Subsidence Monitoring - Nebula Surveyors

Understanding Subsidence: What Are Its Causes?

Subsidence occurs when the ground beneath a property sinks, often causing damage to its foundations and overall structural integrity.
This downward movement can arise from a number of causes, including the shrinking or shifting of soil, fluctuations in groundwater levels, decomposition of underlying organic material, or the compaction of soil due to heavy loads above. These changes may result in the property settling unevenly, which can manifest as visible signs of damage such as cracks in walls, sloping floors, or windows and doors that no longer open or close properly.

Although it is commonly assumed that the weight of a building is to blame, subsidence is typically linked to changes in the ground beneath rather than the structure itself. Factors such as soil composition, surrounding vegetation, and a history of mining activity in the area can all increase the likelihood of subsidence occurring.

Early identification of potential causes, especially through a detailed RICS Home Survey, is essential in preventing long-term issues and informing any necessary remedial work. Clay soils, for example, are particularly prone to shrinking and swelling depending on moisture content, which can be affected by seasonal changes or the presence of nearby trees. Tree roots can extract moisture from the soil, contributing to movement and placing stress on the foundations. Human activity, especially in regions with a mining legacy, can also leave the ground unstable and vulnerable to settlement.

The range of Subsidence Monitoring Services we provide:

Retro Target Monitoring with a Total Station

Retro target monitoring, using a Total Station, is a precise technique for tracking structural movements and deformations in three dimensions. Retroreflective targets are placed on the structure and observed with a Leica Total Station, which measures angles and distances to calculate the targets’ coordinates accurately. This real-time data is vital for monitoring structural shifts, ensuring safety, and guiding maintenance efforts. The method’s high accuracy and continuous monitoring make it an essential tool for detecting even minor movements, helping to identify potential issues early and maintain the integrity of engineering projects.

Crack Monitoring

Demec stud crack monitoring offers a precise, cost-effective way to assess structural movement. Metal studs are fixed on either side of a crack, and the distance between them is measured at regular intervals using a Demec gauge or digital calipers. These measurements track any changes in crack width over time, helping to determine the rate and nature of movement. The technique is non-destructive, discreet, and ideal for sensitive or historic buildings.

At Nebula Chartered Surveyors, we specialise in accurate building movement monitoring. Drawing on our expertise in land engineering and using advanced tools such as Total Stations, we provide detailed movement data to establish whether structural shifts are active or historic.

Level Monitoring for Structural Stability

Level monitoring is essential in engineering and construction for tracking elevation changes that impact a building’s structural integrity. This process measures shifts in foundation levels over time, offering deeper insight than crack monitoring. Using precise Topcon optical instruments (accurate to 0.1mm) and following BRE Digest 386 guidelines, evenly spaced points are set on the building, typically 300mm above ground level, with a datum established in a stable area. As the building shifts, these points move relative to the datum. Measurements are taken every 4-8 weeks for 6-12 months to track subsidence and level differences. This technique is crucial in areas prone to ground settlement or structural subsidence, helping to inform decisions on necessary repairs and safety measures with the use of advanced equipment such as laser levels and GPS systems.