Galway sits on variable glacial deposits—till, sand, gravel, and the occasional pocket of soft marine clay near the Claddagh Basin, all overlying Carboniferous limestone. VS30 values here shift dramatically within a few hundred metres. We have measured everything from 180 m/s in estuarine silts to over 800 m/s on shallow rock at the eastern edge of the city. For any structure over two storeys, the seismic site class drives the base shear calculation in Eurocode 8. A bad assumption—like defaulting to class B—can under-design the lateral system by 30% or more. Our seismic microzonation experience across the west of Ireland confirms that Galway’s ground conditions demand measured shear wave velocity, not tabulated proxies. The MASW method gives us a continuous VS profile down to 30 metres in under an hour per line, with repeatable uncertainty below 8% when processed with proper dispersion curve picking.
VS30 in Galway can flip from 210 m/s to 520 m/s within 40 metres—measured data beats tabulated assumptions every time.
Process and scope
We recently ran a four-line survey on a steep site off Taylor’s Hill Road. Two lines on the upper plateau registered VS30 of 520 m/s—solid class B. The lower line, just 40 metres downslope, dropped to 210 m/s through a buried peat layer; that flipped the entire foundation zone to class D. The design team had to switch from shallow footings to a piles solution with a structural slab, adding three weeks to the programme but sidestepping a differential settlement problem that would have emerged within the first winter. Our standard setup uses a 24-channel seismograph with 4.5 Hz geophones at 2-metre spacing, a 10 kg sledgehammer source, and three shot points per spread to build dispersion images with strong signal-to-noise up to 40 Hz. Processing follows the Park et al. (1999) phase-shift method, with manual fundamental-mode picking verified against forward-modelled synthetics. The result is a layered VS model from surface to 30 metres that feeds directly into the EC8 site classification table.
Local ground factors
The geophone array comes out of the truck in three pelican cases—24 channels, cables, trigger link, and a battery that has to last a full Galway winter day. We lay out on tarmac, gravel, or wet grass; coupling is everything. On the Claddagh side, where the tide pushes groundwater to within half a metre of surface, we swap spike-base geophones for flat-bottom pads and add a sandbag on each to kill wind rattle. The real risk is not the equipment—it is interpreting a dispersion image where the fundamental mode gets pinched by higher modes above 15 Hz. An inexperienced analyst will pick the wrong branch, overestimate VS30 by 20–30%, and push the site into a stiffer class than it deserves. We cross-check every line with at least one passive-source record using the ReMi technique to extend the low-frequency end, confirming the fundamental-mode trend down to 5 Hz. That dual active-passive approach is not optional in Galway; it is the only way to catch the soft layers.
Frequently asked questions
What does a MASW survey cost in Galway?
A single-line MASW survey with VS30 calculation and an EC8 site-classification report typically runs between €1,430 and €3,190, depending on the number of lines, site access, and whether passive-source recording is added. Multi-line or combined active-passive surveys sit at the upper end of that range. All quotes include mobilization within County Galway, data acquisition, processing, and the signed report.
How long does a MASW survey take on a Galway site?
Fieldwork for one 46-metre spread takes roughly 45 to 60 minutes, including layout, shot recording, and pack-up. Processing and reporting add another two to three working days. A two-line survey on a typical half-acre site is usually completed within a single morning on site.
Do you need to drill boreholes to measure VS30?
No. MASW is a non-invasive surface method—no drilling, no excavation. We lay geophones on the ground surface and generate seismic waves with a sledgehammer. The technique works on grass, gravel, asphalt, or concrete, and leaves zero disturbance. That said, pairing MASW with a single borehole or CPT sounding for ground-truthing improves the geological model considerably.
What EC8 site class is typical for Galway?
Galway varies enormously. Ground-moraine till on the east side often gives class B (VS30 360–800 m/s). Estuarine deposits near the Claddagh and along the Corrib frequently fall into class C or D (180–360 m/s and below). We have also recorded class A (VS30 >800 m/s) on shallow limestone bedrock in the Ballybrit area. There is no single typical class—site-specific measurement is the only reliable approach.
Is MASW accepted by Galway City Council for planning?
Yes. MASW-derived VS30 and EC8 site classification are standard in geotechnical design reports submitted to Galway City Council and to An Bord Pleanála. Our reports reference IS EN 1998-1:2005 with the Irish National Annex, which is the regulatory framework for seismic design in Ireland. We also include the raw dispersion images and processing parameters so the data can be independently reviewed.
