11.3.2 Tilt of buildings

Buildings will slant because of ground settlement and thereby be damaged.The tilting of buildings should be monitored during excavation.The tilt of a building can be estimated by the relative settlement between two reference points，using measuring devices for plane surveying such as the level or theodolite，divided by the horizontal distance between these two reference points.If the two reference points are located on the two adjacent columns or foundation footings，the measured value is the angular distortion.Besides，the tilt of a building can also be monitored by using a tiltmeter，the value produced by which is called the tilt angle.

A commonly used device to measure the tilt of a building is tiltmeter（see Figure 11.9），which is similar to the inclinometer in measurement principle.The same as the inclinometer，the tiltmeter contains a sensor，which can be categorized into resistance type，vibrating type，force balance accelerometer type，etc.

Figure 11.9 Photos of tiltmeters

The tilt of a building contains the rigid body rotation and the angular distortion.The value taken by a tiltmeter is the tilt of the structure at the place where the datum plate is fixed.To make the measured value represent the safety of the building，the most suitable location of the datum plate has to be determined according to the structural behavior of the building and the convenience of measurement.The roof and walls are the most commonly selected locations.

With tiltmeter placed，the initial measurement to obtain the initial value to be deducted from the value taken after excavation is started.The difference is the tilt angle of the building at the excavation stage.