DRILL & BLAST

The construction of a new undergroundcasino, excavated below houses datingback to the beginning of the 20th century,has formed a major part of a recentproject to convert the “Fennia” block, in

Kaisaniemi, in the heart of Helsinki, into a largeshopping and entertainment centre. The undergroundpit housing the casino is situated beneath the five-storey “Nikolajeff House”, a structure preserved by theFinnish National Board of Antiquities.

The casino works included tearing down all of theNikolajeff House’s ground floor rooms, load-bearingstructures and original wooden pile foundations, aswell as excavating and building the new premisesbelow the groundwater level. The foundation level ofthe building also had to be lowered by about 16m,into a deep rock pit blasted below the house.

The US$8.5M contract for the foundation works(including rock blasting, underpinning and heavyconcrete works), was awarded in December 2001, withwork starting at the beginning of 2002. A separatecontract for the interior construction and furnishing wasawarded at the beginning of 2003, with workcommencing in March 2003.

The client and owner of the property is Suomi Group(a Finnish life insurance company), the consultant forstructural, rock, geotechnical and foundationengineering was IP Engineering and the contractor forboth the foundation works and the interior constructionand furnishing was YIT Construction.

Existing structures and initial base conditionsThere are a number of existing underground structuresbelow the neighbouring buildings, including subwaytunnels built in the 1970’s, the Kluuvi service tunnel builtin the 1980’s, and the Kluuvi subway station and car

park built in the 1990’s. There is also a 10 screencinema complex under the building next to the casino,with some of its halls located in a deep pit belowground, adjacent to the planned casino.

During the last three years, the Suomi Group has alsorenovated other stores and offices in the Fennia blockand built a 100m long underground passageway, with avarying width of some 3m, from Mikonkatu Street,through the cinema complex, to the Kaisaniemi subwaystation ticket hall. Along the sides of this tunnel arerestaurants and small shops, serving cinemacustomers, subway passengers and office workersvisiting the Fennia block. Above ground, Mikonkatu hasbecome a popular restaurant and terrace street.

Nikolajeff House is founded on the banks of the oldKluuvi bay, made up of thick clay and sludge layers. Theunderlying rock surface slopes towards the Kluuvi basinunder Mikonkatu Street. Below the building, the rocksurface is approximately 4m-7m under the ground floorlevel. However, at the end of Mikonkatu, the rock level

Jet grouted walls Work tunnelsOpen blasting area

45APRIL 2004 Tunnels & Tunnelling International

Gambling with drilland blast in Helsinki

Janne Lehto and Harri Vehmas, of YITConstruction Ltd, and Teemu Rahikainen, of IP-Engineering Finland Ltd, describe the constructionof an underground casino development, directlyunderneath an historic building in the centre ofHelsinki, Finland

Gambling with drilland blast in Helsinki

Above: Open blasting

works underway on the

underground casino

development

Below: Fig 1 - Plan view of

the building, showing the

jet grouted walls, new

concrete beam structure

(brown), work tunnels and

open blasting area

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drops significantly, down towards the basin, to itslowest point 12m below the surface.

The original wooden pile foundations of the househad rotted completely, into a soft pulp-like material,causing partial settlement.

Retaining walls and temporary drill pile workThe casino’s construction pit was supported, and thegroundwater sealed, using jet grouting techniques.From mainly inside the building’s 2.5m high basement,jet grouting was used to construct retaining walls for thesoil masses under the streets and neighbouringbuildings (also founded on wooden piles), and tosupport the perimeter walls of the building (Figure 1).These walls were then anchored to the rock outside thepit using VSL and Ishebeck anchors.

Before tearing down the old foundations and startingexcavation, a massive load transfer beam was built intothe building’s ground floor ceiling, with the help ofpressure casting techniques and self-sealing concrete(Figure 2).

The old building was then supported using 230 x170mm diameter temporary steel piles, drilled into therock. The steel piles were installed around the building’sold load-bearing piles, through the naturally foundedfloor, with the tops of the piles secured to the loadtransfer beam structure. The new piles formed a rigidsleeve that carried the loads during construction.

In order to minimise the risk of building settlement,the piles were drilled with the help of a water-drivendown-the-hole hammer, which releases some of itsdriving water into the ground. To avoid a flushing effecton the old pile-footings, Rotex Symmetric RC drill bitswere used. The exterior walls of the old house were thusresting on the jet grouted walls, with the centre of thebuilding supported by the temporary steel piles.

After finishing the support works, the whole internalstructure below the ground floor ceiling, including theold wooden load-bearing piles and naturally foundedfloor, was demolished.

