wide range of techniques which rely primarily on visual observation with instrumentation toobtain ameasurement of length or determination of position from which movement may be deduced.The methods generally require the presence of an observer on site at the time of measurementwhich can be operated remotely once installed.there is a wide range from which to select the system most appropriate in terms of cost andaccuracy to study effectively the problem in hand..Principles of these techniques are commonto those used in traditional site-surveying techniques familiar to engineers setting out buildingsprior to and during construction or by land surveyors at the planning stage.The additional characteristics required for monitoring the performance of a building structureinclude the ability to refer reproducibly to points on the structure over significant periods oftime, with confidence in the integrity of the points. There is the corresponding need for reliablestable datum points, if absolute, rather than just relative, measures of movement are required.The requirements for accuracy of monitoring are likely to be considerably greater and thereforemore onerous to achieve. The movements expected during a monitoring programme willdepend on the size and type of the building and the cause of movement. The effects offoundation movement on superstructure, dimensional changes caused by environmentaleffects, structural deflection under load or incipient instability may be of quite differentmagnitudes and patterns. Nevertheless, the expected magnitude is likely to be millimetres, upto perhaps 10 or 20, but of course withexceptions, and the accuracy required to be no worse than a millimetre. Ifprogress of the movement is to be followed, particularly if it is to be used totrigger action to safeguard safety as opposed to leading to an understanding ofthe behaviour of the building structure or fabric, or if it is required to determinewhen movement is ceasing, then submillimetric accuracy may be required.The factors to be considered in order to achieve the required accuracy

• The accuracy of the basic survey instruments used.• The precision of any necessary ancillary equipment.• The rigidity of fixing and long-term integrity of permanent reference• fittings to which measurements are to be made.• The repeatability of positioning of all equipment used.• The stability of points taken as data.• The relative position of points to be monitored on the building and of• the datum points.• The effects of meteorological conditions.• The competence of the observers.

Compatibility of all these factors is essential for the attainment of accuratedetermination of position at one time and for the reliable comparison ofobservations made at different times, often at long intervals.When a survey is to be designed to monitor the response to known events,such as adjacent construction work, it is essential that sufficient time is allowedto permit at least two sets of observations to establish datum measurementsbefore possible disturbance takes place.

1.1 Introduction

There are two groups of sensors used for monitoring:• geodetic – measuring georefernced displacements in three

dimensions• geotechnical – measuring relative physical quantities

1.2Geodetic methods

Geodetic monitoring comprises techniques based on opticalobservations which indicate the movement of building by distanceand angles measure in reference to control point. Equipment usedfor such measurement is total station and level but also GNSSreceivers are included into this group.

1.3

Sensors used for geotechical monitoring are inclinometers, piezometers,extensometers.