Seismic Microzonation in Swindon: Ground Response That Actually Holds Up Under Review

Most microzonation studies we deliver for Swindon sites fall between £3,370 and £13,780, depending on the number of measurement points, the depth of investigation, and whether we need to mobilise a CPT rig for liquefaction assessment. A small infill plot needing two MASW lines and one calibration borehole sits at the lower end; a multi-hectare commercial development with 2D basin modelling and multiple deep boreholes runs toward the upper end. We provide a fixed-price scope before any fieldwork starts, so there are no surprises once the geophysics team is on site.

The UK is a low-to-moderate seismicity region, but that does not mean seismic hazard is zero. The British Geological Survey records roughly 200–300 detectable earthquakes in the UK each year, and the 2008 Market Rasen event (magnitude 5.2) was felt strongly across southern England. More importantly, the combination of soft near-surface deposits and a moderate-magnitude event at 20–30 km depth can produce ground motions that exceed the Eurocode 8 Type 2 spectrum at certain periods. Swindon's variable drift cover — from stiff gravels to soft organic silts — means two adjacent plots can fall into different site classes, and the difference in design acceleration is real. Insurance underwriters and institutional investors increasingly ask for site-specific seismic hazard assessments on major projects, even outside the traditional high-hazard zones.

A typical programme runs four to six weeks. The first week covers the desktop study — BGS borehole records, geological mapping, and any existing site investigation data. Fieldwork usually takes two to three days on site for the geophysical acquisition, plus one or two days for any calibration boreholes. Processing and ground response modelling take the bulk of the programme: we run multiple input motions, iterate the shear modulus reduction and damping curves, and produce the response spectra and maps. The draft report goes to the client at week four or five, and we incorporate any comments within a week. If the programme is on the critical path for a planning submission, we can compress it to three weeks by overlapping the fieldwork with the desktop review.

Yes, and we actively encourage it. If you already have SPT borehole logs with good recovery, CPT profiles, or downhole geophysics from a previous phase, we can incorporate those data into the velocity model and reduce the number of new measurement points required. The key requirement is that the existing data must be deep enough — ideally 30 m or more — and well-documented enough that we can verify the stratigraphic picks and the testing standards used. BS 5930 logs with SPT N-values and consistent soil descriptions are perfectly usable. We still typically run at least one calibration MASW line and one check borehole to tie the old data to the new, but the overall programme cost drops noticeably when there is good legacy data available.