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10

PLANNING & DEVELOPMENT _ _ _

Geotechnical Maps of Helsinki and Their Use in Tunnel Planning

U. V. Anttikoski P. J. Raudasmaa Helsinki Geotechnical Department

he City of Helsinki has about 500,000 inhabitants and covers an area of 181 sq. km. The geo­

logical structure of the city can be di­ vided into bedrock and overlying soil deposits. The bedrock is Precambrian and crystalline, whereas the overlying deposits date from the Pleistocene epoch, having been laid down mainly

U. V . Anttikoski is Head of the Geotech­ nical Department of the City of Helsinki. P.]. Raudasmaa is an engineering geologist with the department.

during and after the Ice Age. Helsinki, therefore, manifests very old and very young geological features, but nothing between.

The ground surface level in Helsinki

is + 62 m at its highest point, and the face of the bedrock has been observed to lie at the - 70 m level at its deepest point. The surface level varies substan­ tially from place to place, with rock outcrops common. Although the bed ­ rock contains numerous cracks and zones of weakness it is well-suited for tunnel construction, as it consists mainly

of granite, gneiss, and amphibolite. The present-day rate of land uplift in Hel­ sinki is 3 mm/year.

Clay covers approximately 35% of the city area. The clay is soft with a water content usually of 70-120% and a shearing strength of about 7-20 kPa. Soft clay layers frequently occur in small pockets.

Special problems for geotechnical design are caused by multi-story build­ ings constructed on timber piles; the total number of such buildings is about 100. Also, many rock tunnels and deep

f-----·0"'""5-'k-"'m"------1

- ROCK OUTCROP

, CLAY UNDERLYING LESS

THAN 3m OF Fill E:::::3 GRANITE

GRANOD IOR ITE

FRICTIONAL SOIL ON lilTiiT1TlTITi1 BEDROCK, LESS THAN 3m MORE THAN 3m OF FILL

MICA GNEISS

AMFIBOLITE

tr::.:::::::.l FRICTIONAL SOIL

.1. LEVEL OF LOWER SURFACE

- -- OF CLAY LAYER, m

QUARZ- FELDSPAR GNEISS

WEAKNESS ZONE

Figure 1 . Section of a geotechnical soil map of Helsinki, scale

1:10,000. Only the most important drilling points are marked.

There are 13 types of deposits (original map printed in color).

Figure 2. Section of a Helsinki bedrock map, scale 1:10,000 (orig­

inal map printed in color).

282 U1ulergrowul Space, Vol. 8, pp. 282-285, 1984.

Printed in Lhe U.S.A. All rights reser\'ed.

0362-0565/84 $3.00 + .00

Copyright © Pergamon Press Ltd.

Foundation maps of ground water risk areas are also compiled. These maps are especially important in tunnel planning, since great risks are in­ volved in tunneling where building foundations may be damaged. Data on foundations are given on separate overlays, so that amendments can be made and the overlays copied together with the base map. The base map is at a scale of 1:500, but it is also used in reduced form at a scale of 1:2000.

The geotechnical maps also provide information on ground water levels. In areas with risk- prone timber pile foun­ dations, the Geotechnical Department has monitored the ground water level continuously since 1972. The data are stored in machine-readable form.

Figure 3. Geotechnical map at a scale of 1:2000.

Tunnel Planning in Helsinki

The total length of the hard- rock tunnels in Helsinki is about 130 km. All the tunnels are constructed by the drill-and-blast method, and the cross section is usually 10-20 sq m. The sub­ way system accounts for 10 km of tun­ nels, the rest being service tunnels (see Underground Space 8:1, pp. 4-6). At the present time a tunnel 7 km long is being constructed for district heating pipes, water pipes, and electrical cables, and another tunnel 17 km long for con­ veying purified waste water into the

cellars have been excavated, increasing the risk of the timber foundations de­ caying as a consequence of a reduction in the ground water level.

Geotechnical Maps

In 1955 the Helsinki Geotechnical Department started to compile a sys­ tem of geotechnical maps at scales of 1:500, 1:2000, and 1:10,000. The maps a t scales of 1:500 and 1:2000 are based on transparent overlays and can be

sinki, geotechnical maps based on drill­ ing data are indispensable. The bound­ aries of geological layers have been drawn as contour lines on the basis of the drilling data. Both the drilling points and the lines have been drawn on sep­ arate plastic overlays. The desired combinations of data can therefore be obtained by copying these one on top of the other. The overlays are easy to amend, and so the maps (1:500, 1:2000) are continuously updated.

open sea. The city also has a consid­ erable number of other rock caverns, such as oil storage facilities (600,000 cu m), water tanks, equipment shel­ ters, civilian air-raid shelters, and sand silos-a total of about 1.6 million cu m.

