Adhesion Strength at the Shotcrete–Rock Contact in Rock Tunneling

Abstract

An increasing number of tunnels have been constructed worldwide for a diverse range of infrastructure projects, including highways, subways, railways, and various utilities. As a result, tunnel engineers are increasingly aware of the importance of safe and economic tunnel construction. Peck (1969) addresses the following three issues in terms of tunnel construction: maintaining stability and safety during construction, minimizing unfavorable impacts on third parties, and performing the intended function over the life of a project. Among these issues, the first is directly related to the behavior and stability of the tunnel support system. In rock tunneling, the New Austrian Tunneling Method (NATM) is a widely used tunneling method in which the shotcrete liner acts as the main support. Shotcrete is sprayed on the tunnel excavation surface very shortly after being excavated and supports the ground load in conjunction with the rock bolt and the steel rib. Barrett and McCreath (1995) explained the failure modes for the shotcrete liner (Fig. 1). It has been shown that the behavior of the shotcrete liner is highly dependent on the adhesion strength at the shotcrete–rock contact. For example, if this strength is sufficient enough, then the mode of failure may be a direct shear failure. Otherwise, the shotcrete liner may fail under a flexural condition. In addition to the behavior of structural characteristics, sufficient adhesion strength can merge the surrounding ground and the shotcrete liner into a single body, increasing tunnel stability. Therefore, the adhesion strength at the shotcrete– rock contact can influence the failure mode for the shotcrete liner, which, in turn, can impact the overall tunnel stability. This indicates a need for a better understanding of this adhesion strength. An adhesion strength test method (Swedish Standard 1987) from Sweden drills two circular slots with a doublediamond drill. Here, the inner circular slot is drilled through the substrate and shotcrete, and the test equipment is attached to the shotcrete drill core for the pullout test. The test method can be used only for relatively stiff shotcrete. Brennan (2005) explains a shotcrete bond test method from the International Concrete Repair Institute (ICRI). This method uses a core bit to drill the applied shotcrete and substrate, and then attaches a rigid steel disk to the top of the core by using epoxy. Then, the test equipment applies some tensile load to the disk until it fails. This test method is similar to the direct test of tensile bond strength by the American Concrete Institute (ACI 2004) or the American Society for Testing and Manuals (ASTM 2004). A thin spray-on liner (TSL) has been developed and used in the mining industry since the 1990s. Ozturk and Tannant (2010) compare various adhesion test methods based on TSL and suggest a pulloff test of an elevator bolt (33 mm in diameter) attached to the liner with strong epoxy. This test uses a thin-walled diamond core bit to isolate the test area by overcoring. Ozturk (2012) provides a method for calculating the liner–substrate adhesion M. Son (&) Department of Civil Engineering, Daegu University, Jillyang, Gyeongsan, Gyeongbuk 712-714, South Korea e-mail: mson@daegu.ac.kr