Hard rock tunnel boring: Prognosis and costs

TUNNELS

8c D E E P SPACE

Hard Rock Tunnel Boring: Prognosis and Costs Arne Lislerud Abstract--This paper is drawn from [our project reports, dealing with: (1) hard rock tunnel boring; (2) drill-and-blast tunnelling-prognosis; (3) drill-and-blast tunnelling--costs; and (4) a drillability and drilling rate index catalogue. The project reports are based on jobsite follow-up work o[ both bored and blasted tunnels. This paper summarizes some developments in T B M and cutter design, [actors influencing the boring process and [actors that aI[ect tunnelling costs, e.g. the increased productivity possible with TBMs.

he first tunnel boring in Scandinavia was done in medium to soft ground conditions, i.e. micaschists and limestones. Today, however, most machines bore in hard rock, i.e. granite and gneiss. Table 1 provides a review of tunnel boring machine (TBM) drives in Scandinavia. Drives accomplished by different manufacturers are summarized in Table 2. Table 3 compares drill-andblast to tunnel boring meterage in Norway. Although most tunnelling in Norway is carried out in connection with the construction of new hydro power plants, road and highway tunnel meterage is increasing.

T

Geology--TBM Performance In contrast to drill-and-blast tunnelling in hard rock, tunnelling performance of TBMs is highly dependent on geological conditions. T h e significance of geological and machine parameters in relation to TBM performance is rated as shown in Table 4. The interaction between rock mass properties and machine parameters is described in project report 1-83 of the Norwegian Institute of Technology (1983).

Rock Mass

Jointing

Most rock is jointed or fractured to some degree. Rock mass j o i n t i n g must be m a p p e d and put into use through a workable system developed by the engineering geologist.

Present address: Arne Lislerud, Norwegian Institute of Technology, 7034 TrondheimNTH, Norway.

T a b l e 1.

R e v i e w o f T B M drives in Scandinavia.

Country

Completed

T B M s on jobsite

Faroe Islands Norway Sweden

1 34* 3

1 5 1

TOTAL

38

7

*Includes two midi full-facers. For this study, j o i n t i n g has been classified, with regard to penetration, in the following four groups or classes based on joint openness, roughness, and continuity. (1) S y s t e m a t i c a l l y f r a c t u r e d rock mass: • Parallel-oriented joints (rated

sP)

(2) (3) (4)

Joint

• Parallel-oriented fissures (rated St) • Foliation planes or bedding planes (partings) (rated St). Non-fractured rock (rated St O). Marked single joints (rated ESP). Crushed zones • G r o u n d support work probably will be needed. rating combined with joint

T a b l e 3.

frequency and orientation to tunnel axis provide the basis for calculating the j o i n t factor k~. T h e joint factor k~ for fissures and foliation planes is shown in Fig. 1. Penetration rates are more or less proportional to the joint factor k,.

T a b l e 2. T B M drives in S c a n d i n a v i a by di[[erent manu[acturers.

Atlas Copco (and Jarva) Bouygues Demag Robbins Wirth

8* 2 2 26 7

*Includes two midi full-facers.

C o m p a r i s o n o] drill-and-blast to t u n n e l b o r i n g meterage in N o r w a y .

Cross-section

1983

1984

1985

2 - 10 m 2 10 - 30 m 2 3 0 - 60 m 2 6 0 - 100 m 2 > 100 m 2

32,818 51,603 25,958 2019 1455

17,990 35,740 51,270 1210 950

21,871 34,997 53,643 2108 2990

111,853

107,160

115,509

26,708

15,390

14,136

TOTAL m bored

Tunnelling mid I 'nderground Space Technology, Voi. 3, No. 1, pp. 9-17, 1988. P ] i n w d in ( ;w ar Bill;fin.

