3 Dams on an Island

The island of Jersey is the largest of the Channel Islands that are an archipelago of British Crown Dependencies in the English Channel. Jersey is an island measuring about 120 square kilometers and it lies in the English Channel, about 12 nautical miles (22 km) from Normandy, France, and about 87 nautical miles (161 km) south of Great Britain. It is the southernmost of the Channel Islands, with a population of about 100,000 people.

The availability of clean water is, of course, one of the major issues for islanders. Rainwater accounts for 96% of the main source of water and is collected and stored in reservoirs; the remaining 4% is abstracted from boreholes. Currently, on the island, there are six reservoirs and dams with water treatment plants.

Smartec is monitoring the movements of three critical blocks installing SOFO sensors in drainage holes no longer in use.

Aim of monitoring:

Val de la Mare dam was completed in 1962 and during the last decades, it was suffering due to ASR (Alkali Silica Reaction), important seepages, and differential displacement of some blocks. Smartec, in partnership with Jersey Water, designed and installed a tailored fiber optic monitoring system. The sensors used are very long extensometers based on SOFO interferometry. Each sensor is 10m long and two sensors have been joined together to cover the total distance from the crest to the gallery.

Handois is a gravity dam and Smartec, again together with Jersey Water, decided to install some fiber optic sensors to check:

• the uplift at the concrete-bedrock foundation
• the seepage and water level inside the body of the dam
• the movements of an existing crack

It is extremely interesting to correlate the temperature, crack width, reservoir level, and seepage rate.

In the Grand Vaux dam, Smartec monitored the seepage and the water pressure inside the body of the dam. The sensor used is the FOP pressure gauge.