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Shotcrete FAQs system to the shotcrete, and how are shotcrete-to-shotcrete shotcrete be applied directly to the interior of the rubble wall (gunite) anchors treated in an MSE wall? (which includes small, loose stones; large gaps; and cracks), or would we have to first have the walls repointed and Answer: There are many ways to attach a shotcrete facing smoothed over? to a soil nail shoring system or an MSE wall system. For the attachment to a soil nail wall system, you could review the Answer: Yes, shotcrete would be an excellent method to fill Federal Highway Administration’s “Manual for Design & the voids, open mortar joints, and gun an overlay over the Construction of Soil Nail Walls.” For MSE wall systems, irregular stone foundation. The use of shotcrete would be you should consult with the MSE wall system vendors. dependent on the access and ability of the applicator to safely Shotcrete facing systems are commonly used on both types place the shotcrete. A tight or low crawl space would make of walls, but it is beyond the scope of our association to it difficult. We would suggest cleaning out loose materials provide further guidance. with compressed air and water prior to the shotcrete place- ment. We recommend installing either a 2 x 2 in. (51 x 51 Question: I’m looking for information on the quantity of mm) 12-gauge or a 3 x 3 in. (76 x 76 mm) 11-gauge wire rebound expected when applying shotcrete against soil. We mesh over the stone foundation and gunning the shotcrete in have a W4 4 x 4 in. (102 x 102 mm) layer of mesh 2 in. (51 mm) place to fill in the mortar joints, creating a shotcrete overlay from the soil face that is covered by a 4 in. (102 mm) initial over the entire stone surface. layer of shotcrete. Is there a general ballpark figure that can be used, such as a percent of the total shotcrete placed? Question: We will be shotcreting a pool and the designer has put an expansion joint in the pool going from the top of one Answer: Your question does not indicate the orientation of the wall through the floor to the top of the other wall. It also shows application. If the shotcrete is being applied to a sloped surface a 9 in. (229 mm) polyvinyl chloride (PVC) waterstop in this for a channel or slope the rebound should be incidental. If joint. I have seen this used with cast-in-place concrete, but not shooting a vertical wall, the amount of rebound is relative to with shotcrete. I was wondering if there are any guidelines on the skill of the nozzleman, the quality or nature of the mixture, shooting around a PVC waterstop. the shotcrete process being used (wet-mix or dry-mix), the stability of the wire mesh, and other parameters. The range Answer: This detail is normally only used on very large com- could easily vary from 5 to 20% on vertical walls relative to petition pools on the order of 164 ft (50 m) in length. It takes the aforementioned listed parameters. a lot of skill, technique, and care to properly encapsulate the waterstop and it should only be attempted by a shotcrete con- Question: I have been hired to design a large concrete pit tractor with experience in this application. The successful for a fertilizer plant. The pit will need to be approximately encapsulation of the waterstop is more challenging with the 13 ft (4 m) deep by 55 ft (17 m) long by 15 ft (5 m) wide. The dry-mix process than when using wet-mix shotcrete. The tech- pit will contain water at varying depths and will support niques for the proper encapsulation are generally developed grating covering the pit that will support equipment. The state by the individual shotcrete contractor and there is no specific is requiring the pit slab and walls to be a monolithic pour. “guideline” available for encasing waterstops. Could shotcrete be used in this situation and be considered a monolithic pour? Question: We are, and have been, designing and constructing permanent soil nail and shotcrete retaining walls. Typically, Answer: If the directive from the state is to cast (or shotcrete) our designs consist of a primary nozzle-finished shotcrete facing both the slab and the walls monolithically, this would be a difficult to shore during our top-down construction, followed by a task with either shotcrete or cast concrete. If the directive is to secondary shotcrete facing that is shot and sculpted once the cast the floor monolithically, and then the walls monolithically, full height of the wall has been excavated, drilled, and shot shotcrete could certainly be used and would be considered a with the primary facing. monolithic placement. Once the state’s intent is clarified, this question should be posed to a shotcrete contractor who might We had a comment recently that only the secondary facing be the actual contractor on the project for their input. thickness can be used in our design for the wall’s flexural As this is a fertilizer plant, there may be additional consid- capacity because the shotcrete layers may delaminate. Our erations due to the potentially aggressive nature of the fluids general practice is to pressure-wash the primary nozzle- introduced into this pit. finished shotcrete facing before our approved and experi- enced nozzlemen place the secondary layer. From our Question: We own a 200-year-old house with a rubble experience, this procedure has been very effective and we foundation. The foundation is structurally sound, but needs to have not experienced any delamination between shotcrete be repointed, and some of it has no mortar at all. We would layers on any of the millions of square feet of shotcrete we like to seal it to make it watertight and keep out radon. Could have placed this way. 58 Shotcrete • Spring 2014


2014SprShotcreteEMag
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