R6sum6--Cet article est bask sur les rapports de quatre pro jets traitant les sujets suivants: (1) percement de tunnel dans les roches dures; (2) pronostic des mkthodes tunnelibres par percement et par explosion; (3) co~ts des mbthodes tunnelibres par percement et par explosion; et (4) catalogue d'indexes de possibilitb de percement et de taux d'avancbe de percement. Les rapports de projet sont basks sur le suivi des travaux sur site pour gtla ]ois des tunnels percbs et explosks. Ce rapport rbsume quelques dbveloppements e]Iectuks sur les tunneliers, sur la conception de dkcoupe, sur les [acteurs in[luenqant le procbdb de perloration, et sur les [acteurs qui a[Iectent les co~ts de tunnelage (comme un accroissement possible de la productivitb des tunneliers).

0886-7798/8S $3,00 + .00 P e l g a m o n Plvss p h

9

Table 4.

Rated rock mass and machine factors influencing T B M performance.

Rock mass factors

Machine factors

Rock mass jointing (k,) - - Type and continuity - - Frequency - - Orientation Rock porosity Rock drillability (DRI)

Thrust per cutter (M) Cutter edge bluntness (b~) Cutter spacing (,4) Cutter diameter (d) Torque capacity and RPM The machine's capacity for handling large chips or blocks General solidity against blows and vibrations Cutterhead curvature and diameter (D) Backup equipment

Stress in rock Rock hardness/ abrasiveness (CLI)

the toughness (or lack ot IJriltlem's~)ot ~ertain rock tylWS.

Machine Factors-T B M Performance TBM performance is highly dependent on machine design. Machine factors that influence boring performance are listed in Table 4.

Net Penetration The basic penetration in systematically jointed rock is shown in Fig. 3. Net penetration is found by using the following formula: i = it," k~ (mm/rev.) I = i" RPM • 60/1000 (m/h). The prognosis model for an earlier project report (Norwegian Institute of Technology 1983), did not combine jointed and non-fractured rock in one diagram (see Fig. 1). As a result, the revised prognosis model is based on a different concept, in which nonfractured rock is easily included. The influence of cutter spacing and cutter bluntness also is included. The 1987 project report model is based on normalized penetration tests. A typical test result in non-fractured granite for a 15.5-in. cutter is shown in Fig. 4. The penetration test curve is normalized to a power function: i = (M/MI) ~' (mm/rev.) M = thrust per cutter (kN) M~ = "critical thrust" (the thrust needed to bore 1.0 [mm/rev.]) b = slope of curve (b is an expression for the chipping frequency).

Figure 1. Correction factor k~ as a f u n c t i o n of fissure class and angle between tunnel axis and planes of weakness.

Rock Drillability The ease with which rock can be bored is measured by an indirect method. The Drilling Rate Index (DRI) is a combination of the rock brittleness value ($20) and Siever's miniature drill test (SJ). The test methods are described in the Drillability and Drilling Rate Index Catalogue (Norwegian Institute of Technology 1981). The SJ value expresses rock surface hardness. A useful correlation between SJ and a calculated rock Vickers Hardness from mineral content has been found. T h e SJ value is very useful for determining the degree of rock weathering.

The Sz0 value includes the effect of rock brittleness and, therefore, grain size and grain boundary strength. However, the effect of rock porosity is not included in the test. Recent follow-up work carried out in vesicular basalt on the Faroe Islands shows that porosity in the range 3-t2% has a considerable effect on penetration rates and rock blastability. The bedrock in Norway normally has little or no porosity. The relationship between the compressive strength and the DRI is shown in Fig. 2 for 65 parallel tests grouped according to rock type. The plot shows that the compressive strength used for rating drillability seems to underestimate

10 TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY

Penetration parameters b and M~ are dependent variables. A plot of b as a function of M~ is shown in Fig. 5. Likewise, a raw plot of M~ vs joint factor k~ is shown in Fig. 6. The work of finding the relationships between b , A, d, DRI and k~ to M~ and b in mathematical terms will be completed in 1987. It should be noted that cutter bluntness has its greatest effect on the slope b, i.e. chipping frequency.

Medium-Pressure Water Jets Recent boring tests with mediumpressure water jets (300-350 bar) in nonfractured granite showed little or no effect on boring performance. On the other hand, all of the water introduced by the water jets into the muck resulted in operating problems with the backup equipment.

