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2013FallShotcreteEMag

Pool & Recreational Shotcrete Corner Success in Shotcreting Begins with Forming By James Scott Icessful shotcrete structure, and can be done in grade the pool floor rests upon left in an undis--fundamental, requirement that all organic soiland materials must be removed, with the sub-n the creation of swimming pools, forming isone of the core components to building a suc a variety of ways. Each type of forming has its turbed and compact condition (Fig. 1). Many pros and cons; and there usually is more than one times, a layer of crushed stone is applied over type of forming that will work for any particular the top of the subsoil to provide a dry, stable situation. workable surface, as well as a clean, dense Forming for shotcrete is one-sided and has surface for the shotcrete. The crushed stone performance criteria it must meet. Stability of also serves as a drainage layer under the pool formwork is a major factor that allows for suc- shell. In sites with a rock subgrade, the crushed cessful shotcreting. Form stability means forms stone creates a “cushion” over the rock. An that don’t move or vibrate during the gunning additional benefit of the crushed stone layer is process, and allows fresh shotcrete to be placed that it can be used to fill in voids or remove at optimal density without shifting in the finished unevenness in the excavation process—which concrete structure. Consistent high density of the enables the floor to be shotcreted at a uniform placed shotcrete equates to water tightness of the thickness to meet the design thickness, as pool shell, which is the whole point, after all! dictated in ACI’s “Shotcrete for the Craftsman.” Often, earth is used as the form for shotcrete • In addition to being a form for the floor, the in two areas: horizontal and vertical (that is, floors earth subgrade has to structurally carry the high and walls). loads of the filled pool without excessive settle- • Most pools will have the floor setting on the ments. The soil will be looked at for various in-place soil of the project. It is a basic, but issues when analyzing its suitability for any particular project: soil structure and variation in the soil profile, presence of groundwater, ledge rock, expansive soils, and sloping grades. The geotechnical engineer will often use soil testing to determine the soil’s competence to carry the loads. The structural engineer will consider the bearing capacity and settlement of the soil. • I once asked an engineer, “What is the single most important thing I could focus on to lessen my risk when building a pool?” His answer was simple and telling; he simply said, “Com- pact the last layer of soil exposed by the machine before putting down the crushed stone.” This told me that if I left any disturbed, less-dense soils under my pool, compaction would be taken care of for me by gravity with the weight of the filled pool! In a very real way, the ground must be in a stabilized, compact condition before the shotcrete process. For Fig. 1: The subgrade has been carved to the profile required, including those in the freezing areas, be aware that a trenching of plumbing lines beneath the future pool shell frozen subgrade has an expanded volume that 56 Shotcrete • Fall 2013


2013FallShotcreteEMag
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