On engineering-geological studies concerning the selection of the course of the water tunnel Hausjärvi-Helsinki

Engineering Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

ON E N G I N E E R I N G - G E O L O G I C A L STUDIES C O N C E R N I N G T H E SELECTION OF T H E COURSE OF T H E W A T E R T U N N E L HAUSJ,~RVI-HELSINKI HEIKKI NIINI

Water Division, National Board of Public Roads and Waterways, Helsinki (Finland)

(Received September 12, 1966)

SUMMARY Selection of the most favourable course of the tunnel is carried out on the basis of engineering-geological studies. These aim to predict, in the alternative tunnel courses, the influence of the rock conditions on tunnel blasting, the amount of lining, the site and depth of the adits to the tunnel and the extent of the definitive field studies still to be done before the construction of the tunnel. The economic benefit from the studies can be estimated as a considerable percentage of the total cost of the tunnel driving.

INTRODUCTION The National Board of Public Roads and Waterways of Finland is planning the conveyance of water (more than 10 ma/sec) from Lake P~iij~inne in inner Finland to the Helsinki water-deficit area (see KAJOSAARI, 1965). South from the river Puujoki in Hausj~irvi this will take place along a bedrock tunnel with a length of 60 km and an area of about 10 m 2. The tunnel will be constructed through a migmatitic part of the Svecofennian schist belt in the hard Precambrian bedrock. The geological studies for the selection of the course of the tunnel are carried out in three stages: (1) preliminary map and aerophoto studies by which a relatively small number of tentatively advantageous tunnel courses are chosen for closer examination; (2) field studies, mainly detailed geological mapping along the alternative tunnel courses and seismic soundings at the most critical places; (3) economic comparison on the geological qualifications of the tunnel alternatives.

Eng. Geol., 2(1) (1967) 39-45

40

HEIKKI NIINI

PRELIMINARY MAP STUDIES

It is reasonable to take into consideration all the possible tunnel courses that are less than ca.10~ longer than the straight distance between the terminal or otherwise fixed points of the tunnel. Thus, the breadth of the area under preliminary study between two points must be, at its centre, more than half of the distance between these two points. The most important basic material comprises aerial stereoscopic photographs or maps in the scale 1: 60.000, 1:20.000 and 1: 10.000, geological maps of the bedrock and of the superficial deposits in the scale 1 : 100.000, and topographical maps in the scale 1 : 100.000, l : 50.000, 1 : 20.000 and 1: 10.000. With the aid of this material connected with quite superficial field controls, a general geological picture about the area is formed, especially its fracture tectonics. This takes place roughly according to the principles described, in connection with studies of other areas, in more detail by, e.g., NIINI (1964, pp.36-48) and KORPELA (1966). On this basis it is then attempted to find a small but unlimited number of tentatively profitable tunnel courses that avoid the weakness zones of the bedrock as much as possible, and are situated favourably with respect to the different rocktype areas and the directions of anisotropy (see Fig.l). The alternative tunnel courses, thus chosen, form the study lines of the field studies but they may, in places, still be broad zones.

FIELD STUDIES

Before the alternative tunnel courses can be compared with each other, knowledge about their geological conditions must be considerably increased. This is done by field studies, mainly at the most critical places of the tunnel alternatives, such as deep valleys, esker ridges, tectonic fracture lines, and weathered zones. These studies aim at the following four considerations: (1) The rocks. The bedrock outcrops along and close to the alternative tunnel courses are mapped in detail. Especially as regards the blasting of the tunnel, attention is paid to the structure, mineral composition and texture, disintegration, and anisotropy (directions and development of schistosity, stratification, jointing, etc.) of the different types of rock. (2) The elevation of the bedrock surface. Between the outcrops, the bedrock surface is determined mainly by refraction-seismic sounding with twelve-channel explosion seismographs. Due to the positive results of the investigations of JOHANSSON (1964), GARDEMEISTER (1964) and LEHTINEN (1965), the one-channel hammer seismograph is also used when controlling the thickness of certain shallow formations above the bedrock that are supposed to consist solely of a thin layer of till. On the sites of clayey flat layers above the bedrock, electrical resistivity Eng. Geol.,

