INSTRUMENTATION - THE KEYSTONE OF DAM MONITORING

P. Choquet, Ph.D., First Vice-President, Roctest Ltd., St-Lambert (Montreal), Canada K. Saleh, Ph.D., Head of Civil Engineering Research Group, IREQ, Hydro-Quebec, Varennes

(Montreal), Canada

ABSTRACT Dam monitoring covers all procedures and methods, of which instrumentation is only one aspect. However, instrumentation provides most of the quantitative data and therefore represents the keystone of monitoring. Identifying the different deformation modes involves measuring the geometrical evolution of the dam, starting with the most accessible parts, namely the crest and exposed portion of the downstream and upstream slopes. Where the size of a dam warrants such examination, internal deformation or displacement can be measured in zones judged sensitive. Fissuring zones can be due to the interface with abutments, to surrounding geometrical discontinuities or to the nature of the material in the foundations. They could also be caused by particular structural elements, such as an impervious membrane, toe tunnel or tunnel built under the central core, and interfaces with concrete structures such as spillways. Defining failure mechanisms requires measuring the factors determining mechanical load, starting with the height of the water in the reservoir and the pore pressure at different locations in the embankments and the foundation. The impossibility of measuring pressure at every point means limiting the number of measurements along certain profiles upstream and downstream from drains and natural or man-made cutoff structures. Generally, pressure should be measured at any point where a variation could indicate a potential disorder. The paper anylyses all the main types of dams which can be grouped in three categories: embankment dams (homogeneous, core dams and dams with impervious membranes), concrete dams (gravity, roller-compacted concrete, buttress, arch and multiple arch dams) and masonry dams. For each type of dam, design principles and modes of potential failure are detailed, leading to recommended monitoring systems. The paper concludes with the special case of dam foundation monitoring for both concrete and embankment dams and stresses the importance of automatic data acquisition and processing as a complement to modern dam instrumentation programs. 1. DAM MONITORING Dam monitoring covers all procedures and methods, of which instrumentation is only one aspect. However, instrumentation provides most of the quantitative data and therefore represents the keystone of monitoring. From the monitoring perspective, the life of a dam comprises three phases: construction, initial filling and operation. The objectives of monitoring as well as the load conditions vary

2

with each of these phases. Certain measurements and the corresponding instruments may therefore be designed to supply data only in the first two phases. The frequency of measurements also varies with these phases. Construction and filling are initial load phases during which safety is monitored by making sure that thresholds are not surpassed. During these phases, every step must be taken to control the speed with which stresses are applied, notably in the building of embankments and the raising of the water level. During operation, measurements may be taken at less frequent intervals. However, it is of the utmost importance to consider the number of measurements over several decades. Moreover, detailed observation of sensor response can provide valuable data on the evolution of the safety of a dam and the aging of the components and materials used in its construction. Identifying the different deformation modes involves measuring the geometrical evolution of the dam, starting with the most accessible parts, namely the crest and exposed portion of the downstream and upstream slopes. Where the size of a dam warrants such examination, internal deformation or displacement can be measured in zones judged sensitive. Fissuring zones can be due to the interface with abutments, to surrounding geometrical discontinuities or to the nature of the material in the foundations. They could also be caused by particular structural elements, such as an impervious membrane, toe tunnel or tunnel built under the central core, and interfaces with concrete structures such as spillways. Defining failure mechanisms requires measuring the factors determining mechanical load, starting with the height of the water in the reservoir and the pore pressure at different locations in the embankments and the foundation. The impossibility of measuring pressure at every point means limiting the number of measurements along certain profiles upstream and downstream from drains and natural or man-made cutoff structures. Generally, pressure should be measured at any point where a variation could indicate a potential disorder. The difficulty arising from the obligation not to omit any important location leads to a consideration of overall measurements that have an integrating function., Such measurements can alert the operator should any abnormal variations be detected in a reading. One example would be measurement of flows through a shell or cutoff structure. There are six phases in monitoring a dam:

i. overall design of monitoring ii. definition and practical organization of the system iii. specifications established by the chief engineer iv. quality assurance plan (QAP) set up by the company responsible for implementing the

system v. measurements during construction vi. long-term follow-up during operation.

2. INSTRUMENTATION

Instrumentation is an integral component of any dam, and its insertion in a structure can entail design modifications. Although the formulation of its goals and its definition are the responsibility of the project designers, other players are also involved in implementing dam monitoring systems: the equipment manufacturers, the various personnel in charge of installing the equipment and then taking the measurements, the construction contractors, the engineers, and the owners who will be using the results. Coordinating all these different players is no simple task. Procedures outlined in quality assurance plans (QAP) aim at specific objectives: an operational system, and reliable, stored and interpreted data. 2.1 Selection

