Why is crack width monitoring on its own a waste of time?

Many engineers just fit crack width monitors that measure the change in the width of a crack and then get some test pits dug. For the reasons outlined in the following paragraph this can be very time consuming and non-conclusive.

We believe that this is a very unscientific approach because cracks only tend to change in width when the stresses in the wall reach a certain critical level. As the stresses in the wall force the crack open so they are reduced in intensity. Hence if the crack monitor is put across a crack just after the critical stresses in the wall have been reduced by an increase in crack width it may be a long time before the crack itself moves again. Furthermore a knowledge of how much a crack has increased in width does not give any information on what is causing it. It is a bit like measuring the depth of a dent in a car and trying to work out whether it was caused by bad brakes! This is because the crack is a symptom and not a cause. The cause is normally relative movement of the foundations. Furthermore it is very annoying when gauges are glued across the crack and then another crack appears alongside it!

We are not saying that crack widths should not be monitored - but that their measurement is part of a scientific approach as described below.

What will be involved in the investigation that FEF recommends?

We will probably recommend the installation of a number of movement gauges that will be used to monitor the vertical movement of all the critical wall areas. Please visit this website for more information on these www.movementgauge.com We will install the gauges and provide an automatic level and train you in its use so that you can monitor the gauge movements.

We will also provide you with a vernier gauge to measure the crack widths when you undertake the weekly movement gauge readings.

We can do all these measurements ourselves but it will be more costly.

The report will also recommend a duration period for the investigation. If the movement seems to be an ongoing problem then it will be necessary to excavate test pits with a mini-digger. We will include an assessment of the cost of this in our preliminary report following our free visit.

Dangerous structural situations

We are often called to inspect properties where the structure is dangerous or the cause of the instability is obvious. In these cirumstances we will provide a quotation for undertaking the remedial works during our initial visit.

Where is subsidence and cracking likely to occur in France

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