STRESS ANALYSIS OF FREEZING PIPES BY MODELLING

Using the simulation test facility designed on the basis of similitude principle, the authors of the paper carry out a series of simulation tests and study the influence of effective factors on the stress in the freezing pipes during hoth periods of active freezing and excavating-lining. They obtain preliminarily the regularity of stress variation and discuss the causes of freezing pipe fracture.

Study of the insulation effect of freezing apparatuses

In-situ tests and numerical manipulations were conducted for comparison of superior/inferior quality of casing-tube-vacuumized insulation and polyurethane insulation of fi＇eezing apparatuses in relation to non-uniform cold distribution in the multi-coil fi＇eez- ing layout and the contradiction between large-coil diameter layout of the freezing holes and stability of the headfi＇ame base. Tests show that： （1） under current technology, vacuumized insulation is of short duration due to lack of appropriate applied vacuum; （2） after the freezing pipe is insulated with polyurethane, the temperature gradient is less than no insulation, and the temperature dif- ference between insulated and non-insulated apparatuses is stable as the freezing period increases. Finally, it is pointed out that polyurethane insulation is quite efficient in present technical freezing construction. Keywords： freezing pipe; insulation effect; polyurethane; energy conservation

Feasibility evaluation for excavation of Naghshe Jahan Square subway station by underground methods

In recent years, in reaction to the increasing usage of urban areas, the excavation of underground spaces has been developed. One of the most challenging issues encountered by engineers is the construction of subway stations as large underground spaces at shallow depth with soft surrounding soils. In this paper, Naghshe Jahan Square subway station located in Isfahan, Iran, has been simulated by geomechanical fnite difference method(FDM). This station is located under important historical structures. Therefore, the ground displacement and surface settlement induced by the excavation of the subway station should be strictly controlled. Many of such problems are affected by selected excavation method. For these reasons, different underground excavation methods associated with construction have been studied. In this study, sequential excavation method and large-diameter curved pipe roofng method are used and the numerical results of the two methods are compared. The presence of groundwater table obliges us to choose special techniques for the stability of the ground around the subway station during construction; hence compressed air and ground freezing techniques are utilized in the simulations of the subway station. Finally, after choosing appropriate support systems, the large-diameter curved pipe roofng method with 1.5 m spacing between curved pipes is proposed.