Management of Groundwater

Groundwater Control

General

Groundwater presence should be analyzed during the construction, maintenance and operational phases of the tunnel. Groundwater inflow can lead to the washout of the fine-grained loos material, thereby altering the strength characteristics of the ground and hence influencing the stability of the excavation.

Note: Building-up of water pressure can cause deformation in lined tunnels. This can be a problem in brick-lined tunnels.

During excavation, water in the ground surrounding a shaft or tunnel should be managed effectively. Measures should be taken to prevent the presence of water in a shaft or tunnel from creating unsafe working conditions (BS 6164: 9.1.1).

Several methods can be implemented for ground control, external dewatering, impermeable cover, ground freezing, ground treatment, slurry, and earth pressure balance machines, compressed air, controlled inflow, etc. (BS 6164: 9.1.2, 9.1.3,9.1.4, 9.1.5, 9.1.6, 9.1.7 and 9.1.8).

Case: Diversion Tunnel at Jinping II hydropower station is one of the deepest underground construction projects in China. The overburden depth of the 1.5 – 2.0 kms depth with maximum depth of 2.525 kms. The tunnel was subjected to maximum single point groundwater inflow of 7.3 m3/s. The water infiltration not only affected the structural integrity of the tunnel but also impacted the safety and stability of the lining structures. This caused technical difficulties and delayed project progress (S.Y. Wu, 2007).

MUST KNOW

As According to Alberta OHS Code Part 32, Section 461(5)

An employer must ensure that an underground shaft is provided with suitable and efficient machinery or another device for keeping the shaft free of accumulations of water.

As According to Alberta OHS Code Part 32, Section 464(2)

An employer must ensure that a tunnel is provided with suitable and efficient machinery or another device for keeping the tunnel free from accumulations of water

External dewatering

The water table can be lowered by drainage of water external to the shaft or tunnel. This can be achieved by deep wells, well-pointing for the surface, or by a drainage sump in a shaft or pit, possibly combined with drainage headings.

Controlled Inflow

When water flow is small, it might be preferable to control rather than to exclude. In open-face tunnel, a sub-drain below invert level can be used to draw down the level of the water table. Water flows might be acceptable and should be piped through linings. After construction, such pipes should be sealed up and back grouted or can be accepted as a permanent water inflow, depending on the circumstances (BS 6164: 9.1.8).

Handling of groundwater

Sumps

Invert sumps are liable to be submerged and constitute a hazard to persons walking near submerged sumps. Therefore, the sumps should be securely covered or signed and buried (BS 6164: 9.2.2).

Pumping Capacity

In all shafts and tunnels where there is a risk of flooding, there should be an adequate provision of pumps. If the tunnels are downgraded, standby pumps should be provisioned to mitigate the risk of any pump failure (BS 6164: 9.2.3).

Inundation

The water in general conditions in a tunnel or shaft can be managed by compressed air and pumps, as described in  Chapter 2. Water management is crucial during the inflow of a large amount of water with pressure inside a tunnel orshaft.

The risk associated with inundation should be evaluated at all stages of planning.

In the event of an inundation, there is a serious risk of persons being trapped inside the tunnel, especially if the tunnel rises from a flooded shaft. If space permits, high level walkway should be built to mitigate risks under such a situation (BS 6164: 10.1).

Inundation at the Tunnel Face

In an open-face tunnel driven below water bodies, and or below the water table, there is a risk associated at the face of tunnel. Inundation can be associated by breaking into faults, buried channels, abandoned mine working or other geological features that connect to an underground water source. Breaking into man-made structures, such as wells or culverts can also result in inundation. Therefore, the presence of such features should be determined by careful geological and geophysical studies.

There is a possibility that water flowing back through the tunnel could flood third party assets. Hence, precautions should be taken to prevent this from happening (BS 6164: 10.2.1).

Precautions

In soft ground and hard rock, a closed face machine should be used. When there is a risk of inrush of saturated ground, sand tables should be fitted to the tunneling shield as a precaution.

During subaqueous tunneling, where the cover beneath the water body is found to be inadequate, a layer of clay or other impermeable material should be deposited across the lining, which can act as an impermeable blanket.

For tunnels near the sea level, tide tables should be studied and monitored. Tide gauges should be installed and used in appropriate circumstances.

River level should also be monitored. Arrangements should be made to obtain any flood warnings from the relevant authorities. Where rainfall, snowfall, or melting snow is a significant factor in the risk of flood, the metrological forecast should be regularly obtained and studied (BS 6164: 10.2.2).

Shaft Protection

The shaft should be properly lined and all openings at the top of the shaft should be located above any foreseeable flood water level. When openings into shaft are below the highest foreseeable flood water level are essential, such openings should be made up of appropriate material and equipment, which can be closed in the event of risk of flooding. Such protective work should be planned early in the design stage.

Watertight bulkheads should be installed when more than one tunnel is driven from a shaft. Their installation could give rise to a risk of entrapment in isolation sections, and strict procedures should be adopted to prevent this occurrence.

Precautions When Flooding Threatens

Shafts and tunnels workings should be provided with a communication protocol during the events of flood, and flood warnings should be effective. Immediate action if the shaft is threatened with flooding should include (BS 6164: 10.2.4):

1. withdrawal of all persons from below ground; 

2. closure of all working openings situated below possible flood level, after the evacuation process;

3. arrangements for a continuous monitoring flood level system, with inspection of any points of risk;

4. strengthening of any vulnerable points, using sandbag protection where appropriate;

5. preparation and putting into use of emergency pumps or any other standby plant;

6. preparation of isolation of the electrical supply to the threated workings.

Remedial Actions

The cause of inundation should be, and steps should be taken to prevent further inundation before any re-entry to the tunnel is considered. A protocol for re-entry should be devised which should include (BS 6164: 10.3):

• • safeguarding the structural stability of the tunnel;

  • checking the atmosphere is free from any gases and fit for respiration;

  • ensuring the plant and electrical equipment is safe;

  • confirming all strutting, timbering, walkways, stairways, and gantries are inspected and made safe

Flooding of a Tunnel from a shaft or an Adjoining Tunnel

Identification of Flood Risk

The risk that a tunnel will be flooded from an access shaft or an adjoining tunnel should be considered in advance.

Possible source of flood includes river flood, high tides or burst water undermining any protective works or burst water mains overtopping (BS 6164: 10.4.1).

The risk due to high tides, river flood, and mitigation techniques are discussed in BS 6164: 5.4).

Shaft Protection, Precautions during Floods and Remedial Actions

Shaft protection, precautions during flooding, and remedial actions are all discussed in section 5.4. For more specific notes, refer to sections BS 6164: 10.4.2, 10.4.3, and 10.4.4.

Precautions During Drilling

Precautions should be taken when drilling through the face or lining. Risk assessment of meeting water, gas, or any other obstruction, including unexploded ordnance, should be made before drilling starts (BS 6164: 10.5).

MUST KNOW

Alberta Occupational Health and Safety Code Relevant to the Topic

According to the Alberta Occupational Health and Safety Code (current as of January 1, 2019) an employer must follow the following:

According to Part 32. Section 445

An employer must ensure that an excavation that a worker may be required or permitted to enter is kept free of an accumulation of water that may pose a hazard to the worker.