Introduction:
Subsidence can be a homeowner’s worst nightmare, causing structural damage to buildings and compromising their stability. Understanding the causes and conducting a thorough subsidence investigation is crucial to address the issue promptly and effectively. In this blog, we will explore subsidence investigation, uncovering the various types of tests carried out to determine the underlying causes.
What is Subsidence?
Subsidence refers to the sinking or downward movement of the ground, leading to the settling or tilting of structures built on it. It can occur due to various factors, including natural geological processes, human activities, and environmental conditions. Identifying the cause is essential in implementing the right remedial measures.
Subsidence Investigation:
Subsidence investigation involves a systematic approach to determine the subsidence's causes, extent, and severity. It typically consists of visual inspections both above ground and more importantly below ground through trial pits, geotechnical assessments, and specialised laboratory testing. Here are some common types of tests performed during subsidence investigations:
Site Inspection:
A site investigation specialist visually examines the site and surrounding areas to identify potential factors contributing to subsidence. This includes looking for visible signs such as cracks in walls or floors, uneven settlements, or displacement of foundations.
Trial Pits:
Trial pits are a type of intrusive site investigation used to determine the ground conditions across a site to study or sample the structure and composition of the subsurface. This includes gathering foundation depth, type, and condition information. They are usually carried out using hand tools, but mechanical options can be used for larger pits where access allows.
Borehole Logging:
Boreholes are carried out to collect soil samples at various depths alongside in-situ strength testing. These can be done by hand or with rotary or driven mechanical options. These samples are then analysed and logged to determine the soil type. Borehole logging helps assess the soils, and the samples recovered can then be tested for in-situ moisture content, Liquid and Plastic limits, swell strain characteristics, and levels of desiccation to examine the likelihood of subgrade shrinkage.
CCTV Surveys:
Underground CCTV surveys are an essential tool to check the condition of drainage in the affected area, highlighting damage, root ingress, blockages, and other issues. Hydrostatic drain integrity testing is also helpful in identifying unseen issues within the system.
Watermain:
Watermain integrity testing can be carried out using pressure gauges to detect losses over a period or a listening stick test to identify loss.
Crack and Level Monitoring:
Using digital callipers, cracks in a structure are measured over a set period of time to track the opening and closing for movement to understand the pattern and what might be causing it. Level monitoring surveys are also used to record the movement of the footprint of a property or structure.
Inclinometer Survey:
An inclinometer is a device used to measure lateral movement or deformation of the ground. It consists of a series of sensors installed at regular intervals along a borehole or inclinometer casing. This survey provides valuable data on ground displacement and can help determine the direction and magnitude of any movement.
Soil Moisture Monitoring:
Monitoring the moisture content in the soil is vital, as changes in water levels can significantly impact soil stability. Sensors are installed in the ground at various depths to measure moisture variations over time. This data helps assess the influence of groundwater fluctuations on subsidence.
Laboratory Testing:
Soil samples collected from boreholes are subjected to laboratory testing to determine their physical and mechanical properties. Tests such as moisture content, Liquid and plastic limits, consolidation, Desiccation and shear strength analysis help assess the soil’s behaviour under different conditions, aiding in the identification of potential triggers for subsidence. Root analysis/identification is also carried out to help identify any potential vegetation that could be affecting the movement of a property/building.
Conclusion:
Subsidence investigation plays a pivotal role in understanding the causes of ground movement and implementing appropriate remediation strategies. By conducting a comprehensive range of tests, including visual inspections, borehole logging, inclinometer surveys, soil moisture monitoring, and laboratory testing, experts can gather vital data to unravel the mystery behind subsidence. Armed with this knowledge, homeowners, engineers, and geotechnical professionals can take informed actions to protect structures and prevent further damage.
Remember, early detection and prompt action are key to mitigating the risks associated with subsidence. If you suspect subsidence on your property, it is crucial to consult with qualified professionals who specialise in subsidence investigation and remediation.
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