Implementing an enhanced deformation monitoring system

November 19th, 2018, Published in Articles: PositionIT

The Sri Lankan Authorities recognised that the ongoing monitoring of Victoria Dam is essential in understanding the structural integrity of the dam post construction, as well as to understand how it is behaving in accordance with its original design. It was deemed important to update the monitoring system to capitalise on the latest advances in monitoring technology to provide this information. 

A combination of automated prism monitoring, GNSS monitoring receivers and automated logging of the geotechnical instrumentation were implemented to provide an automated monitoring system, where all of the data is pooled into a central monitoring software environment capable of providing detailed reporting and analysis of the structural deformation.

Background

Victoria Dam is located on the Mahaweli River in the Nuwara Eliya District of Sri Lanka (37 km south east of Kandy) and is located approximately 209 km upstream from the coast. It is a double arch concrete dam, where the maximum height is 122 m, the base width is 25 m thick and the crest width is 6 m thick. The total arc crest is 520 m long. The resulting reservoir has a surface area of 22,7 km² and a total capacity of 722-million m³.

The dam is important for Sri Lanka as it is used for both hydropower and irrigation. The hydropower power station is located a further 8 km downstream from the dam wall and supplies approximately 6% of Sri Lanka’s power.

Fig. 1: The Victoria Dam is a double arch concrete dam with a 520 m long total arc crest.

Fig. 1: The Victoria Dam is a double arch concrete dam with a 520 m long arc crest.

Construction of the concrete dam commenced in 1978 and was completed in 1985, at which time a comprehensive monitoring system was installed and implemented. The requirement for monitoring was included right from the initial design stage and so all of the latest technology at the time was implemented.

Along with a number of geotechnical instruments (which were installed below and within the structure) a complete geodetic system was also implemented. The geotechnical instruments consisted of pendulums, vibrating wire clinometers, vibrating wire strain meters, thermometers, vibrating wire piezometers and drainage flow meters. The geodetic monitoring system consisted of circular targets on the face of the wall (which were monitored by using forward intersecting rays from a network of fixed stations), as well as a series of precise levelling points installed at evenly spaced intervals along the full length of the crest. All of the monitoring up to this point was conducted manually on a periodic basis, from fortnightly on the geotechnical sensors to yearly for the geodetic points.

But over the years, a number of the original instruments has stopped working due to the overall age of the system.

Design approach

Fig. 2: The fully automated geodetic system comprises an automated monitoring of prisms on the dam wall and GNSS monitoring points on the crest.

Fig. 2: The fully automated geodetic system comprises automated monitoring of prisms on the dam wall and GNSS monitoring points on the crest.

The implementation of the new monitoring points and equipment started with a review and assessment of the original data.

After consulting the client, it was decided to install a fully automated geodetic system, consisting of automated monitoring of prisms on the dam wall and GNSS monitoring of points on the crest. Furthermore, the precise levelling points would be resurveyed using a precise digital level.

A vibrating wire piezometer coupled to a wireless telemetry system would also be installed to allow for automated water level readings, would could be used to correlate dam levels with the other measured data.

All of the data from these various instruments would then be automatically collected and stored in a local database, where it could be further interrogated and analysed by the dedicated monitoring software.

Precise levelling and benchmarks

A network of 16 evenly spaced precise level points exist along the crest of the wall. These points consist of a metal housing cast into the dam wall and a threaded insert, which acts as a protective cap. The insert is removed when taking measurements and a brass ball is placed in the void. The existing points are all still in excellent condition and thus would be adopted in the new monitoring system. However, this would be surveyed from now on using a digital precise level. The datum control points, on which the precise levelling is based, also needed further improvement.

Fig. 3: A network of 16 evenly spaced precise level points exist along the crest of the wall.

Fig. 3: A network of 16 evenly spaced precise level points exist along the crest of the wall.

There was only a single primary precise levelling benchmark available as the others had been damaged over time. For this reason, an additional two primary benchmarks were established in the vicinity to improve the level control. The datum was also updated by transferring a level value from the nearest country-wide precise level benchmark to site. Rigorous levelling techniques dictated by the Sri Lankan Survey Office were used to ensure the datum values were updated in an compliant manner. This would also allow the Sri Lankan Survey Office to use these benchmarks as primary datums, should they need to.

Automated prism monitoring

Fig. 4: One of two new automated total stations.

Fig. 4: One of two new automated total stations.

Mini-prisms (24 mm) were installed on the dam wall at positions adjacent to the existing survey targets. Additional prisms were also installed on the crest of the wall adjacent to the overflow gates to provide additional monitoring of these structures. Due to the double-curvature of the wall it was necessary to improvise a suitable but safe method to reach all parts of the wall to attach the prisms to the wall during the installation process.

