10.6.1 Crack Monitors
Measurement of width of the crack developed due tensile failure of the slope is a reliable and inexpensive means of monitoring slope movement. Figure 12 shows two methods of measuring crack widths. The simplest procedure is to install a pair of pins on either side of the crack and measure the distance between them with a steel tape (Figure 12). If there are two pins on either side of the crack, then the linear distance can also be measured to check the transverse displacement. The maximum practical distance between the pins is probably 2 m. Figure 13 shows a wire extensometer that can be used to measure the total movement across a series of cracks over a distance of as much as 20 m. The measurement station is located on stable ground beyond the cracks, and the cable extends to a pin located on the crest of the slope. The cable is tensioned using a proper weight, and movement is measured by the position of the steel block threaded on the cable.
Crack meters is also a very useful tool for early detection of deforming mass movements (Figure 13). It measures the displacement between two points on the surface that are exhibiting signs of separation. The distance between the pins is measured regularly to establish a time-series of the wall movements. Velocity and acceleration indicators can then be established for the time-series. The main disadvantage of this type of monitoring device is the risk involved with personnel making measurements on unstable ground. This issue is of concern for any technique that requires manual collection of the deformation data from the slope failure zone.
Figure 11: Method of monitoring tension wire in the cracks in slope
Figure 12: Pin crack meter measuring crack displacement