The term "Non-Continuous Pressurised (NCP) driving" mentioned in this article refers to tunneling with single-shield hard rock TBM or dual-mode (hard rock/earth pressure) TBM.

Cutter detection

The detection method of the cutter head is different in soft soil and hard rock: in soft soil, the inspection frequency of the cutter head is usually based on the distance (such as 100m/time). When using a slurry shield in hard rock, the frequency of inspections must be increased due to the increased consumption of the cutterhead.

The inspection of the cutterhead of the muddy water shield requires high-pressure entry into the tank, which is a high-risk operation. It is difficult to carry out directly when the water pressure exceeds 6.5 bar, so the high-pressure entry method must be used. Although there are already cases of high pressure entry of about 12bar, the risk is huge; while the pressure in some tunnels is much higher than 12bar, entry becomes more expensive, dangerous and time-consuming.

In a project in Hiroshima, Japan, the construction party used a Robbins muddy water shield with a diameter of 13.7m to excavate in a granite layer. The builders chose the muddy water TBM because they were more experienced with muddy water TBM, although they expected to encounter water pressures of up to 13 bar; the high pressure water area only covered about 5% of the entire length of the tunnel.

The construction party's solution was to grouting on the ground to create a safe area, and the shield did not check the cutter head in the non-safe area to avoid high-pressure intervention. After reaching the safe area, the tool is changed at atmospheric pressure; however, in actual construction, the tool and the cutter head are damaged before reaching these safe areas, resulting in delays in the project.

Any type of TBM wear rate can be high, depending on the abrasiveness of the formation. However, the wear problem of muddy water shields is often more serious than that of hard rock TBMs: the excavated soil of muddy water shields is constantly in contact with the cutter head and cutter, even with muddy water pipelines, transfer points and elbows need to be replaced frequently, and Carry out additional work related to muck removal.

Compared with the mud-water shield, the biggest advantage of using NCP roadheader in rock is that it is easy to inspect the cutter and cutter head, without the need for pressurized cabin, and early detection of cutter head and cutter problems.

formation water

In the event of a sudden influx of water in high water pressure formations, the NCP TBM can maintain pressure with sealable muck chutes on the bulkheads. This type of design can be used as a pressure relief gate in semi-earth pressure mode, which opens by pressure, removing the The muck is sent to the conveyor belt. In extreme cases, the sealed gate can be activated, drilled forward and grouted using a probe/grouting rig to reinforce the formation and seal the water. Additional seals around the main bearing can be filled with pressurized grease, and other vulnerable points can be sealed in the same way.

The multi-mode TBM can realize continuous excavation under high pressure through a long enough screw conveyor: a long enough screw conveyor can reduce the water pressure, and the surface-mounted wear-resistant hard surface can reduce the wear of the screw conveyor.

One example is the Mumbai Metro project, which used two Robbins multi-mode tunnel boring machines. In these machines, the central screw conveyor is able to seal itself and maintain pressure, so the TBM can drive continuously or the screw conveyor can also work continuously:

Close the screw outlet and pressurize the cutter head compartment and screw with water.

The muck gate remains open to allow muck to enter the cutterhead compartment and screw.

When the auger is full of muck, the water is pushed out of the auger and back into the excavation cabin.

If the screw machine is almost full, the muck gate closes and the water pressure inside the screw machine is reduced, emptying it into the spare compartment.

The muck is unloaded from the screw conveyor onto the conveyor belt, the discharge gate is closed again, and the screw conveyor is refilled with water under pressure.

The muck chute door was opened again and the TBM continued to advance.

Formation gases and pollutants

When the muddy water shield tunnels, the underground gas encountered can also be discharged through the muddy water pipeline, which is easier to deal with; and the current NCP roadheader is also equipped with an air inlet, which can discharge the gas from the top of the cutter head cabin and directly introduce it during the tunneling process. in the ventilation system.

Contaminants like asbestos may be more manageable in muddy water pipes, but many other types of contaminants are difficult to separate from muddy water and are therefore easier to handle with NCP boring machines. In addition, in the construction of mud-water shield, the quality of bentonite itself is uneven, and some low-cost materials contain heavy metals, which may be harmful to the environment. At the same time, the mud-water slurry itself is also easy to combine with heavy metals, polluting most of the slurry and making the separation more difficult.

cost comparison

Although hard rock TBM grouting requires time and cost, it is not high compared with the pressurized cabin in mud-water shield excavation. At the same time, pre-grouting has mature experience in tunnel construction at home and abroad.

It is true that the current atmospheric pressure tool change technology has been improved, but this technology generally requires a larger equipment diameter and a deeper cutter head, and such a cutter head structure easily affects soil dumping, increases the number of cutter head maintenance, and often The pressure tool change technology does not solve the problems of cutter head maintenance, replacement of the center cutter head or scraper, etc., which are the parts that wear more during the process of large-diameter hard rock excavation.

In addition, tunnel lining is also a major advantage of NCP tunnel boring machine: in NCP tunnel construction, single-shield TBM is used for grouting before excavation, and the lining can be installed independently after excavation, which greatly saves costs.

The Delaware water diversion tunnel project in New York adopted a Robbins single-shield TBM with a diameter of 6.8m and achieved great success. The speed of the TBM reached 6 m/h. Click to review:

Slurry shield is an effective choice for high water pressure hard rock excavation, but it is not the most economical choice. Compared with hard rock TBM, although the mud-water shield saves the time of post grouting, it also brings additional time investment in pressurized cabin, tool detection and so on. For this reason, in most cases, the NCP roadheader is the better choice. Editor / Zhao E