3

Instrumentation permits a high accurate quantification of certain parameters regarding the behavior of a structure and allows their speed of evolution to be followed. It is possible to observe the stabilization of movements or, in the case of acceleration, to deduce the eventuality of failure. By comparing measured values with values calculated at the design stage, using a reference model, instrumentation enables the monitoring of dam's safety level and the timely implementation of corrective measures. Three major criteria should be considered in selecting instruments: • reliability of measurements obtained (absence of drift, resolution and accuracy) • longevity of the instruments, supported by numerous references • ease of automating readings essential for efficient data gathering and interpretation. The choice of components for the monitoring systems is of the utmost importance, and the following considerations must be kept in mind: - environmental conditions adverse to transportation and installation, which call for simple,

sturdy equipment well protected against risks of overvoltage caused by lightning; - a dam's life span of at least several decades; - replacing equipment sunk into the ground or buried in concrete. This is a last resort, since

it always involves adding new instruments down boreholes, which constitute a discontinuity in the site to be monitored;

- the difficulty of impossibility of resetting instruments, which means they must be built around sensors with virtually no zero drift;

- the great line distance between the sensor and the cable output point, which necessarily implies selecting means of measurements insensitive to line effects;

- developing automated reading procedures, which makes equipment using electric signals an attractive choice,.

It is important to distinguish between "hard-core" equipment, that is, equipment that provides information and cannot be replaced, and components such as cables in tunnels, junction boxes and data acquisition systems. These can be changed or upgraded once the dam is in operation. This publication deals only with static measurement instrumentation, which represents the majority and often the only type of measurements for a dam. Dynamic measurement instrumentation is generally restricted to the addition of seismographs at different levels on a dam, which trigger a series of static instrument measurements, above a specific threshold. Knowledge of a dam's behavior during an earthquake allows the reevaluation of its safety, for the maximum theoretical earthquake intensity selected during the dam's design phase. 2.2 Quality The importance of using sturdy, stable equipment, particularly that which is permanently installed within the body of a dam, cannot be overemphasized. This is especially critical for cells measuring pore pressure, a knowledge of which takes precedence over that of all other factors. The sensors and cables that transmit the information to the outside of the dam need excellent reliability. The amount invested in these is in line with the value of the expected data and the time that obtaining this information will require. The relative value of the individual sensors and cables has to be considered by comparing it to that of the peripheral components (exterior cables, in situ or remote data acquisition) and to the amortized cost of processing the data over several decades. This relative value is small, considering that the sensors and cables constitute the very heart of the systems. Their gross value should not be viewed separately. 2.3 General Layout

4

The general layout of the instruments is based on the vertical or horizontal sections which the designers consider to be of primary importance. The choice of instruments depends on the objectives assigned to monitoring. The first objective is to monitor safety by verifying the proper functioning of the different components. Next, it is a matter of verifying the hypotheses regarding the behavior of the dam, in order to improve future projects. Finally, the major difficulty is to prioritize the parameters considering the overall configuration of the dam, including the specific structural elements and the heterogeneity of its foundation. In dam monitoring, measurements are taken only on parameters deemed significant and at points judged critical. However, experience shows that, most of the time, difficulties spring up where they are not expected, and that it is the local heterogeneity that weakens a structure. If this heterogeneity is not detected or taken into account, then only continuous gathering and interpretation of monitoring data can forewarn the operators of any evolution that could indicate a disorder. Using very high resolution equipment or equipment that is highly sensitive to distant stress changes is one way to compensating for the lack of precision in enumerating the factors and locating zones of risk. 2.4 Cabling Appropriate cable layout is one of the essential conditions if a monitoring system is to last over the years. The layout not only has to take into consideration the geometry and the embankment zones from the perspective of the life span of the dam, but also the conditions in which it was built. The construction phase is one of the most critical phases in the life of any monitoring system. An earth dam is voluminous compared to a concrete dam. Unlike in a concrete dam, tunnels cannot be excavated through an earth dam. At best, a tunnel can be built in the foundation along the longitudinal axis in the case of core dams or along the perimeter of the base of the impervious membrane. The problem is how to minimize the length of the individual cables embedded in the embankment by quickly joining up tunnels and abutments on the banks or downstream slopes, while at the same time crossing the different zones and avoiding stresses due to differential settlement (Figure 1). Installing cable trenches in a few horizontal planes as possible makes it easier to carry out the necessary operations. These considerations are illustrated in Figure 2.

Figure 1: Cabling general layout

5

Figure 2: Cabling principles, zoned earth dam, detail

3. EMBANKMENT DAMS

The architecture and choice of components for monitoring earth and rockfill dams are based on the analysis of the structure's behavior carried out by the designers in developing a project. The designers consider certain modes of deformation and their amplitude, as well as certain failure mechanisms. In the latter case, they also assess the factors determining the mechanical load and potential changes in the materials. Boundaries are a key element in the monitoring process. By boundaries, we mean connections of any king - in the embankment, foundations, abutments, attached structures, and concrete dams that may be combined with embankment dams. The recommendations made in the chapters on foundations and other types of dams should be kept in mind. However, the following points apply specifically to embankment dams:

i. Retaining walls forming connections in earth and concrete dams Pore pressure cells must be used to ensure that this boundary does not constitute a preferential path for seepage flow.

ii. Tunnels under the embankment As in the case of retaining walls, the main problem of tunnels located under an embankment concerns seepage that must be monitored. Monitoring the tunnel itself comes under the heading of monitoring concrete structures and is not discussed in this chapter. Embankment dams use their own weight to resist the water pressure, and overall stability is thus constantly ensured. The stability of the upstream and downstream slopes must be considered separately, in relation to the construction and operation phases, which determine

6

the seepage flow through the embankment. This flow varies with the permeability zoning of the embankment. It is the zoning that distinguishes between the different types of embankment dams.

