Thermisch-hydraulische Simulationen zur Optimierung von Vereisungsmaßnahmen im Tunnelbau unter Einfluss einer Grundwasserströmung

摘要

The ground freezing method has recently been given a boost as a versatile technique to provide artificially frozen ground as impervious and bearing support structures for tunneling applications. Excavation can proceed safely inside the frozen ground structure until the final lining provides permanent support. In contrast to grouting works the freezing method is completely reversible and has no significant environmental impact. Ground freezing is not limited by adverse ground conditions and may be used in almost any soil formation, regardless of structure and grain size. The excavation of cross-passages and the widening of shield-driven tunnel cross-sections to build underground stations are main fields of application. However, often the alleged high costs for the supply of energy are the exclusion criterion for a systematic and scheduled application of the method. The optimization of ground freezing projects requires a reliable prediction of frost propagation as vital design criterion. Existing groundwater flow is the crucial factor and major thermal load that requires intensive consideration for a safe and cost-effective freezing project. Due to the temperature-dependent behaviour of frozen soil, the latent heat released at phase change and the thermal impact of flowing water the analytical approaches known so far for the design of frozen support structures can only inadequately assess the complex phenomena. Only numerical simulations can provide solutions to the coupled heat flow-groundwater problem but are not easily applicable in the majority of cases. Within this thesis the Finite Difference Program SHEMAT (Simulator for Heat and Mass Transfer) is further developed to provide a practical, numerical analysis tool to predict frost propagation of ground freezing projects that are subject to groundwater flow. The main focus lies on sufficiently accurate determination of freezing times based on a few geotechnical parameters only. For this purpose a simplified phase change model is set up to describe the freezing process in fully saturated soils. The unfrozen water content is used as main variable of the freezing stage and allows deriving thermal properties of the soil, namely thermal conductivity and thermal capacity, by mathematical models. By means of experiments it is shown, that the unfrozen water content can be derived from the grain size curve with a high degree of accuracy in case of non-cohesive soils. The numerical model is verified by an analytical approach of frost propagation and by recalculation of two model experiments. The reanalysis of a real ground freezing project shows good results and reveals the essential demand to know the flow conditions on site for reliable calculations. Using the example of the freezing of a cross passage flow-adapted freeze pipe arrangements and optimized operating modes of the cooling units are presented to achieve considerable cost saving potentials. It becomes obvious that the freezing time increases exponentially with flow velocity and different designs may become advantageous for different velocities. Even without additional installation of freeze pipes the freezing time can be reduced by 20% using flow adapted freeze pipe arrangements. Additional freeze pipes on the critical upstream side can reduce freezing times by almost 50%. The most advantageous reduction is achieved by precooling the groundwater by positioning pipes in the upstream. Moreover, the operating mode can be modified by the assembly of pipe groups with respect to their specific thermal impact of the groundwater flow. As a consequence the energy supply can be reduced by selectively charging these groups. The simplified, numerical model presented in this thesis provides opportunities to optimize future ground freezing applications subject to groundwater flow.