Seismic Microzonation Studies in Stoke-on-Trent: BS EN 1998 Site Response & Ground Motion Characterisation

The UK's hazard model assigns Stoke-on-Trent a bedrock PGA of 0.04–0.07 g for a 475-year return period, which seems negligible. The issue arises when soft Trent Valley alluvium—with Vs30 values frequently below 240 m/s—amplifies this motion by factors of 1.2 to 1.8 at periods matching typical building resonances (0.2–0.5 seconds). BS EN 1998-1 requires site-specific analysis where ground conditions fall into Site Class D or E, precisely the scenario across much of the city's valley floor. Additionally, the Coal Authority identifies over 200 recorded mine entries within the city boundary; the interaction between shallow voids and cyclic loading introduces a risk dimension that generic hazard maps cannot capture.

To resolve Vs30 reliably, we typically extend shear-wave velocity measurements to at least 30 metres depth. In Stoke-on-Trent's drift-covered areas, this means penetrating through 8–20 metres of glacial till and alluvium into the underlying Carboniferous sandstone or mudstone. Where bedrock lies shallower than 30 metres, we use the extrapolation procedure defined in BS EN 1998-1 Section 4.1.3. Shallow refraction microtremor (ReMi) surveys coupled with MASW have proven effective for reaching 30 metres in the Trent Valley fill, though sites adjacent to the canal network sometimes require borehole-based downhole methods where cultural noise from road traffic compromises surface-wave dispersion curves.

The investment ranges from £3,060 to £14,460 depending on site area, investigation depth, and the complexity of the geological model. A small plot requiring a single MASW line and one borehole with downhole seismic, processed to produce a site-specific response spectrum, falls at the lower end. Multi-hectare regeneration sites—like the former Shelton Bar works or Festival Park extensions—demand denser geophysical grids, multiple deep boreholes with dynamic laboratory testing, and 2D finite-element site response modelling, placing them toward the upper end of the range. Each quotation is scoped against the specific requirements of the structural engineer's design brief.

We integrate Coal Authority abandonment plans with geophysical survey data—primarily electrical resistivity tomography and MASW—to map zones of potential voiding or collapsed ground. Where abandoned seams lie within 10 times their height below the foundation level, we assess cavity stability under the seismic demand using the limit equilibrium approach outlined in CIRIA C758. The microzonation maps flag 'caution zones' where ground motion amplification may combine with mining-induced ground deformation, and we provide modified response spectra that account for the reduced shear stiffness across fractured rock masses above old workings.