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2014FallShotcreteEMag

Considerations for Underground Training By William T. Drakeley Jr. Tcaused developers to turn an eye to shotcrete substrate and placing the material accordingly.Key ingredients that are a must for anyrock, following joints and fractures in the-he growing demand for increasingly complex and adaptable urban infrastructure has as the most economically viable, technically flex- applicator, subcontractor, or company in the ible, and structurally sound construction method. underground shotcrete world include complete Shotcrete—or sprayed concrete—is the natural familiarity with the following: option for the underground tunnel or mining 1. Underground excavation; environment. It intrinsically allows for the 2. Geologic conditions of ground or rock stabilization of existing structures and accom - formations; modates tight construction schedules through 3. All aspects of the shotcrete process as it relates quick application without the need for forming. to underground support systems; and Furthermore, shotcrete can be applied to variable- 4. Installation techniques, geologic reinforcement, grade substrates such as ore, existing rock, or ledge and all alignment controls (such as rock bolts, without requiring extensive demolition because as lattice girders, welded wire, ground wire, a material, it binds quickly and bears its own weight. pencil rods, and mining straps). Shotcrete has increased in popularity as the With many tunnels or underground infra- preferred method of stabilizing blasted rock structure projects, determining the weight-bearing and covering exposed earth in underground properties and stability of the rock or ground will construction. Because tunnel construction is a determine shotcrete techniques (Fig. 1). Shotcrete multi-phase process wherein excavation may take installation comprises a key support component place in stages depending on the load-bearing to the substrate, following the process of capacities and stability of the substrate, shotcrete identification. A typical shotcrete crew needs to ideally allows for flexibility in the concrete understand this basic premise. Shotcrete appli - application without compromising strength. cators need to know how a site is excavated, what Relative ease of application notwithstanding, the different phases of tunnel rock removal are, underground shotcrete requires a precise set of and at what time drift wall support is needed. skills that are responsive to the underground Jürg Schlumpf and Jürgen Höfler, in their setting. Incorrectly placed materials can lead to handbook Shotcrete in Tunnel Construction,1 disaster, especially in high-risk environments such identify six factors that are of prime importance to as tunnels or mines. the mixture requirements for both dry- and wet-mix For those familiar with the aboveground shotcrete shotcrete processes relating to the workability and process, shotcrete in the underground environment durability of the mixture: high early strength, the will sound virtually the same. For instance, fol - correct set concrete characteristics, user-friendly lowing production of the shotcrete material, the workability (long open times), good pumpability material is transported to the site and conveyed to (dense flow delivery), good sprayability (pliability), the equipment. The material is then sprayed at high and minimum rebound. velocity onto the receiving surface. Given the force Regarding placement, crews must be able to of compaction and the immediate commencement identify the factors that will determine the amount of the hydration process, material may also be of material that can be placed in a single sitting. placed vertically and overhead. The shooting These include the adhesive strength of the concrete, direction should be at a right angle to the receiving the characteristics of the receiving surface, and the surface, whether placed manually or robotically. direction of placement. For instance, when shooting However, in the tunnel environment, it is the a ground or floor surface, the depth of placed substrate—the existing rock or ground conditions— material can be as thick as desired, as long as that should be viewed as the primary construction rebound and overspray are removed. When shooting material. Shotcrete is a key support component. walls, it is preferable to build thickness through a Crews must have the ability to perceive the number of thinner applications or through the reinforcement needs of individual sections of systematic layering of the full thickness from the 34 Shotcrete • Fall 2014


2014FallShotcreteEMag
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