Figure 3: Manicouagan 3 dam – Overall instrumentation diagram 3.1 Homogeneous dams 3.1.1 Design Principles In this type of dam, the zoning consists of installing either a vertical or an inclined chimney drain. Seepage flow through the dam is controlled by the upstream section of the embankment. 3.1.2 Modes of Potential Failure Excluded a priori from this discussion are failures due to submersion, which is by far the most frequent cause of destruction (31% of principal causes). The notion of overall failure or failure by a dam sliding on its foundations also does not apply here. Potential failures develop mainly in the slopes, either during construction, when pore pressures develop as a result of introducing a load too quickly, or during operation, for example in the upstream slope following a drop in reservoir level, or in the downstream slope. Failures may also involve part of the foundations.

7

However, these modes of failure are less to be feared than failure due to internal erosion, which represents the major danger that generally threatens embankment dams. Manifestations of this type of phenomenon are difficult to define, and the resulting failure mechanisms are complex. Instrumentation of these structures may therefore focus not on the initial cause, but on one of its effects (Figure 4).

Figure 4: Embankment dam – Modes of potential failure

3.1.3 Monitoring Systems The indispensable role played by visual observation should once again be noted. The monitoring systems are designed to determine flow networks upstream from filters and drains, verify the efficiency of drainage systems (absence of downstream pressure), monitor pressure values in the foundations and follow the evolution of seepage flows through the embankment at critical points. Figure 5 illustrates these principles. The most important parameters measured are therefore pore pressure, seepage flow, upstream reservoir level and water levels in the abutments and downstream. The measurements obtained allow cross-checking of the design assumptions that helped determine the size of the dams. They are useful in monitoring safety, by ensuring that these measurements do not deviate, over time, from the values corresponding to admissible safety factors (Figure 5).

8

Figure 5: Homogeneous dam – Overall instrumentation diagram The following instruments are used: - reservoir level indicators showing the hydraulic load applied to the structure as a whole.

Depending on site conditions, the downstream level may also have to be measured. - pore pressure cells, (PWS, PWF and FPC-2 piezometers) in the body of the dam,

abutments and foundations. - open piezometers, Casagrande CP type, on the lower portion of abutments and

downstream from dams. - seepage flow meters, at different points, consisting of weirs equipped with level indicators

(NIVOFLO and ultrasonic systems) and TH thermometers. With the exception of open piezometers, all these instruments must allow automatic reading, although this certainly does not rule out selective direct readings. Open piezometers are increasingly automated, through the installation of electric cells in the access standpipes (PWS piezometers). Overall deformation may also be measured, as discussed in the following section on core dams. 3.2 Core dams 3.2.1 Design Principles Stability and impermeability are completely separate functions. Stability is maintained by shells of alluvium or rockfill. Impermeability is ensured by cores made up of a mainly clay-silt mix or with a very broad grain size distribution. Drains and filters installed between the cores, shoulders and foundations provide the necessary transitions in grain size distribution for drainage and for preventing internal erosion. Depending on the nature of the foundations (rock or alluvium), special measures may be taken to connect the core to the foundation with an impervious seal. 3.2.2 Modes of Potential Failure

9

The main risk threatening core dams throughout their life remains submersion. As in the previous case of homogeneous dams, the other failure mechanisms relate to the slopes along with the foundations. Internal erosion in the body of the dam or its foundation is the most prevalent cause of destruction, ahead of mechanical failure. Fissuring in the core may be behind the erosion, and can be avoided by installing elaborate filters. 3.2.3 Monitoring Systems The monitoring systems are designed to verify the efficiency of the cutoff structures by measuring pore pressure. External and internal displacement measurement, together with load measurement, allow the structure's response and, specifically, the risks of fissuring, to be evaluated. The overall evolution of flow through the dam is monitored by means of integrating flow measurements. Figure 6 illustrates these principles.

Figure 6: Core dam - Overall instrumentation diagram For pore pressure and seepage flow measurements, the same instruments used in homogeneous dams are employed. Displacement measurements, which are carried out in conjunction with load measurements, may take on particular importance,. These measurements are used in assessing the evolution of displacement and the corresponding risks of fissuring. The following instruments are used:

i. for surface displacement measurements (crest and slope)

- targets for direct topographic measurements. These same targets may be combined with measurement devices (bench marks) using long base extensometers (CONVEX), in particular for measuring crest deformation. This brings out any zones of tension or compression, as well as potential fissuring.

ii. for measurements of displacement within the dam:

- settlement standpipes using an R-4 torpedo, which give a discontinuous reading of

displacements along a vertical line.

10

- inclinometer tubings using an ACCUTILT RT-20 inclinometer probe, which give a

discontinuous reading of horizontal displacements along a vertical line (settlement inclinometer tubings providing measurements in x, y and z are also available).

