With the unprecedented number of tunnels under construction along the Eastern seaboard of Australia, and with more planned over the next few years, industry professionals are on the lookout for new innovations that will overcome common tunnelling problems and speed up the tunnelling operation.

The water that accumulates within a tunnel creates a number of issues that would traditionally need to be managed in an ad hoc fashion to meet the needs of the day. Often this will involve adding on tanks and processes within the tunnel as site staff chase their tails trying to keep up with both water and mud that is produced as the tunnel progresses. Getting the water to the surface for treatment and discharge off-site can become an overwhelming focus.

This water is produced by both groundwater infiltration and the tunnelling processes themselves. The groundwater can be clean, but it can also contain high quantities of iron and, in some instances, man-made contaminants that are a legacy from past industrial activities at the surface, adding to the complexity of water treatment that is required.

Managing the water appropriately, both within the tunnel and at the surface, is critical for ensuring uninterrupted operation. But to manage the water, you actually need to focus on solids.

Start with solids at the source

Removing water from the tunnel sounds simple enough. However, within the tunnel itself the problem becomes the solids within the water rather than the water itself. Long tunnels require multiple sumps and pumps, long pipe runs and a location where the water must be collected and transferred many tens of meters to the surface. Aside from the multiple locations where pits, pumps and pipes can block, the additional energy required to move the mass of solids within the water imposes a high energy demand on the process.

Once at the surface, these solids choke the water treatment process preventing off-site discharge occurring fast enough to keep ahead of the water building up in the tunnel.

To overcome reliability issues ‘fish tanks’ and sand separators have been tried along the length of the tunnels, with a view to reducing the ongoing problems with the pumping equipment and preventing overloading water treatment systems at the surface.

These ‘fish tanks’ are desludged to the tunnel stockpile which adds tons of water to the weight of the spoil that must be removed to the surface. More truck movements add to the project time and costs, and are another energy intensive practice adding to the carbon footprint of the project.

Rather than moving these solids with the water and coping with the issues that arise, removing the solids from the water within the tunnel and at the source protects the downstream processes and saves on wear and tear, power and labour.

Focus on solids at the treatment plant

Traditional water treatment for tunnels involves an attempt to manage the inevitable solids followed by a clarification step. Sometimes additional treatment is required to ensure the quality is acceptable for discharge, or because a portion of reuse is desirable. By tailoring the treatment plant to be robust against high solid loads there is a greater assurance that it will perform continuously in all situations.

Specifically, targeting iron and other contaminants requires a targeted chemical treatment regime. To be effective without requiring high dosage rates, as much of the solids as possible must be removed from the water stream prior to this chemical conditioning step. Providing a robust system in a compact footprint can be challenging.

A holistic approach

Hydroflux has taken a holistic approach and designed two complementary treatment processes for both in-tunnel solids management and water treatment at the surface.

The TunnelMATE™ is a skid-mounted system that can be deployed into the tunnel to remove solids from water streams and produce a dry and spadable cake – reducing labour, pumping energy, and pump wear and tear. The TunnelMAX™ is a surface-mounted modular treatment system designed to accommodate high solids loads if they occur and otherwise treat the water to a high quality for discharge – providing a compact, robust treatment and dewatering process in one system.

Both systems have been designed for automation, reliability and minimal operator input, and when used in combination on a site, they are a highly effective total solids and water management solution.

TunnelMATE™ systems can be configured for 50 – 500kg/hr of dry solids, and TunnelMAX™ systems can be configured for 2 – 50m3/hr of water treatment. Selection is site specific depending upon intended tunnelling methods, geology and groundwater infiltration rates – contact Hydroflux for assistance in selecting a system to suit your needs.

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