The earthmoving work on the Central Library is still on schedule despite the weather that has been anything but favourable to construction. The “beavers” of E.M. Pekkinen Oy have already finished digging, and excavation has also reached the final stretch. The worksite is at the final bottom depth of three metres below sea level.
When a January blizzard was blowing briskly through Helsinki and the people on the streets quickly found their way indoors, the earthmoving work at the Central Library worksite went on like any other day. Pekkinen’s people are used to working under the open sky, from which all kinds of things fall down.
“If we could make a wish, there wouldn’t be anything falling down from the sky and we’d have mild weather in the winter. Zero degrees Celsius or a little below would be nice winter weather to work in,” say the site engineer Risto Sell and the site manager Kyösti Kontio.
They have been kept waiting for ideal weather. Before Christmas, the worksite was under heavy rain, followed by freezing cold in January; two days of heavy blizzard before mid-January brought dozens of centimetres of snow with it. Few people stopped to study the text and images on the fence around the Töölönlahdenkatu Street worksite.
What to do when snow blocks the front of the site huts? Take out the excavator, of course. Use it to pile the snow on a truck driven behind it and you are back in business. On snow days, keeping the routes passable means clearing the snow non-stop. The subcontractor, Maarakennus Rinne-Laturi Oy, has its roots in Ostrobothnia. There is nothing like getting to clear the front of your porch with an excavator!
Naturally, the excavators were also used for their real purpose at the Library’s foundation pit; in addition to clearing the snow, they were used to load the rock waste after blasting the rock. Hannu Rinne-Laturi is in the driver’s seat.
In heavy cold, even the massive machinery is tested. The worksite machines contain a lot of hydraulics and the hydraulic oil may become stiff in the cold, which prevents the machines from starting. Using strong force against the hard ground may result in hairline cracks even in the bucket and boom of an excavator. People work carefully in the in cold, and when the temperature is below -20°C, the machines rest when necessary.
Blasting holes and explosions that go pop
The Central Library pit was the result of the removal of 20,000 cubic metres of soil. The digging was already finished before Christmas, and even excavating the rock has progressed faster than anticipated at the worksite. In mid-January, the excavation had reached the final stretch: out of the 9,000 cubic metres to be excavated, only roughly one thousand remained.
￼The focus at the worksite was on drilling blasting holes and blasting the rock in small, controlled batches. In practice, the explosions were only small pops that not all neighbours might even notice without the loud whistle, which blows at increasing intervals to announce an upcoming explosion. Still ahead is work such as excavating openings for the bottoms of the lift shafts and the pumping station. The water from the basement sewers will be directed to the pumping station in the future.
The blasting starts by drilling blasting holes into the rock, in which the explosives are placed. The holes are drilled down to the depth of three metres. In other words, the layer to be removed is three metres high.
The blasting holes delineate a rectangular area of ten by two metres. The detonators are numbered and they explode at different times. This controls the direction of the blast. Pure dynamite is used as the explosive. At one time, 50–80 cubic metres of rock are removed.
In the most active phase, there were four or five blasts per day, while at the start of the year there were only three or four per day. The removed rock is taken to Jätkäsaari, where the City of Helsinki takes care of crushing it.
No wonder the work has proceeded so briskly, when even the driller takes a coffee break while walking with the cup in hand. The driller Tomi Röyskö and the master builder Kari Suomala had just enough time for a brief chat between blasts.
Concrete casing keeps the rock together and the water away
The rock around the plot is broken, so the walls have been sealed with a concrete casing. This involves curtain grouting, which means sealing the rock by pumping concrete into the cracks.
The toe beam in the image marks the space between the rock and the supporting wall. In addition, a concrete curtain has been built inside the rock by injecting concrete; it prevents water from migrating under the building. The curtain will go around the whole pit, even though the level of brokenness of the rock varies a great deal. In some places the rock is badly broken, while at others it is more solid. The intention is to make the Central Library basement completely leak-proof, so the casing is constructed very carefully. The cement mix in the cracks in the rock prevents water from entering the area of the building.
“In underground structures such as car parks, curtain grouting is common. In practice, as much concrete as possible is pumped into the cracks in the rock; one crack may take as much as 1,000 kilos of concrete. We have estimated that 100,000 kilos of concrete will be needed,” says Kyösti Kontio.
At a worksite, something that looks unimposing may still serve an important purpose. Here, groundwater has leaked into the vicinity of a toe beam constructed at the bottom of a sheet pile wall, and it has frozen. An oil burner is warming up the air in a tent, keeping the area unfrozen. A working day can literally be spent on melting ice.
At the start of the year, there are a little over 20 people working at the Central Library worksite. In addition to the supervision, the total number of people at the worksite includes construction workers, excavators, people carrying out the injections, machine operators and subcontractors, such as excavator contractors. In the image, you can see the supervisors; from the left: site manager Kyösti Kontio, master builder Kari Suomala, construction worker and student foreman Jarmo Ahokas, as well as the site engineer Risto Sell.
A secret meeting of the beavers behind a security guard? Hardly. The men at Pekkinen talk openly about their work, in which they have years of experience.
At the second gate at the corner of the Sanomatalo building, the official Pekkinen logo has been joined by a cartoon beaver that now also adorns the helmets of the Pekkinen workers. In the drawing, the beaver is studying a construction guide.
In February, the concrete construction of the basement will begin behind the worksite fences. The library basement gets a foundation that makes erecting the walls possible. Hopefully, the worst of the frost and blizzards will be over by then.