- horizontal tubings or equivalent devices, which indicate the evolution of settlement

along an initially horizontal line (profile gages and settlement gages). - extensometric systems, which indicate displacements in x and y on bases (distance

between two measuring points ranging from a few meters to several dozen meters. They are laid out over distances that may cover the entire length of a dam along its axis or, in a transverse direction, the width of a load (ERI 200 embankment extensometers).

- SSG and R4 settlement systems consisting of spaced measuring points, as in

extensometric systems, and generally combined with the latter. Unlike the preceding systems, which involve manual operations that are therefore discontinuous in time, extensometric systems allow automatic data input with a measurement frequency varying according to the range of parameters that affect the measured value. Chains of fixed clinometers (LITTLE DIPPER) inserted in inclinometer tubings can also be read automatically. iii. for load measurements;

Load measurements refer to measurements made using cells that are often called total pressure cells. It is a fact that great difficulties are encountered in stress measurement. This type of measurement is always indirect, and consequently comprises many difficulties in interpretation. In embankment dams, TPC total pressure cells are used, in which the filling fluid pressure is preferably measured by electric sensors, which are easy to read. These jacks, laid out in groups, each have a different orientation. They thus allow an assessment of load variations rather than stress variations. They are useful near the boundaries of zones forming embankments, and provide data on the formation of discharging arches in the core and the associated risk of fissuring. 3.3 Dams with impervious membranes 3.3.1 Design Principles In impervious membrane dams, impermeability and stability are also separate. Stability is maintained by a shell of random fill or rockfill. Impermeability is provided by a thin membrane laid on the upstream slope. This membrane, made of asphalt, concrete or geotextile, drains inward and is connected by an impervious seal, natural or man-made, to the banks and foundations. 3.3.2 Modes of Potential Failure Except for failures due to submersion, failures of impervious membrane dams are the result of internal erosion, mainly in the foundations. They may also relate to the failure of the impervious membrane following differential settlement, which may stem from a variety of causes. 3.3.3 Monitoring Systems The monitoring systems are designed to verify such aspects as the behavior of impervious membranes by measuring displacement and deformation, which could lead to local failures that are the site of seepage through the membranes. Seepage flows must always be measured. Figure 7 illustrates these principles.

11

These systems are always combined with visual inspection, and are designed to: - monitor seepage that may originate in fissuring in the membrane or insufficient

impermeability of the foundation. - measure pressure in the foundations or in mechanically dangerous zones that may be

stressed by seepage. - verify the behavior of the impervious membrane by measuring displacement and

deformation which could lead to local failures that are the site of seepage through the membrane.

The most important measurements are those related to the first two objectives, which involve monitoring the impermeability of the membrane and the foundations, as well as seepage flow and pressure. Seepage flow meters are used, with an effort made to distinguish between seepage from the membrane, the perimetral joint and the foundations, through careful placement of the collector and measurement instruments. These instruments are described in Figure 7. Pore pressure measurement instruments and open piezometers (PWF and FPC-2) are also used.

Figure 7: Dam with impervious membrane – Overall instrumentation diagram

For the behavior of the impervious membrane, whether rigid or flexible, equipment for either selective or integrating measurements is available. The following are used:

12

- ICA-2000, ACCUTILT and R-4 settlement inclinometers - SSG settlement gages - joint meters (for rigid membranes), such as JM, RTF and GEO-D fissurometers - EM vibrating wire extensometers embedded in the concrete (for rigid membranes) - topographic bench marks on the crest and downstream slope of the membrane's

supporting embankment. The body of the embankment may be monitored as detailed in the section on core dams, although this occurs less often. 4. CONCRETE DAMS

Concrete dams may be classified in four main categories: gravity dams, buttress dams, arch dams and multiple arch dams. The concrete may be poured in place or roller compacted. These methods affect instrument selection, but do not alter the monitoring principles, which are based on the nature of the structures. 4.1 Gravity dams 4.1.1 Design Principles Gravity dams are more or less triangular in shape. They rely on their weight for stability. The resultant of this weight, together with the other external forces applied to the structures, must satisfy certain location criteria at the base of the dam. Figure 8 shows the various forces taken into account in analyzing the stability of a gravity dam.

Figure 8: Gravity dam – Applied forces

13

To ensure adequate distribution of the stresses placed by the dams on their foundations, the resultant of the various static and dynamic forces must pass through the center third of the base of the structures. When the resultant R lies between B and C, the entire foundation (AD) is in compression (Figure 9).

Figure 9: Gravity dam – Stable situation

When the resultant R lies between C and D, i.e. in the downstream third of the base, there is a loss of contact between the dam and the foundation of the upstream part, and an increase in stresses at the downstream toe of the structure (Figure 10).

Figure 10: Gravity dam – Unstable situation When the resultant R lies downstream from the toe of the dam, i.e. beyond point D, the structure tilts over. 4.1.2 Modes of Potential Failure Failures of gravity dams occur following two main modes: sliding and tilting. These phenomena may apply to the structure as a whole or, more often, to some blocks only. Understanding the modes of failure helps determine the monitoring necessary to prevent them. Sliding may take place along the surface of the concrete-rock interface (Figure 11) or within the foundation mass, along a surface of lower resistance, such as joints or layers of materials (Figure 12). These planes of potential sliding must be taken into account in designing the structures, and their presence calls for particular instrumentation.

