The geology beneath Hull is dominated by soft alluvial clays and silts of the Humber estuary, reaching depths of 20 to 30 metres before hitting the glacial till or chalk bedrock. For base isolation seismic design, this means the isolation system must work with a relatively flexible soil column — the fundamental period of the structure shifts, and the isolators need to accommodate larger lateral displacements without losing vertical load capacity. We combine this with a MASW Vs30 survey to classify the site per BS EN 1998-1 and confirm the design spectrum, and a pressuremeter test to capture the in-situ modulus decay under cyclic loading.
For Hull's soft alluvium, base isolation seismic design must account for soil-structure interaction that reduces the effective damping of the isolation system by up to 15%.