The Pressures of Tunnel Grouting

Are you afraid of floating or collapsing the carrier pipe during tunnel grouting? What about tunnel and pipe failure resulting in a road or railroad settling, not to mention water, or even worse wastewater, shooting out of the ground onto a road or into a river? These are just some of the dangers of improper grouting of tunnels, proving that tunnel grouting can be just as risky as tunnel excavation itself. If too much grout at too high of pressure is introduced while grouting the annular space between the tunnel liner and carrier pipe, the grout can exceed the allowable strength of the carrier pipe and cause it to collapse. If too little grout is introduced or at too little pressure, the grout will not completely fill the annular space to protect the tunnel after the tunnel liner corrodes. Once the grout begins to harden, chiseling out the grout and carrier pipe may be the only solution, so it is important to get tunnel grouting right the first try. The design team needs to develop specifications to set the Contractor up for success by specifying acceptable grout mixes, densities, pressures, and number of lifts. The specifications should also include methods for preventing the pipe from deflecting or floating and require an overall tunnel grouting work plan. Large diameter pipes likely require multiple lifts and blocking, hold-down jacks, and/or filling the pipe with water to prevent floatation. Contractors need to select and test mix designs for grout, perform calculations for the theoretical volume of grout, and develop a plan for injecting the grout to fill all voids in the annulus. For contact grouting, the Contractor must plan for the installation of grout ports in the primary tunnel liner when the grout is to be injected depending on the type of primary tunnel liner. For backfill grouting, grout may need to be injected at multiple locations along the tunnel to fill all voids and evenly distribute the grout. Planning the backfill grouting operation is crucial, because removing a bad grout job can damage the primary tunnel liner and carrier pipe as well as cost significant time and money. Based on recent experience with water and sewer line projects across Texas, we have developed some best practices when performing tunnel grouting. This paper will discuss those best practices, specification language, example calculations, and tips during submittal review to avoid the costly consequences of improper grouting which will benefit Owners, Engineers, and Contractors.