A Tape Extensometer is a portable device used for measuring displacement between pairs of eyebolts which may be installed on a structure or in an excavation such as a tunnel or shaft. It comprises of a stainless steel measuring tape with equally spaced precision punched holes. The tape winds onto a reel, which incorporates a tape tensioning device and either a digital LCD or dial gauge readout.
Magnetic targets are used to place a magnetic field at specific locations in settlement systems and can comprise of plates, beams and circular collars which house the magnets. Different types are used for various applications including borehole and filling types.
Typically the circular collar type is also often referred to as a “Spider Magnet” which has metal legs to keep it in place during installation in a borehole and subsequent grouting.
Reflective targets are also used for topographic surveying.
Where settlement or heave occurs vertical pipes are subject to the resultant of axial and radial stresses and damage occurs because the axial strain and consequent stress exceeds the buckling strength of the pipe. Typical examples of this are magnetic settlement systems and inclinometer casing.
The axial stress is caused by “skin friction” between the outside wall of the pipe and the surrounding soil as it settles.
If axial movement can be accommodated during settlement then the axial stress is eliminated. This is done by having telescopic joints at various locations along the pipe which can move as settlement occurs. If the settlement is expected to be greater than 2% then it is recommended that telescopic joints should be used.
Temperature effects are a key factor in geotechnical instrumentation and can be divided into two main categories:-
However it is recommended that wherever possible temperature should be measured in the vicinity of each sensor so that correction factors can be applied.
MEMS sensors are relatively unaffected by temperature effects especially as compared to electro levels.
Temperature correction should be applied to sensors which are affected by temperature and this is typically done either by having a thermistor built into the sensor or by placing a thermistor at the instrument location and reading its resistance. The corresponding temperature can be obtained by the Steinhart-Hart Equation.
A thermistor is a type of resistor whose resistance varies significantly with temperature and is widely used for measuring temperature in geotechnical instrumentation.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −50 °C to +150 °C.
They can either be used as a stand-alone temperature sensor or incorporated into other instruments to provide a record of temperature changes.
A tieback is a horizontal wire or rod, or a helical anchor used to reinforce retaining walls for stability. With one end of the tieback secured to the wall, the other end is anchored to a stable structure, such as a concrete deadman which has been driven into the ground or anchored into earth with sufficient resistance. The tieback-deadman structure resists forces that would otherwise cause the wall to lean, as for example, when a seawall is pushed seaward by water trapped on the landward side after a heavy rain.
Grouted tiebacks can be constructed as steel rods drilled through a concrete wall out into the soil or bedrock on the other side. Grout is then pumped under pressure into the tieback anchor holes so that the rods can utilize soil resistance to prevent tieback pull-out and wall destabilization.
Tilt beams differ from tilt meters in that they have a defined gauge length so that changes in tilt can be converted simply and accurately to millimetres of movement. They can also be linked end-to-end to monitor differential movements and provide absolute displacement and settlement profiles rather than point movements.
Tilt meter (in-place)
In-place tilt meters are used to monitor tilt on structures at individual locations and are left in place for the duration of the monitoring. Generally placed on special mounting brackets they can be installed to monitor horizontal and/or vertical tilt and can be either monitor one axis (uniaxial) or two axes (biaxial).
Tilt meter (portable)
Portable tilt meters are used to monitor tilt on structures at individual locations and are used in conjunction with a tilt plate that can be mounted on horizontally or vertically depending on the structure that needs to be measured. Multiple plates are installed on the structure being monitored and a single tilt meter is moved from plate to plate to take readings.
Tilt meter (submersible)
Submersible tilt meters are used to monitor tilt on structures below water (submerged) at individual locations and are left in place for the duration of the monitoring. Generally placed on special mounting brackets they can be installed to monitor horizontal and/or vertical tilt and can be either monitor one axis (uniaxial) or two axes (biaxial).
A transceiver is a device which generates a radio frequency and can be used to send and receive data wirelessly. They can be used in automatic data acquisition systems by linking to a central processing unit (CPU) which controls the intervals at which it transmits and receives data.
A transducer is a device that converts a signal in one form of energy to another form of energy. They can be incorporated into instruments such as piezometers, liquid settlement systems, rod extensometers, crack and joint meters, load and pressure cells. Outputs from transducers in geotechnical instruments are typically vibrating wire, volts and 4-20mA.
A transmitter is a device which generates a radio frequency and can be used to send data wirelessly. They can be used in automatic data acquisition systems by linking to a central processing unit (CPU) which controls the intervals at which it transmits the data to a receiver and/or transceiver at a central location. No configuration over air can be done from a transmitter.
Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources.