Hybrid TBMs: A Clear Solution for Mixed Ground
It has been argued that the majority of good ground for tunneling in the world already has a tunnel going through it. While this might be an overstatement, it is true that more and more of today’s tunneling projects are being proposed in difficult, mixed ground conditions. Hybrid TBMs, specifically EPB/Slurry and EPB/Hard Rock machines, are increasingly becoming the best solution for these challenging conditions.
Hybrid machines have the potential to lower risk and make difficult excavations possible, as long as accurate geologic information is available. For example, a hybrid EPB/Open-Type machine can be optimized towards either end of the scale—depending on whether the majority of the drive is in soft soils or majority in hard rock—to produce the fastest possible advance rate over the entire project. If the tunnel is 80% soft ground and 20% hard rock, the overall machine design will be optimized towards EPB.
But what happens when the divisions are not so clear cut, or when the geology is 50% rock and 50% soft ground? A real example is the upcoming Sleemanabad Carrier Canal project, which will utilize a 10.0 m (32.8 ft) diameter Hybrid EPB/Hard Rock TBM. The 12 km (7.5 mi) long water transfer tunnel in Madhya Pradesh, India consists of shorts sections of hard rock and soft ground. Geology includes clay, gravel, marble, and hard rock up to 180 MPa (26,000 psi) UCS.
The solution at Sleemanabad is to provide three modes of tunneling: pressurized EPB tunneling, non-pressurized EPB tunneling, and open hard rock tunneling. The first half of the tunnel will be excavated using a specially designed screw conveyor, which can operate well in both the pressurized and non-pressurized environments. The oversized screw can be rotated faster to handle short sections of hard rock while minimizing any loss in efficiency.
When a longer section of rock is encountered, the screw conveyor is removed and switched out for the TBM belt conveyor. Additional welded steel is added to the cutterhead to close up the open soft ground design, and the machine operates as a Single Shield TBM.
In other conditions, such as sections of soft ground and rock mixed with sections of high water content, a Hybrid Slurry/EPB Type machine is desirable. The machines can similarly be optimized towards slurry or EPB depending on the amount of groundwater expected. An interesting example of this type of hybridization took place at the South Central Hillsborough Intertie Tunnel in Tampa Bay, Florida, USA. The machine excavated limestone with significant groundwater using both a screw conveyor and modified slurry removal system. See the case study here.
Hybrid machines are becoming more common, and are likely to gain in popularity over the near future. With greater experience will also come greater optimization of conversion times between modes—one of the biggest challenges during tunneling. While these machines do have some negatives, as all machine types do, the solution of Hybrid TBMs in mixed ground vastly outweighs any difficulties over the life of the project.