2(1) (1967) 39-45

SELECTIONOF THE COURSEOF A WATER TUNNEL

41

sounding may also be used. A little overburden drilling is carried out only when controlling the results of the different soundings. (3) Fracture zones. The breadth and the average rock stability of all the assumed breaks, fracture zones and other sites of weakness in the bedrock along the preliminary tunnel courses are determined approximately from the seismic velocities in bedrock and from the bedrock topography. Observing the experiences in seismic soundings of TAANILA (1963, 1965) and KAURANNE (1965), the multichannel seismographs are only used here because of the necessity to obtain the true seismic velocities in bedrock and accurate profiles of bedrock topography, which are very difficult to obtain by a one-channel hammer seismograph. Also, for control, diamond drilling is scantily used at the deepest and most critical places.

'Ylentoia ~ ~

L

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'

'

l

~

) lN

[

. . . . . ~m

X

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F~granite ~felsicschists

~maficschists

I ~ I t h i c k earth layers ~

tunne(atternative

Fig. 1. Generalized bedrock and three preliminary tunnel alternatives near Lake Ridasj~rvi east of the City of Hyvinkaa. The straight-lined oblong areas (depressions) covered by thick loose earth layers represent fractures and weakness zones of the bedrock.

(4) Superficial deposits and ground water. In deep valleys, it is reasonable, for the sake of safety, also to determine the quality of the loose deposits andthe ground-water permeability above the b e a k zones of bedrock. This is because, (in case of a future collapse of the tunnel) besides ground water, the loose earth material may penetrate into the tunnel, as has sometimes happened (see, e.g., MORFELDT, 1965, p.510). Then, in different cases, the extent of the damage can be very different, depending on the porosity, size of grains, consolidating, and other properties of the earth and on the water pressure. The knowledge needed in this stage about the superficial deposits and ground water is obtained by means of various kinds of overburden drilling and seismic sounding, most often in connection with the previously-described studies. Eng. Geol., 2(1) (1967) 39~,5

42

HEIKKI NIINI

ECONOMIC COMPARISON BETWEEN THE GEOLOGICAL QUALIFICATIONS OF THE DIFFERENT TUNNEL COURSES

Such factors as, e.g., the exact situation of the terminal points, the possible future branches and pipe joints of the tunnel with the relevant pumping costs, the ownership of land in.tile different, tunnel courses, the t!me schedule of the future construction of the tunnel;the ~levelopment of the local road network, the future use of the blasted rock, etc., have been shown to be either very insignificant to the selection of the tunnel course or too difficult to be evaluated at all. Thus, for the comparison between the tunnel alternatives, it is sufficient to determine the following four engineering-geological considerations and their financial significance. The influence of the different rocks on the tunnel blasting For this technical purpose the rocks must first be classified also according to their physical properties and directions of anisotropy measured in relation to the directions of the tunnel alternatives. These factors have been shown to be very important in previous similar underground works, e.g., by WAHLSTROM 0948) and STINI (1950), and in Finland by VAH.~SARJA (1964) and LAPPALAINEN(1965). The relative resistance to blasting and, hence, the influence on the blasting cost of the different rock groups thus obtained are estimated as follows: Three factors of each rock group are considered on the basis of the experiences obtained in previous blasting works in similar rocks. The factors are: (a) the drillability and the required drill-hole density and length (per round), i.e., the drilling cost; (b) the consumption (per round) and required type of explosives; (c) the loosening ability of the round. Concerning common rocks, each of these factors may alone cause a ten per cent deviation at the most in the total blasting cost. Thus, e.g., the difference in the blasting cost between a massive felsic intrusive rock and a tough mafic schist orientated roughly parallel to the course of the tunnel (and where the round loosens incompletely and more drill holes and more expensive explosives are needed) has been estimated to more than 20~o of the average blasting cost. Because of the great total extent of the study material of all the tunnel alternatives, the influence of the rock groups on the cost of the tunnel blasting must only be tentatively estimated by means of such simplified calculations and no significant blasting experiments or control tests can be afforded. The necessary lining works at the sites of weakness of the bedrock As examples of similar underground works elsewhere show (see, e.g., ALLONEN, 1965; BREKKEand SELMER-OLSEN, 1965, p.3; and PRIHA, 1965, p.344) it is necessary Eng. Geol., 2(1) (1967) 39-45

