Load & Movement Monitoring

Extensiometers are useful in detecting the dilation of a rock or soil mass. They can also be used to monitor movement on a particular slip plane. Sigra can install a range of extensiometers which can be manufactured to suit the application. These may be tensioned wire or rod and bolt type devices. They can be manually read or fitted with electronic movement detection systems that include LVDTs, potentiometers and load cells which detect the extension of a spring at the end of a tensioned wire. Sigra also uses its own fluorescent gel filled rock bolt extensiometers.

Rock bolt and anchor monitoring serves to indicate whether the load on an anchor is changing. An increase in load indicates that increasing extension is taking place along the rock bolt length and it may be necessary to consider additional reinforcement. A drop in load may be a due to failure of the anchor or its bond and a precursor to catastrophic failure. Anchor monitoring is therefore a very useful insurance policy in critical areas.

Sigra can install load indicators or transducers under the head of rock bolts. These can be of a flat jack – pressure transducer combination or strain bridge form. Alternatively they may simply comprise a group of belville washers under the bolt head which are monitored manually with a dial indicator.

Precision tiltmeters may be used for monitoring ground movement ranging from that induced by hydrofracturing operations to a slowly progressing landslide. Sigra uses precision inclinometers from third party suppliers.

The use of inclinometers that may be raised or lowered in a grooved casing set in a hole is industry standard practice. In addition to this Sigra can fit multiple fixed devices that measure angular tilt within a borehole. These are normally fitted in a soft bentonite cement so that cables from each device do not become cut with small movement.

The location of a failure surface can be very important for slope failures, in knowing the dimension of the caved zone over the back of a stope or a goaf. Sigra can install multiple individual return cable circuits to determine such breakage. Alternatively it can use time domain reflectometry within a cable to determine the break point within a single coaxial cable.