Subsidence Monitoring

What is Subsidence?

Subsidence is the process of ground beneath a property sinking. This change in ground conditions causes the foundations of buildings to move, which in turn can have an adverse effect on walls and general structural integrity. It is particularly problematic when the ground under your property is sinking at different rates. Issues can manifest in several forms including wall cracking, sloping floors & jammed windows.

What causes Subsidence?

There are numerous possible causes of subsidence.

Most common is soil composition. Clay soils; for example, retain a lot of moisture and can do so for many decades. When that clay eventually dries, it will contract. Naturally, anything resting on or in the clay will move with the contraction. Similarly, when clay moistens, it will expand, creating yet more movement.

Another common cause of subsidence is tree roots. Roots are a contributing factor to drying out soil (mentioned above). In the event that foundations are erroneously laid in the vicinity of trees and other vegetation, roots are able to crack foundations and other overlying structures.

Not all subsidence is naturally occurring. Human factors can also be to blame. Subsidence is very common in and around former mining towns. Long abandoned mines can undergo settlement, causing sinkage in the ground above.

Our Subsidence Services

Constructive Evaluation offers a comprehensive suite of services that can be tailor made to suit your requirements.

Crack Monitoring

One very simple and cost-effective method is to monitor cracks over a set period (determined based on condition & severity of defects). To do this we affix a stainless steel Demec stud either side of a crack. Once set, we take a preliminary measurement between the two studs using digital callipers, which are able to give a precise reading to a one hundredth of a millimetre. We will then return at a later date (typically a month later) and measure the distance between studs again. Any expansion in the crack will show up in the measurements. This process is repeated over a period of time until enough data is collected to meet the client’s requirements.

Level Monitoring

Compared to crack monitoring, level monitoring provides more comprehensive data which can give you precise measurements on how much a foundation is moving, if movement occurs at specific times of day & why it might be moving.
We carry out our level monitoring surveys in accordance to BRE Digest 386, using Topcon optical instruments which are accurate up to 0.1mm.
Evenly spaced points are fixed to a building at locations roughly 300mm above ground level. A datum is situated on an unaffected area, within the vicinity of the structure being monitored. As the building moves, the fixed points will move with it. These points are referenced to the datum point. Readings are made every 4-8 weeks over a period of 6-12 months. Readings made against the datum will show a difference in levels and therefore give you accurate measurements of the subsidence over the time period.

Structural Monitoring

Whilst using similar methods to level monitoring, Retro Target Monitoring detects movement across 3 dimensions to an accuracy of 1mm. This is particularly beneficial to clients undertaking deep excavations for projects such as basements. Small targets are fixed onto the structure in question and other buildings in the surrounding area. All of the targets are monitored using a Leica total station and then referenced to known local survey datums. The accuracy and versatility of retro target monitoring; compared to level monitoring, means that repeat readings can be made over a shorter period e.g. weekly.

Contact Us

Constructive Evaluation can provide bespoke monitoring packages for a range of different scenarios. Contact a member of our surveying team to discuss your requirements