Excavation of the clay and soils, a total volume ofabout 7000m3, was conducted in stages, according tothe progress of the demolition works. All thedemolished and excavated material was removedthrough the pre-existing Kluuvi service tunnel, by meansof a 2.5m x 2.5m work shaft, minimising the surfaceimpact of the works.

Blasting of the work tunnels and pillar shaftsTwo 30m long work tunnels, with a cross section ofapproximately 18m2 each, were blasted below theplanned load-bearing pillars (Figure 1). Blasting of thesetunnels was conducted, via the service tunnel,concurrently with the upper jet grouting and temporarypile works. The invert level of these tunnels also formedthe final foundation level of the new building.

While the temporary steel piles supported the house,20 pillar shafts (1.4m x 1.4m) were blasted through themiddle of the pile towers. Before opening up the shafts,a collar casting was made at the base of the steel piletowers, which ensured the rigidity of the pile towers’while blasting the shafts open.

Blast hole drilling was mainly carried out with a smalltwin-boom 18t Tamrock Paramatic H 207 L jumbo andthe rock loaded with a 20t Volvo 120 wheel loader. Theblasted rock was transported using dump trucks, viathe service tunnel. Blast fumes were removed throughan exhaust ventilation set-up and released at roofheight, via a “fabric channel” installed in an old elevatorshaft of a neighbouring building.

Between the new foundation level and the reinforced

concrete beam structure, an 18m high connection wasestablished through the pillar shafts. New load-bearing650mm x 650mm K60 reinforced concrete pillars werethen installed. The prefabricated pillar elements, eachwith a length of 7m, were transported on site throughthe work tunnels and connected together in the shafts.This facilitated the construction of the building’s newload-bearing foundations and the temporary steel pilescould be removed after transfer of the loads.

While the pillar and new entrance level constructionworks were underway, blasting of the rock between thepermanent reinforced concrete pillars also commenced.The lower sections of the pillars were protected from theblasting; with planks in the tunnels and long, narrow,rubber mats, which were trimmed to fit, in the shafts.

The first stage of the open blasting works was to“collapse” the ceilings of the work tunnels, establishing>10m high vertical benches along the tunnel walls.Open blasting with such high faces, up to 18m high,around slender load-bearing pillars, inside a preservedbuilding in central Helsinki, was a task that requiredextreme care and precision. In the tight areas betweenthe pillars and walls, blasting was carried out in 2 - 3

Left: A completed

permanent load-bearing

pillar in its work shaft, the

steel piles and securing

concrete structure having

been already removed

Below: The only access

route to the site is the

connection tunnel, which

leads to the Kluuvi

municipal service tunnel

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separate stages. In the “roomier” area between the twotunnels, the whole 10m face could be blasted in onestage. The blasted rock was used to protect thefootings of the new pillars during the work.

Drill holes for the open blasting were mainly madewith Tamrock Commando 110 drill rigs, and manually intighter spaces using an Atlas Copco RH 57 Pantherpneumatic machine. The blasted rock was loaded by asmall, 360° rotating, tracked Cat 312 B excavator.

Felt and rubber mats were used as blasting mats.However, it was also necessary to conduct individualblasts inside the building without blasting mats,therefore blast design and initiation sequences had toincorporate highly controlled removal of the rockwithout excessive throw.

Explosives were supplied by Finnish manufacturer,Forcit Oy. The tunnel blasting used ammonium nitrateTNT based “Aniitti” 28 x 400mm cartridges as the mainblasting agent. “Kemix” 32 x 1000mm emulsion pipeswere used in the bottom holes. Electric detonators wereused to initiate the rounds, which varied from 2.5m-3m.

For benching, blast holes were drilled to a burden of1m, with a hole spacing of 1m-1.5m. Aniitti 32 x 400 or28 x 400 cartridges were used as a bottom charge andKemix 17 x 1000 pipes as the column charge. In atypical 10m bench, 1-2 cartridges of Aniitti 32 x 400 and3-4 cartridges of Aniitti 28 x 400 were used in thebottom of the hole, Kemix 17 x 1000 as the columncharge and 3m stemming. A typical instantaneouscharge was 2.5kg. In border holes, Aniitti 28 x 400 wasused for the primer and F17 x 460 for a low densitycolumn charge. Millisecond electric detonators wereused with 10g/m detonating cord in the sideholes toensure ignition of F17 x 460 trimming charges.

Special features of the tunnel alternativeWide utilisation of the work tunnels proved to be anexceptionally good solution logistically. However, usingthin, inhomogeneous rock ceilings as temporarysupport structures required careful planning fortemporary reinforcement and seamless co-operationbetween the planning and construction teams.

