Settlement cells or gauges are a suitable tool for this assessment, but how do they work exactly?
Settlement gauges, also known as settlement plates or rod settlement gauges, are typically used to measure the settlement of the original ground surface as an embankment is constructed on top. They consist of a rigid metal or concrete plate that is installed at a specific location on the ground. The settlement of the ground is determined by measuring the change in elevation of a vertical rod that is fixed to the plate and lengthened as the embankment is built up around it.
Settlement cells or liquid level gauges measure the change in elevation at the location of the cell. The installation process involves excavating a pit or borehole to the desired depth and placing the cell at the bottom. The cell is carefully levelled and fixed in place to ensure conformity with the ground and accurate measurements. The cell contains a liquid, connected by a tube to a remote and accessible reservoir of liquid.
The pressure, or head, of the liquid in the cell is recorded and provides a measurement of the elevation of the cell relative to the reservoir. Changes in elevation indicate a vertical displacement but the reservoir must be situated on stable ground or else its elevation relative to a bench mark checked using surveying instruments.
A variation of the liquid level gauge is the full-profile or hydrostatic profile gauge (HPG) where the liquid pressure at multiple points along a near-horizontal tube are measured by means of a probe to obtain settlements along a profile line, such as across an embankment foundation.
Both settlement cells and gauges rely on the principle of relative displacement measurement, measuring change in elevation or position of specific points or depth relative to a reference point. The accuracy of the measurements depends on factors such as instrument calibration, stability of installation, and precise surveying techniques.
To gain a comprehensive understanding of ground behaviour, cells and gauges are often used in combination with other monitoring techniques such as inclinometers, extensometers, or piezometers.
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