The Costs of Tunneling With the drill-and-blast method of

construction it is essential that a tunnel be divisible into suitable portions, with an access tunnel at about the middle of each portion. This permits alternate-

amended continuously (see below), whereas the maps at a scale of 1:10,000 are printed and multicolored (Figs. 1 and 2). The maps are used primarily for city planning purposes; but they are also useful in the planning of rock tunnels, where geological and geotech­ nical data are needed for large areas.

The detailed geotechnical maps (Figs. 3 and 4) provide data on c. 130,000 drilling points, and on the type and depth of drilling at those points. The code next to the drilling point can be used to obtain additional information from the archives of the Geotechnical Department. The most common meth­ ods of drilling are weight sounding with sampling, vane testing, percussion drilling, and core drilling of bedrock.

Since the geological conditions vary considerably on a small scale in Hel-

Volume 8, Number 4, 1984

Table 1. Comparison of the 1982 cost of laying pipe or cable in a street and in a tunnel (1984 US dollars). Price of pipe or cable is included .

Total Cost Dia. perm for Cost Excl. of Pipe or Tunnel per Differ-

Pipe Cable in . m. Pipe or ence Type of Pipe or Cable (mm) Street Cable in Tunnel perm

Water 600 390 145 245

District heating 600 820 420 400

Sewage 600 355 90 265

110 kV electric electric cable connection and communication cables 345 230 115

Local telephone cable (1,200 pairs) 85 60 25

Total 1,050

UNDERGROIJNn SPAr!' 9>l'\

heading tunneling to be used, whereby blasted rock can be loaded and trans­ ported in one drift while drilling is being done in another. Under ideal condi­ tions the cost per meter for alternate-

heading tunneling in Helsinki is about 30% lower than that for one-drift tun­ neling. Ideal conditions means here that the work is not interrupted because of reinforcement and sealing and that

zones of weakness which complicate blasting occur in moderate numbers.

The length of the drift to be driven also affects the cost, of course. In al­ ternate- heading tunneling the opti­ mum length of a drift in Helsinki is about 1 km under ideal conditions. However, lengthening the drift to 2.5 km adds only about 10% to the cost, excluding the cost of excavating the ac­ cess tunnel.

In a study conducted in Helsinki, Finnish contractors have been asked their opinion as to the significance of bedrock investigations for the risk re­ serve when making a tender. The dif­ ference between good and very poor investigations turns out to be about 20% of the tunneling cost. In Helsinki, 3- 5% of the tunneling costs are generally used for planning and bedrock inves­ tigations. It is possible to experience good end results with only a small in­ vestment in investigation and planning largely because geotechnical maps are available for the whole city area.

One of the greatest risk factors in tunneling is grouting, most of which is done in the areas of high ground water risk where the compression of soft clay deposits and the decay of wooden piles can raise the cost of tunneling consid­ erably. The price of the tunnel may be two to three times higher in a ground water risk area than under ideal con­ ditions. Here, again, the availability of good maps reduces the risk to the tun­ nel builder.

The Laying of Pipes in Rock Tunnels

In the Helsinki urban a rea the ren­

Figure 4 . Geotechnical map at a scale of 1:500.

Figure 5. Multipurpose service tunnel (utilidor) under construction. On the left are district

heating pipes; on the right, water pipes. The tunnel cross section is 17 sq m.

ovation and replacement of pipes causes continual digging up of the streets. This, in turn , causes ever worsening traffic congestion and increases the pressure to widen thoroughfares. In a city like Helsinki, however, it is often econom­ ical to lay pipes in rock tunnels (Fig. 5) and thereby avoid these and other open­ cut disadvantages. To date, tunnels have been used only for laying mains; dis­ tribution pipes are still always laid un­ der streets.

In general, the various public utili­ ties (water supply, sewerage, energy supply, telecoms, etc.) operate inde­ pendently, which makes it difficult to convince them to lay their pipes and cables in a multipurpose tunnel, or utilidor. Utilidors have also met with opposition for several other reasons. So the Helsinki Geotechnical Depart­ ment has appointed a working group comprised of members of the various public works departments and the City Planning Bureau to coordinate the planning of technical service networks and to investigate opportunities for co­ operative tunnel construction. The

group meets once or twice a year, and favorable results have been achieved.

