Dam engineering 6

1. PDE201 – Dam Engineering I
Instrumentation & Surveillance
 Instrumentation
There are three main reasons for monitoring the performance of
embankments and their foundations:
1. To observe the performance for safety reasons,
2. To compare the actual performance with that predicted by designer,
3. To improve our knowledge of the behaviour of dams in general.
 Provision of monitoring instruments is an accepted practice in all
dams of any magnitude.
 To provide an indication of the validity of design assumptions,
 To determine an initial datum pattern of performance against which
subsequent observations can be assessed.
 A basic level of instrumentation is frequently installed to monitor
existing dams.
 Instruments may be installed to provide a measure of reassurance
 They serve to detect significant and abnormal deviations in long-term
behaviour of the dam

2. PDE201 – Dam Engineering I
 In existing dams instruments may also be required to record specific
parameters of behaviour in response to an acknowledged or suspected
design deficiencies or behavioral problem.
The primary function of instrumentation may be for:
1. Construction control: verification of critical design parameters
with immediate looped feedback to design and construction,
2. Post-construction performance: validation of design;
determination of initial or datum behavioral pattern.
3. Service performance/surveillance: reassurance of structural
adequacy; detection of regressive change in established
behavioral pattern; investigation of identified or suspected
problems.
4. Research/development: academic research; equipment proving
and development.
There are possible overlap between certain of the functional
classifications.

3. PDE201 – Dam Engineering I
Parameters in monitoring dam behaviour
The most significant parameters are:
1. Seepage and leakage (quantity, nature, location & source);
2. Settlement and loss of freeboard in embankments (magnitude, rate);
3. External and internal deformation (magnitude, rate, location);
4. Pore water pressures and uplift (magnitude, variation);
5. Internal stress or pressure (magnitude).
Certain key parameters are of primary concern regardless of the
type of dam, e.g. seepage and external movement or deflection;
others are relevant to a specific type of dam, e.g. pore water
pressures in relation to earthfill embankments.
Instruments: Design Principles
Monitoring instruments are required to function satisfactorily under
very harsh environmental conditions and for essentially
indeterminate periods of time, possibly several decades. It is
desirable that instruments be

4. PDE201 – Dam Engineering I
1. As simple in concept as is consistent with their function,
2. Robust and reliable,
3. Durable under adverse environmental and operating conditions, and
4. Acceptable for “through-life” cost (i.e. the sum of purchase, installation,
support & monitoring costs).
Types of instruments: Design & operating principles(see pp. 752 - 758, handout)
Instrumentation Planning
The planning and specification of a comprehensive suite of instruments
involves a logical sequence of decisions:
1. Definition of purpose and objectives;
2. Definition of observations appropriate to the dam considered;
3. Determination of the locations and numbers of measuring points for the
desired operation;
4. Consideration of the time period to be spanned, I.e. long- or short-term
monitoring;
5. Consideration of the optimum sensing mode in relation to the desired rapidity
of response, required accuracy, etc;
6. Selection of hardware appropriate to the task as defined under 1 – 5.

5. PDE201 – Dam Engineering I
 Step 3 is one of particular importance and sensitivity.
 Instruments must cover known critical features of the dam, but also be placed at
locations where ‘normal’ behavior may be anticipated.
 In the case of new dams, at least two sections should be instrumented, including
the major section.
 It is good practice to draft an ideal layout in the first instance, and then to
progressively eliminate the less essential provisions until an adequate, balanced
and affordable plan is determined.
 It is advisable to consider instrumentation programs in terms of the overall
system required, i.e. instruments, installation, commissioning, monitoring, and
data management, and interpretation.
Data acquisition and management
 Logical planning of data acquisition and processing is essential if the purpose of
an instrumentation program is to fully realized.
 Unless observations are reliable and the information is interpreted quickly, the
value of a program will be severely diminished.
 Within the operating plan the frequency of monitoring should be determined on a
rational basis, reflecting the objectives and the individual parameters under
scrutiny.

