Soil Liquefaction Analysis in Bognor Regis: Practical Risk...

The coastal plain under Bognor Regis hides a quiet risk. Loose saturated sands and silts, common along this stretch of West Sussex coast, can lose strength rapidly under seismic or dynamic loading. That is liquefaction. And it can turn a straightforward foundation design into a costly failure. Our analysis quantifies this risk using site-specific data. For sites near Aldwick or Pagham, where the water table is high and the geology is complex, a desktop assessment alone is not enough. We correlate field data from CPT testing with laboratory cyclic triaxial to produce a factor of safety you can act on. No generic reports. No unnecessary ground treatment. Just a clear commercial decision tool for your Bognor Regis project.

Liquefaction in Bognor Regis is not about big earthquakes. It is about loose coastal sands, a high water table, and the right dynamic trigger. Know your factor of safety before you pour.

Approach and scope

BS EN 1997-1:2004 requires assessment of seismic soil behaviour for structures in consequence class CC2 and above. In Bognor Regis, that includes most commercial and multi-unit residential builds. Our analysis follows the Youd-Idriss framework, adapted for UK seismicity. We start with SPT drilling to measure blow count and fines content at depth. These numbers feed the liquefaction potential index (LPI). One site, one clear metric. If the LPI indicates risk, we test further. Cyclic triaxial on undisturbed samples gives the residual strength. Then we model settlement and lateral spread. The output is not a textbook chapter. It is a table of depths, factors of safety, and predicted displacement. Your structural engineer gets exactly what they need for foundation sizing or Improvement specification. No fluff.

Site-specific factors

At just 4 metres above sea level, much of Bognor Regis sits on Quaternary drift deposits. Loose sand lenses are common. A site investigation without liquefaction screening is a gamble. The trigger could be a distant Channel earthquake. Or it could be pile driving next door. The result is the same: sudden loss of bearing capacity, differential settlement, structural cracking. NHBC Chapter 4.2 flags liquefaction as a key geotechnical hazard for low-lying coastal sites. We have seen projects where ignoring this risk added six weeks to the programme and tens of thousands in remedial grouting. A £2,000 analysis during RIBA Stage 2 avoids that. It is the cheapest insurance your project can buy.

Technical parameters

Parameter	Typical value
Analysis framework	Youd-Idriss (NCEER) with BS EN 1997-1:2004
Field data input	SPT N60, CPT qc, grain size distribution
Laboratory test	Cyclic triaxial (BS 1377-8:1990)
Output metric	Liquefaction Potential Index (LPI)
Magnitude scaling	Adjusted for UK intraplate seismicity
Settlement estimate	Ishihara & Yoshimine (1992) method
Reporting format	Factual + Interpretative (AGS 4 format)

Related technical services

Screening-level LPI analysis

Ideal for single plots and small schemes. We use existing SPT or CPT logs to calculate LPI at each layer. Report includes factor of safety, settlement estimate, and a clear yes/no on mitigation need.

Full cyclic triaxial programme

For sites with marginal LPI or high consequence class. Undisturbed samples tested under cyclic loading. Outputs residual strength and pore pressure generation curves. Used to calibrate advanced numerical models.

Common questions

Is liquefaction really a risk in Bognor Regis, given the low UK seismicity?

Yes, it can be. The risk is not driven by large crustal faults. It is driven by the combination of loose saturated sands, a high water table, and any dynamic trigger. Even a moderate event in the English Channel, or heavy piling vibration on an adjacent site, can induce excess pore pressure. We have measured SPT N-values below 10 in sands under the coastal plain. That is prime liquefiable material.

What do I do if the analysis shows a liquefaction risk?

We define a clear path. If the LPI is low, we may recommend a stiffer foundation raft with edge beams to tolerate small settlements. If the risk is high, we specify Improvement. That could be vibrocompaction to densify the sand, or stone columns to provide drainage. The report gives your contractor a target depth and acceptance criteria. No open-ended remediation.

How long does a liquefaction analysis take?

A screening analysis on existing logs can be turned around in 5 working days. A full cyclic triaxial programme takes 4 to 6 weeks, including undisturbed sampling, laboratory testing, and reporting. We always align our schedule with your planning submission deadline.

What does a soil liquefaction analysis cost for a typical project in Bognor Regis?

For a screening-level LPI analysis using existing field data, budget between £1,790 and £2,350. A full programme with new SPT or CPT borings and cyclic triaxial testing typically ranges from £2,800 to £3,420, depending on depth and number of samples. This includes the AGS-format digital data and a stamped interpretative report.

Soil Liquefaction Analysis in Bognor Regis: Practical Risk Assessment for Builders