Tampere Water is currently building a trunk main water pipe, using 800 mm diameter spheroidal graphite cast iron, which connects the two waterworks at Kaupinoja and Rusko in Tampere, Finland.
The overall length of the main is approximately 14 km. Trenchless construction methods have been utilised on several sections and Sitowise plc was the main designing consultant and is now working with the final part. The drilling contractor was Lännen Alituspalvelu plc.
Tampere Water is responsible for the supply, treatment and distribution of clean water; the construction and maintenance of networks; the conveyance of wastewater, the maintenance of the sewer system; and the treatment of wastewater in the city of Tampere and in Pirkanmaa region in Finland. More than 250,000 people live within Tampere Water’s operating area. Sitowise is a Nordic expert in the built environment with over 2,000 employees and offers sustainable design and consulting services for projects of all sizes. Lännen Alituspalvelu and its sister company Eesti Horisontaalpuur have a total of 50 employees and form together a €16 million annual revenue. They provide horizontal drilling with all known methods and are specialists in hammer drilling. Lännen Alituspalvelu operates in Finland and Scandinavia, Baltic countries, and Europe with over 30 years of experience.
In an exemplarily cooperation, Tampere Water, Sitowise and Lännen Alituspalvelu have carried out trenchless construction in several locations along the new trunk main. First cooperation took place in 2017 and the latest trenchless construction project was completed in 2021.
Altogether 700 m of horizontal directional drilled (HDD) 800 mm diameter water pipe and three 140 mm diameter plastic casing pipes were installed in two sections. This was done to conserve the environment and because of the challenging ground conditions. Along the pipeline is an adjacent main ditch which meant groundwater levels were quite high. The work also included three hammer drilling installations of 1,200 mm diameter steel casing pipe with the length of approximately 70 m each. The trenchless alternative was deemed suitable from both technical and economic point of view, as the excavation costs would have been high due to soft and wet soil conditions. Excavation would have also created more significant climate impacts compared to trenchless methods.
In the design phase of the work, ground penetration radar was used to investigate three separate alignments at about 5 m apart. It was possible to conclude the distance of soft
soil from the main ditch based on the ground penetration and ground survey results and to design the optimal route for the water pipe installed with HDD. At the site, previous
ground fills prevented the use of HDD further away from the main ditch.
Designs were further developed together as a site service with the contractor, resulting in a smaller number of drilling trenches. The installation and pulling of the pipes were done with a specially made chute, which consisted of a split steel pipe. The installed pipe was pulled towards the mouth of the pilot hole along the chute.
The work progressed with first installing steel casing pipes with hammer drilling under two streets and on the edge of the old ground fill area, after which HDD was conducted through the steel casing pipes. Pipe assembly to sockets was done in the assembly trench in the middle of the drilling line. The assembly trench was also used as a trench for hammer drilling and from this trench the pipe was pulled in two directions in HDD. By utilising one trench for several tasks in the project, excavation and transportation of the soil could be minimised and climate impacts of the construction were decreased.
Bentonite slurry used in HDD was dried after drilling in a specifically constructed land-based pool. The pool was used to filter water through a bank made of crushed stone, and
water also evaporated from the pool over a period of one to two months. After drying, bentonite was so dry it could be mixed with other soil making it unnecessary to transport
it elsewhere to be processed. Overall, the bentonite management in such a manner promoted the sustainability of the project.
This case of trenchless construction was a great example of how cooperation between the work group and stakeholders brings out the environmental, technical and temporal benefits. Therefore, trenchless construction is a very responsible way of civil engineering.