Settlement Cell FAQs
A settlement cell is a device used to monitor settlements in embankments, fills, and foundation soil. It reports settlements of a discrete point, as opposed to a settlement profile. The settlement cell consists of three components: a liquid filled tube, a pressure transducer, and a reservoir of liquid. One end of the tubing is connected to the pressure transducer, which is embedded in the soil. The other end of the tubing is connected to the reservoir, which is located at a higher elevation on stable ground, away from construction activity. The transducer measures the pressure created by the column of liquid in the tubing. The height of the column is equal to the difference in elevation between the transducer and the reservoir. As the transducer settles with the surrounding soil, the height of the column increases and the transducer measures a higher pressure. Settlement is calculated by converting the change in pressure to millimeters or inches of liquid head.
The transducer and tubing are completely buried, unlike rod-type settlement devices, so they do not interfere with construction traffic and are less likely to be damaged. Also, readings are obtained remotely, at the location of the reservoir, again avoiding interruption of construction activity. Tubing can be routed conveniently and does not have to follow a straight line path between transducer and reservoir.
Air bubbles are commonly cited as sources of error, but with proper installation, both factors are easily controlled. By far, the most significant sources of error are barometric pressure (when using a non-vented VW sensor) and temperature variations.
Air bubbles: If air bubbles are present in the liquid, they can cause errors, because air is lighter and compresses more easily than liquid. However, the cell leaves the factory pre-filled with de-aired water. Special headers at the reservoir end of the tubing are overfilled, so the liquid is under pressure and remains under pressure until the final step when the tubing is connected to the reservoir. At connection time, the tubing squirts out liquid rather than sucking in air. Thus, air bubbles are not usually a problem.
Temperature: Temperature problems can be controlled by minimizing the length of tubing that is affected by the ambient temperature. Also the reservoir should be protected from the direct heat of the sun.
Liquid Level Maintenance: It is important to maintain the level of liquid in the reservoir. Typical practice is to replenish evaporated liquid with water each time a reading is taken. To do this, fill the reservoir until the water flows out of the overflow tube. Ordinary water is sufficient. It does not need to be deaired, and it does not need to be mixed with ethylene glycol, since that component of the mix evaporates much slower than the water does.
Barometric pressure: The reservoir is open to atmosphere, so the pressure of the atmosphere acts on the surface of the liquid in the reservoir, and the transducer sees the combined pressure of the column of liquid and the atmosphere. This does not affect the vented VW settlement cell or the pneumatic cell (provided the pneumatic indicator is zeroed at reading time), but it does affect non-vented VW cells. What is the magnitude of these changes? They are many times larger than the resolution, accuracy, and precision of the system. A 1 millibar (0.75 mm Hg) change in atmospheric pressure is equivalent to 10 mm ( 0.4 inches) of water head. Weather fronts bring much greater changes.
|The graph at right shows 1.5 days of data from a settlement cell installed at the Isle of Wight. Atmospheric pressure was recorded by an electrical barometer. You can see how the settlement cell tracks changes in the atmosphere. The offset between settlement reading and the barometer reading is the head of water. As you can see, the settlement cell closely tracks the barometer.|
|The graph at right shows a month of data from a long term test in the laboratory. Barometric pressure was recorded by a VW piezometer. During this month, atmospheric pressure varied over a 20 millibar range and was tracked by an equivalent change (300 mm or 11.8 inches) in head of water even though no settlement actually occurred. Again, the difference between settlement cell reading and the barometer reading is the head of water (about 2.5 meters).|
As mentioned above, if you have a vented VW settlement cell or a pneumatic settlement cell, you do not need to concern yourself with barometric pressure. However, if you have a non-vented cell and wish to monitor small settlements, it is necessary to measure barometric pressure and to compensate settlement readings for changes in barometric pressure. Changes in barometric pressure can be very localized, so it is not sufficient to obtain barometer readings from the TV weather report or from the barometer on the office wall. Barometer readings must be obtained on site at the same time as the settlement reading. If you are using a data logger, then it should be programmed to read a barometer at the same time as the cell. If you are obtaining readings manually, you should use a high-quality, hand-held barometer to measure barometric pressure.
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