14

Figure 11: Gravity dam – Failure by sliding

Figure 12: Gravity dam – Failure by sliding in foundations

Failures by sliding may occur when the design assumptions do not match certain aspects of the actual situation. The presence of zones of low shear resistance that were not detected during site investigation, hydrostatic force greater than the maximum foreseen due to exceptional floods, and insufficient spillway capacity may all cause this type of failure. Failures by sliding may also result from the inefficiency of impervious screens cutting off the permeable, erodible horizons, or inadequate drainage leading to an increase in uplift (Figure 13). Erosion in the foundation mass at the downstream toe, as a result of spillages or faulty spillway design, is also a cause of failure (Figure 14).

Figure 13: Gravity dam – Uplift

15

Figure 14: Gravity dam – Downstream erosion Failures by tilting may occur when the resultant of all the forces applied to the dams does not meet the criteria defined in the preceding section on design principles. This non-compliance means that the design assumptions in some ways do not correspond to the reality, e.g.: - hydrostatic force greater than the maximum foreseen - uplift greater than the assumptions, due to inefficient drainage or cutoff - uplift felt in fissures created in tension zones. High thermal gradients through the

structures may cause fissuring. - change in the profile of the dams' downstream foundations, due to erosion created by

faulty spillway design - inadequate concrete strength due to poor preparation or deterioration of the concrete,

causing failure by compression at the downstream toe in the high-stress zone. 4.1.3 Monitoring systems As is true for most civil engineering structures, monitoring of gravity dams comprises the following two components, in addition to visual inspection (Figure 15): - verification of design assumptions - study of the structures' overall behavior. A) Verification of design assumptions Design assumptions are verified by means of measurements, usually selective, of the parameters applied in calculating the stability of the structures. With reference to the preceding paragraphs, the following list of parameters to be measured can be established: Reservoir levels upstream and downstream from the dams Equipment used - pressure sensors with barometric correction, such as PWS piezometers - floats with encoding system - bubbler systems with surface pressure sensors. Uplift Uplift at the concrete-foundation interface and in permeable zones may form shear planes in the foundations. Instruments must be located so as to provide transverse profiles of uplift for comparison with the design profiles.

16

Figure 15: Gravity dam - Overall instrumentation diagram Uplift is measured using closed piezometers installed in boreholes or standpipes which open onto inspection tunnels equipped with pressure gages or sensors for automatic readings. Equipment used - PWS vibrating wire piezometers - pressure sensors installed in tunnels, similar to the preceding but with screwed coupling

and bleed valve. Concrete stress condition This measurement is designed to verify the distribution of stresses on the foundations and at certain levels in the dams. The downstream toe of the structures, where stresses are highest, must be monitored especially closely. This measurement must be combined with temperature measurement to interpret the results. Equipment used - TPC total pressure cells - SM and EM short base extensometers (surface and embedment type) - thermometers such as the PT-100 thermometer probe and THT thermistor temperature

probe. B) Study of the structures' overall behavior The modes of potential failure help determine the type and location of measurements designed to detect the beginning of any such phenomena. In addition to "geometric" measurements, complete monitoring must include the measurement of seepage flows through the construction joints and foundations. For either sliding or tilting failures, the

17

equipment used must allow the following to be measured with a high degree of resolution and accuracy: - horizontal displacement - vertical displacement - rotation - construction joints and fissures - seepage flows. Horizontal displacement Horizontal displacement is first measured using optical instruments and studs placed along the crest of the dams. This system has some disadvantages, however, such as the considerable time required to take the readings and the absence of automation. These manual measurements must therefore be complemented by automatable high-precision measurements (Figure 16).

Figure 16: Gravity dam – Rotation and displacement measurement Equipment used - direct and inverted pendulums - TELEPENDULUM inductive and RXTX optical reading stations - inclined borehole extensometers such as GEODIS, FIBERBLEX and SAM. Vertical displacement As in the case of horizontal displacement measurement, optical measurements must be complemented by measurements carried out using automatable fixed apparatus. Equipment used - borehole extensometers - NIVOMATIC series of leveling pots - TELEPENDULUM or RXTX reading stations with Z displacement gages. Rotation On the basis of the mass profile of gravity dams, the initial hypothesis may be formulated to the effect that each point in a block is subject to the same rotation. Equipment used - direct and inverted pendulums - TUFF TILT fixed clinometers. Joint measurement The concrete is poured in blocks to ensure that uncontrolled fissures are not created as it shrinks or subsequently during fluctuations in temperature. Gravity dams have joints with

18

sealing strips. The behavior of these joints is monitored by instruments placed inside the structures during construction, or on the surface, in the inspection tunnels. Equipment used - JM-E embedded fissurometers - JM-S and GEO-D surface fissurometers Interpreting the displacement measurements requires simultaneous measurement of reservoir levels and temperatures (water, air, internal). Seepage flows These measurements are essential for assessing the efficiency of the joints between sections and seepage through the foundations. They entail building small spillways that collect seepage from a clearly defined zone. Equipment used - ultrasonic level sensors - NIVOLIC inductive sensors with float. For result analysis, these measurements must be correlated with reservoir level and rainfall measurements. 4.2 Roller-compacted concrete (RCC) dams Roller-compacted concrete dams are gravity dams that differ from conventional concrete dams in the method of placing the concrete. Most of the volume of RCC structures is made up of roller-compacted layers, as in earth dikes (Figure 17). The composition of the concrete varies from one structure to the next, but presents a number of common features, including:

