11.7 Application of Monitoring Systems
In former sections，commonly used monitoring items and instruments have been introduced.The lateral deformations of the retaining structure and soils，the tilt and settlement of the building，the ground settlement，and the heave of the excavation bottom，the uplift of the central post，the strut load，the stress of the retaining structure，and the water pressure within the excavation zone are directly related to the safety of both excavations and adjacent properties.Figures 11.24 and 11.25 diagram the monitoring system of a MRT（Mass Rapid Transit）station in Singapore（Zhang et al.，2018）.
Figure 11.24 Instrumentation layout of the monitoring system（Zhang et al.，2018）
Each monitoring item can be set with the alert and action levels separately.These measured values，and alert and action levels all contribute to the understanding of the existing conditions of the excavation and serve as the basis to judge whether and when emergency measures are to be taken.
An alert level is an important indicator that tells whether the excavation is under abnormal conditions.An action level is the limit indicating the necessity for emergency measures.Under normal conditions，measured values will vary with the excavation depth.Generally speaking，when the measured value exceeds the alert level，it follows that the excavation is under a certain abnormality and engineers have to be cautious and make a judgment of the cause，though the excavation can continue.When the measured value exceeds the action level，it indicates the excavation is under dangerous conditions and has to be suspended right away to take necessary measures to restore the stability.
In fact，there are many objects of measurement related to the safety of an excavation.That some of the measured values grow beyond the action levels does not necessarily lead to the conclusion that the excavation is really under unsafe conditions.To our certainty，however，the higher measured values grow beyond the action levels，the closer the excavation is to failure.Engineers have to make their own judgment of the present conditions of safety on the basis of the relations between the measured values and the alert／action levels and the characteristics of the excavation.
The setting of alert and action levels is based on empirical experiences so far and no generally accepted standard formula is available.In engineering practice，the allowable value is usually taken as the action level and 70%⁃80% of the allowable value as the alert level.The allowable value is a generally accepted physical quantity，when taking the factor of safety into consideration.For example，the allowable settlement at LG2057（see Figure 11.24）is 9 cm，the action level for the settlement is then 9 cm and the alert level 6.3 cm（70% action level）.