Subsidence Monitoring

Subsidence Monitoring

Subsidence results in foundation movements that cause cracks. Cracks in buildings can also be caused by other things. It is therefore important to understand if cracks in a building are actually changing and if so, how fast are they changing, are they getting bigger or smaller?

If crack monitoring is believed to be necessary it is often carried out over a 6 – 12 month period. In a simple case where there is an inappropriate tree or large shrub or perhaps a leaking drain close to an area of minor subsidence then following mitigating action (for example repairing a drain) the cracks are measured to see if the movement stops or at least reduces to an acceptable level that can be controlled without having to resort to foundation underpinning.

If the damage is more severe, if there is possibly a third party recovery or if the pattern of damage is not immediately obvious as subsidence, 12 months crack monitoring will be undertaken.

We sub-contract the monitoring but the results are checked and analysed in-house.

We have considered “remote monitoring” but have concluded this is not as useful or indeed cost effective as having a person visit a site to measure tell-tales. The monitoring visits are an opportunity to obtain information such as fresh cracks, sticking doors or windows. Remote monitoring cannot provide this sometimes valuable input.

As with the rest of the industry, we use metal studs fixed across a crack and then measure the changes with a vernier calliper. Small fluctuations are normal and there is a range of opinion as to what is an acceptable variation and what might be considered to be “significant movement”. This is where our experience counts.

Sometimes, the nature of building movement is not immediately obvious and it is necessary to monitor changes by taking levels and by measuring distortion. In its’ simplest form, this type of monitoring could be achieved with a spirit level although accuracy could be an issue. If level monitoring is necessary, we can undertake this but in most cases, it is more cost effective to use a specialist sub-contractor. Level monitoring is more expensive than crack monitoring, particularly if a deep datum is required. However, as with the crack monitoring, we receive the field data and analyse it in house and then provide the appropriate report.