CASE STUDY
Advanced Satellite InSAR Monitoring of the Central Prestatyn Coastal Defence Scheme
Ground Engineering Awards 2026 Finalist in the Technical Excellence Category
Ground Engineering Awards 2026 Finalist in the Technical Excellence Category
PROJECT IMPACT
Alignment of independent data sets, with Geofem’s integrated analysis showing strong correlation between two independently sourced settlement profiles.
Increased confidence in the findings through corroborated InSAR trend analysis.
Safe reduction of post-lift consolidation period by two months
Accelerated commencement of third-lift placement.
Reduced in situ monitoring requirements, lowering carbon impact and overall project risk.
Early project delivery with tangible real-world impact: Coastal defence scheme completed ahead of schedule protecting 2,297 homes and 86 businesses from escalating storm surge & sea level threats.
Project overview
The Central Prestatyn Coastal Defence Scheme involved the construction of a 1.65 km embankment over very soft, normally consolidated tidal flat deposits to protect communities in North Wales from coastal flooding.
The embankment was designed to be built in three sequential lifts, with consolidation periods required between each stage to manage settlement and excess pore pressures.
To manage ground risk and uncertainty, JBA Consulting partnered with Geofem to enhance the in situ monitoring of embankment settlement.
“Geofem delivered an integrated monitoring solution combining conventional geotechnical instrumentation with high-resolution satellite-based InSAR to support safe and efficient delivery.”
The challenge
The coastal towns of Rhyl and Prestatyn in Denbighshire, North Wales, are increasingly exposed to tidal flooding and storm surge risk. The existing sea defences—built over 65 years ago—are no longer sufficient, with recent storms already flooding nearly 500 properties in nearby east Rhyl. This risk is expected to worsen due to climate change and rising sea levels.
To address this, Denbighshire County Council delivered the Central Prestatyn Coastal Defence Scheme to strengthen the frontage with a new 1.65 km embankment up to 4 metres high.
The scheme was constructed over very soft, low-strength tidal flat deposits, requiring a carefully staged approach in three lifts with consolidation periods between each stage to manage settlement and pore water pressures.
“While initial assumptions suggested around nine months between lifts, ground testing indicated consolidation could take up to three years, creating significant programme uncertainty... These challenging ground conditions demanded tight control of stability, settlement and pore pressures throughout construction, with limited visibility from traditional point-based instrumentation alone across the full 1.65 km alignment.”
The biggest technical challenge was delivering InSAR displacement data within one month of each lift. Instead of time-series InSAR, which provides millimetric accuracy, individual interferograms from SAR image pairs were required, offering centimetric accuracy. However, settlements were less than expected – at less than 1cm per month – threatening the viability of the technique for construction monitoring.
The Geofem approach
The project required a robust monitoring-led approach to reduce uncertainty, support safe decision-making, and enable efficient delivery on highly variable ground.
“Geofem leveraged the power of for·see Motion to provide continuous, site-wide visibility of ground behaviour, allowing settlement risks to be effectively managed while informing construction sequencing decisions.”
Geofem’s advanced satellite-based InSAR monitoring provided an additional site-wide source of embankment settlement data, entirely independent of the settlement plate data throughout each consolidation phase.
“Our team applied a carefully designed InSAR processing strategy, selecting multiple coherent ascending and descending SAR image pairs to achieve sub-centimetre displacement accuracy from individual interferograms. We achieved a displacement accuracy close to that of time-series InSAR. Monthly processing generated high-resolution vertical displacement maps that were independently validated against the in-situ measurements.”
Results
As indicated in the above image, the settlement plate data was highly variable. That alone created too much uncertainty to shorten consolidation times. However, Geofem’s interpretation of the combined datasets showed that the two independently sourced settlement profiles were in broad agreement.
Geofem’s complementary monitoring approach and consistent settlement trends observed through InSAR gave engineers confidence to shorten the post-lift consolidation period by 2 months, resulting in total programme reduction with associated cost saving.
This enabled earlier placement of the third lift in January 2025, safely accelerating the programme while reducing on-site monitoring demands, carbon impacts and project risk.
New coastal defences were delivered by Balfour Beatty on behalf of Denbighshire County Council ahead of schedule, safeguarding 2,297 homes and 86 businesses from the increasing threat of storm surges and rising sea levels.
Geofem Insights
Embankments built on very soft ground cannot always be constructed in one continuous operation because the foundation soils need time to gain sufficient strength to support them. Planned rest periods give time for consolidation to occur, pore pressures to dissipate and the soil strength to increase before the next layer of fill is placed. This staged approach allows construction to proceed safely while making best use of the ground’s improving capacity.
"Geofem Ltd provided us with SAR-derived embankment settlement data from single interferograms following Lift 2, confirmed by terrestrial data."
Faster than expected settlements allowed the early resumption of construction, saving time and cost.
Ian Johnson, Technical Director
JBA Consulting
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