Good vibrations16 April 2001
Vibrating wire sensors have been used to monitor dam safety for a number of years and continue to be used in a variety of applications. Barrie Sellers describes the principle behind the sensors and what makes them so versatile
Growing concern over dam safety has triggered the need for simple and reliable techniques for remote, real-time sensing and monitoring. Vibrating wire (VW) sensors were first used on dams over 60 years ago and despite all the advances in electronics over that period of time they continue to be used in a variety of applications. Combined with the latest methods of remote data gathering, vibrating wire sensors have made the real-time safety monitoring of dams both feasible and economical.
The VW principle and its advantages
The VW sensor is basically a tensioned steel wire which is made to vibrate at its resonant frequency by an electronic coil positioned next to the wire. This same coil, or a second one just like it, in conjunction with a permanent magnet, senses the movement of the wire in the magnetic field and outputs a sinusoidal current pulse of the same frequency as the vibration. The frequency is measured to very high accuracy using a quartz crystal oscillator, which counts the duration of a known number of cycles.
The VW sensor is inherently a mechanical sensor and it is this fact that gives the VW sensor its outstanding long term stability. The zero stability is of the order of 0.025%FS per year.
The frequency output can be transmitted through 5km long cables, without loss or degradation due to resistance changes in the cable and connections. Similarly the output is greatly unaffected by moisture intrusion into the electrical circuitry, which gives the VW sensor a huge advantage over other electrical sensors.
VW sensors are digital sensors; unlike mV and 4-20mA sensors, the sensor output does not require digitising before it can be fed into a data acquisition system. Modern techniques of digital signal processing make it possible to filter out electrical noise emanating from nearby electrical equipment.
Finally, the relative simplicity of the electrical part of the sensor (a coil), when compared to sensors with mV or 4-20mA outputs, confers a greater measure of immunity to voltage surges produced by lightning – an important consideration.
Piezometric levels; pore water pressures; uplift pressures on foundations; the effectiveness of grout curtains and membranes; leakage past clay cores etc, are all measurable using VW piezometers. Pressures acting on the diaphragm produce changes in vibrating wire tension and frequency output.
Piezometers are built directly into the earth or concrete of a new dam; in old dams the piezometers can be installed in existing observation wells and Casagrande style open standpipe piezometers. A miniaturised version (6.5mm diameter) is available for installation in small diameter standpipes.
Multiple piezometers can be installed in a single borehole using a spring-loaded mechanism to hold the piezometer filter stone in close contact with the borehole wall. This allows the borehole to be filled, from bottom to top, with a bentonite grout. No sand lenses around the piezometers are required which speeds up the installation process.
The amount of seepage is measured using weirs or flumes. The water level in these devises is measured to a high precision using a VW weir monitor.
This sensor is interesting because it embodies a pure force transducer. A weight is suspended from the vibrating wire and is partially submerged in the water. As the water level rises and falls the buoyancy force acting on the weight changes, producing a change of tension in the wire. Such a sensor has excellent temperature stability and is capable of measuring very small changes in seepage levels. The sensor is located in a stilling well connected hydraulically to the weir or flume.
Reservoir and tailwater level measurements can be automated by using a submerged VW pressure transducer. Protection is required to prevent the transducer from becoming fouled by dirty water. Corrosion can be prevented by using transducers made entirely from titanium. Barometric pressure fluctuations are compensated for by the use of vented transducers.
VW crackmeters and jointmeters are used to monitor the opening and closing of existing cracks and joints. Three-dimensional arrays can also be set up to monitor shearing motions. Range of movement of the sensor is controlled by a tension spring inside the sensor. This spring is placed in series with the vibrating wire so that as the crackmeter is extended, most of the movement is absorbed by the spring as it pulls on the wire and increases the wire tension.
Long base VW displacement sensors, up to 3m in length, have been used extensively to monitor temperature induced cracking in roller compacted concrete (RCC) dams during construction. They have been installed across crack initiators and construction joints and also in long chains to monitor the location and extent of random cracking.
Many old concrete dams suffer from alkali/aggregate reaction (AAR) which causes the concrete to swell. The swelling can give rise to high pressures in the concrete, leading to extensive cracking and preventing the closing and opening of spillway gates and the turning of generators and turbines. One solution to the problem is to cut a slot in the dam from upstream to downstream to relieve the pressure and prevent further cracking and swelling. The pressure relief and subsequent build up of pressure as the slot re-closes over a period of time, were measured using soft inclusion stress cells (SISC), installed in diamond drill holes in the concrete. Readings can be monitored to help assess the need for re-slotting.
At Roosevelt dam in Arizona, US, the height of this old masonry dam was increased by about 24m by the mass placement of concrete on the downstream side and crest of the dam. The junction between the new concrete and the old masonry was monitored using more than 800 VW embedment jointmeters, with integral thermistors connected to a number of dataloggers. Readings were used to verify the design assumptions concerning recommended block dimensions, lift heights, placement temperatures, cooling requirements and effects of pressure grouting of the keyed contraction joints following
Settlement of earth dam embankments can be remotely monitored using VW settlement systems. A VW pressure sensor measures the height of a liquid column extending from the sensor location to a reservoir situated on stable ground. As the sensor settles, the height of the column increases. Systems of this type are capable of detecting settlements of as little as 5mm over distances of 300m. They have the rare ability to be re-calibrated and checked for zero stability in-situ. A typical performance record is shown below.
Rock deformations are measured using multiple position borehole extensometers (MPBX), grouted into boreholes drilled downwards into the foundation and abutments. The figure on pp35 shows typical deployment locations designed to measure any tendency for the dam to tilt or slide along its base. Move- ments of as little as 0.05 mm are detectable.
Another method of measuring tilting and shearing is to use VW in-place inclinometers or tilt sensors. Strings of these sensors, connected together by articulated rods and installed in boreholes equipped with inclinometer casing, allow a continuous record to be made of shearing type deflections in the foundation and any tilting of the concrete structure. Tilts of as little as 5mm in 50m can be detected.
Old concrete dams, with insufficient margins of safety to withstand seismic loadings, can be strengthened using tie-downs installed in boreholes drilled through the dam into the foundation rock below. Long term loads on these tie-downs are monitored using VW load cells. In this way any loss of tension in the tie-downs, caused by anchor slippage for example, is detected so that appropriate remedies can be applied.
There are many options available to retrieve data in a timely manner. These range from a single channel datalogger accessed periodically by lap-top computer, to multi-channel systems with interconnected dataloggers communicating via land-lines or radio to a remote readout station, where automatic alarm functions provide an early warning of the onset of dangerous conditions.
The data acquisition hardware and software system can be implemented in a variety of ways to allow for more or less of a real-time monitoring implementation. The costs column assumes additional cost over the basic price of the data acquisition hardware.
Dam safety, and hence public safety, is greatly enhanced by the use of automated data acquisition systems: not only are the data gathered in a more timely manner but they are of much better quality since the measurement record is continuous, enabling subtle effects to be seen much more clearly and correlated more confidently with causative phenomena. Using VW sensors helps to ensure that the data gathered are reliable enough to permit the use of automatic alarms, warning of the onset of dangerous conditions.