Observing the appearance of and measuring the change in crack width is a popular technique used to assess structural damage of buildings. Cracks are one of the first signs that there has been movement of the structure due to ground movements.
Typical ground movements associated with demolition and construction activities:
Heave When large structures are demolished there can be an upward movement of the ground beneath nearby structures as a result of soil expanding.
Settlement (also known as compaction) Downward movement as a result of soil being compressed by the weight of a building after construction.
Subsidence The ground beneath a building sinks which usually occurs when the ground loses moisture and shrinks. Areas with clay soil can be more susceptible. Changes to a water course, removing trees and vegetation and collapsing drain can all contribute to subsidence.
Landslip Downward movement of sloping ground.
Cracks can also indicate if there has been stress on the structure due to excessive transient and cyclical vibration. Guidance on vibration from construction and demolition activities is provided in BS5228-2 Code of practice for noise and vibration control on open sites.
Monitoring Crack Movement techniques The traditional method for measuring cracks is by installing a manual crack monitor
Figure 1 Manual crack monitor
Once the monitor is in position across a crack, movement can be read from the device and noted by the engineer during a construction project. This relies on regular site visits and good note keeping.
Automated Crack Monitors
In recent times it has become more popular to use digital crack monitors which can be linked to data loggers for remote access and to send you alarms if there are significant changes. The sensors often record temperature at the same time to display how a crack varies as structures naturally heat and cool. Data is typically displayed on a cloud platform at an hourly resolution for easy and complete analysis.
Figure 2 Ellitrack digital crack monitor and logger
The system should be left in place for as long as possible for you to build a picture of evidence to assess if the movement is still accruing and the direction. Modern loggers can run on internal batteries for several years to remove the need to visit site.
For further details please see https://www.campbell-associates.co.uk/crack-monitor