Transferring pipes from streets into tunnels requires more than just basic geotechnical data and cooperation, however. Information must also be made available on the costs involved. A calculation of certain of these costs for Helsinki in 1982 is given in Table 1. The costs shown are for average con­ ditions. In Helsinki the current cost of excavating with alternate-heading tun­ neling is about $US 500-700 per me­ ter for an 18-sq-m tunnel. If water and district heating pipes (0 600 mm) can be placed in the same tunnel, it is usu­ ally economically feasible to do so, ex­ cept in areas of high ground water risk. Of course, the laying of the pipes must be well-coordinated, and comparative calculations are needed to ensure that the right alternatives are selected.

Geotechnical Maps in Hard-Rock

Tunnel Planning

Hard-rock tunnel planning consists largely of searching for and circum­ venting zones of weakness, and adapt­ ing the rhythm of excavation to pre­ vailing conditions. Investigations and planning go hand in hand. Prognoses of geotechnical conditions in Helsinki are made first with the aid of geotech­ nical maps and then elaborated with stage-by-stage investigations. The aim of the cost calculations is to find the cheapest way of laying the pipes while taking into consideration the magni­ tude of risks involved.

Service tunnels differ considerably in geometry, which imposes different

Figure 6. The third portion of the new Helsinki sewage discharge tunnel. The access tunnel

has been planned in such a way that, after its excavation, three drifts can be driven

simultaneously.

conditions on their planning. The po­ sition of tunnels for water under pres­ sure (raw water, purified sewage water) can be planned relatively freely in both a horizontal and a vertical direction. These tunnels are generally built quite deep (30-80 m) in Helsinki.

Sewage tunnels that operate by grav­ ity, however, impose more restrictions on their elevation because the sewage water must be conveyed to the treat­ ment plants on the sea with a minimum of pumping. With a hard-rock tunnel, the geotechnically most advantageous position along the horizontal plane may be given to the tunnel, which is why many hard-rock tunnels deviate mark­ edly from a straight line. It costs con­ siderably less to construct a hard-rock sewer tunnel than a concrete pipe sewer deep in soft clay deposits.

The costs per meter of the pipes and cable to be laid in hard-rock tunnels for district heating, water, and elec­ tricity are so high, however, that straight tunnels are favored. These tunnels re­ quire the construction of numerous connections to pipes and cables in the streets, which means that geotechnical

conditions have less influence on the location of the tunnel.

Geotechnical maps are indispensable for defining the position of a tunnel both horizontally and vertically. The maps are also needed in establishing suitable places for access tunnels and in giving information on the type of foundations for buildings. This also fa­ cilitates the calculation of ground water and vibration effects. The maps permit the magnitude of the risks involved to be calculated and, thus, either elimi­ nated or minimized. Final Example

The third portion of the sewage dis­ charge tunnel presently under con­ struction in Helsinki is a good example of the coordination of geotechnology and the demands of the tunnel exca­ vation method. Figure 6 depicts the ac­ cess tuimel for this portion of the dis­ charge tunnel and its southern , northern, and eastern forks. The ac­ cess tunnel has been planned in such a way that, after its excavation, three drifts can be driven simultaneously. The southern tunnel, which is in good bed-

rock, has been chosen as the determin­ ing factor for the time schedule. The northern fork of the tunnel is the most risky for the time schedule because of the large amount of grouting to be done there. The eastern fork will also have to cross a difficult zone of weakness, and thus requires reinforcement and grouting. The aim here is to use the resources of the excavation site as ef­ fectively as possible, based first on the information available from the city

geotechnical maps. 0 References Anttikoski, U. V. 1979. The geotechnical

maps of Helsinki. Proceedings of the 4th

Rapid Excavation and Tunneling Conference,

Atlanta 1979.

Anttikoski, U. V., and Raudasmaa, P. J. 1981. The map of building foundations. Pm­

ceedings of the 1Oth International Conference

on Soil Mechanics and Foundation Engineer­

ing. Vol. 3. Stockholm. City of Helsinki Geotechnical Department.

The geotechnical soil, rock and foundation map

sheets of Helsinki, 1955-1980.

Paavola, P. A report on excavation costs of mu­

nicipal tunnels. Geotechnical Department of Helsinki. Pub. No. 27/1982 (in Fin­ nish).

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