In the first part of a three-part article, Ian Hume looks at methods of monitoring structural movements I'MSURETHATCRACKIS GETTINGWORSE! All of us involved in conservation will have been greeted by a building owner who says 'Tm sure that crack is getting worse". But is it? We had a crack in our bedroom ceiling which even I, on fortunately rare sleepless nights, thought was getting worse. We redecorated and the crack has not reappeared. All buildings, not only old ones, move and occupants are frequently disturbed by seeing apparently alarming cracks, leans and other distortions. As the building which you own is financially your most valuable possession, such concern is not unreasonable. However conservation professionals need to sort out the various types of distortions, to determine their causes and to suggest remedies. Sometimes the remedy will be as simple as that to my own problem, a coat of paint. WHY MONITOR? If there iscurrent movement, monitoring is a helpful diagnostic tool to determine directions, speed and magnitude ofmovement. Correct diagnosis of movements is always necessary before the correct solution to the problem can be established. As with the surgery, proper cures can only be effected after correct diagnosis of the disease. In the case of past movements, monitoring can be utilised to confirm that cracks and other signs of distress have reached a point of stability. It can quantify movements (or lack thereof) so that distortions which appear to worsen may be shown to be a figment of an overimaginative mind (oraresultof asleepless night). As an example of this, I was called some years ago now to the Palace of Westminster to look at some cracks which had apparently appeared to widen rapidly as a result of building works. Monitoring showed that tl1ecracks were temperature related, closing when the building warmed (and thus expanded) and widening when it cooled down again. There was certainly movement but, importantly, it was not progressive. The connection between the building works and the size of the fractures was psychological, aggravated perhaps by disturbance of dust making the fractures dirtier and therefore more apparent. Only monitoring could establish the truth. This case brings to mind a further reason for monitoring: that of checking for disturbance caused by adjacent building works. Many years ago there was a proposal for major piling works for a new roll-on/roll-off ferry terminal very close to an English Heritage castle. There were existing fractures and so it was decided to monitor before, during and after the piling. The results of the monitoring showed that movements before the piling were seasonal and not progressive and that the fractures widened significantly during the piling but settled down to being purely seasonal after works ceased. We were able to lay the blame fairlyand squarely on the piling. We also avoided embarking on costly remedial works which would have appeared to have been successful in stopping movement only because the movement had ceased anyway. The simple but accurate monitoring of structural and ground movements is a powerful but much underrated weapon for use in the conservation of the built environment. It can be both straightforward and economic as well as allowing buildings to continue in use without major structural intervention. METHODSAND TECHNIQUES AVAILABLE Structural movements in buildings fall into one of three categories. They may be current, in which case some action needs to be taken; they may be indications of past movements which have ceased permanently; or they may be the result of movements which now dormant but which are likely to restart. In each of these cases monitoring can have a role to play. The most obvious sign, and usually the earliest, is cracking of the building. Ground movements can result in differential settlement, tilting and other movements within the superstructure. In more extreme cases cracking of the ground around a building can occur. This article, and its companions in the next two issues of Context, describe some of the simple methods which can be used to follow such movements in structures. Methods of showing results in an easy-tounderstandform will be suggested. Some of the various high accuracy methods are also discussed but the aim of the articles isto suggest ways ofproducing acceptable results using modestly priced techniques. Some high-accuracy equipment is extremely expensive and its purchase can only be justified by a major organisation with a considerable budget; however, there is scope for large savings in costs by using sophisticated monitoring to prove that vastly expensive remedial works are unnecessary. The methods suggested all utilise equipment which (with the exception of the targets) is brought to the site for the monitoring exercise, then taken away for use elsewhere and brought back for the next cycle of readings. The targets and stations used for the techniques discussed are of minimal cost and are generally unobtrusive. The cost of equipment dedicated to each site istherefore minimal. Sophisticated electronic equipment can be used but will not be discussed in these articles because costs are usually high and a considerable amount of hardware often needs to be dedicated to each specific case. This type of equipment is capable of working to a very high degree of accuracy but, in normal CONTEXT49
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