excavation to each drive.

Construction of the Caledon and Ash Tunnels using the above methods with a TBM would require using two TBMs (one on each tunnel) because of programming constraints for the overall project.

An alternative one-pass lining system, whereby a segmental lining is installed sequentially behind the TBM, was also considered feasible. In this regard, a lining system would provide immediate reck support, long-term reck support and a final lining. This possi- bility was not designed and specified by the Engineer in the Tender Docu- ments for two principal reasons:

1. The specification of a method of construction requiring a relatively high level of technical sophistication and specific expertise may have precluded a number of otherwise experienced Contractors from tendering.

2. The design of the segmental lining system is intimately related to the design of the TBM and cannot be designed in isolation. A design by the Engineer would impose unnecessary restrictions on a tenderer. The selec- tion of the TBM and lining system by the tenderer to suit the tunnel conditions based on the tenderer's specific expertise and preference was considered more appropriate.

Therefore, allowance was made in the tender documents for the Caledon and Ash Tunnels for an alternative, one-pass lining system involving a precast concrete segmental lining to be proposed and assigned by the Tenderer to accommodate criteria established by the Engineer (see section 4).

Such a system was proposed by the successful tenderer, HMC Tunnelling Venture, and is the subject of this paper.

The one-pass lining system has significant programme advantages aris- ing from the simultaneous excavation and permanent lining of the tunnel.

HMC have programmed to construct both the Caledon and Ash Tunnels using a single TBM.

4. Design Criteria for the Segmental Lining

Certain criteria had to be accommodated by the design of the segmental lining system. These criteriawere identified by the Engineer in the tender documents for consideration in the Contractor's design. The criteria are summarized in the following sections.

4.1 Geotechnical Criteria The primary factors affecting the

stability of the excavation profile are the rock mass strength and the induced stresses or depth of overburden related to the K ratio (ratio of horizontal to vertical stresses).

From the observations made at the Delivery Tunnel Test Facility, the principal mechanisms of failure resulting from overstress were inferred as follows. The mechanisms are illustrated in Figure 2.

(a) S h e a r fa i lure o f t h e e x c a v a - t i o n i n m a s s i v e o r f i s s u r e d r o c k . Shear failure can occur as a result of high induced stresses close to the excavation surface. For a K value greater than unity, as shown, shear failure will initiate in the crown and in the invert, leading to a breakout of the excavation periphery.

This type of shear failure will occur where the induced stress exceeds the shear strength of the rock mass. This situation would be equally applicable to the weak deformable mudrocks with low stresses and to the stronger rocks under high cover. Failure will also be influenced by the type of fissuring.

(b) S h e a r f a i l u r e a n d b e d sepa- r a t i o n / s l a b b i n g in b e d d e d f o r m a - t i o n s . In bedded strata there is a potential for bedding plane overstress to occur, particularly where the K ratio is greater than unity.

The effects of shear stress are: • Destressed loosened haunch

zones. • Stress concentration in the beds

adjacent to the crown and invert, leading to strata buckling or bed separation.

This failure can be prevalent in interbedded sedimentary formations.

(c) Spa i l ing o f t h e e x c a v a t i o n p a r a m e t e r in m a s s i v e r o c k . Spalling may occur as a result of high induced stresses around the tunnel opening, giving rise to extensional strain failure either in the sidewall (as illustrated for K < 1.0) or in the face. This type of failure is observed in massive brittle rocks and may be exacerbated by struc- tural features (joints/bedding) in the vicinity of the stressed area.

4.2 Segmental Lining Criteria The excavation of a tunnel with a

TBM and the simultaneous installation of a segmental lining of necessity demands a fully integrated system. From a consideration of the geotechnical and operating conditions, certain requirements were identified by the Engineer with respect to the segmental lining and, accordingly, with respect to the TBM.

4.2.1 Overall System Criteria The proposed system should be de-

signed to:

(a) Provide immediate ground support behind the cutterhead. Therefore, the system shall be fully integrated with the TBM so as to allow rock support close to the cutterhead compatible with continuous machine advance.

(b) Provide a stable invert for the TBM.

