A waste water treatment plant was built on an open space in the North of Spain located on top of recent alluvial deposits ranging in thickness from 10 to 20 m. For the past 40 years blast furnace slag has been depositing on this open space forming a layer which, prior to the start of construction of the treatment plant and precisely coinciding with its siting, was as high as 15 m and this caused consolidation of the alluvial deposits beneath, allowing part of the installations to be built using direct foundations, having taken the precaution of not exceeding the preconsolidation load.

The biodigestor tanks occupy two 62 x 195 m rectangles which when filled can take a maximum depth of 8.8 m of water. These two rectangles are separated by a 23-m wide corridor containing a fill conveying a similar pressure on the ground to that of the tanks under operation, whereby the area of 147 x 195 m can be taken to be under a uniform load. The tanks each consist of three equal deposits built on foundations of 28 concrete slabs, 14 for each of the two rectangles. Before they were built, some 5 m of fill were removed to guarantee that the working loads of the two tanks would remain below the ground's preconsolidation pressure. This article analyses the evolution of the settlement recorded over the 2.5-year period following the filling of the tanks and also reports on the way the structure has behaved.