KBS-3H - Excavation of two horizontal drifts at the Aespoe Hard Rock Laboratory during year 2004-2005. Work description, summary of results and experience

SKB and Posiva Oy in Finland jointly study the possibility to develop a variant of the KBS-3 method for final disposal of spent nuclear fuel. The idea is to make serial deposition of canisters in long horizontal drifts instead of vertical deposition of single canisters in the deposition hole. The studies concerning the horizontal deposition alternative are conducted within the framework of a KBS-3H project, where certain demonstration activities are implemented. A key issue of the running project is to test the ability to excavate the horizontal deposition drifts. The objectives for this work are as follows: To show the feasibility of meeting the geometrical and other requirements; To construct two deposition drifts needed for the later project stages. One drift is needed to demonstrate that heavy load can be transported into the drift. One drift is needed to demonstrate that a plug (bulkhead) can be constructed by low-pH shotcrete; To evaluate the applicability of selected excavation methodologies for realistic repository conditions, and based on the experience in the project define need for technical developments/improvements. To meet the objectives, two deposition drifts were excavated at the Aespoe Hard Rock Laboratory during the period October 2004 to February 2005. One horizontal drift was 15 m in length and one 95 m in length. Both drifts were excavated to the diameter 1.85 m using horizontal push-reaming technology by adapting conventional raise-drilling equipment. The drifts were excavated in good rock conditions where no rock support or grouting was needed for feasible excavation or are needed to operate the drifts. SKB and Posiva have stringent geometrical requirements for the excavated drifts and the conclusions concerning compliance with the requirements are: Length: The project met this target. Two drifts were excavated, 15 m and 95 m respectively in accordance with the initial plan. Diameter: Actually it was not easy to measure the diameters of the drifts. The few measurements by the tape method show full compliance. Measurements by a dummy supercontainer in the 95 m drift show that around 90% of the 356 measurements were within the diameter limit 1,840-1,850 mm. Maximum value measured was 1,855 mm and minimum value 1,835 mm. It seems that there is systematic difference between measurements at the rear and at the front of the dummy, so the conclusions concerning these diameter measurements are preliminary. Inclination: The measurements show that the vertical inclination for the 95 m drift is within the limit 2 ±1 deg. To simplify mucking by flushing, the inclination should be minimum 2 deg. Deviation of the pilot hole: For this project it was decided that the end of the pilot hole for the 95 m drift should be within 22 cm of the theoretical line. The actual measurements show -61 cm deviation in the vertical direction and 11 cm deviation in the horizontal right direction due to the non-functional active steering. The stated requirement was not met, but the pilot hole was straight enough for the dummy of the super-container to be pushed to the end of the drift. The requirement should be restated as deviations horizontally would be worse than deviations vertically. Steps: A few steps in the drift surface have been recognized in the short 15 m drift and in general the data show steps < 5 mm in accordance with the requirement. However the measurement methods are not good enough to corroborate compliance for all drift surface area. Roughness: Roughness of the drift surface should be <5 mm, but the measurement methods are not good enough to corroborate compliance for all drift surface area. In general the surface is smooth. The measurement using profiler shows that the requirements were met for the data collected. The measurements based on evaluation of the gauges at the dummy show very few data points outside the range 5 mm. Straightness: Waviness or deviation from the centre line should be <2.5 mm over a distance of 6,000 mm. No measurement or evaluation methods were tried to examine the requirement; instead the dummy was manufactured to prove that a super-container with the same size would fit into the excavated drift without being stuck. The test verified that a super-container will fit into the drift. As part of the project scope, measurement technology was partially developed and tested. Besides the geometrical requirements other requirements were met. The method for excavation is feasible and safe with respect to occupational safety and environment. The method is also reasonably efficient in spite of that neither the equipment or working procedures are optimised. Maximum daily advance rate during reaming was in the order of 6 m per 12 hour day. The main conclusion is that horizontal push-reaming can produce 95 m drifts in good rock that likely will meet the requirements for operational and long-term safety. The technology would also be applicable for 300 m long drifts provided that technology is developed and tested to drill straight enough pilot holes. More experience is needed to understand for what rock conditions additional pre-grouting would be necessary before the drift is reamed and for what rock conditions the drift might need temporary support during construction and before emplacement of the super-container. Several suggestions for additional technical development and improvements have been identified in addition to the developments during the project