Crackmeters on Turtle Mountain

What is a Crackmeter?

Crackmeters (or crack gauges) are thin steel rods anchored on both sides of a crack wall to measure how the cracks pull apart when the mountain slope moves. The most sensitive of the different monitoring tools on the mountain is a vibrating-wire crack gauge that can detect changes as small as ten microns - the thickness of a human hair.

Crackmeters on Turtle Mountain

Turtle Mountain has 22 active crackmeters on the western side of the south peak. The crackmeters are in sets of three at six different locations, placed at different angles across the crack. These orientations detection true direction and magnitude (vector) of the mountain's movement.

The ends of a crackmeter are secured in 12.5 mm wide holes drilled into the sides of the fissure. The length (including extension rods) is about one metre. The cabling runs through flexible conduit back to the datalogger at the weather station, which records and stores the data until telemetered back to the Frank Slide Interpretive Centre.

Readings are taken every 15 seconds and average once an hour before transmitted to the data centre at the Frank Slide Interpretive Centre.

Overall, the crackmeters have functioned reliably since their initial installation in the fall of 2003, but there were some negative effects from snow and ice on the instruments. Although we built metal roofs over the crackmeters to protect them from snow, the combination of blowing wind and snow caused snow and ice to accumulate underneath the housing and on the crackmeter rods. This caused significant damage to the instruments and required annual repair and maintenance. In summer 2007, we upgraded the roofs and didn't notice any snow accumulation issues during the winter.

Results

Through monitoring, we have noticed a consistent width change of the cracks each year due to temperature changes. In the spring and summer, the rock expands, making the cracks narrower. In the fall and winter, the rock contracts, causing the cracks to become wider.

We have measured width changes of approximately 0.4 mm during each cycle. Total crack widths range between 1 and 1.5 metres at the crackmeter locations. Daily temperature changes have also caused width changes (around 0.02 mm); however, we have not recorded a permanent change after every cycle. Electrical noise has caused alternating changes up to 1.5 mm as shown below. We believe this is from lightning strikes.

Graph shows typical crack width change versus time for crackmeters at the south peak of Turtle Mountain. Click on graph to enlarge.

Installing crackmeters on Turtle Mountain. Click on images to enlarge.