A pair of 5.3-mile-long railway tunnels were constructed through the Hallandsasen ridge in southern Sweden. The tunnels have a diameter of nearly 35 feet. A special challenge for the project is a section called the Mölleback zone. This zone consists of several subsections – some made up of clay and silt and others of fractured rock, gravel and clay. The fractured rock sections are characterized by high water flows – up to 230 gallons per minute – and pressures up to 12 bar. The whole Mölleback zone had to be frozen in order for the tunnel-boring machine (referred to as TBM or the mole) to be able to penetrate. Each tunnel required 16 freeze holes with a length of 330 feet, evenly spaced on a perimeter with a 26.4-foot diameter. Drilling was done in two main steps – steered drilling with a mud motor (6.8 in. OD) and installation of glass fiber casing (6.4 in. OD).
Actually, before the steered drilling was done, a Wassara W80 hammer was used to drill three grout holes – more than 400 feet long – in each tunnel. This was necessary in order to stabilize the formation.