43

SELECTION OF THE COURSE OF A WATER TUNNEL

to line such tunnels for a length of ca. 1 0 ~ on an average. Because a more detailed prediction of the need for lining is impossible, the required lining works are only divided into two groups, heavy and light. According to the examples of the water-tunnel works of the city of Helsinki (see SARASTE, 1966), the light lining mainly means single concrete-spraying and the heavy one consists of thick concreting with a steel mesh and with or without injection or roof bolting. The amount of the lining needed is chiefly calculated from the true seismic velocities in bedrock. The prediction of the lining works also includes the preliminary determination of the direction of the tunnel through the very weakness zones. As to the present tunnel plan, the calculated lining cost has remained at 5 - 6 ~ of the total blasting cost in the most favourable tunnel courses but has risen to some 15~o in the less favourable preliminary alternatives.

The sites of the adit shafts or inclined tunnels For deep water-conveyance tunnels of this kind, it has been theoretically calculated that differences of some ten metres in the depth of the adit shafts or tunnels have much more influence on the total cost of the tunnel driving than differences of the order of 1 km in the distance between two adits. Thus, it is more reasonable to search for the most favourable sites of the adits on the topographic-geological grounds in order to get them as short and economic as possible than to stick to any exact adit distance fixed beforehand. Because the suitable sites of the adits usually number more than is needed, their most convenient combination must be experimentally determined. Owing to the depth of the pressure line, the Hausj~irvi-Helsinki tunnel will, in places, be built so deep that the cost of the adits has been calculated to more than 20~o of the total cost of the tunnel section in question. Although, therefore, the sites and depths of the adits have been outlined rather minutely already for the comparison between the tunnel alternatives, they need not be considered definitive as regards the more detailed future planning.

Estimate of the definitive field studies for each tunnel alternative Concerning such alternative tunnel courses as are poor in bedrock outcrops and to a great extent consist of thick earth layers, especially eskers, the cost of the mere geological study still to be done (chiefly diamond drilling) forms a considerable cost factor compared with the costs of the tunnel driving proper. CONCLUSION

Concerning the selection of the course of the Hausj~irvi-Helsinki water tunnel, an example is presented in Table I. Its purpose is to show the numerical performance of the comparison between the different engineering-geological factors,

Eng. Geol., 2(1) (1967) 39-45

1,800 1,000 7,560

800 850 900

8,200 500

1,200

20

1,9 200; 130; 100

1,500

1,200

810 470 1,280

-

750

400 1,500

4,760

700

a The costs have been estimated in Finnish marks (mk).

Total

Additional field studies Seismic sounding Diamond drilling Total

Adits (with all costs depending on the depth o f the adits) Three adits, total length Estimated risk of downward blasting of the tunnel Total

Light Heavy Total

Lining

Massive granite Leptite, tunnel at right angles to stratification Amphibolite, tunnel at right angles to schistosity Migmatitic schists, tunnel at oblique angles to schistosity Amphibolite, tunnel parallel to schistosity Total

Blasting

1,100

610 230 840

2,700 8,220

1,800

-

860

2,860

m

6,500 550

2,600

8,590 =: 119 ~

15 100 115

25 1,825

1,800

325 705 1,030

5,620

850

1,440

-

3,330

1,000 m k

Middle

Western in

Unit price

Factor~Tunnel alternative (ink~m)

SECTION

A SIMPLIFIED E C O N O M I C C O M P A R I S O N B E T W E E N T H R E E A L T E R N A T I V E C O U R S E S OF T H E T U N N E L

TABLE I

Eastern

1,600

1,000

630 130 760

7,620

-

1,100

650

5,870

m

3,200 200

8,650 = 120 ~

10 75 85

50 1,370

1,320

245 345 590

2,430 6,605

1,530

-

645

2,000

1,000 mk

KALLIO-YLENTOLA1

250 195 445

5,480

7,180 ~ 100 of

5 20 25

30 1,230

1,200

i

88O

490

4,110

1,000 mk

0.3

17

77

%

z z

[]

4x 4~

SELECTION OF THE COURSE OF A WATER TUNNEL

45

b u t at the same time it should show, although more generally, the great e c o n o m i c i m p o r t a n c e of the geological studies in c o n n e c t i o n with the selection of the courses of long t u n n e l s in the areas of P r e c a m b r i a n bedrock.

