|Vertical Displacement Gauge|
Probing of the active layer assumes that the underlying permafrost is neither aggrading nor degrading over time. In an effort to monitor these processes across the study area, we are suggesting that vertical displacement gauges (VDG) be installed at 6-10 locations on the grid. A description of the alpha VDG model, installation procedures, and measurement is described below.
VDG Description and InstallationThe gauge is constructed from steel wire (~2 mm diameter), wood, and a rubber stopper as shown in the figure. One end of the wire is attached to a bolt which serves as an anchor, and the apparatus is inserted into a small-diameter borehole that penetrates about 30 cm into the upper permafrost. Installation should occur in late summer when the thaw depth is maximized. The borehole is backfilled with a slurry of cuttings generated during the drilling operation. Several days should be allotted for the wire and anchor bolt to refreeze in the permafrost.
The wire extends above the ground surface. A round piece of thin plywood, 20 cm in diameter, is painted white and a small hole (5 mm) is drilled in the center. This is placed over the wire and lowered to the ground surface. Finally, a rubber stopper, with a small hole in the center, is placed over the wire. The hole should be just large enough to allow movement of the stopper on the wire; care should be taken to ensure that the hole is not too large. This is a critical step; experimentation with wire gauge and stopper hole size is required.
The end of the wire should be looped and marked with a colored flag. It should extend about 30 cm beyond the top of the stopper.
Once the base of the gauge is frozen into the permafrost, the wire should be pulled taut and the wood place firmly on the ground surface. The rubber stopper is pushed down to the surface of the wood. There should be enough resistance that the stopper does not move along the wire when released. The distance between the end of the wire and the top of the stopper is recorded.
VDG MeasurementDuring winter, the ground surface will heave and the wood/stopper apparatus will be displaced up the wire. When the ground thaws in summer, the wood will be lowered with the ground surface. The stopper, however, will remain at its displaced position on the wire (if the wire/stopper contact is not too loose). The distance between the end of the wire and top of the stopper should be recorded to determine the seasonal displacement. At the end of the thaw season, the wood and stopper should be reset to the ground surface, and the distance recorded.
In this manner, the seasonal displacement is measured. Furthermore, because the device is anchored in the permafrost, it serves as a benchmark. Repeated annual measurements will indicate if subsidence/aggradation is occurring. By placing 6-10 of these inexpensive gauges across the site, we will be able to determine the overall vertical stability of the site.