Major faulting in London: relating tectonic-scale processes to site-scale engineering geology

Critical infrastructure projects in London increasingly encounter unexpected ground conditions caused by major faults in the near surface geology. Their presence challenges the traditional interpretation of minimal faulting, which stems from limited exposure and structural information, and historical misinterpretation. These major faults are now recognised as both a geological unknown and a geotechnical risk since their origins, extents and architectures are poorly constrained. This research investigates major faults in London to characterise them structurally and determine their engineering geological impact. Direct observations and indirect evidence are coupled to overcome information deficiencies, provide fault analyses, generate geological models, map faults indirectly, and assess fault reactivation and inheritance mechanisms. This investigation shows that Alpine reactivation of basement Variscan and post-Variscan faults caused propagation into overlying Late Cretaceous-Palaeogene cover through a series of en échelon Riedel shears and reversed faults. Fault compartmentalisation offset the ground into blocks and subtly influenced certain sedimentological and ongoing hydrogeological processes. Significant transpressive and transtensive shear zones developed where major faults interacted in the cover through linkage and/or confined block shearing. This research demonstrates that the London Basin is a structurally complex product of Alpine intraplate tectonism. The region is redefined here to reflect the differing Alpine responses of underlying basement domains, with the Variscan Front repositioned northward in light of London’s Variscan fault network. The identified local tectonism and regional partitioning cause lithological, mechanical, and hydrogeological properties to vary at the site-scale. Consequently, major faults inconsistently affect the ground across London to generate often unique local engineering geological impacts. Both a ground investigation workflow and fault zone categorisation criterion are proposed to improve their identification and engineering geology characterisation. This research has revealed how Alpine-triggered widespread but locally complex fault propagation in London has contributed to subsurface geological complexity to the detriment of ground conditions.Open Acces