Types of Instruments

  MASONRY AND CONCRETE DAMS INSTRUMENTS

Instruments are classified based on principle of working such as 

  1. Electrical Type-
    a) Resistance Type :-Work on electric resistance. 
    b) Vibrating wire principle :-work on the principle that frequency of vibrating wire depends on the tension in wire and therefore on strain
  2. Mechanical type -
    a) Uplift pressure cells-
        1) Box type 
        2) Bowl type
    b) Plumb line : Deflection of wire is measured to a reference point.
  3. Optical Type :-
    Wild T3 Theodolite and Wild N3 levels are standard instruments.

EARTHEN DAM INSTRUMENTS :

  • Pizometers- It indicates the magnitude and distribution of pore pressures and their variations with time and also pattern of phreartic line zones of potential piping,and effectiveness of under seepage control measures.

Pizo meters Types are 1) Open stand pipe 2) Twin tube type hydraulic 3) Electrical Type 4) Pneumatic

  • Earth Pressure Cells - Used in monitoring soil stresses in the dam core and filter where there is likelihood of development of tensile stresses, which may eventually result in cracks.

Earth Pressure Cells types are
1) Electrical 2) Hydraulic

  • Settlement Instruments:- It determines vertical movement within embankment and settlement of foundation and identify the zone of excessive and uneven settlement which may lead to stability problem and cracking.

Settlements are measured by:
1) Cross arms 2) Settlement plugs, etc.

  • Pore Pressure measurements:- It is used for the purpose of monitoring pore water pressure in the body of the dam. They are based on vibrating wire principle.
  • Seepage:- Seepage water from the foundation drainage gallery is measured by installation of 'V' notches.

Analysis of Instrumentation data and preparation of behavior reports:--

Object of Data processing -
1.Immediate objective being assessment of the data to detect sudden changes requiring urgent attention.
2.Summaries and presentation of data to understand and compare with the predicted behavior of the dam and appropriate action to be taken, if called for. 
 
The Govt. of Maharashtra, Irrigation department have distributed the works pertaining to instrumentation among various Organisation --
 Central Design Oraganisation-
  1. Preparing proposal for instrumentation in new dams to design requirement.
  2. Preparing and furnishing Designers Operating Criteria and relevant design parameters of normal behavior to field officer and DSO.
   Maharashtra Engineering Research Institute :-
  1. Procurement, Installation and Commissioning of all special and sophisticated instruments.
  2. Identification of abnormal and deficient working of instruments and setting right deficiencies.
  3. Annual inspection,checking and calibration of all instruments and furnishing calibration details to field officers and DSO.
  4. Imparting training to field officers and staff about maintenance and operation of instruments.
  5. Collection and processing of data for improving quality and utility of data.
Dam Safety Organisation :-
  1. Analysis and interpretation of instrumentation data and monitoring behavior and status of the dam.
  2. Periodical inspection of instruments and monitoring deficiencies
  3. Suggestion of proper instruments where ever necessary in consulting with MERI & CDO
  4. Guidance to field officers regarding collection and recording of data.
  Field Officers :-
  1. Operation and maintenance of instruments as per guidelines.
  2. Procurement and installation of instruments in consulting with MERI. Nashik
  3. Taking observation & recording instrumentation record.
  4. Submission of data in prescribed form.
100 Instrument Analysis Report has been prepared by this organization based on data received from field officers and circulated to all concerned every year. 
 
Summary and conclusions
  1. The mortality of Instruments in Maharashtra is very high and from the data it is seen that 58% of instruments not functioning properly.
  2. Flooding of the gallery is the major problem in the instrumentation functioning and is responsible for malfunctioning of the instruments.
  3. Stress meters and Strain meters have shown large failure ratio in masonry dams and their installation needs to be restricted to dams prone to seismic activity only.
  4. Performance of thermometers is better where readout unit assemblies and cables are not subjected to moisture due to flooding.
  5. Plumb line generally give consistent and reliable observation even when subjected to adverse environmental conditions. However the installations take long time and observations are started quite late when reservoir loads are built up and hence initial observations of unloaded conditions cannot be known.
  6. The uplift monitoring is very important from the safety point of view. However chocking of uplift pipes is frequent and data is not available.
  7. Incase of dams, construction is quite slow and it takes years to come the top. The activities are highly labour intensive and great amount of human and machinery interference. This creates frequent disturbance to cables and PVC pipes and instruments leading to possible failure and malfunctioning.
  8. Due to complex configuration of pipes and cables, if no proper case is taken lead tags are lost, it becomes very difficult to identify the pipes and cables, and hence the whole exercise becomes failed.
  9. Efforts should be towards automat and use of latest but reliable technology for a long term feedback from instrumentation, As far as possible instruments having ISO 9000 certification should be adopted / purchased.
  10. Frequency of observation required from the instruments should be clearly elaborated and as far as possible data from instrumentation should be collected at central rooms where experts on instrumentation can analyses the data in the best possible ways.
  11. Seismic instruments are most essential for monitoring local and regional seism city, the safety of dams site. It provides valuable data base which in turn could be helpful in evaluating the safety of dams.