The weathering products of Tertiary basaltic rocks in Lower Silesia comprise clays that contain various amounts of smectite-group minerals, illite, kaolinite, quartz and coarser fragments of non-weathered rock. Deposits of weathering products whose volume is of economic importance occur in Krzeniów near Złotoryja, Męcinka near Jawor and Dunino near Krotoszyce. The expansiveness of these weathering products is evaluated using laboratory analyses of swelling and shrinkage and the empirical nomograms of van der Merwe and Seed. The measured results of swelling of products dominated by smectite-group minerals, and of halloysite weathering products, indicate that the swelling depends on mineral composition. The smectite clays (Krzeniów and Jawor-Męcinka) display swelling strain ep in the range 20.3-31.8% whereas, for the halloysite products (Dunino), the range is 10.0-18.3%. The lowest linear shrinkage characterised the samples from Jawor-Męcinka and the highest, the weathering products with 79-81% clay fraction from Dunino. Most of the basaltic weathering products fall in the van der Merwe nomogram fields of low and medium potential expansiveness. High and very high potential expansiveness is shown by the Dunino weathering products. On the Seed nomogram, the weathering products plot within the field of low expansiveness except for some high and very high expansiveness values from the uppermost halloysite horizons at Dunino.
