CASE STUDY
Retrospective InSAR Displacement Analysis of a River bridge, Pacific Highway, New South Wales
Ground Engineering Awards 2026 Finalist in the Technical Excellence Category
PROJECT IMPACT
Retrospective displacement evidence, leveraging for·see Motion. Geofem applied InSAR analysis to reconstruct a continuous history of ground movement, providing displacement insights during periods when conventional survey monitoring was unavailable, including the Christmas shutdown period.
Improved forensic understanding, with Geofem helping establish when movement began via satellite-derived movement data, and whether movement coincided with the construction activities of fill placement or pile removal.
Reduced dispute uncertainty via independently derived displacement evidence, providing a clearer factual basis for assessing competing causation arguments in a contractual/legal dispute.
Engineering-led interpretation combining InSAR displacement analysis with civil/geotechnical engineering judgement to distinguish potentially reliable displacement trends from construction-related noise or incoherent data.
Independent assessment: Geofem interpreted the InSAR data without prior knowledge of the dispute, thereby ensuring a truly independent assessment.
Greater confidence in decision-making, with the work supporting a more informed settlement of the dispute by introducing objective displacement data that had not previously been available
Project overview
"Geofem was engaged by Brisbane-based geotechnical consultancy AGTRE and renowned geotechnical expert Dr Burt Look to undertake a retrospective InSAR displacement analysis of an existing bridge, its abutments, and surrounding riverbank areas during construction of a new adjacent bridge along the Pacific Highway in New South Wales."
The project aimed to provide independent movement data where conventional survey records were incomplete or unavailable, particularly during a critical period when bridge movement was identified, but monitoring had ceased during the Christmas shutdown.
Using archived SAR satellite data, Geofem performed a retrospective InSAR analysis to assess construction-related impacts and provide an independent assessment of historical bridge displacement and surrounding ground behaviour.
The analysis was undertaken to support the client’s forensic review by providing a concise, independent and reliable assessment of historical displacement trends and construction-related movement behaviour.
Determining the timing and likely causes of movement affecting the southern abutment of the existing permanent bridge presented a complex forensic challenge.
A gap in conventional survey monitoring during the Christmas shutdown period meant that critical displacement data was unavailable, making it difficult to establish when movement occurred and whether it was related to temporary pile removal, abutment fill replacement, riverbank instability, sensitive clay behaviour, tidal effects or other contributing factors.
The bridge is located within a geotechnically complex environment, underlain by Holocene sediments including soft/firm clay and interbedded loose silty sand and soft/firm clay.
Potential scour and erosion influences, and a history of movement affecting nearby structures added to the project complexity.
Additional challenges included interpreting satellite data within an active construction environment, where surface changes and earthworks can make satellite signals harder to interpret.
The investigation required an independent assessment that combined displacement evidence with construction history, geotechnical conditions, and wider site observations.
The Geofem approach
Through the application of for·see Motion, the Geofem team undertook a retrospective InSAR analysis, obtaining and analysing satellite radar data to assess displacement of the existing bridge, abutments and riverbank areas before and during construction. The InSAR data provided movement information at approximately two-week satellite pass intervals and indicated movement beginning during fill placement and before removal of the temporary piles.
The satellite line-of-sight measurements were interpreted alongside the construction sequence, survey records, filling history, pile removal records, and available geotechnical evidence.
Because construction works and earthworks can affect coherence, Geofem used a combination of analysis methods and engineering judgement to identify reliable displacement information and dismiss unreliable data. Using InSAR to measure ground displacement with millimetric accuracy and engineering insight to distinguish the causes between the various construction activities and complex hydrological and geotechnical conditions Geofem provided an independent, additional source of displacement information in a case where conventional instrumentation and survey records were incomplete.
Results
Geofem’s InSAR analysis, powered by for·see Motion, provided valuable displacement information for a critical period when no survey data were available.
By combining satellite remote sensing with civil engineering expertise, we developed a continuous record of movement across the bridge and, abutments and surrounding riverbanks, during construction and extending two years prior to construction.
The satellite data indicated that 10mm of bridge pier and abutment movement occurred during fill placement and only 2mm during the key stage of temporary pile removal, providing an important timing factor in the forensic assessment.
This timing evidence helped challenge a single-cause interpretation based only on temporary pile removal and supported a broader assessment of possible contributing mechanisms.
Geofem’s analysis helped establish a more reliable picture of bridge displacement over the full assessment period by combining satellite remote sensing with civil engineering interpretation.
The resulting displacement evidence became critical to the dispute because reliable, independently determined movement data were available for the first time.
The dispute was settled in a more informed manner, supported by objective movement evidence rather than relying solely on assumptions, incomplete monitoring or model-based interpretation.
By applying for·see Motion and combining advanced InSAR analysis with engineering expertise, Geofem provided an independent, evidence-based understanding of historical ground and structural movement that was previously unavailable.
The retrospective assessment helped clarify the timing and nature of displacement, supported evaluation of competing causation scenarios, and provided the objective evidence required to inform a more confident resolution of the contractual dispute.
Geofem Insights
A key lesson from this case is that FEA models should not be regarded as proof of a particular mechanism simply because they can replicate observed movements. Multiple underlying processes may produce similar deformation patterns, and alternative explanations must be carefully considered.
The InSAR displacement data obtained here highlights that the timing for the theorised sequence of events – “proved” by FEA – was wrong. This data, along with site records, further ground investigation and engineering judgement found an alternative explanation to be far more plausible.
"I have used Geofem to carry out InSAR analysis for a legal case study in NSW, Australia."
InSAR movement data over time was provided when no survey data was available over a Christmas period at a construction site. InSAR provides valuable insights in forensic engineering to assess causes of failures and damages to infrastructure and construction, and I highly recommend Geofem in this regard.
Dr. Burt Look OAM, Geotechnical Engineer
AGTRE
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