Investigation on the Effect of Cyclic Moisture Change on Rock Swelling in Hydropower Water Tunnels
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TECHNICAL NOTE
Investigation on the Effect of Cyclic Moisture Change on Rock Swelling in Hydropower Water Tunnels Lena Selen1 · Krishna Kanta Panthi1 · Maximiliano R. Vergara2,3 · Mai Britt Mørk1 Received: 15 May 2020 / Accepted: 29 September 2020 © The Author(s) 2020
Keywords Rock swelling · Cyclic moisture change · Oedometer test · Water tunnel · Rock texture
1 Introduction Swelling of rocks is often forecasted by the content of swelling clay minerals or anhydrite; however, the intensity of the expansion and subsequent swelling pressure cannot be attributed only to these rock constituents (Ruedrich et al. 2010). Rocks show a wide range of grain sizes, porosities and fabrics which can act as controlling factors on the swelling behavior upon ingress of water (Russell 1982; Olivier 1991; Dick and Shakoor 1992). In addition, the interaction between the rock material properties and the constructionspecific exposure to degrading agents controls important swelling characteristics of the weak and weathered rock mass. The surrounding rock mass on the periphery of hydropower water tunnels is unloaded and drained during tunnel construction and then the tunnel is exposed to cyclic wetting and drying processes during the operational lifetime of the project (Selen et al. 2019). This may lead to time-dependent changes in the rock mass properties aggravating risk on tunnel instability. Hence, the extensive moisture fluctuations are special features of hydropower water tunnels compared to other tunneling projects which seldom are addressed in laboratory testing procedures. Several laboratory testing methodologies have been developed to determine the swelling behavior of rocks and the induced pressure from the rocks on their surroundings, as * Lena Selen [email protected] 1
Department of Geosciences and Petroleum, Norwegian University of Science and Technology (NTNU), Trondheim, Norway
2
Tunnel Engineering, Ed. Züblin AG-Zentrale Technik, Stuttgart, Germany
3
Institute of Soil Mechanics and Rock Mechanics, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
tunnel linings. According to Grob (1972), the swelling strain can be expressed as a function of the swelling pressure and is expressed as a linear relationship in a semi-logarithmic diagram. The so-called “Grob`s law” can be assessed by use of oedometer tests and assumes that the material behaves linear elastically and increases in volume as the applied stresses decrease (Einstein 1994). ISRM (1989, 1999, 1979) has suggested different oedometer swelling tests, including a maximum swelling pressure test. The test measures the maximum pressure induced by a prepared rock disk after water immersion when volume expansion is hindered. However, the long-term swelling characteristics of a weathered rock mass surrounding a water tunnel may not be addressed using this method due to the intricate interaction between the rock properties and the cyclic moisture change during operation. This manuscript investigates the interaction between cyclic wetting and dryi
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