ACKNOWLEDGEMENTS The i n s p i r a t i o n a n d the advice given to the a u t h o r by Prof. A. Mikkola, Institute of Technology, Otaniemi, a n d Lic. Tech. E. Kajosaari, N a t i o n a l Board of Public R o a d s a n d Waterways, Helsinki, are gratefully acknowledged. REFERENCES

ALLONEN,T., 1965. Silvola-Vanhakaupunki raakavesitunnelin louhinta (The blasting of the water tunnel Silvola-Vanhakaupunki). Paineilmauutiset, 4: 2-5. BREKKE,T. L. and SELMER-OLSEN,R., 1965. Stability problems in underground constructions caused by montmorillonite-carryingjoints and faults. Eng. Geol., l(1): 3-19. GARDEMEISTER,R., 1964. Saimaan kanavan rakennustyOh6n liittyvistii tutkimuksista (On studies concerning the construction of the Saimaa canal). Pioneeriupseeri, 2(9): 12-15. JOHANSSON, S., 1964. On the hammer-seismograph model FS-2 and its use in Finland. Geoexploration, 2: 185-194. KAJOSAARI,E., 1965. Etelii-Suomen vedenhankinta. Summary: Water supply of southern Finland. Rakennustekniikka, 7-8: 514-519. KAURANNE,L. K., 1965. Vasaraseismiset luotaukset rakennusteknillisiss~i pohjatutkimuksissa. Summary: Hammer seismic soundings in building geological exploration. Rakennustekniikka, 2: 114-116. KORPELA,K., 1966. Rakennusgeologinen tutkimus kaavoittajan apuna (Engineering-geological study helping the planner). Suomen Kunnallislehti, 4: 252-254. LAPPALAINEN,V., 1965. Rautatiesuunnittelu rakennusgeologisena kysymyksen~i (Planning of railway tunnels as an engineering-geological problem). In: Rautatieteknikot ja rautatietekniikka 1956-1965. Rautatien Teknillisten Yhdistysten Liitto r.y., Helsinki, pp. 165-171.

LEHTINEN,A., 1965. Om seismiska hastigheter erhiillna med hammarseismografen MD-1 vid projektering av j~irnviigar. Geologi (Helsinki), 17(9-10): 136. MORFELDT,C. -O., 1965. Projektering av bergtunnlar. Viig- och Vattenbyggaren, 12: 509-512. NIINI, H., 1964. Bedrock and its influence on the topography in the Lokka-Porttipahta reservoir district, Finnish Lapland. Fennia, 90(1): 1-54. PRIHA, S., 1965. Kalliotunnelit vedensiirrossa. Summary: Conducting water through rock tunnels. Rakennustekniikka, 5: 340-344. SARASTE,A., 1966. Helsingin kaupungin kunnallisteknillisist~i kalliorakennust6ist/i (On the rock construction works of the City of Helsinki). Geologi (Helsinki), 18(3-4): 33-34. STINI, J., 1950. Tunnelbaugeologie. Die geologischen Grundlagen des Stollen und Tunnelbaues. Springer, Wien, 366 pp.

TAANILA,P., 1963. Vasaraseismisten laitteiden soveltuvuudesta maa- ja kallioperiitutkimuksiin. Summary: On the applicability of hammer seismic apparatus for ground investigations. RakennusinsinO6ri, 11 : 446-452. TAANILA,P., 1965. Investigations of construction sites for hydro-electric plants by the seismic refraction method in Finland. Geologiliiton Julkaisuja, 4:18-27. VXH)~SARJA, P., 1964. Tarkkuuslouhinnasta (On precision blasting). Geologi (Helsinki), 16(9) 135-138. WAHLSTROM,E. E., 1948. Application of geology to tunneling problems. Trans. Am. Soe. Civil Engrs., 113(2357): 1310-1321.

Eng. Geol., 2(1) (1967) 39-45