In Kilkenny, the ground beneath a site is rarely as uniform as the planning drawings suggest. We frequently encounter a layered sequence of stiff glacial till over karstic limestone, and occasionally deep pockets of alluvium along the River Nore floodplain. A standard borehole log tells you what the soil is, but it does not tell you how the ground will shake during a seismic event. That is where a shear wave velocity profile becomes essential. By running a MASW survey across the site, we measure the travel time of surface waves to derive Vs values down to 30 metres. The resulting VS30 figure feeds directly into the Eurocode 8 ground type classification, which determines the seismic design spectrum for the structure. For projects near the medieval city centre, where vibration sensitivity is high, the non-invasive nature of the method avoids any risk to adjacent heritage fabric while still delivering the stiffness data the structural engineer needs.
A VS30 number alone is not enough; understanding the velocity contrast between layers is what reveals the real seismic amplification potential of a Kilkenny site.
Methodology and scope
A recent project on a sloping site off the Callan Road illustrated the method's value clearly. The developer had assumed a uniform ground type B across the parcel, but the MASW line revealed a sharp lateral transition: the upper slope sat on very dense till with Vs30 above 400 m/s, while the lower third of the site, closer to a buried stream channel, dropped into ground type C territory with Vs30 around 250 m/s. This kind of variation is not unusual in Kilkenny, where glacial deposition and subsequent river erosion have created a patchwork of stiffness conditions. The survey used a 24-channel seismograph with 4.5 Hz geophones spaced at 2 metres, recording for roughly 20 minutes per line. Processing involved dispersion curve picking in the frequency-velocity domain and iterative inversion to produce a 1D shear wave velocity profile at each midpoint. The final report included a 2D cross-section of Vs values along the survey line, with the site classification boundaries clearly marked. For deeper stratigraphic control, we sometimes pair the MASW with an SPT drilling campaign to calibrate velocity layering against known lithology, particularly where the bedrock surface is irregular and needs confirmation.
Local geotechnical context
Kilkenny sits on Carboniferous limestone overlain by glacial tills of varying thickness, and the National Annex to Eurocode 8 assigns a reference peak ground acceleration of approximately 0.04g to the region. While this is low by global standards, the local site effects can amplify ground motion significantly if soft deposits overlie stiff material. The classic risk scenario is a layer of loose alluvial silt or soft clay 5 to 10 metres thick sitting directly on limestone bedrock. The impedance contrast at the soil-rock interface traps seismic energy, increasing the spectral acceleration at periods that coincide with the natural frequency of low-rise and mid-rise buildings. Without a measured VS30, the designer must assume a conservative ground type, often leading to an unnecessarily expensive lateral load system. An MASW survey removes that guesswork, replacing the default ground type E assumption with a measured classification that can reduce design seismic forces by 20 to 30 percent when the data supports a stiffer category.
Frequently asked questions
What does a MASW survey in Kilkenny typically cost?
For a standard VS30 classification survey on a typical residential or commercial site in Kilkenny, the cost ranges from €1,440 to €2,450 depending on the number of test lines, site access conditions, and whether 2D cross-sections are required. A single-line 1D survey for a house or small extension sits at the lower end. A multi-line 2D survey across a larger development, with full inversion processing and a comprehensive report, falls toward the upper end. All quotes include mobilisation within the county, data acquisition, processing, and a signed report ready for submission to the building control authority.
How does MASW compare to a seismic refraction survey for getting shear wave velocity?
Seismic refraction measures P-wave velocity and only works when velocity increases with depth, which is not always the case in Kilkenny's glacial sequences where a stiff till can overlie softer weathered limestone. MASW measures Rayleigh wave dispersion, which is directly sensitive to shear wave velocity regardless of the velocity layering. It also works on concrete and asphalt surfaces, making it practical for urban sites where refraction would require planting geophones in soil. The trade-off is that MASW has a slightly coarser lateral resolution, but for VS30 classification it is the more solid and widely accepted method under Eurocode 8.
Can you run a MASW line inside a building or on a paved surface?
Yes, MASW works well on hard surfaces including concrete slabs, asphalt, and compacted gravel. The geophones are coupled to the ground using base plates with a thin layer of viscous couplant, and the sledgehammer source is struck directly on the surface. This is a key advantage for sites in Kilkenny city centre where the area of interest may be a courtyard, a warehouse floor, or a car park. The survey does not require any drilling or excavation, so it causes zero disturbance to the surface and leaves no trace after the equipment is removed.
