When originally commissioned in 1993, the Viikinmäki sewage treatment plant in Helsinki, Finland, was the largest underground facility of its kind. The plant has proved extremely successful, serving a population of 750,000 inhabitants. However, the combined effects of stricter nitrogen treatment requirements and a large increase in population, have lead to the current construction of a 200,000m3 extension to accommodate new denitrification filters and extra treatment lines.
The path to ‘centralisation’
Helsinki’s first wastewater treatment plants were built at the beginning of the last century. As the city’s population grew over time additional plants were added to the system, until the beginning of the 1970s when a total of eleven plants were in operation. Due to the age of the older facilities, this figure had dropped to eight by the mid-1980s.
In 1984, the Helsinki City Organisation was restructured. As part of this reorganisation, the branch of the City Engineer`s office responsible for sewer works and sewage treatment plants was united with Helsinki Water to create Helsinki Water and Sewage Works. At the time, the managing director of Helsinki Water was Dr Seppo Priha. The idea of sewage treatment centralisation for the area had been around for some time. But when Priha initiated a thorough feasibility study, examining the possibility of replacing all the existing plants with one central underground treatment plant, centralisation became a reality.
The aim was to construct a single facility capable of raising treatment efficiency, while also meeting strict environmental restrictions. As well as the new central treatment plant, the plan included nearly 20km of new sewer tunnels. Treated water would be discharged out to sea through an outfall tunnel built in the mid-1980s.
The city council approved the project in 1986, and construction of the Viikinmäki plant started early the following year. The project investment was huge, with final costs totalling nearly US$212M, including sewer tunnels. Helsinki Water and Waste Water Works covered this cost through regular budgets, balanced by increased sewage fees.
Design of Viikinmäki
The main consulting engineers for the project were Plancenter Ltd for wastewater treatment and Soil & Water Ltd for sludge treatment and energy recovery. Altogether, seven consulting firms were involved in the design. Several factors favoured Viikinmäki as the site for the new plant. The area was undeveloped, it was high enough to allow a single pumping stage from treatment to outfall (even though it was underground), and it was centrally located in relation to the older treatment plants. The efficient layout of the plant, and the favourable construction properties of the hard granite and mica gneiss bedrock, also helped to ensure a cost-effective solution. Furthermore, the rock also provides a stable ambient temperature for the treatment plant all year round, despite the adverse surface conditions during winter.
Rock engineering for the plant was unusually challenging, mainly due to the sheer size of the scheme (the total volume excavated during construction was in the region of 1,000,000m³) and the widely varying bedrock conditions. The 17m-19m span of the process halls, separated by 10m-12m thick pillars, is very large in relation to the overburden. Rock cover over the plant is an average of 10m, with the surface area designated to residential housing developments. In dipping areas where the underground halls are very near the surface, with a rock cover of only 4m in some areas, housing is not permitted. These areas are also characterised by weakness zones that vertically traverse the rock caverns. The zones consist of clay filled fractures and intersect the underground halls at lengths of 15m-25m. According to the Grimstad and Barton (1993) classification, the Q-values in these areas vary between 0.1 and 10 (very poor-fair), with minimal water seepage. In most other areas of the rock mass however, the Q-value generally rated 10-100 (good-very good).
Excavation and monitoring
The size of the project required optimisation of worksite arrangements and a high importance was placed on logistics and the disposal of excavated rock. Teräsbetoni acted as the main contractor, working with approximately 100 sub-contractors. Lemminkäinen (now Lemcon) won all five excavation contracts. The plant’s caverns were excavated using drill and blast, and were reinforced with grouted rebar bolts and steel fibre reinforced shotcrete. During the course of the project approximately 55,000 bolts of 25mm diameter were installed to a typical depth of 4m, and a total area of 199,000m² was shotcreted. A network of drains was installed under the shotcrete to channel dripwater into the treatment process. Similarly, drainage mats were placed between the rock and the concrete walls of the basins, with a layer of macadam. Water leakage from the entire treatment plant has been extremely low, only about 200l/min.
Extensive rock mechanics calculations were performed at the planning stage to estimate the amount of rock movement, if any, that excavation would produce. Every effort was made to ensure the stability of the rock, and to predict any changes likely to be caused by excavation. Several extensometers were used to measure movements at critical points in the roof and pillars during excavation and construction. The loads placed on the reinforcement and changes to the stress state of the rock were also monitored. Precision levelling was used to monitor movements in the rock surface above ground. The calculated and measured results corresponded closely with each other showing typical settlement of 0mm-8mm. Rock movements continue to be monitored regularly in the area of the extension.
The new extension
As foreseen, the environmental impact caused by wastewater treatment in the city was greatly improved by the new central treatment plant. Development of city infrastructure resulting from the project was also extensive, including new housing developments on the old treatment plant sites. For nearly half a decade the treatment plant served mainly to reduce biochemical oxygen demand (BOD) and phosphorus from waste water, but in 1997 extra equipment was installed to facilitate 50% nitrogen removal.
However, since original commissioning in 1993, the population in the area has increased by some 10,000, while the treatment requirements have become stricter. Therefore Helsinki Water decided to embark upon an extension project to meet the stricter treatment process requirements, especially with regard to nitrogen levels that now need to be reduced by 70%. A new 200,000m³ extension will house de-nitrification filters and additional treatment lines.
