Our company is carrying out a tunnel project in rather poor geological conditions, including water seepage and poor rock, with wire mesh and two layers of steel mat. What is the reasonable rebound percentage in such conditions?

Shotcrete rebound varies for many different reasons, many of which you mention in your question. The water seepage must be controlled or the shotcrete will likely not adhere to the surface and will slough off as the water saturates the fresh shotcrete. Accelerator will help, but it is difficult, if not impossible, to achieve good results against a seeping surface. ACI 506R-05, “Guide to Shotcrete,” estimates approximate range of shotcrete losses from 10 to 30%. Some other factors affecting the percentage of rebound are:

Mixture design

  • Shotcrete process (wet- or dry-mix)
  • Concrete mixture design and materials (for example, microsilica will tend to create less rebound; more than 30% coarse aggregate can cause more rebound)
  • Plastic concrete properties (air content, slump)
  • Nozzleman competence
  • Vertical placement generally has less rebound than overhead
  • Thickness of buildup per layer

Reinforcing grid

  • Size and spacing of reinforcing
  • Stability of reinforcing grid

We are, and have been, designing and constructing permanent soil nail and shotcrete retaining walls. Typically, our designs consist of a primary nozzle-finished shotcrete facing to shore during our top-down construction, followed by a secondary shotcrete facing that is shot and sculpted once the full height of the wall has been excavated, drilled, and shot with the primary facing. We had a comment recently that only the secondary facing thickness can be used in our design for the wall’s flexural capacity because the shotcrete layers may delaminate. Our general practice is to pressure-wash the primary nozzle-finished shotcrete facing before our approved and experienced nozzlemen place the secondary layer. From our experience, this procedure has been very effective and we have not experienced any delamination between shotcrete layers on any of the millions of square feet of shotcrete we have placed this way. If installed correctly with our general practice, is there any reason the shotcrete layers would delaminate? If not, have any studies been done to prove this to our reviewer?

All of your points are valid, but the Engineer of Record or the owner makes the final decision on recognizing a composite system or ignoring the value of the initial layer. As your experience shows, shotcrete provides an excellent bond between freshly placed layers and properly prepared concrete or shotcrete substrates. There are many articles available in the Shotcrete magazine archives—found on our website, —that may provide the designer or owner more information to allow them to make their design decision.

We are developing a tunnel. At the tunnel portal (entrance), we have high walls around the portal about 60 ft (18 m) tall. They will have an inner structural shotcrete layer (4 in. [102 mm]) and outer architectural shotcrete (12 in. [305 mm]). Between the structural shotcrete and rock/soil, we have a drainage system to handle the groundwater. At the same time, we may have water at the top ground surface that will drain from top to bottom of the wall. The owner didn’t want to make the water flow as a sheet over the wall surface. We proposed an inlet and vertical 6 in. (152 mm) pipe drop from the top to bottom and band to a ditch at the base of the wall. Can we locate the 6 in. (152 mm) pipe between the structural shotcrete and the architectural shotcrete?

The Federal Highway Administration’s “Manual for Design & Construction of Soil Nail Walls” should address this issue. Many soil nail wall systems incorporate a drainage ditch at the top of the wall that catches the runoff and takes it to the ends of the wall. Your concept of a catch basin and drain between the layers is not something we have seen in the past and we are not qualified to express an opinion on this. We have seen systems with catch basins at the top of the wall and the drains behind the initial layer of shotcrete requiring notching the subgrade. To answer your question, yes, a 6 in. (152 mm) pipe can be fully encased in shotcrete between the layers. Complete encasement of an embedment of this size needs an experienced shotcrete nozzleman with properly sized equipment, appropriate concrete mixture design, and a trained shotcrete crew. The issue of appropriateness of the approach is better answered by a licensed professional engineer familiar with soil nail systems or retaining walls, and shotcrete/concrete.

We are using wet-mix shotcrete for culvert linings, with an existing corrugated steel plate pipe stream culvert. The pipe is 96 in. (2438 mm) long and deteriorated. There is a water diversion, but there is a pressure gradient forcing water through the voids. Any ideas on leak repair procedures?

