Eurotunnel and Hach Lange tackle groundwater monitoring

The groundwater which collects around the Channel Tunnel transport system needs to be monitored before being discharged to the sea. Clive Murren, project manager for Hach Lange, describes how systems were developed to safeguard against contamination

Natural groundwater collected around the tunnels operated by Eurotunnel is continuously monitored before being pumped to the coast and discharged into the sea.

Six multi-parameter water quality monitoring systems have been custom-designed by Hach Lange to ensure that there is no potential for harmful water to enter the pipelines.

Each monitoring system is connected to a sophisticated data collection and alarm
system, which is able to divert water into vast underground sumps if alarm conditions occur. To-date, no such emergency has taken place.

Commenting on the new monitoring system, Eurotunnel senior technician specialist Michael Edwards, says: "The two major benefits are improved reliability and a lower maintenance requirement."

The Channel Tunnel is 50km long, with the 39km undersea section making it the longest undersea tunnel in the world. The Eurotunnel system consists of three separate tunnels: two rail tunnels through which the trains travel, and a central service tunnel.

This safe haven is used for maintenance and evacuation, and is linked to the rail tunnels every 375m. On average, the tunnels lie 40m below the seabed of the English Channel.

The service tunnel is maintained at a higher air pressure, and thus remains free from smoke and fumes in the event of an incident. It provides access to a range of assets which ensure safe and efficient operation of the tunnel. This includes the water monitoring systems, the pumps and pipelines.

The water that seeps down to the tunnels is a mixture of groundwater and seawater. It is collected at six drainage stations and is continuously monitored.

The main purpose of the monitoring system is to protect the enormous pumps (capable of almost 1,000m3/h) and pipes from corrosive attack. It also serves to ensure that water discharged to the sea is not harmful to the environment.

The early monitoring system suffered from a number of problems that largely resulted from blockages in the small pipes that passed water to the sensors. And Hach Lange was tasked with the development of a more reliable, less labour-intensive system.

The Hach Lange and Eurotunnel engineers decided that a flow-though holding tank would resolve potential problems with blockages; large-bore pipes could be employed and sediment could be removed easily. In addition, the latest sensor technology meant that the requirement for recalibration was much lower.

Each of the six flow-through tanks contains sensors for conductivity, turbidity, dissolved oxygen, pH, Redox and temperature and data is transferred to a PLC that is programmed to raise alarms when pre-specified conditions occur. If an alarm is raised, all water is immediately passed to an underground storage sump. It remains in quarantine until tested and passed as fit to be allowed into the pipeline. The monitoring system returns to normal once water quality levels leave the alarm condition. Any quarantined water can then be removed by bowsers.

In the early years, water was passed though a WwTW near Dover. But the water quality was found to be consistently good, so the works was decommissioned. And water is now passed directly to the sea under a discharge consent from the Environment Agency.

Kevin Rivers, senior mechanical and electrical (M&E) technician at Eurotunnel,
was responsible for the configuration of the monitoring and control system. Comparing the Hach Lange system with its predecessor, he says: "The new system is far superior because it is more reliable, it requires less maintenance, and is easier to operate.

"The water quality monitoring sensors are connected to SC100 controllers which are plug and play. All you do is tap the serial number into the controller and it starts to monitor correctly automatically. The reliability of the new system means we no longer experience false alarms, which is a major benefit. There are strict procedures in place before an M&E team can enter the tunnel to investigate an alarm. And, coupled with the amount of time it takes to drive to the monitoring equipment, false alarms are very costly.

"We estimate that the new system requires about one quarter of the maintenance that was previously necessary, which saves a great deal of time and money," Rivers says.

"One of the reasons for this is the new luminescence dissolved oxygen sensor, the LDO, which employs an optical monitoring technology that does not require recalibration. We simply change the sensor cap every year. In order to prevent sensor fouling, we have fitted a compressed-air system that automatically cleans the sensor heads."

Michael Edwards says: "While the tunnel construction was carefully engineered, the water seepage levels have been significantly lower than was originally estimated, and the three quarantine sumps have enormous capacity. So, the whole monitoring and control system is running significantly under capacity.

"However, this provides the reassurance that we would be more than capable of handling any emergency. Furthermore, the system that we have developed with Hach Lange has helped to radically reduce the running costs while significantly improving reliability, which is good news for everyone."

These installations have been successful because the technological requirements
identified by the Eurotunnel team coincided with the development of new monitoring equipment.

As a result, the instrumentation has been deployed in a customised monitoring system, designed to overcome previous problems. Plug-and-play technology coupled with advanced low-maintenance sensors enabled Hach Lange to help develop and supply a customer-specific solution.

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