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2017SummerShotcreteEMag

| COMMITTEE CHAIR MEMO ASA Underground Committee The Underground Committee has developed a new seminar for underground shotcrete “Shotcrete, the method of spraying concrete onto a surface, is ideally suited for the support and construction of underground excavations in earth and rock structures. The pneumatic projection of shotcrete onto a surface at high velocity provides specific quality enhancements that interact with the ground surface and By Axel Nitschke prepared substrates, providing superior bond characteristics; increased density; and resultant strength, durability, and toughness.” “Underground construction and shotcrete application are unique and very demanding. The primary focus is worker safety and the need to provide immediate and effective ground support.” “Early 8-hour to 1-day strength is critical to the performance of underground shotcrete. Much of the shotcrete is applied overhead to irregular tunnel profiles immediately following blasting or other modes of excavation.” “The use of accelerating admixtures is a unique feature of underground shotcrete application in that it provides a means of controlled and rapid strength gain immediately following application.” “The systems of batching and handling of shotcrete materials admixture, and placing equipment require experienced supervisory shotcrete and production personnel.” “Quality assurance, quality control, and the associated inspection and testing activities are equally important in achieving a successful shotcrete program.” The aforementioned quotes are from the Introduction and Scope chapter of ACI 506.5R-09, “Guide for Specifying Underground Shotcrete.” I spend most of my professional career in conventional tunneling, primarily in soft ground or weak hard rock, where shotcrete is the key support element for what tunnel builders call “ground support” and miners call “ground control.” I like the term “ground control” a lot because, if done properly, shotcrete gives you a key tool to “control” the ground. It is important to realize that immediately after the excavation of a new round of tunnel—especially in soft ground or weak hard rock—the ground conditions deteriorate over time to a varying degree. This fact was labeled by our predecessors as “stand-up time.” “Stand-up time” is still widely used by tunnel builders and miners as a qualitative and descriptive assessment of the ground conditions, which defines the time span after the excavation until the tunnel would eventually become unstable if unsupported and would eventually collapse. Stand-up time also gives you guidance about the timeframe that allows the miner to safely install the required support measures. Shotcrete is one key support element to arrest the deterioration and significantly improve the actual stand-up time by sealing the ground surface and providing temporary and/ or final support in combination with or without other support measures. Therefore, proper knowledge and experience about shotcrete at all project levels is essential for underground applications. The ASA Underground Committee is a diverse group of contractors, engineers, and suppliers centered on the use of shotcrete in tunneling and mining. The mission of the Underground Committee is “to educate and promote the use and proper application of shotcrete to the underground construction and mining industry.” In October 2015, the Underground Committee committed to two objectives to achieve this goal: 1. Develop a PowerPoint presentation to “promote and educate” the use of shotcrete based on ACI 506.5R, “Guide for Specifying Underground Shotcrete.” 2. Present at seminars, by webinars, at conferences (WOC, SME) or to specific clients (DOTs, government agencies, contractors, A/E companies). During the last 1-1/2 years, the committee has worked on the development of the presentation. The presentation includes contributions of numerous members of the committee and was reviewed, finalized, and balloted within the Underground Committee in March 2017. Beginning April 2017, the presentation was balloted by the ASA Board of Direction and was accepted without comments on May 3, 2017. This date is important for the Underground Committee twofold: it marks the completion of the first objective but, more importantly, it provides the Underground Committee with a product and tool to reach out into the industry to present at seminars, by webinars, and at conferences or to specific clients to work on the committee’s second objective. The purpose of the presentation is to act as a door opener to professionals who have no or limited experience with shotcrete, but are tasked to specify, design, inspect, or 6 Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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