Chalk is weak, friable, pure limestone; when fresh, UCS = 5–27 MPa; but porosity is 30–50%, so UCS reduces to 50–70% when saturated.
It may have solution features, caves and sinkholes, but these are generally on a smaller scale than in strong limestones.
Weathering of chalk by frost action is severe, to produce weak rubbly debris. This commonly reaches depth of 10 m in Britain due to periglacial weathering during the Ice Ages.
Putty chalk and fine-grained rubble chalk are thixotro pic when saturated, and turn into slurry when disturbed. Should not be excavated or handled in wet winter months, but can be used as fill when dry.
Pile driving in chalk creates slurry at tip; this stabilizes when left undisturbed, so piles may carry higher working load if left for a time after driving. Settlements in chalk are often lower than expected, as rock strength increases under steady load.
Driven concrete piles have ultimate end resistance of N/4 MPa, where N = SPT. Risk of solution cavities below pile tip means that load is best attained by skin resistance, with ultimate values of about 30 kPa on displacement piles and 150 kPa on cast-in-place piles.
Liquefaction failure of putty chalk occurs where it is saturated along route of concentrated drainage flow and can then fail into a cavity beneath, which is usually a mine, gull or cave.
Ground collapses at Norwich and Bury St Edmunds (in East Anglia) and at Reading are mostly related to old mines below soakaways or drain failures; some are collapses of clay-filled pipes within the chalk.
Good surface drainage and ban on soakaways are necessary in chalk areas, especially where voids may exist, in areas with a history of mining or with known caves, or along cambered escarpments with gulls.