Instrumentation for Monitoring and Safety of Hydraulic Structures
Instrumentation which is essentially a technology of measurements and vital in all scientific investigations helps in monitoring and evaluating the performance of dams during their construction as well as during their operation. Instrumentation helps in checking the theories used in design, in validating and improving upon the design principles and discarding the erroneous concepts. Future behaviour of dams can be predicted and suitable remedial measures can be undertaken to strengthen them. Instrumentation also helps in verification of new construction techniques and to build greater confidence among engineers responsible for maintenance and operation of dams.
The instruments and instrumentation systems which used to be most often hydraulic, mechanical, pneumatic and electro-mechanical are gradually getting transformed into electronic ones as they facilitate use of electronic data loggers and computers. At major projects, where the instruments are installed in large numbers, it is desirable to go in for automation so that the results from instrumentation data could be made available as quickly as possible for evaluating the health status of dams and for taking suitable remedial action, if warranted.
The failure of dams in the world is approximately one in 185; but it could be more in future due to faster pace with which dams are being constructed. While failure of dams cannot be completely avoided, it is possible to reduce the effect of their failure on public life and property, if advance information and warning could be provided by monitoring the dam behaviour based on the instrumentation data and timely measures are initiated in the form of strengthening of dams or disaster management. Instrumentation can also form a basis during legal proceedings, claims etc. after the failure of a dam.
A number of higher and higher dams are being constructed in the relatively unstable Himalayan geological formations. These dams have high risk consequences and therefore may need to be adequately instrumented. The instrumentation of foundations should, therefore, be extensive so that adequate foundation treatment is ensured after receiving feedback from them.
For successful implementation of instrumentation, it is necessary that the instruments and the instrumentation system chosen should be sufficiently sensitive, accurate, reliable and durable. Additional care should be taken while selecting instruments, that are buried and can not be retrieved later for servicing. Again proper study and experience is required to be able to understand as to what parameters are required to be monitored for planning and placement of instruments. Instrumentation with respect to its location in Dam need to be absolutely thorough so that areas critical for stress determination are fully covered. The designer should infact specify the critical points which need to be constantly monitored and the frequency at which the collection of data and monitoring is to be done.
2.0 TYPE OF INSTRUMENTS
About forty yearas ago, geomechanics was still a new science and market for instrumentation hardly existed. Instruments were installed only to monitor any special problem encountered during construction. Today geomechanics has matured as a science and instrumentation is invariably specified in every project and is recognised as a necessity in dams. Instruments are now typically installed, read and interpreted by specialised instrumentation engineers rather than by manufacturers.
Instrumentation technology has advanced very rapidly during the recent years and it has become more secure with more complex devices becoming quite common for use in dams. The instruments available since the beginning of the era of instrumentation can be classified into four categories, based on their principles of working viz.: Mechanical, Hydraulic, Pneumatic and Electrical/Electronic. Initially mechanical and hydraulic instruments were used extensively for instrumentation. However, with the passage of time and advancement of technology, pneumatic and electrical/electronic instruments have become popular. The period since when these dam instruments are being used abroad and in India is given below.
Instruments Since when in use (years)
technology abroad India
- Mechanical 60 45
- Hydraulic 60 40
- Pneumatic 45 30
- Electrical/electronic 45 15-25
The mechanical,hydraulic and pneumatic type of instruments are simple, rugged, reliable, cheaper and easy to operate but they have lower response and lower accuracy. The electrical/electronic instruments are highly sensitive and have high resolutions.
Off late the use of Electrical/Electronic type instruments are in vogue and these are being used extensively in instrumentation of dams. The electrical/Electronics type of instruments include unbonded resistance type, bonded strain gauge type and vibrating wire type.
Due to high rate of mortality among various types of instruments, erratic behaviour and lack of proper calibration, the results given by most of the instruments cannot be fully relied upon except in the case of vibrating wire type instruments.
Bonded strain gauge type instruments are suitable for surface installations and for short term observations. The unbounded resistance type of instruments, though have long term stability but they suffer from zero shift, cable resistance variation are sensitive to temperature changes, moisture movements and have short life. Thus, their long term reliability is questionable.
