Some mines have used chain link mesh in shotcrete in severely deforming ground and claim that it is better in holding the ground than mesh after large deformations, in which the shotcrete sustains major cracking with deformations. Other than for such unusual applications, we do not recommend the use of chain-link reinforcement in shotcrete. It cannot be fixed “tight” and as such is susceptible to vibration and movement during shooting, resulting in shotcrete sloughing and formation of voids in the shotcrete. Also, the mesh interconnections are conducive to the formation of voids during shooting. Additionally, there don’t appear to be any consistent material standards on the strength, flexibility, or brittleness of the steel (or other materials) used in the fencing material, so a designer has no way to establish the tensile or flexural strength of the concrete sections. In brief, don’t use chain-link mesh if you want to produce quality, durable shotcrete.
We have a concrete tunnel repair project, where expansion joints in a 10 x 10 ft (3 x 3 m) tunnel need to be repaired. The joints are on the top, bottom, and both sides of the tunnel; therefore, we have vertical and overhead applications. The detail calls for deteriorated concrete to be removed to a depth of about 6 in. (152 mm), existing reinforcing steel to be cleaned and preserved, and old waterstop removed and new PVC waterstop installed. How far can shotcrete be pumped for an application in a tunnel? We are looking at about 200 ft (61 m) for the shotcrete to be pumped from the supply to the repairs. Is this constructible?
Yes, this is definitely a great application for shotcrete. Either wet- or dry-mix shotcrete can easily be used in tunnels with hose lengths of 200 ft (61 m). An experienced shotcrete contractor will be able to select the appropriate process based on the site, availability of material, and their particular equipment and trained shotcrete crews.
I am lowering the roof of the draft tube on a hydropower dam. The work is all overhead and has a slope to it. The new roof profile will be lowered from 2 in. (51 mm) (initial edge) to 6.25 ft (approximately 2 m) thick. The plan is to use rock anchors to transfer the load to the existing concrete, and tie in a reinforcing bar grid at the lower section of the new roof profile (4 in. [102 mm] cover). Shotcrete is being planned for the infill material. The traditional ACI 318 design method was used for sizing reinforcing bar. The concern I have in the design is the application of the shotcrete. The plan is to allow the contractor to install a wire mat (or reinforcing bar mat) approximately 2 in. (51 mm) from the existing concrete roof, then apply shotcrete until reaching the reinforcing bar location, up to 6 ft (1.8 m) thick, then apply the finish layers. I have concerns about how thick shotcrete can be applied overhead. I have received feedback from some shotcrete companies that one can apply up to 4 ft (1.2 m) thick overhead layers, yet others say to never apply more than 4 in. (100 mm) layers. I also have concerns of delamination between the shotcrete and the existing concrete during the installation process, and potential shotcrete falling under its own weight in the thicker locations. Do you know of any situation where shotcrete was installed overhead to thicknesses of 6 ft (1.8 m)? How thick can the shotcrete be before additional reinforcement is required to hold it for overhead applications? Is there a recommended maximum thickness for overhead application of shotcrete being placed before additional reinforcement is required? For the area that is 6.25 ft (2 m) thick, should I be using multiple layers of reinforcement (or fiber reinforcement) to prevent fallout?
This is a very challenging potential installation and there could be several potential approaches. For the shotcrete to bond to the existing concrete, the existing surface should be properly prepared, removing any unsound concrete, then roughened and cleaned to allow for a good bonding interface. You mentioned using rock anchors or bolts. These should be installed before any shotcrete.
For each 6 in. (152 mm) layer, a layer of welded wire reinforcement or structural fibers should be used. These, in conjunction with the rock bolts, should ensure the stability of each layer of the shotcrete from falling.
The surface reinforcement should not be installed before most of the area is within 6 to 8 in. (152 to 203 mm) of the final surface.
There is currently similar thicknesses being placed on the East Side Access in New York City to build back the overbreak for the initial tunneling to the “A-line” or profile that was intended for the mining.
We have a client who is looking to make a relatively deep cut in a mixture of soils and rock (approximately. 50 ft [15.2 m] high, maximum; the structures will be placed on a pad at the bottom of the cut). We are looking to provide a shotcrete facing for the entire cut area. The upper portion of the cut will be in soil; therefore, the design of a soil nail wall with temporary and permanent facing in the soil region seems to be relatively straightforward using design guidance in FHWA publications, Geotechnical Engineering Circular #7, and some software programs. Significant portions of the exposed cut face, however, consist of nondurable bedrock (claystone). We want to stabilize this area with shotcrete to prevent weathering and the generation of overhang conditions where the claystone is overlain by a more durable sandstone. I have been unable to find design procedures or guidance on specifying shotcrete (thickness, reinforcement type, etc.) and whether or not rock bolts should be used. If so, how do you select the size, spacing, resin type, etc.?
Soil and rock stabilization is an excellent application for shotcrete. However, ASA as an association does not provide engineering design. We recommend consulting with a geotechnical engineer familiar with the local geology and soil conditions to evaluate potential lateral earth forces from the claystone. Once potential loads are established, a consulting engineer experienced with shotcrete in soil nailing applications will be able to design the soil nail facing. You can check our online Buyers Guide to find a consulting engineer experienced with shotcrete.