Torque Demand Required torque to rotate the cutterhead depends on: • Cutterhead diameter. • Number and position of cutters. • Thrust per cutter. • Cutter coefficient k.

Volume 3, Number 1, 1988

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T h e cutter r i n g life p r o g n o s i s m o d e l was developed by b a c k m a p p i n g tunnels and correlating geological c o n d i t i o n s to o b t a i n cutter r i n g life. A F O R T R A N p r o g r a m u s i n g data from the cutter shift reports determines the average cutter r i n g life a l o n g the tunnel. T h e expression used is:

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V o l u m e 3, N u m b e r 1, 1988

• Degree of j o i n t i n g . • Skidding. • M u c k removed from invert. In p r i n c i p l e , the cutter coefficient is the r o l l i n g resistance of the cutter. However, the b u g g y wheel effect is also evident, i.e. a larger disc results in a s m o o t h e r

(m)

Because cutter r i n g life Lh is an average for the cutterhead, the effect of the ratio of center and g a u g e cutters to face cutters has been included. T h i s ratio is the basis for the T B M d i a m e t e r correction factor k~,. Cutter c o n s u m p t i o n in m s is determ i n e d by six factors: ( 1) A m o u n t of wearable steel on discs. (2) T i m e - d e p e n d e n t rock p o w d e r abrasion on steel discs. (3) R o c k hardness, w h i c h indirectly determines where edge wear will take place, e.g. at the tip, sides or both. (4) R o c k mass j o i n t i n g and rock b o u n d a r y zones. (5) Cutterhead curvature and diameter. (6) Penetration rate ( m / h ) . T h e Cutter Life I n d e x (CLI) includes both rock hardness and rock abrasiveness on disc steel (see Fig. 8). T h e prognosis for average cutter r i n g life in h o u r s is given by the f o l l o w i n g e q u a t i o n (see Fig. 9): Lh = kd • C L I • k," kQ" kRPm Mineral c o m p o s i t i o n m u s t be taken into c o n s i d e r a t i o n for rocks h a v i n g Q < 14%. I m p r o v e m e n t s are still needed o n the prognosis m o d e l in this area. Recently there has been a renewed

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

11

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Thrust per cutter (kN/cl

Figure 4. Interpretation o] a T B M penetration test.

12

TUNNELLING AND UNDERGROUND SPACE TECHNOLOf;Y

V o l u m e 3, N u m b e r 1, 1988

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Volume 3, N u m b e r l, 1988

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

13

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curves).

(1) Major machine breakdown, e.g. main bearing failure. (m) Ground support work. Operations a - d above are (anne(ted with the boring process itself, whereas e-k are related to the remaining tunnelling operations. Operation 1 and m are not included in the machine utilization percentage, but rather are included as additional time on the total project time-plan. A unit time for the listed operations can be determined. These unit times (h/kin) seem to vary little with regard to TBM diameter; rather, they vary with crew motivation and quality of jobsite administration. Table 5 provides an example of how to predict normal machine utilization through the use of unit operation time, given the following assumptions about the tunnelling methods to be used: Diameter of TBM 4.5 m Penetration rate 2.0 m / h Cutter life 70 m ~ Boring stroke 1.5 m Regrip time 5 min Cutter change and inspection time 40 min./cutter Backup equipment 2 track. Trackless transport of muck is possible and desirable for large-diameter TBMs, i.e. diameter > 7.3m. Martin and Wallis (1985) have described muck transport on the Floyfjell twin-bore highway tunnels.

