Frequently Asked Questions
Why is there no fixed datum?
A fixed datum is rarely relevant. The building, unless piled, will be moving in conjunction with the upper surfaces of the earth under the influence of the moon and tides, the water content of the clays and temperature effects. The Engineer only needs to know if one element of the building is moving relative to another. It is relative movement that usually causes damage.
There may be instances however where movement of the building relative to say the crust of firm clay floating on silts and peat is relevant (e.g. Hull area of Great Britain). A concrete post fixed into the required layer can then carry a movement gauge to give an idea of whether a building is sinking through the crust.
Why is precise data on the foundations needed?
Remedial alterations to foundations can have major repercussions on a building and attached structures and these are not always beneficial. Such expensive and drastic works should only be considered if the movements in the existing foundations are clearly understood.
Might cracks result from other causes?
Diagnosis of structural distress needs to address all causes of damage to a building. Movement in foundations often triggers other damage or vice versa. By far the most common co-existing distress is from eaves spread, as the movement in the wall tends to release horizontal restraint to the roof rafters where horizontal forces are usually 30% of vertical dead load. However, it is possible that break-up in the integrity of a wall from eaves spread can also encourage differential movement along the wall, so the two often go together.
In cases of major movements, intra-leaf splitting of the wall often occurs, particularly in stone walled buildings with rubble cores containing few headers. Splitting of the wall is often easy to deduce in walls with large movements and many cracks but is a particular danger in walls that look sound. Instability and sudden collapse during remedial works can be unexpected.
Should cracks still be monitored?
Cracking might not just be caused by movement in the foundations, so there are advantages in also monitoring cracks where complex movement is suspected.
Should cracks still be measured?
Even in simple cases, the crack widths need to be measured as they represent the actual damage. Use of a crack width gauge, vernier callipers or Moiré is recommended, and the exact position of the measurement should be recorded. For further details see Crack Monitoring
Can the foundations be assessed using only crack monitoring?
Using cracks to monitor foundations is like examining dents in a car to assess the brakes. Cracks can have numerous causes. The author has found that movement (or lack of movement) in cracks rarely indicates movement in foundations. This is illustrated in the case studies.
Are Movement Gauges any use if it is thought that the cause of movement is not necessarily related to the foundations?
One of the major advantages of the Movement Gauges is that results are nearly always useful. Many Engineers will have come across annoying and embarrassing instances where monitored cracks have stopped moving and new cracks opened right alongside the old crack, potentially invalidating months of monitoring. On the other hand, while Movement Gauges have often produced unexpected results, the movements, or the lack of movements, are still valid. Despite decades of experience, Robert admits that initial assumptions over foundation movements are often proved incorrect. Proof that foundations are not moving is likely to be useful.
Are there unusual Health & Safety issues?
The outstanding results from initial trials from 1996 indicated that the movement gauges could not only provide early and quantitive data on foundation movements, but the reading of movement gauges were considerably safer than crack width gauges. Cracks tend to open at high level, so monitoring often involved working from ladders with the associated risks and costs. Once set up, movement gauges could be read by a single surveyor in most circumstances saving manpower costs. The use of laser levels even allow the movement gauges to be set up by one person if necessary.
The use of glue and working on a building site are the obvious risks to be assessed.
What skills are needed to use Movement Gauges?
The required skills can be gathered from the "Instructions for Setting Up Movement Gauges". A knowledge of an optical site level is essential. An Engineer needs to advise on the location for the gauges.
I used a staff as a change-point to move the level from the side to the front of a house. Why were the results inconsistent?
While the movement gauges should give results to 0.1mm, a normal site staff can really be only read to 0.5mm at best. As the staff is not fixed, swivelling it or crunching a grain of sand beneath it could result in a further error of 0.5mm.
Do not therefore mix the gauges and a staff. Fix two gauges to the two faces of the building at a corner. Unless there is a crack at the corner, the two gauges will move "as one" and can therefore be used as a change point. As it is only canges in level that are being measured, there is no need to even measure the differences between the two gauges at the corner.