Microtunnel Boring Machines (MTBM)
Reinventing Pipeline Installation: The Direct Pipe Revolution
Desalination, Coastal Solutions, and AVN Machines: Enhancing Water Sustainability
Microtunnel Boring Machines (MTBM)
Microtunnel boring machines (MTBM) are similar to larger tunnel boring machines (TBMs). The MTBM and jacking frame are set up in a shaft at the required depth. The operator monitors the MTBM's location, orientation, and hydraulic devices via a computer console, a CCTV camera, or a Gyro unit. Some systems use video cameras in the jacking shaft and at the separation plant. Gyro Control has generally replaced cameras for location and digital feedback.
In most microtunneling operations the machine is launched through an entry eye and pipes are pushed behind the machine. This pipe jacking process is repeated until the MTBM reaches the reception shaft at the far end. The speed of the advancing machine is limited to the speed at which the pipe is inserted into the entry eye via the hydraulic rams in the jacking frame.
AVN Machine
Herrenknecht AVN or Slurry Machines with a cone crusher are all-rounders for safe tunneling in the diameter range of 0.4 to around 4 meters. These microtunneling experts show their full potential, particularly in non-accessible machine sizes. The slurry-supported excavation concept makes it possible to use these machines in all kinds of ground conditions, ranging from silt to clay to incohesive soils, and further to gravel and rock. Depending on the diameter, this is possible both in pipe jacking and the segmental lining.
AVN Machines belong to the category of closed, full-face excavation machines with a hydraulic slurry circuit. The soil to be excavated is removed using a cutterhead adapted to the respective geology. This makes it possible to use the machines in almost all geological conditions. In soft soils and mixed geologies, standard or mixed ground cutterheads are used, while a rock cutterhead with disc cutters is used for tunneling in stable rock formations. A cone-shaped crusher inside the excavation chamber crumbles stones and other obstructions to a conveyable grain size while tunneling and advancing. Afterward, the material falls through openings similar to a strainer in front of the suction port and is removed through the slurry line and the suspension. The excavation diameter can be enlarged using an upsize kit and a modified cutterhead. This means that the AVN Machine can be used to tunnel different diameters and for different types of pipes.
In cohesive soils, in particular, keeping the crusher cone clean and avoiding clogging is a special challenge. Based upon years of practical project experience, Herrenknecht has developed a range of water nozzle options, which can cope with different geological requirements. Additional water can be injected to clean the system using high-pressure nozzles integrated into the cone crusher. These jets cut loam or clay in the excavation chamber avoiding clogging. The medium-pressure nozzles use the standard slurry ports. Changing the size of the nozzles can adapt the suspension pressure and optimize the material flow. A newly developed, optional camera system in the excavation chamber provides the operator with an additional tool to visually monitor wear and tear and verify the situation inside machines with a more than 1.2 meters diameter.
Pipe Jacking
AVN Machines are often used in pipe jacking operations. The maximum drive length depends on several factors such as skin friction, available jacking force, or the nominal diameter of the jacking pipes. Automatic lubrication systems reduce the friction between pipe skin and soil by injecting bentonite into the annular gap. In the range of diameters that allow man-entry, it is possible to use additional hydraulic intermediate jacking stations. These are installed in the pipe string at intervals that are defined specifically for each project. The necessary thrust force is reduced and distributed by dividing the pipe string into individual sections. This means that the distance between the launch and reception shaft can be more than 1,000 meters, depending on the project conditions and the number of intermediate jacking stations.
Installation of the Pipe String
A launch shaft is built out on the land side of the pipeline. Due to project-specific conditions the dimensions and the design of the launch pit may vary, economic considerations being a key factor. A thrust wall is being constructed to create resistance to jack the pipe string ahead. The correct pipe jacking direction at the start of tunneling is achieved by positioning guide rails in the launch shaft on which both machine and pipes can be aligned exactly.
High-performance hydraulic jacks are used to press the jacking pipes through the ground. At the same time, a steerable Micromachine excavates and removes the soil. The remote-controlled Herrenknecht Microtunneling machine is operated from a control panel in a container that is located aboveground close to the launch shaft. This guarantees high safety since no staff has to work in the tunnel during tunneling. The Micromachine’s position is monitored by a remote-controlled surveying system. During the construction phase, the necessary tunneling forces increase constantly due to the skin friction on the growing length of the pipeline already installed. Intermediate jacking stations and an automatic bentonite lubrication system are installed to reduce the tunneling forces and to better distribute them. This results in a reduction of the forces of the hydraulic main jacking stations acting on the launch pit and the thrust wall. Tunneling tolerances achieve values of less than 30mm. The seaside end of the pipeline is closed with a bulkhead equipped with a valve.
Advantages
• Usable in almost all ground conditions and with high water pressures
• Highest tunneling safety due to hydraulic tunnel face support
• Ideal technology for tunneling in the non-man-entry diameter range (Microtunneling)
• Precise control even with small radii in vertical and horizontal curves
• Very long pipe jacking stretches of more than 1,000 m are possible with the use of intermediate jacking stations
Excavation of the Recovery Zone
For the recovery of the Micromachine, a pit or depression must be excavated from the seabed which is big enough to allow the recovery. This can be done, for example, using an excavator or a suction pump.
Recovery Using a Floating Crane
A floating crane is positioned at the place from where the Micromachine is to be recovered. There are lifting eyes on the upper side of the Micromachine. The Micromachine’s lifting eyes are connected to the crane with the help of a spreader beam. Divers make all necessary connections. Then the floating crane lifts the Micromachine onto a boat or a pontoon and the machine is brought to land by a barge or a boat.
El Marsa Desalination Project by RADCO
In this example, a Micro tunneled pipeline connecting a caisson adjacent to the Mediterranean Sea to a submerged intake screening system at 12.5 m depths. The intake tunnel comprises 240 m of 1516 mm ID Steel casing micro tunnel section joined to the intake screening system with a submerged 12.5 m, 1,200 mm GRP pipeline.
RADCO was responsible for designing and administrating the El Marsa desalination project.
RADCO used a Herrenknecht AVN 1200 MTBM system with a mixed ground cutter head to contend with the mixed face geology. The system includes an MT875K jacking frame with 700 tons of thrust capacity, a control container with the operator control console, power distribution center, and MTBM drive motor, a series of pumps to assist with excavation and slurry circulation, and a Derrick slurry separation plant.
Conclusion
There are no impairments, neither for the coast nor for the people living there. The method is environmentally friendly and the tunneling work does not depend on climatic conditions, such as waves, storms, etc. In addition, the installation of the pipe string can be carried out using tunneling technology on an onshore site. There is no hazardous handling of the pipe string during installation or floating and in connection with expensive offshore equipment and diving teams.
Herrenknecht Microtunneling technology allows for the construction of long sea outfalls which can be jacked deeply beneath the seabed – under almost all geological and topographic conditions. Herrenknecht machines are very suitable for rocky/mixed grounds, and also underwater. Horizontal Directional Drilling and outfall tunnels which are built with the segment lining method, the Direct Pipe® method, or with pipe jacking technology (Microtunneling) do not depend on the costal conditions, i.e., outfalls can be driven even on rocky or steep coasts.