Subsidence monitoring can take many different forms; from simple measurements with a crack gauge to continuous recordings with a wireless crack movement recorder, with built in data logger. Each system comes with its own strengths and weaknesses, which must be considered in order to apply the most appropriate and effective technique. Considering that most monitors can now easily be sourced online, it is imperative that monitoring should only be completed by a genuinely experienced professional to ensure that data is collected and recorded accurately.
An example of poorly installed and recorded monitors:
Readings from crack monitors inspected at a property in Roscrea, Co. Tipperary in 2019 would suggest that all cracks have closed by a distance of over 25 mm! This is just one example of how poorly installed and recorded monitors provide no data whatsoever and can be a complete waste of time and money for the consumer. Subsidence monitoring should be performed by qualified, experienced professionals for accurate results
Crack Width Monitors / Tell Tales
Crack width monitors or ‘tell tales’ are the most common form of subsidence monitoring used in Ireland today. Invented by Englishman Roger Johnson in the 70’s (a Chartered Civil and Structural Engineer and Chartered Surveyor), this device has been an invaluable tool for subsidence experts. Unfortunately, many manufacturers have breached this patent and continue to sell similar forms of crack monitors, often in acrylic, a cheaper and weaker form of plastic. Although these monitors are relatively easy to install, many are fitted and recorded incorrectly without the assistance of experts.
The ‘standard’ monitor consists of two plates, which are independently fixed to either side of the crack whilst overlapping for part of their length. The bottom plate is calibrated in millimetres by means of a grid and the top plate is transparent and marked with a red hairline cursor; the plates are fixed so that the cursor is in line with the centre of the scale.
Readings can be taken at defined intervals to determine if the crack is still opening, closing or even cyclic. Horizontal and vertical readings can be taken to an accuracy of +/- 1.0 mm and by interpolation to +/-0.5 mm.
The ‘plus’ series monitor has three extra features. The first is the addition of four (rather than two) pre-set pegs, which allows for quick and easy fixing whilst spigots attached to each plate can be measured with a vernier calipers within 0.1 mm. However, the pre-set pegs do not always allow the cursor to be square to the grid so accurate recording on installation is paramount.
Crack monitors are not without their disadvantages – tiny vertical movements can be difficult to measure, particularly if time is a concern. Furthermore, they can be somewhat unsightly and may be prone to vandalism depending on their location.
There are other forms of crack monitoring which are less commonly applied. Level monitoring is rarely used in Ireland, particularly in domestic houses and is usually completed by specialist surveying companies. Monitoring points are fixed to the building and base readings referenced to a datum point, are recorded. Subsequent readings are taken at agreed intervals, for example, every three months. Level monitoring can measure vertical movements to ±0.1 mm, which may not have any actual effect on crack width. Special steel discs can be fixed to walls and the distance measured by inserting the jaws of a caliper into the small cavity on each disc. This system is less obtrusive than tell tales but will only tell the degree of movement as opposed to the direction.