Seismic Microzonation Mapping for Bognor Regis and the Arun Coastal Plain

The ground conditions between Aldwick Bay and the town centre of Bognor Regis shift more than most engineers expect. Residential plots near the seafront often sit on raised beach deposits and wind-blown sand, while sites closer to the A29 corridor encounter the London Clay Formation at depth, with a complex cover of brickearth and head deposits. These contrasts mean that two structures just a kilometre apart can experience fundamentally different seismic demands during a low-probability event. Microzonation work in Bognor Regis has to capture the transition from dense gravels in the marine terrace to softer alluvium in the Lidsey and Felpham lowlands, because shear-wave velocity profiles can double or halve across a single postcode. We run the lab programme that feeds these maps: resonant column tests, cyclic triaxial suites, and bender element measurements on undisturbed samples, all tied to borehole geophysics that we cross-check with a MASW survey where access permits, and with in-situ permeability testing when pore-pressure response needs to be calibrated against stratigraphy.

Site response in Bognor Regis can shift from stiff gravel to soft clay within twenty metres of spread; microzonation maps that boundary so the foundation design doesn't guess.

Approach and scope

The most common mistake we see in Bognor Regis is treating the entire site as a single NEHRP site class based on one borehole log, without accounting for lateral variation in the drift geology. The raised beaches of the West Sussex Coastal Plain are notoriously patchy: a three-metre-thick lens of sandy gravel can pinch out within twenty metres, exposing underlying soft clay that drops the Vs30 from class C to class D. A proper microzonation workflow starts with a dense grid of geophysical soundings, followed by selective sampling at layer boundaries. In the lab we run undrained cyclic triaxial at confining pressures that match the in-situ overburden, then derive modulus reduction and damping curves specific to each unit. For granular interlayers we complement the programme with grain-size analysis to confirm gradation, because even a small silt fraction changes the pore-pressure generation rate during cyclic loading. The output is not a single spectrum but a map of spectral acceleration contours that the structural engineer can query by foundation footprint.

Site-specific factors

We reviewed a six-storey residential scheme off Chichester Road where the initial desk study assumed a uniform stiff clay profile across the footprint. When the microzonation survey came back, the southern third of the plot was underlain by soft silty clay with a Vs of 170 m/s, while the northern portion registered 380 m/s in dense gravel. The structural model had to be re-run with two separate ground-motion inputs, and the foundation system switched from isolated footings to a piled raft tied together with grade beams to handle differential spectral displacement. Ignoring that lateral contrast would have concentrated drift demand in a single column line. Bognor Regis has enough drift variability that a single-site-class assumption is a liability; the microzonation map is what catches the boundary before the concrete is poured.

Technical parameters

Parameter	Typical value
Vs30 classification range	180–500 m/s across Bognor Regis drift units
Mapping grid resolution	25–50 m for urban plots, 100 m for greenfield
Cyclic triaxial confining stress range	50–400 kPa per overburden
Modulus reduction curves	G/Gmax and damping ratio per PI and confining pressure
Liquefaction susceptibility (granular layers)	Assessed via gradation, fines content, and cyclic stress ratio
Target return period (seismic hazard)	475–2475 years per Eurocode 8 site-specific study
Amplification factors (Fa, Fv)	Mapped per 0.2 s and 1.0 s spectral periods

Related technical services

Site-specific seismic response analysis

One-dimensional equivalent-linear and non-linear ground-response modelling using DEEPSOIL or STRATA, calibrated with laboratory modulus reduction and damping curves measured on undisturbed samples from each stratigraphic unit identified beneath the Bognor Regis site.

Laboratory dynamic soil testing

Resonant column and cyclic triaxial test suites on cohesive and granular materials, including bender element Vs measurements, strain-controlled loading from 0.001% to 1% shear strain, and pore-pressure evolution monitoring for liquefaction assessment in saturated sands of the coastal plain.

Common questions

What is seismic microzonation and why does Bognor Regis need it?

Microzonation divides a town or site into zones with different ground-motion response characteristics. Bognor Regis sits on the West Sussex Coastal Plain, where raised beaches, brickearth, alluvium and the underlying London Clay create sharp lateral changes in stiffness. A single seismic hazard value for the whole town would be misleading; microzonation gives the structural engineer spectral accelerations that reflect the actual ground column beneath each part of the site.

How is a microzonation map produced?

The process starts with a dense grid of geophysical measurements (MASW, downhole seismic, or crosshole) to capture shear-wave velocity profiles. We then take undisturbed samples at key layer boundaries, run dynamic laboratory tests to determine modulus reduction and damping curves, and build one-dimensional site-response models. The results are interpolated into contour maps of PGA, spectral acceleration at 0.2 s and 1.0 s, and site class per BS EN 1998-1.

What ground conditions in Bognor Regis most affect seismic response?

The critical factor is the contrast between the stiff gravels of the raised beach deposits and the softer silty clays in the Lidsey valley and former marsh areas. Even thin soft layers can amplify ground motion at certain periods. Depth to the London Clay also matters: where the clay is shallow, the site tends to be stiffer; where it is overlain by thick alluvium, the fundamental period lengthens and spectral accelerations can increase significantly.

Can a microzonation study help reduce foundation costs?

The reference range for this service in Bognor Regis is £3.090 - £13.440. The final price depends on the project scope and volume.

What does a microzonation study cost for a site in Bognor Regis?

For a typical Bognor Regis site, a microzonation study including geophysical survey, targeted sampling, dynamic laboratory testing, and site-response modelling ranges from £3,090 to £13,440 depending on the number of measurement points, the depth of investigation, and the complexity of the drift geology across the footprint.