On the level13 April 1998
A simple but effective monitoring technique uses liquid levels to monitor dam movement
Tianshenqiao, the world’s largest rockfill dam, is under construction in China. It will be 178m high, and requires 17.68km3 of compacted sandstone rock fill. Professor Pedricto Rocha Filho, head of the Civil Engineering Department of the Pontifical Catholic University of Brazil, is monitoring the dam using a simple but effective system involving around 70 electro-levels (precise tilt sensors), supplied by Construction Monitoring Control Systems (CMCS).
An electro-level is a precise tilt sensor. It consists of a sealed glass tube partially filled with a liquid (iodine and ketone) with a high electrical resistance. Three or four electrodes fitted in the longitudinal axis of the tube measure the resistance of the liquid in each half. As the tube is tilted, the amount of electrolyte changes and so does the resistance. The linearity and sensitivity of the electro-level depends on both the type and amount of electrolyte and the curvature of the tube. The devices can be extremely sensitive — in very stable locations they can and have been used to observe earth tides due to the moon.
The benefit of using electro-level systems to assess engineering performance is that they are easily fixed to the material or structure. In soils they can be used to determine shear modulus; when clamped on to beam elements they can be used to calculate the deflections, bending moments, shears and loads on those elements.
The authors first used electro-levels to monitor the shear strains produced in the soil around vertical and laterally loaded piles. The work provided accurate performance data that was used to understand interaction within pile groups and also to check the new (at that time) computer-aided design methods such as finite elements. The electro-levels were able to determine deflection of the piles, as well as bending moments and load reactions. The electro-levels were compared with electrical resistance gauges.
Lateral loading tests, again comparing results from electro-levels and electrical resistance gauges, were later carried out on I-section steel piles in France to investigate soil reaction loads. The load was calculated by fitting a curve through the results (with 16 monitoring points, a sixth order polynomial was found to give the best fit). With the equation of the curve it was then a relatively simple matter to integrate for deflections and repeatedly differentiate for bending moment, shear and load. Results from both sets of instruments were very good. The advantages of the electro-level system were its much higher sensitivity than the ERS gauges, its low relative cost and the fact that it could be removed and used again.
Having proved itself, the electro-level system was extended to more complex building structures.
London has several brick-built embankment-supported reservoirs. The owner, Thames Water, is using electro-levels to provide detailed in-service performance data on the movements and strains in these structures. These automatic systems, connected by modem, are monitored daily and are very cost- effective, providing on-line information at a fraction of the cost of conventional manual surveying, without the need to empty the reservoir.
Currently electro-levels and crack monitors, battery-powered and accessed by modem, are being used to monitor movements within earth dam structures as part of an assessment programme for Yorkshire Water. At the Tianshenqiao dam (TSQ 1) twenty +/-6° electro-levels have been installed along the bottom of a section. Each is mounted in a 40mm diameter waterproof circular case so that they can be adjusted on site. It is important that the electro-levels are firmly fixed to the subject. At TSQ-1 they are bolted to the surface of the completed concrete membrane. This method (as opposed to burying them in the membrane) has been adopted to reduce the disruption to the construction process as much as possible. Fixing intimately to the most critical element of the structure will undoubtedly provide the most pertinent information on the membrane performance.
Rockfill dams are popular because they can be constructed quickly. They can also be made from local materials and are therefore relatively cheap. The secret of their success is, however, support in the long term of the waterproof concrete membrane. Determining how to design the dam to prevent rupture or cracking of the concrete membrane is all-important, and to improve the design it is essential to monitor the membrane’s performance.
Traditionally, horizontal movements of the rock fill have been monitored by burying steel plates fitted with a horizontal plastic tube in the dam wall during construction. An inductive coil sensor pushed up the tube is used to detect the initial, and subsequent, horizontal location of the buried plates relative to a reference on the downstream face of the dam.
Changes in vertical measurements are determined from water overflow levels, also referenced to the downstream face. The water levels consist of a buried open-ended tube in the rock fill with the other end of the tube fitted in the gauge house. When the free end of the tube is lifted the water level will settle to the level of the buried open end.
Measuring movements of a few centimetres from distances of 200-300m, offers obvious inaccuracies. Another problem is time: taking the readings of even a small number of instruments can take several hours to complete. During filling, the rate of monitoring could be an important factor. Several load and unload cycles may be needed to check the initial performance of the membrane. When using electro-levels the rate of reading is limited only by the method of data recording.
Professor Rocha’s long term objectives are to show through simple but effective monitoring that performance of these giant structures can be improved. He says it is important to concentrate on the upstream compaction of the rock fill – the use of a finely graded well-compacted fill can significantly improve the support of the membrane.
Monitoring by electro-levels on the Xingó dam, in Brazil, by Professor Rocha, has shown that the technique of curve fitting through the results can provide valuable data at a fraction of the cost of conventional instrumentation. Hori-zontal movements of the crest calculated from the electro-levels have compared very favourably with movements monitored by conventional surveying results.
The most exciting information coming from the electro-levels is, however, long term movements along the whole length of the submerged concrete membrane. The use of a limited number of electro-levels (hand-read at present) with curve fitting procedures and mathematical models, offers a more cost-effective method of determining the performance of these large structures than has previously been available.
A simple assessment of the likely effectiveness of the electro-level system deployed on the TSQ-1 dam can be made by comparing crest movements monitored by standard surveying with those calculated from the electro-levels. During the initial impounding trials the surveyed movements were of the order of 40cm, compared to about 38cm from the electro-levels. Because of the additional information about the movement of the membrane below the water level these results are considered to be very encouraging.