Charging and blasting stages were planned inweekly meetings between contractor and designer.This ensured that numerous details, work sequencesand reinforcement requirements were taken intoaccount in this very complicated operation. The shapeof the rock ceiling, and hence the structural model, wasconstantly changing as the construction workprogressed. The reinforcement structures had toensure, in all imaginable situations, a margin of safetyof no less than 2 beneath load bearing structures, andelsewhere no less than 1.8, against collapse of the rockceiling.

Vertical movement of the rock ceiling, and thebuilding resting on it, was monitored weekly during theworks from approximately 100 detection points, byprecision levelling. More frequent monitoring wascarried out near the active work areas. Temporaryreinforcement of the rock ceiling was carried out usingpre-tensioned steel and cable bolts.

Work tunnel/pillar shaft alternativeThe work tunnel and pillar shaft solution offered severaladvantages over conventional open blasting. Tunnelblasting enabled removal of more than a third of thetotal volume of rock, 8800m3, before open blasting waspossible. This facilitated simultaneous construction inseveral areas, i.e. tunnel blasting could be conductedin parallel with piling and demolition works, whichsignificantly accelerated the construction works in an

Fig 2.1: Connection to the Kluuvi service tunnel from the eastern side of the building

Fig 2.2: The load transfer beam structure, drill piles and work tunnels

Fig 2.3: Blasting and opening the pillar shafts between the drill piles

Fig 2.4: Mounting the element pillars via the work tunnels and casting the footings

Fig 2.5: Final open blasting between the reinforced concrete pillars

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extremely demanding time schedule situation. With careful planning of drilling and charging, it was

possible to blast significant volumes of rock with openblasting, which was one of the key advantages of thework tunnel/pillar shaft alternative. Although thesmallest explosions in the open blasting stage werejust “one bore firings”, average round volumes werebetween 40m3-70m3, and it was possible to completeseveral of these during one day. The largest volume ofrock blasted in a single stage was 300m3.

With a conventional work procedure, the temporarysteel piles would have been required to support theloads during the whole open blasting phase. As thesteel piles were in contact with rock due to be blasted,the risk caused by blast vibration would have beenextremely high. In the pillar shaft solution, thepermanent reinforced concrete pillars were alreadysupporting the building during the open blasting phase.The risk caused by rock blasting was thus remarkablylower, as the load-bearing structures were stronger andalso isolated from the rock being blasted.

Community relations and monitoring of worksDuring construction, from pile driving to rock blastingand heavy concrete work, the neighbouring buildingswere all in operation. Traffic on Mikonkatu street andVilhonkatu street, the cinema centre in direct contactwith the blasting site, as well as offices and anaccommodation business in its upper floors, and theFennia restaurant directly above the works, were inconstant use. Furthermore, there is a neighbouringRadisson SAS Hotel, which was renovated some yearsago. Using flexible work shift arrangements,continuous noise measurement and minute-preciseblastings, during the cinema screens’ intermissions,disturbance to the neighbourhood was reduced to aminimum.

It was also possible to limit blast vibrations and noneof the 10 vibration detectors placed in the neighbouringbuildings registered values above the preset limit of 20-45mm/s (depending on the distance from the blasting).In addition, maximum vertical acceleration values were0.25G for un-isolated computers and vibrationsensitive devices and 1.5G for isolated computers.

Despite an exceptionally dry period, groundwaterconditions remained constant during the works, and no

compensation was necessary. Water leaks into the pitwere minimal, as the rock was systematically curtain-grouted before commencement of the open blasting.

The renovation of the building’s upper floors wascompleted, forced by the project schedule, ahead ofthe foundation works. This work sequence required theutmost care in the construction of the undergroundpremises. The building was constantly monitored forpossible settlement during the numerous load transferperiods. The works were carried out with only minimaldamage to the renovated premises.

Completion of the Grand CasinoThe blasting and underpinning works for the casinowere completed in April 2003, after which the interiorconstruction and furnishing contract commenced.

The new Grand Casino opened its doors for first timeon All Fools’ Day (1 April) 2004. The property, includingthe new Grand Casino, will now continue to beoperated by the Finnish Slot Machine Association, RAY(Raha-automaattiyhdistys), which distributes all of itsprofits to social and welfare programmes (believed tobe the only company of its kind in the world to do so).

Construction of the casino completes the renovationof the whole Fennia block, providing its 21st centuryowners, with pleasant, traditional and extremely high-quality premises. T&T

Left: Open blasting works

well underway

Above: Fig 3 - Finished

section of the building’s

underground extension

Below: The fully completed

casino ready for its grand

opening on 1 April 2004