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Construction NDT

News on Non-Destructive Testing

Welcome to Construction NDT for September 2013. This month we are talking about structural health monitoring and Geotechnical testing.

We begin by discussing the operating principles, strengths and limitations of vibrating wire sensors.

Our second article presents sample situations where vibrating wire sensors and other sensors have been combined to collect data to ensure excavation or repair are conducted safely.

Finally we are excited to be attending Concrete 2013 in QLD and hope to see many of our clients there.

Vibrating Wire Sensors

Instrumentation for structural health monitoring or geotechnical applications can require a variety of different sensor types.

Vibrating wire sensors measure the change in tension of a length of steel wire. This wire is fixed at two points and if set in motion will vibrate at a natural frequency controlled by the tension. The simplest application of this principle is to fix the sensor between two points on a structure and to measure strain as related to natural frequency change when the points expand or contract.

Vibrating-Wire

Measuring the natural frequency requires that the VW gauge is set into motion and the vibration converted to an electrical signal. Both may be achieved using an electrical coil close to the steel wire. A voltage pulses sent to the coil will create an electromagnetic field that attracts the wire and set it into motion. This motion will induce an alternative current signal in the coil at the same frequency as the wires vibration.

VW-Sensor


To measure other parameters than strain the VW sensor needs to become more complex. For example a simple pressure sensor can be constructed by fixing on end of the wire to a diaphragm that will be displaced by pressure differentials. This construction could be used with a reservoir to measure vertical displacement or be the pressure sensor used with a piezometer to measure pore air pressure or pore water pressure. It is also common to include a temperature sensor such as a thermistor as part of a sensor package so temperature variations can be corrected for.

VW-Pressure


Vibrating wire sensors greatest advantage is measurement of frequency rather than resistance or voltage. Frequency signals may be carried effectively over very long cable lengths and face little difficulty if there is ground leakage, damaged cable insulation [unless the cables corrode completely] or similar risks. Electromagnetic interference may be a concern but can usually be reduced through the use of shielded cable and site selection.

The biggest risk with vibrating wire sensors are long term zero drift. Wire corrosion is one source of drift but unlikely provided sensors are built to eliminate galvanic corrosion and properly sealed. Greater concern is drift caused by wire creep under continuous tension, which may be reduced by pre-aging the sensors, typically using high temperature cycling will eliminate many stresses in the sensor and reduce installed zero drift.

Examples of Vibrating Wire Sensors and Application

Sensor  Application
Piezometer  Measurement of Pore water pressure in fully or partially saturated soils
Extensometer  Monitoring linear displacement
Pressure Cell  Monitoring pressure bearing on tunnel lining
Anchor Load Cell  Monitoring tensions applied anchors or direct compressive loads
Crack Meter  Specific extensometer format designed to monitor linear displacement along cracks or joints in structures
Liquid settlement system  System which uses VW sensors and a column of water to measure vertical displacement in earthworks

 

 

 

 

 

 

References

Dunnicliffe, J, 1993. Geotechnical Instrumentation for Monitoring Field Performance. 1st ed. New York: John Wiley & Sons Inc..

Instrumentation for excavation and repair

Instrumentation for Concrete Repair

Safety during construction projects is important. In one particular repair project it was determined that the stability of the walls of a coal dumper were at risk during hydro-demotion in preparation for patch repairs. The removal of concrete using hydro-demolition could potentially have weakened the structure and put workers at risk.

Engineers determined that strain gauges could be used to monitor for any movement in the structure at selected points. The gauges installed were Vibrating Wire Surface mounted strain gauges with grouted mounting blocks. The installation required the gauges to be set at mid frequency due to the unknown nature of any forces that could be unleashed. The strains could have been negative or positive, so flexibility was important in the set up.

A Campbell Scientific CR800 data logger was used with the system programmed to give an alarm when strain limits were met at the gauges. Back up batteries were used and the system was plugged into mains power to trickle charge the data logger. This meant that even in an extended power cut the data logger would still operate.

Instrumentation for Excavation – Case Study

Project Name: Museum of Human Evolution, Burgos Project Year: 2004-2005

Project Overview: Work on the site began in 2004 and the Museum of Human Evolution was inaugurated on July 13, 2010. Its foundation is based on the archaeological site of Atapuerca, located 20 km east of Burgos. The Atapuerca site has been designated a UNESCO World Heritage Site.

The specialist geotechnical company KellerTerra were contracted to construct the deep foundations which would be used to support the structure and for underground parking and the lower levels of the museum.

Part of this deep foundation required the construction of a diaphragm wall to a depth of approximately 20 metres.

Monitoring done: Data on diaphragm wall deflection was regularly monitored to ensure construction quality and the safety of adjacent buildings, particularly important given its high density urban setting. Due to the depth of the excavation and the proximity of ancient buildings, extensive monitoring was necessary within the diaphragm walls by means of embedded vibrating wire strain gauges and inclinometer casing. From this the load on the diaphragm wall and deflection could be measured. Tie back anchors were monitored using vibrating wire anchor load cells.

All strain gauges were connected to a data logger and monitoring point, a web based data visualisation software where the data could be viewed through the internet. Pre-set trigger levels were set and a regime of alarms developed as part of the construction safety system.
Inclinometers were also installed around the excavation to measure any movement of ground.

VW-Installation

List of products used:

  • VW embedment strain gauges: to monitor strain within the cage
  • Inclinometers & inclinometer casing: to monitor movement in and around the retaining walls
  • VW anchor load cells: to monitor the load within the ground anchors
  • Geologger data logger: to capture data, store it and upload to the internet
  • Monitoring Point: visualisation software to provide information in real time

Parts of the images used in the article “Vibrating Wire Sensors” use elements from Wikimedia Commons which may be found at the following addresses:http://commons.wikimedia.org/wiki/File:Simple_sine_wave.svg / http://commons.wikimedia.org/wiki/File:Coil.gif / http://commons.wikimedia.org/wiki/File:Thermoresistor.svg

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