Helmut Hass

Tunnels and deep shafts will be built, where and when they are necessary. There are no problems, only challenges and solutions.

file under: Design-Build, Facilities, Geotechnical, Government, Water

  • Q. What is ground freezing and how does this ground stabilization method work? A. Soil below the groundwater level contains water, which is extremely stable and watertight in frozen condition. The water is frozen via one or more rows of vertical, horizontal or inclined pipes installed in the ground. By the constant flow of a coolant (e.g., brine or liquid nitrogen) through the pipes, heat is withdrawn from the soil and a ring of frozen soil occurs around the pipes. Like the cement in concrete, the ice bonds the soil particles together, imparting strength and impermeability to the frozen soil mass.
  • Q. Where is ground freezing best applied? A. Typical applications are the sinking and lining of shafts up to more than 600 m deep; deep excavations; tunneling under the protection of a structural and watertight frozen soil body; cross-passages between shafts and tunnel tubes; large open excavations; and retaining walls. It is also used for temporary soil improvements under foundations, temporary sealing of leakages, and temporary water cut-off for connections at the interface between existing and new underground structures.
  • Q. What are its benefits? A. Ground freezing is not the cheapest method for ground stabilization, but definitely first choice in difficult situations. This is because it is safer than all other ground stabilization methods with regard to water tightness and structural support. In addition, it is environmentallyfriendly because no barriers (e. g., diaphragm walls, chemical, other grouting material) remain in the subsurface environment after the application.
  • Q. Do you have a favorite project? A. All projects are different, from tunnels in difficult ground conditions and unstable buildings, to leaking walls. Every project has its own challenges, but there is one I will never forget—the famous 800-year-old Leaning Bell Tower of Pisa in Italy. At the end of the 1980s it was leaning so much that it had to be closed for visitors. To stabilize the building’s inner structure, form a slight reversion of the leaning and subsequently stabilize its position, and produce a safe foundation, we constructed a new circular base around the tower’s old foundation. Ground freezing was applied for the temporary stabilization during the construction.
  • Q. What is your personal relationship with ice? A. Well, when I was a boy I used to skate in winter. Today, my favorite frozen product is vanilla ice cream.
  • Helmut Hass, CDM Smith vice president, Dipl. -Ing., possesses a wide range of geotechnical engineering experience and is world-renown ground freezing expert.