The UNIS Borehole Jack;: Description, fieldwork and new classification system

Abstract

The increasing interest in the Arctic region due to exploitation of natural resources requires methods for estimating design loads on offshore structures. The borehole jack (BHJ) is an ISO approved tool for assessing the in-situ confined compressive strength of ice. On request from the Norwegian University of Science and Technology (NTNU), a BHJ was made by M-Tech and delivered to the University Centre in Svalbard (UNIS) in 2010. This work presents a technical description of the UNIS-BHJ as well as calibration instructions and experimental setup. A classification system of stress time curves has been developed with focus on post-peak stress behavior. Three field campaigns have been conducted, two in Van Mijenfjorden (first-year level ice) and one in the Barents Sea (young and rafted ice) in March and April 2012. The new classification system proved convenient when used for classifying the results of these experiments. An advantage is that the system is applicable for all BHJs, hence allowing comparisons of different works to be made regardless of the BHJ used. Investigations of the spatial variation of borehole (BH) strength were also done. Sampling areas of sizes 100 by 100 m and 10 by 10 meters were established in Van Mijenfjorden, where both concluded mean BH strength of 16.8 MPa with STD of the larger area 1.9 MPa and the smaller 0.3 MPa. Another three sampling areas of sizes 20 by 20, 4 by 4 and 4 by 4 m were established in the Barents Sea, with BH strengths 13.0, 11.1 and 14.0 MPa and STDs 3.2, 2.0 and 2.4 MPa.