11.2 Element of a Monitoring System

Monitoring field performance is basically the measurement of the physical quantities of some objects，such as deflection，stress and strain.The common monitoring items in excavations are（a）movement of the structure or soil，（b）stress or strain of the structure or soil，and（c）water pressure and level.The monitoring objects of movement include the lateral deformation of the retaining structure and soils，the tilt of the building，the settlement of the ground surface and the building，the heave of the excavation bottom and the uplift of the central post，etc.The measurements of stress and strain include those of the strut load，the stress of the retaining structure，and the earth pressure on the wall.The measurement of water pressure refers to the water pressures within the excavation zone，outside the excavation zone，and on the retaining wall.

In principle，the types of monitoring instruments in an excavation are not special.As long as they meet the budget and the criterion of accuracy，any instruments or methods are feasible.Most of the instruments can be divided into electronic and non⁃electronic types.The electronic type is more sensitive and easy to read.They can also be divided into an automatic or semi⁃automatic monitoring system.Nevertheless，the precision is easily influenced by the installation process and the surroundings.The durability of electronic instruments needs examination because an excavation usually needs to be monitored over a long period.Thus，when installing an electronic instrument in the field for long⁃term monitoring，durability needs special attention.

Generally speaking，the instruments for plane surveying，such as tapes，levels，and theodolites can be used for the measurement of movement.The measurement of the tilt angle can also resort to those for plane surveying（same as above）or electronic instruments.Electronic instrument sensors include strain gauges and force balance accelerometers，etc.The most common instruments to measure stress or strain are the strain gauge or the electronic transducer that takes the strain gauge as the measurement unit.This chapter will omit general instruments for plane surveying.Please refer to related books on plane surveying if interested in the subject.