Instrumentation for Monitoring and Safety of Hydraulic Structures

DAM INSTRUMENTATION

1.0      INTRODUCTION

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

——————————————————

  1.     Mechanical               60               45
  2.    Hydraulic                  60                40
  3. Pneumatic                 45                30
  4. 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 :

  1. a) Splicing  of  instruments  leads  can  be  readily performed with little or no adverse impact upon the long-term performance of the system.
  2. b) The vibrating-wire cable can withstand abuse  during   the construction process and still function properly.
  3. c) When properly protected against lightening (primarily   by deep burial and adequate shielding of cables), vibrating wire   instruments have proved to be highly reliable.
  4. 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

3.Vibratingwire   piezometers

4.Unboundedelectric resistance piezometers

5.Bonded Electric  resistance piezometers

6.Multipoint  piezometers  with packers

7.Multipointpiezometers  surrounded with grout

8.Multipointpushin piezometers.

9.Porous tube piezometers

10.Slottedpipe  piezometers.

11.Pore pressure cells.

 

  1. Seepage 1.Buckets and stop watch

2.Weirs

3.Flumes

4.Flow meters

5.Velocity meters

6.Geophysical seepage monitoring

7.Water quality meters

8.Resistivity test

 

 

III.Strain                                                   1.Elastic  wire   strain  meters

2.Vibrating  wire   strain  meters.

3.Reinforcing meters

4.Nostressstrainmeters

 

IV.Stress                                                   1.Gloetzl Cell

2.Carlson Load Cell

3.Vibrating  wire   stress meters.

4.Flat jacks

 

V.Relative movement across

Joints(Between Blocks)                        1.Joint meters

 

 

VI.Displacements                                     1.Multipoint extensometers

2.Whitemore gauges

3.Crackmonitoringgauges                                                                                           4.Calipers                                                                                                                    5.Micrometers

6.Dial gauges

7.Vibrating wire settlement sensors

8.Internalverticalmovementinstallation

9.Inclinometers.

 

VII.Deformation                                       1.Multipointboreholeextensometers

2.Foundation   deformation gauges.

3.Tunnel type gauges

 

VIII.Deflection/Surface movements          1.Plumblines

2.Tilt meters

3.Embankment measuring points

4.Structural measuring points.

5.Surveyingtechniques.

(a)Triangulation

(b) Trilateration

(c) Collimation

 

IX.Temperature                                        1.Resistancethermometers

(Surface & Dam Body)                       2.Vibratingwire thermometers.

3.Thermisters

 

X.Seismic                                                   1.Geophones(For monitoring micro seismic                                                                                activities)

2.Seismograph(Strong motion monitors)

  1. Structural Response Recorders

 

4.1       The various parameters which are required to be monitored in   concrete/masonry and gravity dams are:

  1. a) Pore water pressure/uplift pressure measurement in dam foundation   and abutments.
  2. b) Seepage
  3. c) Strains in dam module
  4. d) Stresses between dam and its abutments or foundation or in a   dam body.
  5. e) Relative movements across joints(between monoliths)
  6. f) Displacements
  7. g) Deformations
  8. h) Deflections
  9. i) Surface movements
  10. j) Temperature
  11. k) Seismic monitoring

4.2       The various parameters to be monitored in Earth and Rockfill   dams for judging their performance are

  1. a) Ground water/pore water pressure
  2. b) Seepage and quality of water
  3. c) Settlements in foundations soils below dams
  4. d) Surface movements,  vertical,  horizontal,  rotational  or   differential movements.
  5. e) Seismic monitoring

 

CASE STUDIES:

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.

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