Project Detail
  • Wastewater Collection & Treatment

    Prague Sewer Protection

    Ďáblice, Prague, Czech Republic

    Ďáblice (“she-devil”) is a small enclave in District 8 (Fig 2) of the Czech Republic’s capital Prague. Aside from its intriguing name, the community is known for its many sightseeing wonders, including the Ďáblice Observatory (Fig 3) on Ládví mountain, Prague’s highest point at 1,178 ft (359 m) above sea level. 

    While the area’s elevation provides excellent views of the sky and surrounding lands, it also presents a challenge for municipal services such as sewer and water. One of Ďáblice’s oldest sewer lines was built in the 1970s and featured a series of short tunnels connected by six narrow drop shafts connecting each tunnel, forming a staircase-like structure descending down the steep hillside (Fig 4). 

    Unfortunately, this configuration resulted in frequent drop-shaft clogging and constant maintenance. Eventually, the structure began to experience accelerated deterioration brought on by abrasion and microbiologically induced corrosion (MIC). 

    Replacement Required In 2013, Prague-based KO-KA s.r.o. engineering firm was engaged by the city to design an alternative structure capable of providing a more controlled and maintenance-free descent for wastewater flow from the elevated community (Fig 5). The resulting design, features a main 20-foot (6 m) diameter drop shaft at the highest point connecting to a new transfer tunnel at nearly 120 feet (37 m) below the surface. 

    The main drop shaft (Fig 6) features both a spiral chute for controlled day-to-day wastewater descent and an overflow system for storm water acceptance (Fig 7). Storm water is redirected to a central drop shaft, which temporarily allows the overflow to freefall to the transfer tunnel (Fig 8). 

    The new 820-foot (250 m) long transfer tunnel connects with the pre-existing tunnel in order to continue the journey to the city’s main wastewater treatment plant located on an island in the Vltava River. The new 1.45% inclined transfer tunnel provides a less aggressive wastewater flow than the previous system, resulting in lower emission of hydrogen sulfide (H2S) gas and less abrasion. 

    Extra Protection Specified
    One design challenge was the protection of the interior of both of the new shafts. The continual spray and splashing that occurs within the wastewater spiral chute and the extreme spray that takes place during storm overflow can cause MIC and other forms of infrastructure degradation. 

    To protect the new cast-in-place main drop shaft and smaller (13.8 ft/4.2 m diameter) maintenance shaft (Fig 9), KO-KA specified that the structures should receive a coating of Xypex Concentrate (Fig 10) for deep-penetrating crystalline waterproofing and protection. 

    According to KO-KA project engineer Petra Bařinová, “Extra protection was needed due to the aggressive environment caused by sulphate and sulphuric acid attack. We chose Xypex Concentrate and Modified for long-term protection of the concrete without the risk of peeling or other imperfections that can occur with membranes and conventional coatings.” 

    KO-KA has specified Xypex products on many other challenging projects throughout the Czech Republic, including utility tunnels and wasterwater system construction/rehabilitation. In fact, KO-KA specified a special concrete-based injectable mix that was used to stabilize many Prague sewers (Fig 11) that were damaged by historic 2002 flooding in the city. Xypex Admix C-1000 NF was a key part of the sewer rehabilitation mix. 

    Xypex Concentrate contains a mixture of Portland cement and active proprietary chemicals that diffuse into the concrete substrate and react with water and the constituents of hardened concrete to cause a catalytic reaction. This reaction generates a non-soluble crystalline formation throughout the pores and capillaries that exist in all concrete. 

    In this way, water and other liquids are prevented from penetrating the concrete from any direction. Laboratory studies have shown that the crystalline formation of Xypex Concentrate can penetrate up to 12 inches (30 cm) below the concrete surface, providing deep permanent protection. 

    An additional coating of Xypex Modified was also specified to reinforce Xypex Concentrate. Xypex Modified chemically reinforces Concentrate to produce a harder finish that resists even extreme hydrostatic pressure, seals hairline cracks up to 0.5 mm, resists aggressive chemicals, and cannot be punctured or torn. 

    Spray Application Saves Time
    Construction of the new wasterwater collection and transfer system for Ďáblice was carried out from 2016 through August 2020. The application of Xypex Concentrate and Modified took place in early 2020 and was conducted by Nekap s.r.o., the Xypex distributor and a registered applicator for the Czech Republic. 

    Both Xypex Concentrate and Modified can be applied by brush or sprayer to saturated surface dry (SDD) concrete. The products were applied primarily by sprayer (Fig 12) in the Ďáblice project. According to Nekap director and Xypex regional representative Jan Mandelik, the firm’s crew applied Xypex Concentrate and Modified over a four-day period using sprayers mounted on a 20-ft (6 m) extension poles (Fig 13). 

    The specially-designed spray tools helped save approximately two weeks of labor had the job been done using scaffolding and brushes and conventional sprayers. Approximately 20,000 ft2 (1,900 m2) were treated with Xypex products, protecting the two shafts from internal wastewater spray and external groundwater intrusion. A total of 3,329 lbs (1,510 kg) of Xypex Concentrate and 2,513 lbs (1,140 kg) of Xypex Modified were applied. 

    “These structures are designed to last 100 years,” Mandelik notes. “Xypex Crystalline Technology has been in use for more than 50 years on projects like this around the world. Engineers, contractors, architects and applicators specify Xypex because it can enhance the longevity of almost any project involving concrete structures.”

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