Overview
Load cells are the main monitoring instrument used for direct measurement of load in structural members of geotechnical projects. Load cells are placed on the structural elements in the way that the structural forces pass through the cells and can be thus measured and monitored. There are different types of load cells, among others:
- Mechanical cells
- Hydraulic cells
- Vibrating wire cells
- Electrical resistance cells
- Cable tension meter
The main difference is in the measurement technology and range. Vibrating wire load cell is one of the types commonly used in geotechnical monitoring due to its accuracy and long-term performance.
Application
The main applications of vibrating wire load cells in geotechnical monitoring are, among others:
- Monitoring loads in rock bolts and anchor on the excavation faces
- Monitoring loads in tie-backs and bolts of retaining structures
- Monitoring loads in bracings and cross struts of diaphragm walls
- Monitoring loads in steel ribs of tunnel supports for particular purposes
- Load testing of piles and rock bolts or anchors
- Monitoring temporary and permanent loads in pre-stressed anchors
- Long term performance monitoring of tie-down anchors in dams
Installation and operation
Vibrating wire load cell consists of a cylinder of high strength steel with often 3 to 6 vibrating wire transducers. It is based on the vibrating wire technology in which force is measured using a wire that vibrates at a high frequency. The applied external force changes the tension on the wire and consequently changes the frequency. The amount of the applied forces in then determined by measuring the frequency.
Readings are done from each individual transducers and then averaged and used in conjunction with appropriate calibrating factors. The use of multi sensors allows to obtain an accurate reading and account for the effect of eccentric loading. The signals from the cell are transmitted to the readout unit using a multi-conductor shield protected cable.
The load cells are available both in solid and annular models. The annular model has a central hole to accommodate anchors, rock bolts and tendons. When used to measure loads in tieback and rockbolts, the load cell is placed between the bearing plates at the head of the bolt.
The capacity of the load cells depends on the application and may vary between 100 to 10000 kN. Usually the size of the cell increases with the capacity.
Advantages and limitations
Advantages:
- An efficient way to monitor the load in structural elements
- High accuracy o Long-term performance
- Multiple gage system allowing to account for eccentric loading effects
Limitations:
- Not suitable for dynamic loading
- Special manufacturing technologies required to minimize the zero drift [1]
- Need to lightning protection should be evaluated [1]
References
- John Dunnicliff, 1994, Geotechnical Instrumentation for Monitoring Field Performance, Wiley-Interscience, ISBN: 978-0-471-00546-9