Casing is designed to deform with movement of the adjacent ground or structure. The useful life of the casing ends when casing deforms so much that the inclinometer probe cannot track the grooves of the casing. Continued movement of the ground eventually pinches or shears the casing, preventing passage of the inclinometer probe.
The Limits Table
The table shows tracking limits and passage limits in terms of “curvature.” Curvature is the change in the reading from one depth to the next. Thus the values in the chart will not appear directly on your readout.
Suppose we have a metric inclinometer probe. At 20 meters, the A-axis reading is 1010. At 19.5 meters, the reading is 3010. This change is quite large. Are these readings valid or has casing curvature forced the probe out of the grooves?
First, find the curvature: 3010 – 1010 = 2000. Next, find the tracking limit for the A axis in the table below. Suppose we have large diameter casing. The tracking limit is 2393. The curvature of 2000 is smaller than the tracking limit, so we can assume the reading has not been degraded by tracking problems.
However, suppose we had medium diameter casing, where the tracking limit is only 1550. The curvature of 2000 exceeds the limit, so it is likely that the reading has been degraded.
The tracking limits shown below are the limits for perfect tracking, in which the wheels travel on the bottom of the casing groove and not on the walls of the groove. The absolute limits of tracking are larger than stated here, since it is possible for the wheels to travel on the walls of the grooves.
Note: The values shown for tracking limit and passage limit are not direct readings. They are the change in reading over one gauge length of the probe. When tracking limits are exceeded, readings will degrade and eventually will not be reliable. In most cases, passage limits are 2 times larger than tracking limits.