What is Subsidence and What Causes It?

Subsidence, the downward shifting of ground beneath a structure, affects building foundations and compromises structural stability. This condition often results from various factors like soil shrinkage, changes in the water table, decay of organic materials, and soil compression from heavy buildings, leading to uneven ground settlement. Such disparities can cause physical damage including cracked walls, uneven floors, and stuck windows. Contrary to popular belief, subsidence is more related to soil changes and erosion beneath the structure rather than a building’s weight. Influences like soil type, local vegetation, and previous mining activities heighten subsidence risks. Identifying its causes early, particularly in RICS surveys, is crucial for damage prevention and appropriate corrective measures. Common causes include the nature of soil, especially clay, which contracts and expands with moisture levels, and tree roots that can dry out soil and damage foundations. Additionally, human activity, particularly in historical mining areas, can lead to ground settlement and subsidence.

The range of Subsidence Monitoring Services we provide:

Retro Target Structural Monitoring

Retro Target Monitoring using a total station is a sophisticated method widely employed in engineering and surveying to track structural movements and deformations with high accuracy across 3 dimensions.

This method involves the strategic placement of retroreflective targets on the structure being monitored. These targets are then observed using a Leica Total Station, a process that ensures high precision.

The Total Station measures angles and distances between its position and the retro targets, allowing for precise calculation of the targets’ coordinates in three-dimensional space. This data is crucial in assessing how a structure shifts or deforms over time. The method is especially valuable in monitoring buildings for safety and maintenance purposes. Its accuracy and reliability make it an indispensable tool in ensuring the structural integrity of various engineering projects. The use of a Total Station for retro target monitoring provides real-time, continuous data, which is vital for detecting even the slightest movements, aiding in the early detection of potential structural issues.

Level monitoring is a crucial component in engineering and construction, essential for providing detailed data that is vital in assessing and maintaining a building’s structural stability. This process entails accurate measurements of elevation changes over time to gain an insight into the movement of foundations, including the timing of these shifts. More comprehensive than crack monitoring, it is performed in compliance with BRE Digest 386 using Topcon optical instruments, which are precise up to 0.1mm. The technique involves placing evenly spaced points on a building, typically about 300mm above ground level, and setting a datum in a stable area nearby. As the building moves, these points move relative to the datum. Measurements are recorded every 4-8 weeks for a duration of 6-12 months, effectively highlighting level differences and accurately tracking subsidence over time. Level monitoring is indispensable in situations of ground settlement and structural subsidence, particularly in large-scale construction projects and areas susceptible to geological changes. The data gathered is crucial for informed decision-making regarding necessary interventions and safety measures, utilising state-of-the-art equipment like laser levels and GPS systems to ensure maximum accuracy.

Crack monitoring with Demec studs is a precise and cost-effective method used in buildings. This technique involves affixing high-precision metal studs on either side of a crack and measuring the distance between them over time using digital calipers or a Demec gauge for high accuracy. Measurements are taken at regular intervals to track changes in a crack’s width, providing vital data on the rate and pattern of crack movement. This information is crucial for evaluating structural integrity and making informed repair decisions. The method is non-destructive, minimally invasive, and particularly suitable for historic structures, making it a popular choice in structural health monitoring.

At Andromeda Chartered Surveyors, we provide an array of precise building movement monitoring services. Leveraging our comprehensive expertise in land engineering and employing advanced technologies such as Total Stations and various land engineering techniques, we are adept at accurately recording building movement data. This allows us to determine whether the movement is ongoing or has been present for an extended period.

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