The vibrating wire instruments are now increasingly being used in dam instrumentation. These instruments are reliable, sensitive, accurate, durable and can be used with modern data loggers and computers. In fact with vibrating wire instruments, instrumentation can be completely automated and these can be read and interpreted at even far off Central control rooms through satellites. Other reasons for selection of vibrating wire technology are :
- a) Splicing of instruments leads can be readily performed with little or no adverse impact upon the long-term performance of the system.
- b) The vibrating-wire cable can withstand abuse during the construction process and still function properly.
- c) When properly protected against lightening (primarily by deep burial and adequate shielding of cables), vibrating wire instruments have proved to be highly reliable.
- d) Vibrating-wire instruments require no maintenance and can be quickly and easily read at central reading
Selecting vibrating-wire technology for piezometers, settlement sensors, total pressure cells, strain gauges and joint meters permits the same terminal switching stations and readout equipment can be used for all these instruments.
Vibrating wire instruments are however, affected by temperature changes. For this reason, each instrument includes a thermistor so that the temperature changes to be noted and compensated for.
As regards, determining the number of instruments and their exact type or location, their determination is primarily a matter of experienced judgement.
3.0 DEVELOPMENT OF INSTRUMENTATION TECHNOLOGY IN INDIA
Instrumentation Technology in India can be traced back with the establishment of Engineering Research Institute in the Irrigation Depts of States with the assistance of National Physical Laboratory for transducer development.
Further, after Independence, a large number of high dams were taken up for construction. For some of the dams, Consultants from USA and European countries were involved, with the result that the State-of-art of dam instrumentation, as available in advanced countries could be introduced. However, indigenously manufactured instruments have a very high rate of mortality and could not be relied upon. Added to this, the instrumentation of dams was not carried out in right earnestness with the result instrumentation suffered and only a few instruments yielded reliable data.
The vibrating wire type instruments, no doubt enjoy advantages but they are costly and most of their components are imported. A number of Indian firms have of late entered into collaboration with foreign partner and have started producing vibrating wire instruments. It is suggested that only those firms which supply vibrating wire instruments with ISO 9000 certification be used for dam instrumentation.
Moreover due to lack of co-ordination between the construction contractors of dams and manufacturers/suppliers of the instruments, the programme of installation of instruments and their accessories and the successful interpretation of instrumentation data could not be achieved. It is therefore, suggested that the procurement, installation and successful operation of instruments should be a part of the main contract.
Central Water Commission has prepared guidelines for BIS code on standardising the dam instrumentation. Still lot of work in standardising the dam instrumentation system need to be done.
In India we have 277 dams above a height of 30m and another 116 dams are under construction(total 393 dams), out of which only about 149 dams are known to be adequately instrumented.
4.0 PARAMETERS TO BE MONITORED IN DAMS:
The various parameters to be monitored and measured in the dams are: Uplift, Pore pressure, stress, strain, joint movements, horizontal and vertical displacement, foundation deformation, deflection, surface movement, seepage, temperature and seismicity.
Various instruments used for monitoring these parameters in dams are tabulated below:
PARAMETERS INSTRUMENTS WHERE APPLICABLE
I.Uplift/pore 1.Twintube Hydraulic piezometers
water pressure 2.Pneumatic piezometers
4.Unboundedelectric resistance piezometers
5.Bonded Electric resistance piezometers
6.Multipoint piezometers with packers
7.Multipointpiezometers surrounded with grout
9.Porous tube piezometers
11.Pore pressure cells.
- Seepage 1.Buckets and stop watch
6.Geophysical seepage monitoring
7.Water quality meters
III.Strain 1.Elastic wire strain meters
2.Vibrating wire strain meters.
IV.Stress 1.Gloetzl Cell
2.Carlson Load Cell
3.Vibrating wire stress meters.
V.Relative movement across
Joints(Between Blocks) 1.Joint meters
VI.Displacements 1.Multipoint extensometers
3.Crackmonitoringgauges 4.Calipers 5.Micrometers
7.Vibrating wire settlement sensors
2.Foundation deformation gauges.
3.Tunnel type gauges
VIII.Deflection/Surface movements 1.Plumblines
3.Embankment measuring points
4.Structural measuring points.
(Surface & Dam Body) 2.Vibratingwire thermometers.
X.Seismic 1.Geophones(For monitoring micro seismic activities)
2.Seismograph(Strong motion monitors)
- Structural Response Recorders
4.1 The various parameters which are required to be monitored in concrete/masonry and gravity dams are:
- a) Pore water pressure/uplift pressure measurement in dam foundation and abutments.