Figure 17: Roller-compacted concrete dam - low cement content with added fly ash - low moisture content - uniform grain size distribution with a wide variety of aggregate types. The RCC layers are approximately 30 cm thick and are bound together by a layer of mortar over all or part of their surface. To control fissuring due to shrinkage and fluctuations in temperature, vertical joints are made using different techniques, such as inserting vertical sheets of plastic, as the roller-compacted layers are built up.

19

The upstream and downstream faces that serve as forms are made of either precast concrete or concrete cast and vibrated with forms. Their generally lower cost, due to their great speed of execution, explains the fact that this type of dam is in ever growing use in many countries. RCC dams further offer the significant advantage of being able to stand up to spillages when steps are taken to prevent erosion at their downstream toe. Some structures are even designed to act as backup spillways in case of exceptional floods. 4.2.1 Design Principles The stability of these structures is provided by their weight, as a result of their trapezoidal or triangular shape with chimney. The same stability criteria related to the resultant of the applied forces, as described in the section on gravity dams, may be used for RCC dams. 4.2.2 Modes of Potential Failure Modes of overall failure for RCC dams are similar to those for conventional concrete gravity dams, namely: - failure by sliding - failure by tilting. In addition to these phenomena affecting the structure as a whole, local disorders may stem from the particular design of RCC dams. These specific problems are: - infiltration between two layers of roller-compacted concrete, which may lead to water

pressure behind the downstream, or even upstream, face in the event of rapid emptying - inefficient bond between the faces and the body of the dam - uncontrolled fissuring outside the vertical construction joints. The detection of excessive infiltration has led to grouting work in a number of existing RCC dams. 4.2.3 Monitoring Systems Monitoring of RCC dams is essentially the same as for concrete gravity dams (illustrated in Figure 15). Measurement of internal stresses are not of any real use, outside of specific research programs. On the other hand, incorporating instruments to measure fissuring in the structure and joint opening is of definite interest. This measurement is done using the following equipment: - EM short base extensometers - ERI long base extensometers - TH and PT-100 thermometers - JM-E embedded fissurometers - PWS and PWF vibrating wire piezometers. Figure 18 sums up the various measurements carried out for an RCC dam.

20

Figure 18: RCC dam - Overall instrumentation diagram 4.3 Buttress dams 4.3.1 Design Principles Compared with gravity dams, buttress dams have higher internal stresses and also place greater stress on the foundations. They call for more thorough monitoring than gravity dams, but a similar analysis of stability. Hollow gravity dams are a variant of buttress dam. This type of dam may be considered a "lighter" version of the gravity dams discussed in the preceding section. Instead of having a continuous virtually triangular section, buttress dams consist of an upstream slab supported on buttresses which are more or less triangular in shape and which bear the hydrostatic force of the reservoir. Each buttress thus forms a kind of gravity dam which carries an additional hydrostatic force transmitted by the adjacent slabs (Figure 19).

21

Figure 19: Buttress dam 4.3.2 Modes of Potential Failure Modes of failure in this type of dam are essentially the same as for gravity dams, namely sliding or tilting. The analysis of modes of failure presented in the section on gravity dams consequently applies to buttress dams. Another mode of failure specific to this type of structure is failure of the impervious slab. This failure may be due to an excessive internal stress level in the concrete or to unacceptable differences in horizontal and vertical deformation between two adjacent buttresses. 4.3.3 Monitoring Systems The instrumentation required to verify the design assumptions and study the structures' overall behavior is similar to that previously described. This instrumentation is naturally placed in the buttresses. The main differences concern the importance of the measurements of displacement of each buttress and measurements of internal stresses in the slab to ensure that none of these parts is in tension. Additional equipment, such as JM or GEO-D short base surface extensometers, may be installed on the downstream face of the slab. 4.4 Arch dams 4.4.1 Design Principles Arch dams have much thinner cross-sections than do gravity dams. They comprise a double radius of curvature, and are convex facing upstream so as to transmit the load of the hydrostatic force onto the abutments and foundations. Internal stresses in the concrete and exerted on the rock are much higher than for other types of concrete dams. Analyzing these stresses demands a thorough knowledge of the modulus of deformation of the surrounding rock. The stresses determined by analytic calculation must be less than pre-determined values, depending on the quality of concrete. No zone should be in tension (Figure 20).

22

Figure 20: Arch dam 4.4.2 Modes of Potential Failure The stability of arch dams is based mainly on the competence of the rock that forms the abutments. Failures in these structures are mostly caused by sliding in one part of the foundations along a plane of low shear resistance. This displacement may be initiated by the creation of strong uplift in the foundations at the upstream toe of the dams. Erosion at the downstream toe due to spillage or faulty spillway design may also lead to failure in arch dams. Downstream displacement of the toe of the dams gives rise to fissuring in a zone at the toe and downstream from structures in tension (Figures 21 and 22).

