In-situ ground investigation forms the very backbone of geotechnical engineering in Hull, delivering direct measurements of soil and rock behaviour without the disturbance that plagues laboratory samples. This category encompasses all field-based testing methods that assess the engineering properties of the ground in its natural state, from strength and stiffness to permeability and density. For a city built largely on soft, compressible alluvial deposits alongside the Humber Estuary, understanding these properties at their true in-situ condition is not merely academic — it is a fundamental requirement for safe and economical design. The data gathered through these techniques directly informs foundation design, earthworks specifications, and groundwater control strategies, making it an indispensable phase of any intrusive investigation.
Hull’s geological setting presents a classic example of a post-glacial sedimentary environment that demands a rigorous in-situ approach. The near-surface geology is dominated by the Hull Formation — a sequence of soft, normally consolidated clays, silts, and peats deposited during the Holocene transgression. These estuarine alluvium soils are notoriously variable, often containing organic layers and lenses of loose sand, and they exhibit low bearing capacity and high compressibility. Beneath this, the glacial till of the Skipsea Member provides a more competent stratum, but its depth and consistency can vary sharply across the city. Standard penetration tests and cone penetration testing are essential here to profile this challenging sequence, while a field permeability test (Lefranc/Lugeon) becomes critical when assessing the silty and sandy layers that can act as confined aquifers, directly impacting excavation stability and dewatering design.
The regulatory framework governing all in-situ work in Hull is firmly rooted in British Standards, most notably the BS 5930:2015+A1:2020 Code of practice for ground investigations, which is the definitive guide for planning and executing field tests. This is complemented by BS 1377 for soil testing methods and BS EN ISO 22475-1 for sampling and groundwater measurement. Crucially, the execution of any in-situ test must align with the requirements of Eurocode 7 (BS EN 1997-2:2007), which mandates that the selection and interpretation of field tests are based on a thorough understanding of the ground model. For a project in Hull, compliance is not just about following a procedure; it is about demonstrating that the derived characteristic values are representative of the site-specific conditions, a principle that underpins the entire geotechnical design process as verified by the supervising engineer.
The types of projects in Hull that rely on comprehensive in-situ testing are as diverse as the city’s built environment. Large-scale regeneration schemes along the waterfront, such as the Fruit Market or the Yorkshire Energy Park, require deep profiling with pressuremeter tests to design piled foundations that can bypass the weak alluvium and socket into the underlying till. Infrastructure projects, including the A63 Castle Street improvement works, depend on rigorous in-situ density and CBR testing to verify the performance of engineered fill and pavement layers. Even smaller residential developments on the city’s outskirts necessitate dynamic probing and trial pitting to assess the risk of shrinkable clays or to design soakaways, where a field permeability test is a statutory requirement under many sustainable drainage (SuDS) approvals. Each project, regardless of scale, uses the in-situ suite to translate the geological model into a set of reliable design parameters.
An in-situ test measures soil or rock properties directly in the ground without removing the material from its natural environment. This preserves the ambient stresses, moisture content, and fabric that are inevitably altered during sampling and transport. For sensitive soils like Hull's soft estuarine clays, lab tests on even high-quality samples can underestimate settlement, making field tests the more reliable basis for design.
A typical investigation in Hull’s alluvial soils would centre on the Standard Penetration Test (SPT) and Cone Penetration Testing (CPT) for profiling and strength assessment. These are routinely supplemented by field vane tests in the soft clays to measure undrained shear strength. Permeability testing, such as the Lefranc method in boreholes, is also common to assess the drainage characteristics of silty and sandy layers for foundation and basement design.
BS 5930:2015+A1:2020 provides the overarching code of practice, dictating everything from the spacing of exploratory holes to the selection of appropriate test methods based on the ground conditions. It sets strict requirements for equipment calibration, test execution, and the recording of data. Adherence ensures the investigation is systematic and that the results are robust enough to satisfy the observational method and Eurocode 7 design requirements.
While in-situ testing is invaluable, it is rarely used in complete isolation. A robust ground investigation integrates field tests with a program of laboratory classification and strength tests on recovered samples. The in-situ data provides the continuous profile and 'undisturbed' behaviour, while lab tests allow for correlation and a deeper understanding of the soil's composition. This combined approach is the standard of care for managing Hull's variable ground conditions.