- b) Seepage
- c) Strains in dam module
- d) Stresses between dam and its abutments or foundation or in a dam body.
- e) Relative movements across joints(between monoliths)
- f) Displacements
- g) Deformations
- h) Deflections
- i) Surface movements
- j) Temperature
- k) Seismic monitoring
4.2 The various parameters to be monitored in Earth and Rockfill dams for judging their performance are
- a) Ground water/pore water pressure
- b) Seepage and quality of water
- c) Settlements in foundations soils below dams
- d) Surface movements, vertical, horizontal, rotational or differential movements.
- e) Seismic monitoring
A few case histories are presented below which indicate as to how the instrumentation has helped in measuring and monitoring the behaviour of dams and structures and in implementation of the remedial measures for safety of the dams etc.
a) In June 1985, a big land slide with a slide mass of 30 million cubic meters took place at Xintanzhen town in China. This town is situated on the bank slope of the Yangtze River, about 70 km upstream of Gezhouba Project or 27 km upstream of Three Gorges Dam It was due to perfect instrumentation and monitoring work, such as alignment system, levelling measurements, triangulation, bore hole observation etc., that it was possible to forecast much in advance that such a landslide is inevitable. In compliance with this advance forecast, the authorities evacuated 481 families consisting of 1370 people well in time and human casualties were completely avoided. This landslide destroyed 95% of the old Xintanzhen town. It is reported that when slide mass plunged into the river, the surge was as high as 40m.
b) The 82m high Bhandardara Masonry Dam, in the State of Maharashtra over River Pravara, a tributary of River Godavari, having a total length at top of dam as 507m was completed in the year 1926. This dam was operated for about 43 years without any problem or distress. In 1969 it was noticed that a heavy sheet of flow of water @ 0.62 cumec(22 cusecs) was gushing out from the contact plane between masonary and the rock foundation at a distance of about 70m from the centre of the After investigations, remedial measures were undertaken to repair and strengthen the dam and to stop excessive seepage. While undertaking the remedial measures various instruments were also installed to monitor the effectiveness of the strengthening of dam and remedial measures undertaken.
The results, as gathered from the instruments installed in the dam, showed very encouraging results and effectiveness of the remedial measures like decrease in seepage from 0.62 cumec(22 cusecs) to about 5 litres/sec(0.005 cumec or 0.18 cusecs), reduction in uplift pressure in dam foundation, reduction in deflection of dam from 10.30mm (before undertaking remedial measures) to 4.50mm, reduction in tilt from 72 seconds to 28 seconds.
c) Fontenelle Dam, a zoned earthfill structure in United States, constructed by USBR and completed in 1964 has a crest length of 1652m and a structural height of Immediately after construction, when the filling of the reservoir of the dam was done in 1965, a section of the right embankment near the right abutment collapsed due to excessive seepage and piping. The excessive seepage was under significant hydraulic pressure and it eroded the embankment material along the foundation which was highly jointed and untreated.
After completion of the remedial measures to strengthen the embankmet, reinforcing it with grout curtain near the abutments, the reservoir was filled up. While executing the remedial measures, extensive instrumentation of the embankment in the form of observation wells, installation of piezometers stand pipes, seepage monitoring devices, uplift/pore water pressure instruments etc. was done to monitor and measure the behaviour of the dam. Although the dam functioned satisfactorily till 1982, once again, distress condition were noticed(primarily due to extensive instrumentation), in the form of excessive seepage, piping, increase in the rate of settlement etc. During this second distress condition in the dam, further instrumentation like temperature monitoring, foundation settlement monitoring, ground water flow monitoring, embankment measurement points etc. were adopted, in addition to earlier instrumentation which was also increased.
The extensive instrumentation and monitoring program helped to avoid an emergency situation in 1983. The monitoring of seepage, piping and structural behaviour of the dam could help in identifying areas of potential problem and timely remedial measures were undertaken to avoid any major failure of the dam.
The examples quoted above merely indicate that even dams/structures which would have been operated successfully without any incident or distress for many years, are also susceptible to serious problems and distress thus reinforcing the need and necessity of extensive instrumentation and monitoring their behaviour to be able to detect distress conditions in these dams/structures and take suitable remedial and precautionary measures in advance to avoid heavy loss of human life and property.