(c) Provide s t rain/convergence compatibility (probably anisotropic) with the rock mass to ensure acceptable working stresses within the lining system while providing adequate rock support.

(d) Provide a final hydraulic profile in terms of diameter and roughness, as required to achieve the specified water transfer.

(e) Provide long-term protection from erosion of the rock surrounding the lining.

4.2.2 TBM Criteria In view of the integrated nature of

the construction, it is appropriate to consider the design criteria of the TBM in relation to the segmental lining. The criteria are as follows:

(a) Machine cutting and thrust requirements shall recognize the occurrence of weak and potentially "sticky" siltstones and claystones,high-strength dolerite in dykes or sills, high-strength metamorphosed rock associated with dolerite intrusions, and mixed-face conditions along the alignment.

(b) Alignment control, with particular reference to mixed-faced conditions and the requirements for proper segment installation, shall be a key feature of the machine design.

(c) Thrusting and alignment systems and segmental linings shall be designed to ensure that no damage occurs to completed rings due to the thrust, torque and alignment forces.

(d) Shielding arrangements shall recognize the occurrence of potentially squeezing tunnel conditions and the requirement to protect potentially de- gradable rock and/or provide immediate ground support behind the cutterhead. Long fixed shields shall not be used.


ZONE OF OVER STRESSING/ SHEAR FAILURE

K>I,O

SIMILAR

SHEAR

A FAILURE OF EXCAVATION PERIPHERY

A PRINCIPAL FAILURE MECHANISM

- MASSIVE ROCK

SHEAR FAILURE OF EXCAVATION PERIPHERY - BEDDED ROCK A PRINCIPAL FAILURE MECHANISM

V ~ EXTENSION FRACTURING

( ~ , ~ ~ Kd,O

A SPALLING OF EXCAVATION PERIPHERY IN MASSIVE ROCK

A PRINCIPAL FAILURE MECHANISM

LEGEND ; MAXIMUM PRINCIPAL STRESS DIRECTION

t::::=-. MINIMUM PRINCIPAL STRESS DIRECTION

Figure 2. Principal failure mechanisms resulting from overstress.

(e) The design shall recognize the need for early support placement (in the form of the segmental lining) in overstressed rocks under moderate or high cover sections.

4.2.3 Lining Criteria The tender documents indicated a

preference for a lining system whereby the precast segments are expanded against the rock. The expanded-type lining would provide immediate ground support and a limited degree of radial confinement. Such an expanded lining would be designed to accommodate convergence of the excavation and the induced forces. The following criteria were considered applicable:

(a) In stronger rocks and under lesser cover where lining stresses are evaluated to be acceptable solid (wedge block) expansion systems may be used.

(b) In weaker rocks, particularly under high cover where allowable stresses in solid linings may be ex- ceeded, expansion systems that allow subsequent closure of the ring to control lining stresses within acceptable limits shall be incorporated in the lining system.

(c) The use of compressible joint packing shall be subject to satisfying all requirements for operation of the tunnel under internal pressure.

(d) Expansion systems shall be designed to induce the maximum rea- sonable prestress in the ring, subject to the requirement that the ring not be overstressed at any time.

Rock/lining interaction analyses using analytic techniques would be necessary to evaluate lining stresses.

The worst case ground condition to be catered to would be weak claystones at depths in order of 300 m. Particular attention would also have to be taken of shear failure anticipated in the bedded formations (see Section 4.1(b)), which could give rise to differential loadings on the liningifnot adequately constrained.

5. Design and Construction of the Segmental Lining

The Contractor proposed an alternative one-pass lining system accord- ing to his expertise and previous tunnelling experience. The system, which is very similar to that used on the Grauholtz Tunnel in Switzerland, is illustrated in Figure 3a.

It did not comprise an expanded system, as proposed by the Engineer in the tender documents, because of the concern regarding possible rock break- ing out in the crown and haunch areas, with slabbing and bed separation. It was envisaged that this would cause difficulty in expanding the segmental ring for two reasons:


uJ u

o z w l z

"~AOs~ ~ SEGMENT PRECAST CONCRETE

GROUTED ANNULUS

THEORETICAL TUNNEL AXIS

Figure 3a. Precast concrete segment lining.