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Ca Icerous

shale

schist

Gr"een Phyllite

Mica

schist

Mica

gneiss

Boring

Gr"ani te g n e i s s gneiss

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rqi te , uuart2 dtor'i tel tKondhje 10

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Figure 8. Cutter LiJe Index (CLI) Jar various rock types. interest in i m p r o v i n g ring life by using carbide disc cutters. Unlike the case with steel disc cutters, carbide cutter f a i l u r e f r e q u e n c i e s are i n f l u e n c e d immensely by utilized thrust. T o help solve this problem, research and testing of new steel alloys in discs are increasing. For discs to be used successfully in hard and abrasive rock, the steel disc must be capable of withstanding high cutter loads without c h i p p i n g , while at the same time keeping the edge width as narrow as possible to enhance penetration. Machine Weekly

Utilization-Advance

Gross advance rate, expressed in meters per week as an average for a

longer period, depends on the net advance rate and the number of boring hours during that period. Machine utilization is net boring time expressed in percent of total tunnelling time. Total tunnelling time includes: (a) Boring. (b) Regrip, including collaring (28 min. each time). (c) Inspection and change of cutters. (d) Service and maintenance of TBM and backup equipment. (e) Waiting for muck cars. (f) Ventilation. (g) Installation of track (normally no waiting involved). (h) Maintenance of track. (i) Electrical installations. (j) Travelling time, change of shift.

14 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

vs Drill and Blast

In c o m p a r i n g boring with drill-andblast tunnelling in hard rock, each has some specific advantages and disadvantages. T o fully utilize the advantages of each tunnelling method in construction planning, it is important that the different methods be taken into account at an early stage (see the Holandsfjord power plant example, below). Comparative advantages and disadvantages of the two methods are listed in Table 6.

Tunnelling

Costs

It is difficult to write a short, general and interesting comparison of tunnelling costs. Each project is unique, with specific considerations that must be taken into account. T h e Holandsfjord hydro power plant project illustrates the different construction, ownership and operating costs that must be optimized in p l a n n i n g a major hydro project. 2

Holandsfjord Hydro Power Plant Holandsfjord is the largest project of the Svartisen Hydro Power Scheme, with an installation of 2 x 300 MW and an average production of 2077 G W h / y r . T h e 7040-m-long headrace tunnel has

Volume 3, Number 1, 1988

Figure 9. Cutter ring li]e as a ]unction o] the Cutter Li]e Index (CLI) and correction ]actors k, and kQ

Volume 3, Number 1, 1988

TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY 15

Table 5. Exarnple of determining machine utilization for a project through the use of unit operation times. Operation Boring

Unit time (h/kin) 1000 + 2.0 -- 500 1000 (5 + 60) - 44 1.5

Regrip Cutter change and inspection Downtime TBM Downtime backup equipment Miscellaneous downtime SUM Additional downtime - - Main bearing failure - - Ground support in: Granite and gneiss Continuous spelling Crushed zones

1000

"'(40~'~

4.52 _- 151 \ 60 / 4 x 70 95 115 105 1010

Machine

utilization (%) 49.5 4.4 15.0 9.4 11.4 10.4 100.1

65 5-20 30-80 per zone

Table 6. Comparative advantages and disadvantages of drill-and-blast vs tunnel boring methods. Drill and Blast

T u n n e l Boring

Advantages:

Advantages:

• Versatile equipment, can be easily allocated to dissimilar jobs. • Relatively low capital costs. • Little or no machine risk. • Easier to tunnel through difficult crushed zones.

• Greatly reduced cross-sections for hydro power, sewer and water tunnels. • Rapid excavation; shorter construction time results in lower interest for the client. • Low ground support costs on the average. • Short (< 700 m) and small (< 1.5 m) nearly horizontal tunnels can be easily bored with raise boring equipment,

Disadvantages:

Disadvantages."

• Uneven tunnel contour, with loss of head in unlined hydro power, water and sewer tunnels. • Higher average ground support costs, especially for poor jobsite client administration. • Medium to low advance rates.

• High capital costs (although the importance of capital costs can be reduced by leasing). • Long delivery time for new machines if suitable refurbished machines are not available. • Heavy equipment, time- and costconsuming startup.

• Problems with ventilation of small and long tunnels.

• High geological risk with regard to advance rates, cutter consumption and costs. • Major machine downtime risk, such as main bearing failure. • Although ground support costs are low on the average, tunnelling costs can be exceedingly high and the going difficult in bad crushed zones if the contractor is not well-prepared or motivated to handle these situations.