Figures 21 and 22: Arch dam – Mode of failure The other causes of failure in arch dams are mainly related to deterioration of the concrete during exceptional floods or spillages. Significant seasonal fluctuations in the thermal gradient between the upstream and downstream faces are also a cause of fissures in the arches, and may lead to their destruction. 4.4.3 Monitoring Systems The monitoring of arch dams, like that of other dams, comprises: - verification of design assumptions - study of the structures' overall behavior. Because of the much higher level of stresses, both in the dams themselves and in the abutments, the instrumentation of arch dams must be very carefully planned and denser than that of other types of concrete dams.

23

A) Verification of design assumptions In the analysis of arch dams, the stress condition and associated deformations at each point are determined on the basis of the geometry of the structures, the external forces applied, the mechanical properties of the concrete and the deformability of the abutments. The instrumentation must allow the water level, uplift and stress condition to be verified. B) Study of the structures' overall behavior The general principles described earlier for monitoring the other types of concrete dam apply here as well. Special attention must be paid to foundation deformations as discussed in Chapter VI. The basic measurements also cover: - horizontal displacement - vertical displacement - rotation - opening of construction joints and fissures - seepage flows. Equipment used The same equipment is used as mentioned in the preceding section on gravity dams. A high degree of resolution and accuracy in the integrating equipment used to measure deformations, such as direct and inverted pendulums, is of prime importance. 4.5 Multiple arch dams This type of dam is a combination of arch dam and buttress dam. There are very few of them around the world, and monitoring them will not be discussed in this book. Their instrumentation incorporates all the measurements previously described, with particular emphasis on the following measurements (Figure 23):

Figure 23: Multiple arch dam – Daniel-Johnson instrumentation

24

- rotational displacement of the buttresses by sliding and settlement, using direct and

inverted pendulums and borehole extensometers - distribution of stresses and temperatures in the different arches, using embedded short

base extensometers and thermometers - uplift beneath the buttresses, using piezometers. Understanding the behavior of these dams calls for a simultaneous analysis of all data and the establishment of correlations which cannot be made without automated data acquisition and processing and expert systems of analysis software. These analyses provide a good understanding, after several years of observation, of a structure's evolution for a given reservoir level and temperature. 5. MASONRY DAMS

Most masonry dams were erected before the intensive use of concrete became established in the building of large structures. They are nearly always of moderate height and require very good foundation conditions. These structures demand constant monitoring and, in comparison with concrete dams, show a greater number of cases of failure. Generally speaking, the monitoring of these structures was limited to optical level measurements; no instrumentation was installed during construction. Any instrumentation has been added, as needed, during maintenance. 5.1 Design principles Trapezoidal in shape, these dams rely for stability on their weight, as do concrete gravity dams. The interstices between the blocks forming the body of the structure are filled with mortar, which may represent 30% to 35% of the total volume. The faces that act as form work are made up of roughly textured ashlar with headers as anchors. In some dams, these faces are made of concrete. The rules concerning the resultant of the different forces exerted on the structures are the same as those defined in section 4 on concrete dams. 5.2 Modes of potential failure Because of their construction method, in ashlar masonry, masonry dams present the following characteristics which differentiate them from concrete gravity dams: - absence of construction joints like those found between sections in concrete dams - absence of inspection or drainage tunnels, with some exceptions - the possibility of water infiltration into the actual body of the structures, through defective

mortar - uplift behind the upstream and downstream faces. The first two modes of failure, namely sliding and tilting, are the same as for concrete gravity dams. The comments made in Chapter IV on these modes of failure apply to masonry dams, as well. In analyzing these modes of failure, the effect of the potential presence of water inside the structures must be taken into account. Such infiltration, which may stem from the foundations or from fissures through the upstream face of the dams, must be considered in the analysis of the structures' stability. Measuring uplift with piezometers selectively located along the rock-masonry interface may not suffice for determining the structures' hydraulic condition. Due to internal infiltration, the following types of failure must be added to the overall modes of failure already described (Figure 24):

25

Figure 24: Masonry dam

- separation of faces, particularly during rapid emptying - local failures as a result of disintegration of mortar. 5.3 Monitoring systems Monitoring of masonry dams is essentially the same as for concrete gravity dams. It therefore comprises: - verification of design assumptions - study of the structures' overall behavior, in relation to modes of potential failure. The differences between concrete and masonry dams call for a different monitoring approach with respect to the following points, however: - significant internal stresses cannot be measured in masonry dams - integrating measurements of displacement are preferred in masonry dams, because of

the specific behavior of the blocks - detailed observation of the downstream face is needed, allowing local infiltration to be

detected - also required is detailed observation, through the eventual installation of fissurometers, of

the state of fissuring in the upstream face when the reservoir level is lowered. 6. FOUNDATIONS

