Simplified Hydraulic Jacking Test to Assess Fracture Normal Stress for Unlined Pressure Tunnels: A Field Experiment Using the Rapid Step-Rate Test

• Published in: Rock mechanics and rock engineering Vol. 56; no. 1; pp. 647 - 668
• Main Authors: Ødegaard, Henki, Nilsen, Bjørn
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 2023; Springer Nature B.V
• Subjects: Boreholes; Civil Engineering; Core analysis; Core sampling; Coring; Cycles; Earth and Environmental Science; Earth Sciences; Field tests; Geophysics/Geodesy; Hydraulic fracturing; Hydraulics; Hydroelectric plants; Hydroelectric power; Hydroelectric power plants; Jacking; Normal stress; Original Paper; Pressure; Stress; Stress measurement; Test procedures; Tunnels; Unlined pressure tunnels; Hydraulic jacking; Hydropower; Rock stress measurements; Field testing
• DOI: 10.1007/s00603-022-03063-8
• ISSN: 0723-2632

This paper describes an experimental campaign where a simplified test method. The Rapid Step-Rate Test (RSRT) has been investigated in a full-scale field setting. The test, originally developed to assess rock stresses for pressure tunnel considerations, had in an earlier laboratory study demonstrated a promising ability to assess the magnitude of fracture normal stress. The field campaign presented herein was therefore aimed at assessing the field applicability of this test. The field experiments were conducted inside the pressure tunnel of the Løkjelsvatn Hydroelectric Power Plant in Norway, under field conditions identical to those for which the test originally was developed. A total of 29 individual tests were conducted in 7 boreholes, providing 20 test cycles with interpretable test results. The stress estimates made from the test cycles not affected by the near-field stress of the tunnel showed good correlation with the magnitude of minimum principal stress found from a preceding hydraulic fracturing and over-coring stress measurement campaign. The experience from the field campaign suggests that the RSRT can represent an efficient and reliable method to assess the magnitude of minimum principal stress, particularly useful for the final design of unlined pressure tunnels. Highlights A first field verification of a new flow-controlled hydraulic jacking test protocol, the Rapid Step-Rate Test, is presented The test protocol enables estimation of fracture normal stress through analysis of the pressure decline curve during fracture closure stages The onset of fracture closure can be identified as a distinct brake of slope, passing from an initial linear pressure decline to a more gradual decline