" • ' 4 • " ' . . " m

GROUTE3 ANNULU. c

, 4 ' , 4

11" .

- - -- LONGITUDINAL JOINT

- - CIRCLE JOINT

• a " . 4

• . , . . . . .

GROUTED ANNULUS

Figure 3b. Longitudinal section of segmental lining.

1. Expanding the lining over a void in the tunnel periphery would cause distortion of the segmental ring and difficulty in building of the ring.

2. The occurrence of a deformed tunnel periphery due to shear failure precluding the full expansion of the lining without causing distress to the segments.

The lining system as proposed has the following features:

(a) Each segmental ring is made up of 5 No segments and one keystone.

(b) Each segmental ring is 1.40 m long.

(c) Each segmental ring is stag- gered by 36 ° relative to the previous one (see Fig. 3b).

(d) The segments are equipped with compressible Phoenix gasket seals (see Fig. 4) to prevent erosion of mate- rial from behind the segmental lining. This also provides a relatively watertight lining.

(e) The s e g m e n t a l r ing is as- sembled within the tailshield of the TBM and is bolted together to compress the seals in the longitudinal joint (see Fig. 4). As each segment is located in position, it is held against the previous segmental ring by thrust rams on the back of the TBM. This process also serves to compress the seals in the circle joint and maintain the shape of the ring. The segmental rings are also bolted together to provide additional security of the segments.

(f) The tailshield of the TBM over- laps a section of the previously erected ring, providing complete protection during ring building.

(g) Early grout injection of the annular gap around the segmental ring is

COMPRESSED PHOENIX GASKET SEALS

R : 1350

94 F

R = o{)

N

Figure 4. Typical longitudinal joints.

9 6 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 9, Number 1, 1 9 9 4

carried out to ensure early rock support and, thereby, to reduce the span of unsupported rock between the face and the fully grouted segmental lining. A typical profile of the grouted annulus is shown in Figure 7. The design of the

annular grout had to accommodate the immediate loads imposed by the self weight of the segmental ring, rock loads and associated TBM back-up machine loads. In order to test the grout's capa- bility in this regard, a number of large

1389.27

470 470

- -BOLT HOLES FOR FIXING OF RAILS DURING CONSTRUCTION

P 350 --!=1 350 :[ ]e

I

e)

DIRECTION OF

OF DRIVING LI TUNNEL ~j

L ou:I,N HOLES

-h o~ c

I •

I hJ. RECESS TO RECEIVE CONNECTING BOLT FROM ADJACENT SEGMENT BOX-OUT FOR INSERTION OF CONNECTING BOLT TO ADJACENT SEGMENT

Figure 5. Developed plan for a typical segment.

scale tests were carried out at CSIR, whereby a segment with grout behind it was subject to loading up to 80 tons.

This load represents:

Self weight of segment ring: 12.5 tons

Weight of TBM back-up: 5.0 tons

Rock load (equivalent to a block 4.5 m x 4 . 0 m x 1.4m@ density 2600 kg/mS): 65.5 tons

TOTAL 80.0 tons

Deformations of the segment under this load ranged between 5.5 nun and 17 mm for the various grout mi~es. Less deformation could be expected in the tunnel because the loadings would be spread over a greater area of the lower portion of the segmental ring than the area of one segment.

(h) The longitudinal joint is pro- filed to accommodate a 1350-ram ra- dius on the contact surface of one segment while the other contact surface is flat. This arrangement allows for deformation of the segmental ring under loading, without causing spalling due to stress concentration (see Fig. 4).

(i) The segment rings are tapered in the vertical plane so that by rolling the rings relative to each other, it is possible to closely follow the direction of the TBM. This is necessary for making adjustments when driving in a theoretical straight line, as well as on a curve.

Typical dimensions and a plan view of a segment showing the layout of the bolt connections are given in Figure 5.

It was appreciated that under the worst ground conditions, overstressing of a stiff/rigid lining system would occur (see Sections 4.2.3(b) and (c)). The overstressing was accommodated to a degree in the design, by providing a grout with a low final strength of 2 to 3 MPa. Notwithstanding this arrangement, should conditions occur such that the liningis unacceptably overstressed, causing doubt with respect to the long- term durability of the tunnel, provi- sion has been made in the sizing of the tunnel for casting an inner cast-in-situ concrete lining in overstressed sections.

