Seismic Microzonation Studies in Swansea: BS EN 1998 Ground Classification

For a typical Swansea development site, the cost ranges from £3,170 for a single MASW line with basic Vs30 reporting to around £13,670 for a comprehensive programme including multiple MASW transects, seismic CPTu calibration, and one-dimensional site response analysis. The exact figure depends on site access, the number of survey lines required to map lateral variability, and the depth of investigation needed to reach engineering bedrock.

The Swansea area spans three main ground types. The Coal Measures sandstone and mudstone ridges in the north and west of the city typically classify as type B (Vs30 360–800 m/s). The glacial till and dense alluvial gravels found in intermediate zones often fall into type C (180–360 m/s). The soft estuarine clays and silts along the Tawe corridor and SA1 waterfront can classify as type D (below 180 m/s), particularly where the soft layer exceeds 20 metres in thickness without an intervening stiff horizon.

BS EN 1998-1 is mandatory for structures in importance classes II, III, and IV across the UK, and the standard requires site-specific ground classification unless the site can be reliably assigned to ground type A or B using existing data. Given the geological complexity of Swansea with its coal measures, alluvial valleys, and mining legacy, a desk study alone is rarely sufficient to justify a ground type B classification, making field-based microzonation the prudent and often the only defensible route to compliance.

Fieldwork for a standard microzonation programme consisting of three to four MASW lines and two seismic CPTu soundings typically requires two to three days on site. Data processing, dispersion curve picking, inversion modelling, and report preparation add approximately ten to fifteen working days. Sites with complex geology or restricted access may require additional survey time.

Shallow abandoned mine workings, common in the Morriston and Landore districts, introduce low-velocity voids and fractured rock zones that can alter shear wave propagation and create localised amplification anomalies. We address this by running MASW transects at multiple orientations to detect azimuthal velocity variations and by combining the geophysical results with historical mine abandonment plans and rotary open-hole drilling where necessary. The resulting ground model explicitly includes the depth and extent of worked seams and their influence on the effective Vs30.