The treatment process used is a typical activated sludge process. Wastewater organics are degraded by digesting sludge first in four digesters, and then combusting the digester gas to generate heat and electricity. The energy recovered adds up to roughly 50% of the total electricity consumption and 100% of the heat requirement of the entire plant.The estimated total cost of the project is approximately US$47M, which will be financed through sewage fees levied by Helsinki Water.
The main designer and process designer for the new works is Plancenter Ltd, with Helsinki City Geotechnical Division undertaking the tunnelling design and engineering firm Rockplan Ltd working as subconsultant. In addition Gridpoint Ltd carried out the 3D stability calculations.
Construction of the new works
Space for two new 17m-19m wide x 12m-20m high x 180m-260m long treatment lines have been excavated parallel with the old existing ones, as well as a 230m² denitrification filter cavern and 200m² related facility cavern, positioned transversely at the ends of the sewage treatment lines. Only one of the treatment lines will be completed in this construction phase, the other finished at a later date. The new works also include a 5m wide x 5.5m high x 170m long access tunnel and other connections. As the existing plant employs gravity discharge, the new facilities had to take elevation into account. Lack of space necessitated the filling of some 4,200m³ of disused sewer tunnels and access adits with concrete in order to rehabilitate the rock for re-use. In addition, utilisation of the rock mass close to the margin of the hill has been necessary. Residential houses, a school and streets are also being constructed in unison with the sewage plant, imposing limitations on underground blasting and support work.
Rock Mechanics Technology Ltd was selected to measure and calculate the rock stress using an overcoring technique. Rock sample drilling, measure-while-drilling (MWD), TV imaging, seismic sounding, and radar were also employed. Propagation of blasting vibration was studied using test blasts. The two principal stresses were found to be horizontal, with magnitudes of 8MPa and 3MPa. The third principal stress is vertical, and its magnitude is 1MPa. All principal stresses are compressive.
The main civil engineering and excavation contracts for the new works were awarded to General Engineering Company YIT, with eight separate subcontractors also employed on site. The underground construction works, valued at US$6.6M, began in May 2000 using an Atlas Copco Rocket Boomer XL3 C with ABC Total. The Boomer drilled 250,000m of blastholes, with an average penetration rate of 140m/percussion hr.
To minimise safety risks and vibration damages on existing structures, the responsibility of the blasting was assigned to one organisation. Helsinki City Geotechnical Division, with subconsultant Kalliotekniikka Oy, managed the timing of blastings, mapping the possible damages in structures before and after excavation, sealing of vibration sensitive devises, and measurements of blasting vibrations. Because of the proximity to the operating sewage plant, flexible airtight blasting barriers of wooden beams, secured by steel cables, were installed. However, airborne detonation pressures on the existing caverns were underestimated and the isolating walls had to be reinforced several times with profile steel.
The most critical point of advance was through the fractured weakness zone (Figure 2). These (previously discussed) zones affected two of the 17m wide caverns and a 5m wide service tunnel, all with only 2m-3m of rock cover. In these areas 20m long and 6m long, grouted rock bolts were installed in the crowns and walls, from two directions in a fan shape. Excavation then advanced in small increments, with immediate shotcreting and rock bolting (4m long bolts). In areas without any reinforcement, the crown area was restricted to about 15m².
Systematic monitoring of the movements of the rock mass, and the rock quality, are being carried out by Suomen Malmi (SMOY) and Geotek Oy, while groundwater level monitoring is the responsibility of Helsinki City Geotechnical Division.
The excavated rock has been crushed and used in construction works, where it replaces natural gravel. Due to logistics, crushing was initially carried out on the surface, but as soon as possible, crushing operations were relocated to a tunnel. Some 500,000t of rock was crushed underground using a mobile crusher. The crusher capacity of 2,000t/day over two shifts limited the rate of rock excavation, despite best efforts to optimise worksite arrangements.
Progress and conclusions
Drill and blast has been used for both underground and surface excavations. Underground, extensive lining and support works were required whilst above ground construction of houses and a bridge was underway. All this has been carried out at the same time and, in addition, the existing underground wastewater plant had to remain in operation. Final designs, especially those related to access, had to be extremely accurate, as alterations during construction could impact upon the construction schedule. However, despite these huge logistical pressures, construction progressed successfully and excavation of the new extension was completed on schedule, in January 2002.
No major problems have arisen from the coexistence of the treatment plant works and neighbouring housing works, and no major damage was caused to the existing structures by blast vibrations. Detailed preplanning and tight cooperation during the excavation phase between the owner, designer and contractor ensured the advance through the critical weakness zone progressed smoothly.
The quality of the rock in Helsinki provides excellent opportunities for underground cavern construction. In this particular case, the ability to locate the central sewage treatment works within city infrastructure, as well as the favourable insulation properties of the rock, has provided a model of construction and operation cost efficiency. The procurement cost of underground space in this case is less than US$47/m³.
The denitrification filter caverns are due to be commissioned in September 2003, and the additional treatment lines in March 2004.
Related Files
Fig 2 – Longitudinal cross section of one of the new treatment line caverns, showing the construction sequence through the critical weakness zone