This chapter discusses the main steps involved in monitoring the foundations of concrete, masonry and embankment dams. These foundations may be of alluvium or rock. The designers must exercise the utmost caution in planning foundation monitoring systems, more so than for the bodies of the dams themselves. Few general rules exist in this regard, since each dam is an individual case. The nature and geological structure of the site are what determine the monitoring systems' architecture. This issue is sufficiently important to warrant repetition. That is why foundation monitoring has already been touched upon in the preceding chapters on concrete, masonry and embankment dams. Experience to date indicates that the following points must be monitored, particularly in foundations and abutments: - concrete and masonry dams: shear resistance and risk of internal erosion - embankment dams: risk of internal erosion. We will first look at concrete and masonry dams, and then embankment dams. We will attempt to provide some general rules, while also emphasizing the need for adaptation,

26

particularly in the area of research, and the consideration of local heterogeneities, which are always difficult to foresee or imagine. 6.1 Concrete and masonry dams To monitor shear resistance, movements at the base of the structures must be followed. Long-term displacement measurement may, in fact, provide data on slow drifts, indicative of the existence of pathologies relating to decreased shear resistance. Inverted pendulums and borehole extensometers may be used for this purpose. These instruments are installed in the lower portion of the foundations, as well as abutments and banks. Boundaries of all kinds constitute particularly dangerous zones and in themselves represent heterogeneities. The location and number of measurement points, which may be increased during the dams' life, depend principally on the results of the geological and geotechnical site analyses and of the transmission of stresses by the structure. The risk of internal erosion is always difficult to assess. These risks are monitored by an indirect procedure, through measurements of pressures upstream and downstream from grout curtains or natural cutoffs, flows in drains, and water levels in open piezometers immediately below the dams and in the banks. Figures 25 and 26 elaborate on this broad outline. They also underscore the great freedom which designers have in terms of the number and location of sensors.

Figure 25: Gravity dam – Foundation monitoring

27

Figure 26: Concrete dam – Foundation monitoring The equipment used is basically the same as that described in the chapters on the bodies of the dams, and comprises: - pore pressure cells (PWS, PWF and FPC-2 vibrating wire piezometers) - seepage flow meters - inverted pendulums, in an adapted foundation version of the direct pendulums used in the

body of the dams - GEODIS and SAM borehole extensometers - NIVOMATIC series of leveling pots

28

Specific equipment for rock foundations includes long base borehole extensometers, designed to detect movements of small magnitude. Mobile extensometers differ from fixed ones, in that the latter are more precise and facilitate data acquisition when they are equipped with electric sensors. This equipment must have a high degree of resolution and great stability since, in order to obtain useful data, reliable measurements must be taken well before any deformation or displacement reaches a critical level. In addition, regular data acquisition is the natural complement of high-resolution equipment. The curve which it yields constitutes a signal which may prove very productive. 6.2 Embankment dams Here, whether the foundations lie in upper layers of alluvium or in rock, the basic risk is related to internal erosion. As in the case of concrete and masonry dams, the method is designed to ensure that the flow through the foundations does not present any anomalies. As always, the difficulty consists in not leaving out any hypotheses and having a fairly tight network of instruments available. Measurements of pressure, using cells, of water levels in open standpipes, and of seepage flows may be helpful in monitoring this type of dam. Figures 27 to 31 illustrate different examples of foundations. They outline the application of these various principles, without entering into great detail, however, unlike any list of actual cases, which is always of limited exemplary value.

Figure 27: Homogeneous embankment dam – Foundation monitoring

Figure 28: Core dam with perimetral tunnel – Positioning of pore pressure sensors

29

Figure 29: Core dam with foundation tunnel - Positioning of pore pressure sensors

Figure 30: Impervious membrane dam with toe tunnel – Positioning of pore pressure sensors

Figure 31: Impervious membrane dam with cutoff wall – Positioning of pore pressure sensors

30

7. CONCLUSION This article sets out to present the main points of instrumentation for the principal types of dam, in static mode. The detailed data sheets and instruction manuals for the instruments must be read for a more thorough understanding of the uses of the equipment and its installation methods, in order to plan adequate monitoring systems. Since each dam and its foundations represent an individual case, it is essential that a dialogue be established between the designers of the structures and the instrumentation specialists, before a site’s instrumentation is determined. As a result of the considerable research and development effort undertaken by Roctest over the years, the equipment involved undergoes constant study designed to improve its performance. These improvements are especially notable not only in the area of improved instruments but also in automatic data acquisition and related software. Current trends in the field of instrumentation emphasise the search for higher resolution and precision, providing more accurate measurements and permitting rapid detection of any behavioural anomalies. Increasing the life of these instruments through strict quality control is a constant concern, moreover, with special attention paid to protection against overvoltages due to lightning. Finally, virtually no instrumentation is considered complete without automatic data acquisition and processing, which increases the reliability of the results, yields more detailed analyses, with alarm thresholds, and unquestionably enhances dam safety. A new range of fully optical sensors, the fruit of a major research program in response to current demand, has just been brought out. Its initial applications in the field of dams should soon be made, thanks to the considerable technological benefits offered by the fiber optic sensors. In their desire to maintain their leadership in the monitoring of large-scale structures, Roctest is devoting substantial means to developing its new SENSOPTIC fiber optic product line. In closing, it should be recalled that dam monitoring is a key component of dam safety. Because the failure of a dam can lead to human as well as economic disaster, no compromise in regards to instrumentation quality or reliability should be made.