Drilling and associated drillhole measurements of the pilot hole ONK-PH10

The construction of the ONKALO access tunnel started in September 2004 at Olkiluoto. Most of the investigations related to the construction of the access tunnel aim to ensure successful excavations, reinforcement and sealing. Pilot holes are drillholes, which are core drilled along the tunnel profile. The length of the pilot holes typically varies from several tens of metres to a couple of hundred metres. The pilot holes are aimed to confirm the quality of the rock mass for tunnel construction, and in particular to identify water conductive fractured zones and to provide information that could result in modifications of the existing construction plans. The pilot hole ONK-PH10 was drilled from chainage 3459 to chainage 3639 in March 2009. The length of the hole is 180.00 metres. The drilling was done as orientated core drilling. The deviation of the drillhole was measured during and after the drilling phase. Electric conductivity was measured from the collected returning water samples. Logging of the core samples included the following parameters: lithology, foliation, fracturing, fracture frequency, RQD, fractured zones, core loss, and weathering. The rock mechanical logging was based on Q-classification. The test to determine rock strength and deformation were made with Rock Tester -equipment. Water conductivity of the fractures or fractured zones was measured by Posiva Flow Log equipment. The measurements were done in two phases. During flow measurements also grounding resistance electric conductivity and temperature were measured. In flow logging test, sections of 0.5 m with increments of 0.1 m were used. Water loss measurements were conducted in the hole section 3.70-180.00 m dhd. Geophysical logging and optical imaging of the pilot hole included the fieldwork of all surveys, the integration of the data as well as interpretation of the acoustic and drillhole radar data. One of the objectives of the geochemical study was to get information of the composition of ONKALO's groundwater before the construction will disturb the chemical condition. The groundwater samples were collected from the open hole without any packers. The collected groundwater samples were analysed in different laboratories. (orig.)