Pro1plan Consultants

One of the greatest challenges for tunneling work is the unpredictable nature of the ground and hence it is not uncommon to see delays in tunneling work. The ground is non-homogeneous but actually stratified with different soil types. The soil condition can change abruptly along the tunnel alignment, hence the behavior or soil movement can be difficult to tell when tunneling through. Faced with this unpredictable nature of the ground, the tunneling rate is also difficult to forecast.

In this article, I will dwell into the various challenges encountered in boring tunneling projects.

Geology and Hydrology

In the case of mining through porous soil with high water table, a huge amount of groundwater can be drawn out during tunneling work. The sudden loss of a huge amount of groundwater can cause ground settlement, thus destabilize nearby structures to the tunneling work. Hence, tunneling along the ground with porous soil and high water table needs to slow down to grout the leaking zone and installing of re-charge wells to compensate for the loss of groundwater.

At times where the TBM is stuck in water-bearing soil or adjacent to the water body, then CHI has to be done under compressed air mode which is something tunnel engineers will like to avoid.  Doing CHI under compressed air is a dangerous affair as the crews have to work in confined space under high compressed air mode.

Progress of tunneling can also severely slowed down when passing through soils with varying ground conditions such as faults, weakness zones or soft soil combined hard rock. This is commonly known as mixed-face conditions.

In the case of unstable soil condition in front of the cutter head, during mining, TBM machine can get stuck. Stabilise the rock surface underground can take months before resuming the mining work, albeit slowly.

During tunneling, ground settlements or soil movement may happen which can severely affect the integrity of the structure above the ground. Hence, it is important to take precautions before the TBM passes under or near any of these structures.

Underground Utilities and structures

Prior to tunneling, the structures at the vicinity of the tunnel alignment has to be studied closely. In the case when tunneling near the tunnel of MRT tunnels, permissions have to be sought from relevant authorities.

A pre-con survey needs to be conducted on the nearby building to check the integrity of these structures priors to the Tunneling. Also, instrumentations need to be installed on these buildings and grounds along to monitor the impact on the building and surroundings during tunneling. At times, measures such as installing of recharge wells to prevent excessive groundwater loss or soil improvement such as Jet Grout Piling (JGP) needs to be carried out to stabilize the ground or probing/reinforcing the structures r may need to be done before and during tunneling work.

Choice of TBM

The geology of the ground affects the choice of TBM. In Singapore where the ground is non-homogenous, 2 types of shield TBM are commonly being used namely the Slurry TBM and Earth Pressure Balance Shield (EPB TBM).

For Slurry TBM, the excavation face is supported by pressurizing bentonite in cutter head chamber. Circulation fluid flushes out the muck. For EPB TBM,  the excavation face is supported by pressurizing soil inside the cutter head chamber.

The relationship between the soil granularity of the soil and the choice of  TBM  is as shown below.

Downtime and operation of TBM

The  productivity  of the TBM depends on minimising the downtime of the TBM. Downtime of the TBM can be due to:

• Frequency of Cutter Head Intervention (Planned and Unplanned)

Due to the wear and tear of the cutter heads, it has to be replaced every now and then. In the case of planned CHI, after certain specified length of tunnelling, the operation of the TBM is halted for replacing the cutter heads. This specified length is a function of the cutter head’s specification and the hardness of the ground. In the case where the ground is more abrasive, then the cutter heads need to be change more frequently.   In the case of unplanned CHI, the cutter heads need to be replaced before the planned CHI due to far worse  anticipated abrasive soil conditions.

• TBM break down

Just like driving a motor car on the road, the TBM can break down due to failure in the TBM components.  The frequency of breakdown tends to increase with the duration of the TBM service.

• Delay in the delivery of PC Segmental linings

Due to poor logistic planning, the PC Segments can be delayed from reaching the front of the TBM for installation. Delays can be due to the slow production of the segment, break down of back-up cars that carries the PC segments into the tunnel, breakdown of gantry cranes that lower the PC segments into the tunnels or poor productivity of crews.

• Disposal of the mined material/Efficiency of Slurry plant

For EPB TBM, the disposal of mined materials is done by muck-up trains or by conveyor belt transversing between the TBM and the shaft. Then the mined materials are being lifted up to the ground surface by a gantry crane or using a clamshell bucket if the shaft is not too deep (less than 30m)

If the disposal of mined materials do not pace with the tunneling, then the tunneling rate will hampered.

At times, due to lack of space to dump the mined materials, the operation of TBM has to be suspended.

In the case of slurry TBM,

priors to disposals of soils, the mined material has to be treated in the slurry plant. When the slurry plant choked up, the operation will be halted.

Alignment of the tunnel

Alignment of the tunnel can also influence the tunnelling rate of the TBM, During negotiating curve, the tunnelling speed needs to be slowed down. This is similar to negotiating curves while driving a car on the road.

In  Singapore Cable Tunnel Projects, to minimize tunneling disturbance to buildings above, the tunnels were being built in underneath and along the carriageway. Due to this constraint, some of tunnels alignment between Kallang Equipment Building and Paya Lebar Ventilation Building had the tightest curves (75m radius) for a 7m diameter TBM to date in Singapore.

Conclusion

It is important to take study the geology profile of the bored tunnels in detail when developing the project schedule. It should never develop a schedule with an assumed uniform rate. The bored tunnel activities should be developed in greater detail in corresponding to the geology/hydrology of the ground. Based on the geology/hydrology of the ground, the tunneling rates should also factor in the frequency and duration of Cutter Head Intervention.

With the anticipated tunneling rates, it is important to ensure there are enough workers and resources to dispose of the excavated materials and delivery of PC segments. In the case of slurry TBM, the sizing and operation of the slurry plant are very important so as to ensure the smooth mining of bored tunnels.