The recent Position Statement #2, “Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes,” published by the ASA Underground Committee, pointed out many aspects critical to successful performance and raised some potential issues affecting the placement.1 In the position statement, specific techniques are presented to prevent problems such as delamination, voids, or fallouts. In the discussion, the potential issue of steel fiber-reinforced shotcrete (FRS) causing damage and potentially puncturing the membrane was raised. From the experience of the committee and the available information, it was concluded that: • The forces acting on the fiber are not strong enough to push the fiber into the membrane; and • The fibers tend to orient parallel to the membrane on impact, thus reducing the risk of damage. In parallel, a research project on this subject had been undertaken at Université Laval’s Shotcrete Laboratory, with the results only recently available. This article presents the results of this investigation.2 It is intended to support ideas presented in the ASA position paper and to help in the decision-making process when dealing with waterproofing membranes and FRS in underground projects.
Fiber-Reinforced Shotcrete Applications and Testing Overview
The addition of fibers to concrete and mortars as reinforcement is not a new concept. The ancient Egyptians used straw to reinforce mud bricks for use in structures like the core walls of the pyramids. During the first century AD, the Romans incorporated horsehair fibers in the construction of structures like the Coliseum to help prevent drying shrinkage cracking of the concrete. In the modern era, the first scientific studies on the use of steel fibers to reinforce concrete date back to the 1960s and 1970s.1,2 The use of steel fiber-reinforced shotcrete (FRS) was first introduced in the 1970s.3 The first documented use of FRS was in 1973 by the U.S. Army Corps of Engineers in a tunnel adit project at the Ryrie Reservoir in Idaho. Soon thereafter it became well recognized that soil and rock excavations could effectively be stabilized with steel FRS and its use and acceptance increased globally. In the mid-1990s, the use of macrosynthetic fibers in shotcrete was developed and has increased with particular success in temporary support in underground mines where large deformation capacity is desired. Since the 1970s, thousands of projects have been successfully completed using fibers as reinforcement, including shotcrete, slabson-ground, composite steel decks, slabs-on-pile, and precast elements.
Maryland Purple Line Plymouth Tunnel
The Plymouth Tunnel is a 1020 ft (311 m) sequential excavation method (SEM) tunnel located in Silver Spring, MD, that makes up a portion of the Maryland Transit Authority’s (MTA) Purple Line light rail connecting the existing metro lines around Washington, DC. The Purple Line Transit Constructors (Flour/Lane/Traylor Joint Venture— PLTC) is the Lead Contractor with the Traylor personnel self-performing the excavation and lining work for the Plymouth Tunnel.
Can shotcrete be considered as structural concrete with wire mesh and rockbolts in tunnel linings?
Shotcrete is a placement method for concrete. It is routinely used for a wide variety of structural applications. It has been used for both initial and final linings in tunnels where it is commonly reinforced with wire mesh, fibers, or reinforcing steel. You may want to review our past Shotcrete magazine articles on tunnel shotcrete at https://shotcrete.org/archive-search/
using keywords such as “tunnel,” “underground,” and “linings.” Also, we have two position papers from our underground committee: “Spraying Shotcrete Overhead in Underground Applications,” and “Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes,” that you may find informative. Also, ACI 506.5R-09, “Guide for Specifying Underground Shotcrete,” can provide insight into topics important for using and specifying underground shotcrete.
We have a question about the shotcrete setup strength for the exclusion zone in underground shotcrete work. New York and others are requiring a shotcrete exclusion zone (an area excluded from personnel) based upon either time or strength. The UK asks for an engineered approach to this minimum strength. Do you have any information on this?
The minimum strength for safety must be established by the designer based on the specific structural and geotechnical aspects of the project. The minimum strength value may also be influenced on whether using fiber-reinforced shotcrete or plain shotcrete. The early-age strength was tested with a Meyco Needle Penetrometer after creating a time (early) strength curve with minimum tests at 10 minutes and 30 minutes with Needle Penetrometer; then at 3 hours and 6 hours with Hilti Studs; then at 1, 7, and 28 days with cores. Thus, the curve was calibrated for the specific mixture and environmental conditions. It was also useful to identify when early strength (and potentially long-term strength) was lacking.
I am a structural engineer and we have recently begun work with a shoring contractor. We have been designing soil nails, micropiles, soldier piles, and so on with temporary and permanent shotcrete facings. The contractor has requested that some of our future designs use chain-link mesh in lieu of welded wire mesh, particularly in temporary situations with walls under 10 ft (3 m). I understand that chain link is a cost-effective alternative and, according to the contractor, handles the shotcrete well. Do you have any experience with this type of design/installation process? Can you point me to any literature on the use of chain-link reinforcement in shotcrete walls?
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.