The latter two disadvantages often result in a need for extra edits and jobsites.

16

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

V o l u m e 3, N u m b e r 1, 1988

a drill-and-blast, 87-m 2 cross-section. The headrace tunnel is to be driven from two adits, one just above the power station, the other at intake Storglomvatn (see Fig. 10). Adit Storglomvam is necessary to permit the startup work on SVARTiSEN GLACIER

•

m.o.h,

~

Table 7. Comparative costs o[ drill-and-blast vs tunnel boring [or the Holandsljord project. Optimum cross-section Drill & Blast Boring Construction time Drill & Blast Boring

5.5 years (both agg•) 3•2 years (agg• 1 ) 4.9 years (agg. 2)

~

STOR- < 0 GLOM" VATN

Construction costs

• t*OO / "

:

st.

<

.o

F=87 m = o r

2x6.1m

¢

TB

5820

43590O

20~

(3(3

2X" . . . . .

Adit Holandsfjord Jobsite, roads, adit Tunnelling towards Storglomvatn Ground support Tunnel to pressure shaft, including ground support

Drill and blest 87 m 2 (mil. Nok)

TBM 2.~6.1 m (mill. Nok)

17.5

20.8

66.2 23.2 29.7

93.4

Gate installation

STORGLOM" VATN ~ "

~ - -

87 m 2 2 • E~ 6.1 m

7.5 29.7 5.0

g = 87m~

Adit Storglomvatn Jobsite, roads, adit, gate installations

Gate house

ADIT

25.3

HOLANDSFJORD

Sum, jobsite costs General costs I

"HOLAND~

169.9 59.9

156.4 57.9

Interest during construction

41.3

23.9

period Capitalized loss of head costs

55.2

38.0

318.3

276.2

Figure 10. Plan o[ the Holandsljord hydro power plant.

TOTAL COSTS (in million Nok)

the gate to begin within a reasonable time. This adit lies in a very harsh climatic zone and difficult roadb u i l d i n g terrain. T h e geologic conditions, which include a rock mass of micaschist and micagneiss with high horizontal tectonic stresses, are unfavourable for drill-andblast tunnelling. Therefore, a tunnel boring alternative was introduced to reduce construction time and ground support costs. The alternative called for dividing the cross-section in two, and boring one tunnel at a time. T h e site geology favors boring, which will permit high weekly advance rates. Gate installation will begin after first bore is completed. T u n n e l 2 will be bored while aggregate 1 is being installed. Construction of the Holandsfjord headrace tunnel is scheduled to begin in 1988. The headrace tunnel will be bored using only one 8.5-m TBM due to

changes in income and operating conditions of the power plant• Table 7 provides a projected cost analysis of the Holandsfjord project, showing comparative costs for drilland-blast vs tunnel boring methods. []

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Acknowledgments We hereby thank the major Norwegian contractors, the State Power Board, the Road Department, SEV and TBM manufacturers represented in Norway for their help and financial support over a n u m b e r of years. None of these project reports could have been undertaken without their wholehearted assistance.

References Martin, D. and Wallis, S. 1985. Cut and thrust at Bergen. Tunnels dr Tunnelling 17: 10, 14-18.

Norwegian Institute of Technology. 1981. Drillability, drilling rate index catalogue. PR 8-79. Trondheim, Norway: Norwegian Institute of Technology. Norwegian Institute of Technology. 1983. Hard rock tunnel boring. PR 1-83. Trondheim, Norway: Norwegian Institute of Technology. Norwegian Institute of Technology. 1984a. Drill and blast tunnelling--prognosis.PR 5-83. Trondheim, Norway: Norwegian Institute of Technology. Norwegian Institute of Technology. 1984b. Drill and blast tunnelling--costs.PR 6-83. Trondheim, Norway: Norwegian Institute of Technology.

Notes An updated catalogue, to be published in 1987, will include data available for personal computers. 2 For general meteragecalculations refer to Norwegian Institute of Technology project reports 1-83 (boring), 5-83 (drill and blast) and 6-83 (drill and blast).

TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY 17