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Salt seepage sparks urgent investigation

News

URGENT SITE investigations have been carried out in Blackpool after fears that large solution features between two new massive stormwater storage tanks could threaten their integrity.

Concerns were raised after salt saturated groundwater seeped into one of two 42m deep, 40m diameter tanks built last year by foundation contractor Bachy Soletanche as part of North West Water's Bathing Improvement Programme. The tanks are founded 15m above a thick salt deposit.

Working for the water company and its supervising engineer Bechtel Water Technology, geotechnical contractor Fugro used cross-hole and down-hole geophysics to investigate if voids had formed in the salt horizon. Fugro's plan was to establish ground conditions around the tank and to see if these were unaltered beneath.

Six boreholes were drilled around the tank and sonic, natural gamma, density and temperature/ conductivity logs used to provide detailed geological profiles.

A series of cross shots were then carried out using a borehole sparker and a string of 10 hydrophones. The results were compared with the down- hole sonic logs and showed that the velocities were in line with the variation expected from the anisotropy of the ground.

It was concluded that no sizeable voids were present along the paths of the cross firing and that segments of tank foundation between the firing lines were too small to hold significant voids.

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