Ground freezing of weathered rock in the Mölleback zone at the Hallandsås project : Calculations for optimal freezing design

The Hallandsås project has a long history, after failed attempts to finish the tunnels during the 1990s Skanska-Vinci HB took over the project in 2004. At present time a shielded Tunnel Boring Machine (TBM) called Åsa, which can handle the varied geology at Hallandsås, is used. The TBM also builds a watertight lining behind her. The existing railway over Hallandsås is a single track which is windy and steep. Due to this both speed and load are limited and on top of that only four trains per hour can pass Hallandsås. When the two tunnels are ready for traffic in 2015 the capacity will increase to 24 trains per hour, and the trains can drive at full speed and load. Hallandsås is a horst and was formed approximately 70 million years ago. The movements in the rock have caused poor zones in the rock, which is heavily weathered and carries a lot of water. One of these zones is the Mölleback zone and to be able to penetrate this zone with the TBM effectively, it has been decided that the rock has to be frozen. Artificial ground freezing for stabilization has been used for over 100 years. There are two main methods available. The most common and oldest is the indirect method where brine is circulating in freezing pipes. The second method is the direct method where liquid nitrogen is used as a cooling agent. At the Hallandsås project the indirect method is used. In the eastern tunnel 125 m was frozen, but since a lot of problems, with a lot of water and heavy treatment from the TBM were encountered before the frozen area was reached, it has now been decided to try to freeze 200 m in the western tunnel. The freezing is performed in two steps, Stage 1 is 85 m and Stage 2 is 125 m the overlap is 10 m. To be able to drill the freezing holes in Stage 2 a pilot tunnel is excavated through Stage 1. In this thesis calculations are made to get an optimal design for freezing rime of Stage 2. Both freezing and drilling time must be considered. The freezing time is calculated in a FEM-program called JOBFEM and the drilling time is calculated from the data from the drilling entrepreneurs. In total 15 different designs are considered, the aim is to get a total drill and freezing time under one year. The pilot tunnel will be 5 m in diameter and due to the poor conditions it is not possible to enlarge the drilling chamber (were the drilling work is carried out) for Stage 2. The holes in Stage 2 must therefore be drilled in fans (conical shape formed by the freezing pipes) to freeze the whole length to the 7.3 m radius which is the criterion to drive the TBM through Mölleback zone. Since 125 m is going to be drilled some of the holes must be drilled with steered drilling. The rock mass in Stage 2 consists of two different porosities, the first 60 m have a porosity of approximately 15% and the last 65 m have a porosity of approximately 3%. The analysis in this thesis shows that three fans will be necessary to reach the goal of approximately 6 months freezing time. It is important that the second fan of around 65 m covers the porosity change in the area to reach the freezing time. The fastest result (336 days) considering both freezing and drilling time is reached when 10 pcs. of 125 m long, 12 pcs. 65 m long and 10 pcs. of 30 m long freezing pipes are used. In addition a freezing pipe should be installed in the pilot hole (centre of the ring) to reach face stability. There will be little space to install observation holes, were the temperature is controlled during the freezing process, but at least one hole should be drilled and follow the TBM criteria at a radius of 7.3 m. Temperature monitoring is carried out to every meter so it is known when the freezing criteria is reached. The calculations also show that when a freezing hole is placed in the pilot hole it should not be a problem to reach face stability and therefore, an observation hole is not necessary inside the circle.Validerat; 20120120 (anonymous