This was done by manufacturing a purpose-built gantry, which was suspended from an overhead crane. The gantry was of sufficient length to allow the installation engineer to reach the wall even at the most concave part of the wall.

In addition to the monitoring prisms, 62 mm prisms were installed as part of a control network for the total stations. Care was taken to ensure these locations were on structures outside of the zone of influence of the reservoir, as well as ensuring excellent geometry for the control. A total of four control points per station was established to ensure that the control was geometrically sound.

Two existing monitoring station pillars on opposing banks were selected as the positions for the new automated total stations. The reason for utilising two fixed survey positions and not one is because this allowed for all parts of the wall to be monitored, even when the overflow gates were open. Using a single station would result in certain prisms not being monitored during this critical time as they would become obstructed by the cascading water. Each total station is housed in a protective glass enclosure, which was designed to be independent of the mounting pillar, i.e. the housing does not come into physical contact with the pillar.

The total station, which is mounted on the pillar, is then coupled to a power supply, including backup batteries and a WiFi antenna, all housed in a stainless-steel protective cabinet installed adjacent to the pillar. The data is transmitted automatically to the control centre during each round of measurements. The mounting pillars contained Kern tribrach adaptors as this was the original instrument used to conduct the initial monitoring. It was a requirement by the client to not have these disturbed, so special mounting plates had to be sourced to act as an adaptor to allow for the new automated total station to be mounted onto the Kern tribrach.

GNSS monitoring

Fig. 5: A piezometer attached to a wireless datalogger and transmitter was also installed in the reservoir against the dam wall close to the centre of the crest.

Fig. 5: A piezometer attached to a wireless datalogger and transmitter was also installed against the dam wall close to the centre of the crest.

GNSS monitoring positions on the crest of the dam where selected to provide long-term static observations and to provide redundancy to the prism monitoring. Co-located monitoring prisms were also installed beneath each GNSS antenna to allow for the position of the GNSS antenna to be independently verified using the measurements from the total stations. Each GNSS monitoring station transmits data to the control centre over a WiFi datalink. Additionally, each GNSS monitoring station, much like the automated total stations, is coupled to a backup power solution, allowing for the data to be collected even if the primary power is unavailable. The primary GNSS base was installed at the control centre, from which the baselines are calculated via post-processing.

A piezometer attached to a wireless datalogger and transmitter was also installed against the dam wall close to the centre of the crest. A piezometer capable of measuring water level deflections with a range of up to 70 m was used. The wireless datalogger transmits the readings hourly to the control centre, where the data is automatically recorded into the monitoring database.

Control centre

The control centre is located on high ground overlooking the dam wall, offering line-of-sight to almost all parts of the dam structure.  A suitable area in the dam’s control centre was made available. Here, a dedicated server specific to the monitoring project was installed. The cabinet housing the GNSS base station and associated electronics was also installed adjacent to this sever. This server connects to all of the data feeds via a WiFi bridge and wireless gateway. The server is also connected to the dam’s internal IT network, allowing the dam engineers to access the data whenever required.

Monitoring software

Dedicated monitoring software not only validates and stores each measurement automatically, but also performs all the required calculations mandated by the client to conduct their analysis of the movement. A user-friendly and intuitive web interface allows multiple users to access the data simultaneously and to perform the reporting required. In addition to this, alarm levels can be set to ensure no large-scale movements go unnoticed.

In addition to collecting all of the data automatically from the various new instruments, it is also possible to input all of the historical data recorded to date, allowing the data to be analysed over the full lifespan of the structure, all within a single platform.

Shortly before the installation of this new geodetic system, the available geotechnical instrumentation within the dam structure were upgraded to an automated logging system. The data obtained through this method was then integrated into the monitoring database, allowing the dam engineers to incorporate the data from all of the sensors into a common database and analysis tool. The incorporation of these sensors added over 470 instruments into the analysis software.

The resulting system now provides data at the following frequencies:

  • Prisms on the dam wall – every three hours (taking approximately 20 minutes for each instrument to complete a round).
  • GNSS measurements – post processed every three hours.
  • Water level readings – every hour.

Conclusion

The implementation of an automated monitoring system now allows for the Victoria Dam engineers to focus on the long-term structural stability of the dam using significantly more detailed data obtained automatically from an array of instruments. This steady stream of data provides information on the behaviour of the dam throughout the year, which was previously not achievable with the older system. The dramatic seasonal variation that affects this dam can now be fully assessed without compromise on data frequency and quality.

Contact Adrian Jonson, Optron, Tel 021 421-0555,ajonson@optron.com