Extensometers on Turtle Mountain

What is a Surface Extensometer?

Rock mass displacement is directly measured over longer distances using surface extensometers, which span distances of several tens of metres crossing numerous fissures.

Surface extensometers have a head assembly anchored into bedrock on one side of the surface of a crack, and an anchor end on the opposite side. The head assembly contains a weight connected to the anchor end by a steel cable and is suspended over a transducer.

Movement of the rock mass causes a change in position of the suspended weight, which is converted to displacement measurements through the transducer. This is then read and stored by the datalogger before being telemetered back to the Frank Slide Interpretive Centre. Readings are made hourly. The datalogger/telemetry system is also used by the tiltmeter system.

We selected extensometer locations so the head assemblies (upslope end) and anchors (downslope end) were installed in exposed bedrock, with the extensometer cable roughly parallel to the possible direction of movement.

Extensometers on Turtle Mountain

In October 2004, we had four surface-mounted extensometers installed. We installed a fifth extensometer in 2006. We expect the sensitivity of these extensometers to be about one to two millimetres. Each of the extensometers was custom made and installed by DGSI.

Results

No daily or seasonal changes can be seen in the displacement data. However, we have identified three significant, localized displacements:

1. A 4.5 mm extension on extensometer EX-2.
2. A 20.1 and 5.9 mm displacement recorded on EX-2 and EX-3, respectively.
3. A gradual displacement of 1.3 mm measured on extensometer EX-4 during late October 2006.

A detailed analysis of the data reveals that, in all cases, displacement is strongly related to cold weather. For the June and early September 2005 displacements, we recorded heavy freezing. Furthermore, we measured a dramatic temperature drop during the late October displacement event.

Plot of displacement versus time for extensometers at the South Peak of Turtle Mountain. Click on graph to enlarge.

Extensometers on Turtle Mountain. Click on images to enlarge.