Settlement cell

Settlement cell is a single point monitoring instrument used to measure the vertical displacement of soil or rockfill at a given subsurface point. The system can be used to measure vertical displacements at the locations beneath the embankment and structures which is in general inaccessible to other types of monitoring instruments.

The main application of stellement cells is geotechnical moniroting are:

• Measuring vertical displacement beneath and/or within the embankments
• Measuring vertical displacement beneath and/or within earth and rockfill dams
• Measuring vertical displacement under foundation of heavy structures and buidlings
• Monitoring the vertical displacement in subsidences
• Deformation monitoring due to tunnelling
• Settlment monitoring due to dewatering or consolidation

The settlement cell consists of a water reservoir, a pressure cell, and a wiring and tubing which connects the cell to the reservoir. The reservoir and the tubing are filled with de-aired water. Vertical displacement of the cell relative to the reservoir results in a liquid pressure change measured in millimeters of head water.
The reservoir is installed on a stable ground usually out of the construction area at a higher elevation than the pressure cell. The cell is then installed at the construction ground surface typically before filling or embankment. The tubing filled with de-aired water links the reservoir to the cell. Accordingly, the cell pressure transducer measures the water pressure in the tubing created by the different in elevation of the reservoir and the cell. Any vertical displacement of the cell, settlement or heave, will causes an elevation difference between the cell and the reservoir, hence, a change in measured pressure. The measured pressure is then transmitted to the data logger via the wiring system and the vertical displacement is determined by converting the pressure in millimeters of water head.

Advantages:

• The installation is relatively simple and does not require a borehole
• The system including the cell and tubing is usually roboust, stable and reliable in long term
• No major interference with construction activities as the reservoir and reading station can be installed outside the construction area  
• The measurements are almost unaffected by lateral displacements

Limitations:

• The measurements can be affected by the water temperature. Solution: open-air length of the tubing should be reduced as much as possible, and in hot climates, the reservoir should be placed in shadow
• The measurements can be affected by the atmospheric pressure. Solution: The readings should be calibrated by measuring and eliminating the barometric pressure variations at site. 
• By its nature, it gives only information about a single point. Therefore several cells will be required if multi-point monitoring information is required, e.g settlement of different layers in an earth dam.

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