The segmentally lined tunnel had to be sized to provide the same hydraulic capacity as the 4.5-m-diameter cast- in-situ concrete lined tunnel. Account had to be taken of the anticipated aging or roughening of the lining as a result of attack by the soft water being conveyed in the operating tunnel. The following aspects were considered:

(a) The standard of finish of the precast segments and the quality of concrete would be higher than that achieved in a cast-in-situ lining. This is achievable by the use of high preci- sion steel segment moulds and the cast-


ing of concrete under controlled factory conditions. The surface of the segments would be accordingly more du- rable and less susceptible to "aging" or roughening than a cast-in-situ concrete.

(b) The joints between the segments with steps and inherent irregu- larities would cause disturbances to the water flow.

(c) The construction of an inner cast-/n-situ concrete lining in distressed areas would reduce the hydraulic capacity of the tunnel.

To accommodate the above factors, the internal diameter of the segmentally lined tunnel was increased to 4.6- m diameter (cf 4.5-m-dia. cast-in-situ lining).

The system therefore satisfied, to a large extent, the design criteria as listed in Section 4.2.1.(a) to (e) above, and as summarized below:

The tender period for the Caledon a n d Ash Tunnels (Contracts TCTA-11 and TCTA- 12 ) w a s six months. During this period, tenderers who wanted to submit an alternative one-pass lining tender were also required to prepare for submission a basic tender compris- ing either a TBM or drill-and-blast excavated tunnel with a cast-in-situ concrete lining.

During this period, the Contractor was only able to carry out a prelimi- nary rock-lining interaction analysis, which gave an indication of the ad- equacy ofthe proposed system. During the construction period, HMC has carried out a number of subsequent com- puter modelling and statical analyses to confirm the design. Such analyses have taken account of the discontinuities anticipated in the sedimentary rock formations and which have been identified as a principal method of failure (see Section 4.1).

Table 2. Design criteria for the lining system.

Design Criteria

Provide immediate ground support behind the TBM.

Provide a stable invert for the TBM.

Provide strain/convergence compatibility (see also Section 4.2 (1)).

Provide a final hydrualic profile in terms of diameter and roughness.

Design Solution

Grouting of annulus around the segmental lining immediately behind the tailshield.

The early strength of the grout designed to withstand machine and rock loads.

Final strength of grout allows for limited deformation of rock.

• The steel moulds for the segments provided to high tolerances.

• Specified ring build tolerances.

•Intemal diameter of tunnel increased.

Provide long-term protection from The segments are equipped with erosion of rock surrounding the lining. Phoenix gasket seals.

Table 3. Conditions for which the segment designs were developed.

Segment Design Application

Lightly reinforced segment (100 kg/m 3 ) Principal design condition: handling and durability.

Medium reinforced segment (130 kg/m 3 ). Intermediate condition.

Heavy reinforced segment (160 kg/rn 3 ). Poor ground conditions: early backfilling of segments critical to prevent ongoing instability of rock surround.

Modelling of rock discontinuities is inherently fraught with difficulties with respect to establishment of cor- rect rock mass parameters, and is only as good as the input parameters. Con- sequently the analyses can only give a better understanding, rather than a finite definition of the load conditions. The principal objective of the analyses was to provide a predictive model that could be refined by monitoring the performance of the constructed lining.

In order to accommodate the long lead time required to produce segments, to ensure that there was sufficient re- serve to support the TBM during its operation, it was necessary to carry out the design of the segments in parallel to the rock/lining interaction analyses.

Because of the difficulty in assess- ing the geological conditions within a shielded TBM, where rock exposure is very limited, it was considered feasible to consider only three segment types to cater to the varied ground conditions. Accordingly, segments with different levels of reinforcement were designed to cover three broad areas of construction conditions.

The segment designs were developed for the conditions listed in Table 3.

