Experimental observations that simulated active‐layer deepening drives deeper rock fracture

The impact of changes in active‐layer thickness on the depth of pervasive macrofracturing (brecciation) in frost‐susceptible bedrock is unclear but important to understanding its physical properties and geohazard potential. Here we report results from a laboratory experiment to test the hypothesis that active‐layer deepening drives an increase in the depth of brecciation. The experiment simulated active‐layer deepening in 300 mm cubic blocks of limestone (chalk) and sandstone. Temperature, surface heave and strain at depth were measured during 16 freeze–thaw cycles. Macrocracks photographed at intervals were digitally analyzed to visualize crack growth and to quantify crack inclination and length. In chalk, an upper horizon of macrocracks developed first at about 100 mm depth in a shallow active layer during cycles 1–8, followed by a lower horizon at about 175–225 mm depth in a deeper active layer during cycles 9–16. The longest cracks (>35 mm) were most common at inclinations of 0–30° from the horizontal, and numerous cracks