Installing a shotcrete lining requires a somewhat dry substrate and certainly is not compatible with running water. The water needs to be blocked or diverted.
A means of blocking the inflow is to inject a swellable urethane grout through the openings in the existing pipe. The grout, if done properly, will expand upon contact with water and seal the outside of the pipe. Another means of diverting the water is to install drainage material over the inflowing area to collect the water and remove it from the pipe. The shotcrete can then be applied over the drainage material.

I am an engineering technologist working on a landslide project where shotcrete had been applied to stabilize the sandstone head scarp at the crest of the slope. The shotcrete was applied in 1998. After a recent inspection, it was noted that the surface of the shotcrete had some cracking in some sections. How can this be repaired? Can the cracks simply be filled with a grout/mortar mixture of some sort or do the cracked sections have to be removed entirely and shotcrete be reapplied?

Shotcrete can and has been used to overlay previously installed shotcrete or concrete that has cracked over time. It would be advisable that you engage an engineer knowledgeable in geotechnical engineering and concrete properties to formalize a solution. It is important that the cause of the cracks be determined and adequate reinforcing be designed to ensure that the cracks do not propagate through the overlaid shotcrete.

We have a 17 mile (28 km) long TBM tunnel for water that will drive our underground powerhouse. Is there a recommended shotcrete surface texture we could use? Our contractor is using 0.31 in. (8 mm) aggregate, but they are getting an undulating surface. Can you provide some clarity as to what we should ask our contractor to try and achieve?

Shotcrete can be applied with many different textures. The nozzle finish shown is very rough, even for a natural gun finish. Nozzle finishes can be done smoother than this. Another technique would be to use a broom to make it smoother after it is shot. Other finishes include wood float, rubber or sponge float, broom, and smooth trowel finishes. There are many examples of finishes shown in articles in Shotcrete magazine.

I am an engineer working on a project involving shotcrete and earthwork. The shotcrete that was placed has some expansion cracks, which we expected. I would like to know the best way to repair them. Is there some type of waterproof coating/grout that can be applied between the cracks? Part of the cracks will be continuously under water. The shotcrete is the surfacing material for a diversion ditch at a mine, and we need to recommend some remediation solutions to our client.

There are many products in the marketplace for repairing cracks. Because shotcrete is simply a method for placing concrete, any method for concrete crack repair would be applicable. It would be wise to use a product that filled the cracks and is able to tolerate thermal movement in the future (not a brittle product). Many injectable polyurethane grouts can accomplish this. Surface-applied coatings would need an adequate thickness and elasticity to tolerate moving cracks. We suggest that you contact one of our corporate members who is familiar with your area and get their specific advice. Please refer to “ASA’s Buyers Guide”.

I am a structural engineer working on underground structures such as tunnels and caverns. I would like to know the permissible shear strength of shotcrete to be taken for M30 Grade SFRS (M30 = 30 MPa [4350 psi] at 28 days). I would like to know more about its other properties, as well.

Shotcrete is simply a placing method for concrete. Thus, the in-place material properties are essentially the same as cast concrete. A specific value for the shear is beyond the scope of our Association because many design and material properties can affect the shear capacity. We would suggest you engage a Professional Engineer who specializes in Underground Shotcrete. You should consult our Buyers Guide to find such a consultant. ACI 506R-05, “Guide to Shotcrete”, would be a helpful primer to learn more about shotcrete.

I am evaluating a community in central Colorado that contains shotcrete slope reinforcement ranging from 14 to 44 ft (4 to 13 m) in height. Assuming the installation met all required guidelines, what should I anticipate as a useful life for this product?

Shotcrete is a method of placing concrete and properly placed shotcrete should have a service life similar to cast concrete. Generally, concrete structures in normal environmental exposures are expected to have service lives from 50 to 100 years. With particular attention to materials and construction methods, some concrete structures, such as the new San Francisco Bay Bridge, have been designed for a service life up to 150 years. The first step in achieving a long-lasting, high-quality installation is to engage a highly qualified and experienced shotcrete contractor. There are many other factors that influence service life, including using the right mixture design for the anticipated exposure conditions.