Abstract
To avoid hydraulic failure of unlined pressure tunnels, knowledge of minimum principal stress is needed. Such knowledge is only obtainable from in situ measurements, which are often time-consuming and relatively costly, effectively limiting the number of measurements typically performed. In an effort to enable more stress measurements, the authors propose a simplified and cost-effective stress measuring method; the Rapid Step-Rate Test (RSRT), which is based on existing hydraulic testing methods. To investigate the ability of this test to measure fracture normal stresses in field-like conditions, a true triaxial laboratory test rig has been developed. Hydraulic jacking experiments performed on four granite specimens, each containing a fracture, have been performed. Interpretation of pressure-, flow- and acoustic emission (AE) data has been used to interpret fracture behaviour and to assess fracture normal stresses. Our experimental data suggest that the proposed test method, to a satisfactory degree of reliability, can measure the magnitude of fracture normal stress. In addition, a clear correlation has been found between fracture closure and sudden increase in AE rate, suggesting that AE monitoring during testing can serve as a useful addition to the test. The rapid step-rate test is also considered relevant for field-scale measurements, with only minor adaptions. Our findings suggest that the RSRT can represent a way to get closer to the ideal of performing more testing along the entire length of pressure tunnel, and not only at key locations, which requires interpolation of stress data with varying degree of validity.
Highlights
The main objective of experiments described in this paper is linked to a general challenge encountered during final design of pressure tunnels in hydropower; how can the magnitude of minimum principal rock stress be assessed, to a satisfactory degree of reliability, along several kilometres of unlined pressure tunnel? In the early 1900s, prior to the establishment of reliable methods for measuring the underground state of stress, engineers designing the first pressure tunnels made their stress estimations based on simple calculation of overburden weight, as described by Schjerven (1921) and Berkey and Sanborn (1922)
It is generally accepted that reliable data on the magnitude of minimum principal rock stress is absolutely required to ensure a safe design of unlined pressure tunnels, and that in situ stress measurements are essential in providing such information
The first is a rapid variant of the step-rate test, hereafter termed the rapid step-rate test (RSRT), where water is injected in a series of pre-determined flow steps, and the corresponding response in injection pressure, Pinj is registered
Summary
In the early 1900s, prior to the establishment of reliable methods for measuring the underground state of stress, engineers designing the first pressure tunnels made their stress estimations based on simple calculation of overburden weight, as described by Schjerven (1921) and Berkey and Sanborn (1922). This approach, Today, it is generally accepted that reliable data on the magnitude of minimum principal rock stress is absolutely required to ensure a safe design of unlined pressure tunnels, and that in situ stress measurements are essential in providing such information. This is a risky approach, as experienced for instance in the recent failure of the Bjørnstokk HPP in Norway (Nordal et al 2018)
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