We will now look in outline at the waterproofing concept selected at the OL3 nuclear power plant in Finland.

Buildings always conduct their loads via base slabs into the underlying rock. The difference in heights between the rock excavated and the underside of the building floor slab, which may be considerable in some cases, is made up for by using concrete with light constructive reinforcement.

The nuclear island structures below the 0.00 m level are made partly as white tanks, restricting crack widths accordingly (<0.2mm in part). A largely stress-free support for the main buildings was achieved, and waterproofing was also provided against seepage and stratum water which can get to the structure via cracks and spaces in the rock in the shape of an external black waterproofing on all vertical wall surfaces from below the top of ground height up to +0.60 m, and all horizontal upper structural surfaces, such as upper duct connections below ground height.

The waterproof is built up as follows:

Waterproofing on walls below ground surface level

— plastic-modified bitumen undercoat 0.2-0.3 l/m2 (cold)

— one layer of plastic-modified bitumen strip.

Additional thermal insulation

— Polystyrol XPS foam, compressive strength >180kN/m2

— thickness depending on where applied, between 100 and 160 mm.

Waterproofing upper structural surfaces below ground surface level

— sloping concrete at least 2%, at least 100 mm thick, with constructive reinforcement (steel mat c/c100, d = 6 mm),

— plastic-modified bitumen undercoat 0.2-0.3 l/m2 (cold)

— one layer plastic-modified bitumen coating.

Additional thermal insulation

— Polystyrol XPS foam, compression strength >250kN/m2

— thickness 140 mm.

The equalising concrete has drains fitted to drain the joint between it and the rock. This drainage was tailored to suit the local rock structure (depths/interfaces): it leads any water which occurs into the drains surrounding the building, which in turn lead it to the pump shafts from which the water is then pumped out.

The drains around the building consist of perforated PP pipes DN 2 x 110 or 200 in drainage gravel 6/16, enclosed in geotextile wool. In areas concreted against vertical rock surfaces, drainage mats are used to ensure that the rock surface is drained.