11.1 Introduction
Though in the past theories of excavation analysis and design were not advanced，with conservative design and shallower excavation depths，few construction calamities occurred.In recent years，with the high development of construction activity，excavation depths grow deeper and deeper.What's more，these excavations are usually located in densely populated areas and more and more problems of property damage are encountered.Though the technologies of analysis，design，and construction have improved significantly，they are not necessarily capable of coping with changes during excavation.Thus，a well⁃arranged monitoring system will be helpful not only to monitor the excavation conditions but to ensure the safety of excavation.
Generally，the objects of an excavation monitoring program can be summarized as the following：
（1）To ensure the safety of excavation
To ensure the safety of excavation is the first thing for an excavation monitoring program.Before a calamity happens，there will be some signs，such as an extraordinary increase of the wall or soil deformation or stress.A monitoring system can issue an immediate warning to help engineers adopt effective measures to forestall a calamity when these signs appear.
（2）To ensure the safety of the surroundings
Excavations are usually located in busy commercial areas or densely populated residential areas.It is almost impossible to avoid influencing the surrounding buildings，underground pipes，public utilities，and pedestrians.The designer has to consider all these factors and establish a monitoring system to ensure the safety of people and property during excavation.
（3）To confirm the design conditions
Since the existing analysis theories are not satisfactorily mature and the geological investigations can not fully represent the in situ conditions and the complicated construction environment，the analysis results do not necessarily meet the actual conditions.Back analyses based on monitoring results can help modify and correct the original design，reduce the cost，shorten the excavation period，and change the basis of design.Further，they can serve as a reference for similar designs in the future and help enhance the excavation techniques.
（4）To follow long⁃term behavior
An important construction project finished，the monitoring system can be retained for long⁃term follow up，studying whether the long⁃term behavior of the case conforms to the original hypotheses.
（5）To supply factual materials for legal judgment
Information obtained from monitoring systems，along with the construction records，can serve as evidential data when a calamity or property damage occurs.Understanding the true causes of the events can avoid unnecessary disputes and help the restoration and compensation work.