We are replacing an undersized box culvert carrying a creek under a road with a vehicular bridge. To reduce excavation limits, we are using top-down caisson wall construction with shotcrete facing between caissons for abutments and wingwalls. The shotcrete will be placed in lifts as soil is excavated between abutment/wingwall caissons. The architectural pattern for the face of the abutments and wingwalls is a rectangular pattern of an indented, V-shaped notch. The notches have a maximum depth of 2 in. (51 mm). The structural portion of the shotcrete wall will be 12 in. (305 mm) thick with steel reinforcement. Can this horizontal and vertical V-notch pattern be formed or stamped into the face of the structural wall (with additional thickness as required for pattern) in one wall placement? Or does the pattern have to be a separate placement after the structural wall is cured? If this is done in two placements, I assume that we would need reinforcing bars from the structural portion of the wall into the architectural placement and reinforcement within the architectural placement to lock it in place. What is the minimum required thickness of the architectural layer to account for reinforcing bar embedded from the structural layer and the required reinforcing bar in the architectural layer?
There are many ways to approach this situation. It would be difficult, but not impossible, to install all of the work in a top-down sequence and end up with an architecturally uniform surface.
Approach 1: Install a minimal initial layer top-down with either fibers or welded wire reinforcement. Install dowels from the caissons into the structural facing layer. Install the facing from the bottom up with preplaced V-strips to make the pattern. Finish to the outermost face of the detail strips. Alternately tool the details, but likely more like 1 in. (25 mm) instead of 2 in. (51 mm).
Approach 2: Install the structural wall top-down, encapsulating the outer reinforcing steel to a plane at the depth of the detail strips. Prepare the surface by sandblasting or water blasting to create a favorable bonding surface. Install detail strips to the face of the roughened wall. Place and finish the finish layer to the depth of the detail strips.
If the base layer is properly prepared, the bond should be very good and adding dowels would be redundant. There is nothing wrong with redundancy and if so, the minimum layer thickness would be 2 to 3 in. (51 to 76 mm).
The nature of this work will mandate the use of a highly qualified shotcrete subcontractor who has experience in installing similar-quality architecturally significant walls.
We are involved in the design of a hydro project in a section of a water-conveyance power tunnel; we are considering using shotcrete reinforced with welded wire reinforcement as a final liner. In this particular section, the tunnel is under an internal water pressure of 189 psi (1.3 MPa) and water velocities in the range of 16.4 ft/s (5 m/s) can be expected. We have not found any examples of such a design/use at this water velocity and are concerned about long-term durability and potential erosion of the shotcrete and entrainment of fragments into the turbine/powerhouse. Would you have any information regarding the ability of shotcrete to resist water erosion, particularly at 16.4 ft/s (5 m/s)? (Any examples would be appreciated.) What additive can be used to reduce the porosity of the projected mixture?
Shotcrete is simply a placement method for concrete, so characteristics of concrete that are resistant to erosion are equally applicable to shotcrete. ACI 210R-93, “Erosion of Concrete in Hydraulic Structures,” has guidance on flow characteristics that lead to erosion of concrete. Also, ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” Sections 4.6.2 and 4.6.3, also provide guidance on concrete mixture characteristics helpful for protecting against cavitation erosion. Properly designed shotcrete mixtures can easily meet the ACI 350 4.6.3 concrete requirements.
In 2000, Rusty Morgan compiled a list of some 37 water supply tunnels that had been lined with shotcrete (a copy of the data sheet can be supplied upon request). Shotcrete was not the final lining in all of these tunnels and not all the inverts were lined with shotcrete. The evaluation does not document the water velocity in these tunnels, but could be ascertained by contacting the project owners.
It should be noted, however, that the 16.4 ft/s (5 m/s) water flow rate is not particularly fast. The water velocity needs to be in excess of 39.3 ft/s (12 m/s) before cavitation erosion can be expected (refer to A. M. Neville, Properties of Concrete) and cavitation would be the most likely cause of erosion of the concrete surface.
Supplemental cementitious materials including microsilica, fly ash, and slag will generally reduce the porosity of the hardened concrete. Microsilica is used in many shotcrete mixtures, as it helps to reduce rebound, as well as gives the fresh concrete better adhesion and cohesion that can allow for thicker or overhead placements.
We would like to place 4 in. (100 mm) thick shotcrete reinforced with welded wire reinforcement and anchoring bolts in a water pressure tunnel. The water velocity would be between 10 and 16.4 ft/s (3 and 5 m/s). We would like to know if there is a possibility of erosion or cavitation of the shotcrete at this range of velocity. It is mentioned in our concrete manual that cavitation and destructive erosion begin when water velocities reach about 40 ft/s (12 m/s). Because the roughness of the shotcrete surface is higher than the concrete surface, is erosion more likely to occur? Do you know what may be the maximum water velocity acceptable for reinforced shotcrete?
Shotcrete is a method of placing concrete and the surface finish can be as smooth as that of cast concrete. Even with a nozzle finish, shotcrete erosion or cavitation should not be an issue at the stated velocities. Examples of smooth shotcrete surfaces can be found in many Shotcrete magazine articles and in particular (“Restoring the Century-Old Wachusett Aqueduct”).
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.