6. PDE201 – Dam Engineering I
 Detailed prescription of periodicity is a question of common sense & engineering
judgment
 An excess of data will prove burdensome & may confuse important issues;
 Too little information will raise more question than it resolves;
 Excessive complexity in a system, whether in terms of equipment or the operating skills
required, similarly diminishes its utility
 A balance is always required – and the system must remain sufficiently responsive &
flexible
 The monitoring routine should provide for observations at the different seasons & with
significant changes in retained water level.
E.g.Table: Representative monitoring frequencies for embankments
Parameter Frequency
Water level Daily wherever possible
Seepage Daily or weekly
Piezometers Once or twice weekly (construction) to 3 to 6 monthly
(routine)
Settlement-
deformation
Daily (suspected serious slip) to 3 to 6 monthly
(routine)

7. PDE201 – Dam Engineering I
Surveillance
 Dams all type require regular surveillance if they are to be maintained
in a safe and operationally efficient state
 Primary objectives of surveillance program is to minimize the
possibilities of catastrophic failure of the dam by the timely detection of
the design inadequacies or regressive changes in behavior,
 It also assists in the scheduling of routine maintenance or, when
necessary, of major remedial works.
 Surveillance embraces the regular and frequent observation and
recording of all aspects of the service performance of a dam and its
reservoir,
 It includes:
 Routine observation and inspection
 The monitoring and assessment of seepage & instrumentation data
 The recording of all other relevant information, including hydrological
records

8. PDE201 – Dam Engineering I
For the purpose of describing necessary surveillance activities, many of
which are interdependent, five critical phases in the life cycle of a dam must
be identified:
1. The design or pre-construction phase
2. The construction phase
3. The period of first reservoir filling – when the impact of of hydraulic
loading and reservoir-induced seepage effects can be initially observed &
assesses
4. The early operational period of the dam
5. The subsequent aging of the operating structure and its infrequent
exposure to extremes of hydraulic and external loading

9. Recommended location of embedded instruments in arch dams (elevation looking
upstream)

10. Recommended location of
embedded instrumentation
in arch dam (section at
crown cantilever)

11. Typical layout of plumblines
in arch dam

12. Layout for joint
meters

13. Type of Instrument
During
Construction1
During
Initial
Filling
First 2
years of
Operation
Next 5
years of
Operation
After 7
years of
Operation
Plumblines and
optical plummets
N/A weekly monthly quarterly semi-
annually
Inverted Plumblines weekly weekly monthly quarterly semi-
annually
Inclinometers weekly weekly monthly quarterly semi-
annually
Extensometers weekly weekly monthly quarterly semi-
annually
Joint meters weekly weekly biweekly quarterly semi-
annually
Triangulation N/A weekly monthly quarterly semi-
annually
Trilateration N/A weekly monthly quarterly semi-
annually
Strain meters weekly2
weekly monthly quarterly semi-
annually
"No-Stress" strain
meters
weekly2
weekly monthly quarterly semi-
annually
Stress meters weekly2
weekly monthly quarterly semi-
annually
Weirs, etc. N/A weekly weekly monthly quarterly
Open piezometers monthly monthly quarterly quarterly semi-
annually
Closed piezometers weekly3
weekly monthly quarterly semi-
annually
Uplift pressure
gauges weekly weekly biweekly monthly quarterly
Thermometers weekly4
weekly monthly monthly quarterly
Recommended
Readout
schedule

14. PDE201 – Dam Engineering I
Plumblines: Plumblines and optical plummets measure bending, tilting, and
deflections of concrete structures. Conventional plumblines are suspended from
the top of the structure and extend down to the lowest readout gallery.
Inclinometer: Inclinometers are used to measure angles from vertical. They can be
used both in the concrete mass or extended into the foundation. Extending the
inclinometer into the foundation can provide information on a potential sliding
plane being investigated.
Extensometers and Inclinometers: Extensometers and inclinometers should be
installed into the foundation as early in the construction as practical, preferably
before concrete placement, to determine deformations in the foundation due to
construction activities. The location of the extensometers should correspond to
the arch elevations for the other instrumentation groups. The total length of the
extensometer should be between 25 and 50 percent of the height of the
dam.
Joint meters: One or two joint meters are required in every other monolith joint at
the midheight elevation of alternate grout lifts.
Triangulation and trilateration targets: Targets should be placed at the crest and
at one or more points on the downstream face. The targets should correspond to
the location of the plumblines.
Temperature: Thermometers should be installed at locations to verify the thermal
gradient through the structure and to obtain the temperature history of the
concrete.

15. PDE201 – Dam Engineering I
Stress/strain Clusters: Clusters of strain meters, "no-stress" strain meters, and
stress meters should be positioned along four or five arch elevations that
correspond to arch elevations used in the design and analysis.
Seepage: Initially, seepage through the joints and through the drains can be
measured at two weirs, each located to collect seepage through the drains along
each abutment. Measurements of individual drains should also be made on a
regular basis. Additional seepage monitoring points can be added after the initial
reservoir filling, if the need arises.
Pressure: Three uplift pressure groups (standpipe or closed piezometer groups)
should be located in a similar manner as the plumblines. There should be at
least four uplift pressure measuring points through the thickness of the dam in
each group with a spacing between points of no more than 30 feet.