Energetische Optimierung von Vereisungsmaßnahmen im Tunnelbau

The artificial ground freezing method has been used in a great number of underground construction projects under geologically and hydrologically complex subsoil conditions. The main advantage of the freezing method can be seen in the functions of the frozen ground that has an increased strength and is impervious to water seepage. Despite numerous advantages the use of the freezing method in practice is often deferred because the energy consumption is assessed inaccurately due to the lack of effective and reliable calculation approaches. Therefore the energy consumption and the related energy costs are expected to be excessively high. Former investigations at the Chair of Geotechnical Engineering at the RWTH Aachen University dealt with the optimization of the freezing time subject to water seepage. To increase the use of the freezing method in practice not only the freezing time but also the refrigeration capacity and the related energy consumption need to be optimized. Therefore, a holistic optimization of artificial ground freezing applications regarding both freezing time and energy consumption has been carried out considering the freezing phase as well as the operating phase in this thesis. To ensure a realistic determination of the refrigeration capacity the interaction of all components (soil, groundwater, freeze pipe) involved in the freezing process needs to be considered. This requires numerical simulations. Due to the lack of appropriate approaches for the determination of the refrigeration capacity of artificial ground freezing applications two numerical solution models have been developed as part of this thesis. Both a simplified approach and a detailed numerical model have been implemented in the general three-dimensional coupled heat and flow transport code SHEMAT-Suite. The simplified approach considers the influence of the surrounding soil on the refrigeration capacity only. The detailed numerical model considers not only the freeze pipe and the heat transport and flow processes inside the freeze pipe but also the coupling with the surrounding soil. The detailed numerical model is based on the theory of thermal resistances and has been developed in cooperation with the Geophysica Beratungsgesellschaft mbH. First, the mathematical formulation and the numerical implementation have been verified for both approaches. Based on this both numerical solution models have been validated by the recalculation of a laboratory model test performed at the ETH Zurich. The results of the simulations showed that both the temperature distribution in the soil and the refrigeration capacity can be determined satisfyingly with both numerical models. To illustrate the practicality of the detailed numerical model a real artificial ground freezing project has been simulated. The results of this simulation are promising especially considering the background of complex boundary conditions in practice and their insufficient detection. Based on the validation of the numerical models the decisive influencing parameters on the freezing time and the refrigeration capacity during the freezing phase for the freezing of a crosscut have been investigated. Thereby the groundwater flow velocity has been identified as the decisive influencing parameter on both the freezing time and the energy consumption. Furthermore, the soil properties e.g. the porosity show a great effect on the freezing time and the energy consumption. This underlines the need of a reliable subsoil investigation. The influence of the properties of the freeze pipe is low in comparison to the influence of the soil properties. However, the properties of the freeze pipe and the refrigeration plant can be optimized prior to the execution of the artificial ground freezing application. In conclusion, several approaches for the energetic optimization of artificial ground freezing applications during freezing and operating phase have been examined and evaluated by menas of extensive numerical simulations. The results showed significant reductions of the freezing time and the energy consumption during the freezing phase by using the pre-cooling effect of additional freeze pipes in the upstream of the frost body. To optimize the energy consumption during the operating phase different operating modes such as an increased supply temperature or an intermittent mode of the refrigeration system have been examined. The necessary adjustments of the program code SHEMAT-Suite for the simulation of an intermittent mode have been realized within this thesis. The results show that significant savings of the energy consumption can be achieved by the use of both operating modes. However, the intermittent mode is the most effective during the operating phase due to the individual allocation and control of freeze pipe groups that enables the simple control of critical areas in the frost body. Hence, the intermittent mode can not only achieve the highest energy savings but also limit the further frost body growth effectively.Within this thesis numerical solution models for the realistic determination of the refrigeration capacity of artificial ground freezing applications considering the heat transport and flow processes inside the freeze pipe and the soil have been developed. By means of the developed and validated planning tool a holistic optimization of artificial ground freezing applications including freezing time and energy consumption can be achieved in the planning phase. These optimizations can lead to significant savings in freezing time, energy consumption and construction costs. Finally, not only the economic efficiency but also the systematic use of the artificial ground freezing method in practice can be increased