Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 13—Replacement of Conduitsmicrotunneling, only a very short time is required to erode soil at the face and causelarge settlements. The operator must vigilantly control slurry circulation andmachine torque to avoid such unacceptable events (USACE, 1995d, p. 4-44). Figure189 shows a microtunneling installation when an operator stopped advancement.The selection of the microtunnel liner material is critical. The liner material must bestrong enough to withstand the jacking forces and other anticipated loads that theliner could be subjected to during the installation and life of the liner. The linershould be watertight and able to withstand internal and external water pressures.The liner should also be able to withstand any external earth and grouting pressuresexerted on it. Once installed, the liner should be strong and durable enough toprovide dependable service for the life of the project. Liner material considerationsconsist of welded steel pipe, reinforced concrete steel cylinder pipe, plain reinforcedconcrete pipe, HOBAS pipe (a composite fiberglass pipe) and HDPE pipe. Weldedsteel pipe is the liner material used on most projects. Steel pipe is very suitable forjacking and generally requires a relatively thin-walled section compared to concretepipe, which reduces the required tunnel excavation to install the steel pipe liner.Welding can be used to repair any damage to the steel pipe and for attaching fittings.The steel pipe has a relatively smooth exterior, which helps reduce the frictionbetween the excavation and the liner as it is being jacked into place. The steel canalso be designed to withstand all anticipated internal and external loads. If the tunnelboring bends slightly, the steel liner could buckle as it is forced around the bend. Ifthe liner does buckle, it will be very difficult to repair.Another consideration of the microtunneling technique is the selection of the linerjoints. The joints should have adequate compressive, bending and tensile strength towithstand the forces required to jack the liner into place. Once installed, the jointsbetween each section of microtunneled liner need to be able withstand both internaland external water pressures without leakage. The joints need to be perpendicular tothe centerline of the pipe, square, and snugly fit, so the conduit will be straight andeasy to steer with the tunnel boring machine. Once installed, the joints of the linerare generally exposed to very minor additional stresses.Because the pipe can “set” or “freeze” in a location if jacking operations are stoppedfor any length of time, a continuous installation process may be necessary. Commonpractice dictates the use of hydraulic jacks with a capacity greater than anticipated inorder to avoid this situation.Once the liner is installed, the annular space between the excavation and the tunnelliner is generally grouted in a continuous operation. As the grouting procedure takesplace, the lubricating fluid is forced out of the annular space. The grouting isintended to reduce or eliminate potential seepage along the tunnel liner. Thedesigner should explicitly specify the requirements for the liner installation andgrouting. In addition, the contractor should be required to submit for approval by339