Dense networks of dilated fractures occur locally in the upper 5–15 m of bedrock in basement gneisses in eastern Sweden. Near Forsmark, pre-existing sub-horizontal fractures have been jacked open and filled with water-lain sediment, likely during the latest Weichselian glaciation. Despite extensive previous research, it is uncertain whether subglacial hydraulic jacking led to the generation of new fractures, in addition to reactivation of pre-existing ones. Re-analysis of historic photos from excavations near the Forsmark power plant indicates formation of two types of new fracture. Firstly, rock fragments were broken off the main fracture surfaces as existing fractures were jacked open. Secondly, fracture analysis shows that whilst few subvertical fractures occur above tight sub-horizontal fractures, a higher density of vertical fractures occurs above dilated sub-horizontal fractures, suggesting new formation. We apply a model of beam failure theory, borrowed from structural engineering, to constrain potential new fracture generation, using assumptions based on measured water pressure fluctuations from beneath the Greenland Ice Sheet. This modelling shows that beam failure is a plausible mechanism for the generation of new vertical fractures during a subglacial water fluctuation cycle under a range of realistic glaciological conditions. This implies that hydraulic jacking can result in further in situ disruption and brecciation of the shallow rock mass, decreasing the rock mass strength and increasing its hydraulic conductivity. Altogether, hydraulic jacking of existing fractures and the formation of new vertical fractures results in effective subglacial mechanical weathering of the shallow rock mass.