Vented Settlement Cells
Applications
Vented Settlement Cells are used to monitor settlement and heave in soils. Typical applications include:
- Monitoring settlement or heave in embankments and embankment foundations.
- Monitoring subsidence due to tunneling and mining.
- Monitoring consolidation under storage tanks.
- Monitoring settlement due to dewatering or preloading.
- Monitoring settlement in fills.
Operation
A vented settlement cell consists of a liquid reservoir, liquid-filled tubing, and the settlement cell, which contains a pressure transducer. One end of the tubing is connected to the settlement cell, which is embedded in fill or installed in a borehole. The other end of the tubing is connected to the reservoir, which is located away from the construction area.
The transducer measures the pressure created by the column of liquid in the tubing. As the transducer settles with the surrounding ground, the height of the column is increased and the settlement cell measures higher pressure. Settlement is calculated by converting the change in pressure to millimeters or inches of liquid head.
Advantages
The reservoir and readout station can be located away from the construction area. The cell and tubing are buried and do not interfere with construction activities.
Vibrating wire cells can be automated. Tubing lengths of up to 300 m (1000 feet) are permissible with VW cells.
Limitations
Temperature changes in reservoirs and above-ground tubing can affect the repeatability of readings.
Non-vented VW cells require corrections for barometric pressure, so barometric pressure must be recorded.
Vented VW cells automatically compensate for changes in barometric pressure, but desiccant chamber must be maintained.
Vented Settlement Cells Specifications
| SYSTEM CONFIGURATION | |
|---|---|
| Non-Vented Cells | Components include the non-vented cell, settlement plate, tubing, signal cable, reservoir, readout or data logger, and a barometer |
| Tubing and Signal Cable | Tubing and signal cable run from the buried cell to the reservoir at the surface. Lateral runs up to 300 m (1000') are possible if tubing is buried to minimize temperature changes and deviations from the upward slope of the tubing are kept very small |
| Reservoir | The small reservoir accommodates one settlement cell. The level of the liquid in the reservoir must be maintained regularly |
| Dessicant Chamber | Required for vented cells only. Prevents moisture from entering cable and vent tubing |
| Barometer | Required for non-vented cells only. Non-vented settlement cells are sensitive to atmospheric pressure, which can vary as much as 0.5 psi during the course of a day. This is equivalent to 350 mm or 14"of water head. Barometer readings are required to correct settlement readings for changes in atmospheric pressure. Barometer readings must be obtained on site at the same time as the settlement reading. Barometer readings from weather radios and airports are not adequate for this purpose. If your system is automated, order the barometer for data loggers. It has an accuracy better than ±20 mm (± 0.8 inches) of water head at temperatures from - 25 to 50 °C. Check for a similar accuracy if you purchase a portable barometer |
| Readouts | The VW Data Recorder is recommended for manual readings. The CR1000 data logger or VW MiniLogger can be used for automated readings. See separate data sheets for features and specifications |
| Performance Specifications | Range is the maximum vertical distance between the buried cell and the reservoir. Resolution is based on readings obtained with a VW Data Recorder. Calibration accuracy is determined during calibration. Repeatability depends on proper installation of cell, tubing, and maintenance of the reservoir level, maintenance of deaired fluid, and application of any appropriate corrections |
| VENTED VW SETTLEMENT CELL | |
|---|---|
| Vented VW Settlement Cell | 52612420 |
| Sensor Type | Vented vibrating wire sensor with built-in thermistor. Settlement cell includes settlement plate, reservoir and dessicant chamber. Tubing and signal cable ordered separately. Sensor automatically compensates for changes in barometric pressure |
| Range | 14 m (47') |
| Resolution | 0.025% FS |
| Calibration Accuracy | ± 0.1% FS |
| Repeatability | ±0.25% FS to ±1% FS |
| Dimensions | 64 x 280 mm (2.5 x 11") |
| Materials | Stainless steel and PVC plastic |
| TUBING AND SIGNAL CABLE | ||
|---|---|---|
| Settlement Plate | 51410100 | 305 x305 mm ( 12 x 12") steel plate helps maintain required upright orientation of cell |
| Desiccant Chamber | 52612495 | Prevents moisture from entering cable and vent tubing. Desiccant can be renewed in an oven. For one cell. 108 x 108 x 64 mm (4.25 x 4.25 x 2.5") |
| Extra Desiccant Pack | 02540003 | Anhydrous calcium sulfate in moisture proof container. Sufficient for one chamber |
| Small Reservoir | 51419500 | Accommodates one settlement cell. Includes mounting hardware, two quick-connect plugs, and small bottle of de-aired liquid |
