Although ground freezing had been widely used in Europe since the late 19th century for the sinking of deep mine shafts, the technique was largely ignored in the United States. In fact, only two projects were completed between 1888 and 1954. A major drawback was the high cost and construction time of the hangar-sized freeze plants required, limiting the use of freezing to long-term projects on open sites. But with the advent of mobile freeze plants in the 1970s, Moretrench was quick to recognize the potential in the urban marketplace and established a ground freezing division in 1976. Industry acceptance was initially slow, but from small beginnings ground freezing has grown to be a core technology, with a number of notable complex and challenging projects to the company’s credit on major water systems and tunneling projects in North America. Recently Moretrench was called in to go further afield to Argentina, where a particularly persistent issue had effectively halted the completion of an access shaft for the Sistema de Potabilización Area Norte that will transport and treat raw water from the Paraná River, providing potable water for 2.5 million residents in Northern Buenos Aires province.
The project’s Access Shaft #3 had experienced persistent soil and groundwater ingress through the earth support slurry wall panels during excavation, which several remedial efforts over time, including dewatering and various grouting programs, had failed to resolve. The shaft was eventually backfilled with concrete to above the tunnel crown to allow tunneling to proceed. Moretrench was contracted to design and furnish a ground freezing system that would provide a watertight barrier and allow the general contractor to complete the shaft. Moretrench also provided on-site supervision during system operation, and monitoring through to final liner installation.
All ground freezing equipment, including pipes, fittings, tanks, instrumentation and data acquisition system, and two mobile freeze plants with a combined refrigeration capacity of 281 tons, was shipped from Moretrench’s New Jersey headquarters. The installed system consisted of 48 vertical freeze pipes around the shaft perimeter, 25 angled freeze pipes installed around and beneath the tunnel, and seven temperature monitoring pipes. After nine weeks of freezing, closure was verified by temperature and piezometer data, allowing the prime contractor to install the base slab reinforcement and the final concrete liner. The shaft remained dry and stable throughout.
For a comprehensive discussion of this challenging project, see Argentine Shaft Saviour by Moretrench Director of Ground Freezing, Joseph A. Sopko, Ph.D., P.E.