The amount of reinforcement in the heavy segment is the maximum that could be reasonably considered in a factory production operation. The amount of reinforcement in the light segment was that required to allow for the handling loads, while the medium segment accommodated an intermediate amount of reinforcement.

The design logic tha t has been adopted and that will continue during the construction of the segmental lining is represented in Figure 6.

Monitoring of the lining performance is an important element in the design. All segments have been equipped with recesses to accommodate the end of a convergence rod, designed to measure diametrical convergence and also chord lengths. The rod will only be suitable for recording the larger deformations. Cer- tain segments will be equipped with vibrating wire strain gauges for more accurate measurements.

6. TBM Configuration In response to the TBM criteria

listed in Section 4.2.2 (a) to (e), two basic types of TBMs were considered by HMC. One was a relatively short (approximately 7.5-m-long) single- shield machine; the other was a more sophisticated and longer (11 m) double- shield machine. The double-shield machine was finally selected by the Contractor for the following reasons:

(a) The ability to articulate the TBM along its length facilitates steer- ing, providing greater cutting and alignment control.


REFINE AND OPTIMISE (.

~EOLOGY/ COVER

GEOTECHNICAL ANALYSIS

SEGMENTAL DESIGN

CONSTRUCTION TECHNOLOGY

Figure 6. Design logic diagram.

CONSTRUCTION AND LONGTERM LOADINGS - DISTRIBUTION ALONG TUNNEL

,IMI.I SEGMENT TYPES[,,, "

I REFINE

CONSTRUCTION

I " I LINING

I MONITORING

AND( OPTIMISE

(b) The s ingle-sh ie ld mach ine thrusted directly offthe segmental lining, whereas the double-shield machine is equipped with grippers, allowing the TBM to thrust off the rock. Only in very poor ground conditions, which prevent adequate gripping, will it be necessary to thrust off the segments. This will allow for better segment alignment and reduce potential damage to

segments by excessive or eccentric thrusting.

(c) The double-shield machine allows concurrent boring and erection of segments. The faster progress could compensate in part for the disadvan- tages of a longer machine and conse- quent longer unsupported rock span.

(d) The amountofovercutrequired in negotiating a curve is reduced.

Table 4. Parameters of the TBM.

Normal boring diameter 5380 mm

Overcutting facility: • 1st setting • 2nd setting

5400 mm 5420 mm

Cutterhead drive power: • 6 x 250 kW power packs • Revolutions • No. of cutter discs for hard rock

(Cutterhead also equipped with drag-bits and picks for softer ground)

• Maximum thrust per cutter

1500 kW 0 to 8.6 rev/min.

42 No.

220 kN

Telescopic thrust rams: • Thrust (max.) • Stroke length • No. of rams

30,000 kN 1530 mm

12 No.

Gripper system, rear shield: • Clamping force 37,500 kN

Segmental lining thrust rams: ° Thrust (max.) • Stroke length • No. of rams

25,000 kN 1900 mm

20 No.

The Engineer considered that the length of the TBM being equivalent to twice the tunnel diameter was a prac- tical limit. The TBM has been designed to bore through the varied ground conditions anticipated. The main parameters of the machine are shown in Table 4. [ ]

7 . C o n c l u s i o n

The excavation and lining of the Ash Tunnel commenced on 15 May 1992, and by 11 August 1992, 570 m of tunnel had been completed. This was a learning period; at present, an advance rate of more than 20 m per day is being achieved consistently. The high- est advance rate during this period was 31 m per day- -a rate that com- pares with the target rate following the learning curve of 27 m per day.

There have been a number of teeth- ing problems with the lining system; these are progressively being solved and the system optimized. The geological conditions at present are fair, and the system has yet to be tested in the more adverse conditions.

8. Acknowledgments The authors wish to thank the fol-

lowing organizations for permission to publish this paper:

Trans-Caledon Tunnel Authority Highlands Deliver Tunnel Consult-

ants (Ninham Shand Inc., Van Niekerk Kleyn & Edwards, Keeve Steyn Inc., Steffen Robertson & Kirsten)

HMC Tunnelling Venture (Hochtief, Marti, Concor).


Documents

Design and construction of a segmental lining for a machine-bored tunnel: Delivery tunnel North, Lesotho highlands water project