Drilling and the Associated Drillhole Measurements of the Pilot Hole ONK-PH5

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-PH5 was drilled from chainage 991.4 to chainage 1194 in January 2006. The length of the hole is 202.64 m and the vertical depth of the hole from zero level is about 88.2...107.5 m. The aim during the drilling work was to orient core samples as much as possible. The deviation of the drillhole was measured during and after the drilling phase. One steering operation by wedging was made at the hole depth of 128.58 metres. 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 tests to determine rock strength and deformation properties were made with a Rock Tester-equipment. Due to high inflow (c. 200 L/min) mainly from the depth section 56...58 metres no geophysical surveys were carried out in the hole. Flow logging was carried out only from 58 metres to the bottom of the hole. Difference Flow method was used for the determination of hydraulic conductivity in fractures and fractured zones in the drillhole. The overlapping i.e. the detailed flow logging mode was used. The flow logging was performed with 0.5 m section length and with 0.1 m depth increment. Flow-through packer measurements was used to give background information for the grouting design. 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.