Liquid Nitrogen Freezing Method The liquid nitrogen (LN) freezing method is performed by distillation of the air as the nitrogen is transported by truck to the site into special tanks, where it is kept at a tem- perature of -196°C (-320 °F) and at a pres- sure not exceeding 2 to 3 bars (29 to 44 psi). Usually, the LN is stored on site in double- walled storage tanks (under vacuum), where it retains its physical properties until use. The high cryogenic efficiency of the LN allows for the quick formation of a frozen soil structure in only 4-8 days, depending on soil conditions and distance between pipes, and the possibility of difficult situations, such as the presence of underground water streams. The cryogenic output of the LN is 39 kcal/l (118,000 btu/cu yd) for the specific heat vaporization (at -196°C), and 27 kcal/l (82,000) for the heat absorbed by the gas between -196 and -60°C (-320 and -76°F); this is the normal outlet temperature of the gas. Brine circuit in Wislostrada Tunnel A basic requirement for brine freezing is an absolutely tight circuit. If this were not the case, the brine would leak into the ground to be frozen and, due to its high salt content, decrease the melting point of the soil. In this case, the frozen body would not reach the required strength and the whole freezing process could be endangered or even fail. Temperature Monitoring System Control of the ground temperatures was performed using direct reading of chains of thermo-couples introduced at different positions into special holes (thermometer pipes). The thermometer pipes were placed at different sections and distances from the axis of the freezing pipes in order to verify with the highest possible precision the thermal gradient of the frozen ground. The ground, brine and exhaust nitrogen temperatures were automatically recorded every hour. All recorded data (34 thermometric pipes) for a total of 3,880 m (12,800 ft) of drilling. The other two working areas were the west and east shafts. All the holes drilled for these two shafts are located 9 to 18 m (30 to 60 ft) below water level. Including both shafts, a total of 457 holes (19 for thermometric pipes) with a length varying between 5 and 19 m (16 to 62 ft) and a vertical angle of up to 34° were performed, for a total of 7,370 m (24,200 ft). In order to create a continuous frozen wall, all the pipes were drilled using a complex arrangement with very little deviation tolerance. Accurate inclinometer measurements were performed in each pipe using the Maxibor system. Due to the presence of nonhomogenous soil and obstacles such as steel, boulders and wood, a few additional pipes were installed to avoid the risk of possible windows in the frozen wall. The number of additional pipes was less than 10% of the total foreseen in the design. Once all the freezing and thermometer pipes were installed, and the inclinometer measurements and water pressure tests to check the water tightness of the pipes were completed, the freezing stage was started from the west station in both the north and south tunnels. The freezing stage was carried out using LN in order to quickly reach the required frozen wall thickness. The maintenance stage was performed by circulating brine in the same pipes previously used for LN freezing. During the maintenance stage, LN was Brine Freezing method This is a closed method where, inside dedicated freeze units (chillers), the brine (a solution of calcium chloride and water) is cooled to a predefined temperature and afterwards pumped through the freezing pipes in a closed circuit and back to the chillers. The primary cooling agent, usually ammonia, circulates inside the freezing plant. The brine is used as cooling agent that circulates through the freezing pipes (secondary cooling agent). By changing salt concentration in the solution, different melting points can be achieved. 72 • DEEP FOUNDATIONS • JAN/FEB 2015 were transmitted to a central read-out unit and recorded as well as made visible in graphical and numerical form on a dedicated website for authorized staff. Execution of the Works The works covered three separate zones; one inside of Wislostrada Tunnel, where slightly inclined drillings had been performed, and one in each of the two side shafts (west and east), where both horizontal and inclined drillings against water pressure were performed. The first work area was the Wislostrada Tunnel. A total of 223 pipes were installed pumped every 3-4 days over a period of 12 hours, in order to maintain the design average temperature inside the frozen body. During the freezing stage with LN, soil temperatures on the order of -40° to -60° were reached. The thermometric pipe strings were located at a distance between 30 and 60 cm (1 to 2 ft) from the closest freezing pipe. Using data collected from the pipe strings and the correlation between distance and ground temper- ature, it was possible to estimate that the frozen wall created had a thickness of 1.8 to 2 m (6 to 6.5 ft). Once the cutting of the diaphragm walls of the shafts was completed, the operation of swapping from the LN to brine circuit, using the same pipes for both
