DS13-11

DESIGN STANDARDS

EMBANKMENT NO. 13DAMS

CHAPTER 11 INSTRUMENTATION

UNITED STATESDEPARTMENT OF THE INTERIOR

BUREAU OF RECLAMATIONTECHNICAL SERVICE CENTER

DENVER, COLORADO

UNITED STATES DEPARTMENT OF THE INTERIOR

Bureau of Reclamation Denver Office

Denver, Colorado 80225-0007

TRANSMITTAL OF DESIGN STANDARDS

Standards Number and Title:

Change No. 13(11)-8

Design Standards No. 13 - EMBANKMENT DAMS Chapter 11 - Instrumentation

Insert Sheets: Remove Sheets:

Chapter 11 (26 sheets) Chapter 11 (DRAFT)

Summary of Chancres:

Incorporation of comments and corrections received during the trial period for the draft of this chapter.

Approved:

Assistant Commissioner Engineering and Research

Julv ~1, 1990 Date

(To be filled in by employee uho files this change in the appropriate standard.)

The above change has been made in the Design Standards

Signature Date

DESIGN STANDARDS NO. 13

EMBANKMENT DAMS

CHAPTER 11 - INSTRUMENTATION

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF RECLAMATIONASSISTANT COMMISSIONER

ENGINEERING AND RESEARCHDENVER, COLORADO

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EMBANKMENT DAMS Chapter 11 - Instrumentation

CONTENTS

Paragraph Page

INTRODUCTION

11.1 Purpose ........................................... 111.2 Scope ............................................. 2

A. Analytical Reasons ......................... 2B. Predictive Reasons ......................... 4C. Legal Reasons ............................... 4D. Research Reasons ............................ 4

11.3 Deviations from Standard .......................... 611.4 Revisions of Standard ............................. 711.5 Minimum Instrumentation ........................... 711.6 Instrumentation Selection and Layout .............. 811.7 Safety Evaluation of Existing Dams ................ 10

PRESSURE MEASURING DEVICES

11.9 Verifiable Engineering Concepts ................... 10A. Ground-water Levels and Pore

Pressure Observations ..................... 10B. Partially Saturated Soils ................... 12C. Pore Water Pressures Associated

With Dams .................................. 13D. Time Lag in Ground-water Observations ....... 13E. Total Earth Pressures ....................... 14F. Earth Pressure Against Rigid Boundary ....... 16

11.10 Types ............................................. 18

SEEPAGE MEASUREMENT DEVICES

11.11 Verifiable Engineering Concepts ................... 19A. General ..................................... 19B. Failure Modes and Criteria .................. 19

11.12 Types ............................................. 22

MOVEMENT

11.13 General .......................................... 24

Chapter 11 - Instrumentation EMBANKMENT DAMS

DS-13(11)-8 - 7/1/90 vi

CONTENTS - Continued

INTERNAL MOVEMENT MEASURING DEVICES

11.14 Verifiable Engineering Concepts ................... 25A. Vertical Movement ........................... 25B. Horizontal Movement ......................... 26

11.15 Types ............................................. 26

SURFACE MOVEMENT MEASURING DEVICES

11.16 Verifiable Engineering Concepts ................... 2711.17 Types ............................................. 27

VIBRATION MEASURING DEVICES

11.18 Verifiable Engineering Concepts ................... 2811.19 Types ............................................. 29

VISUAL INSPECTIONS

11.20 General .......................................... 29

DATA ACQUISITION, PROCESSING, AND REVIEW PROCEDURE

11.21 General ........................................... 3111.22 Data Acquisition .................................. 3111.23 Data Review ....................................... 3211.24 Possible Actions .................................. 34

APPENDIX

ReferencesTablesExample


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11.1INTRODUCTION

PURPOSE

.1 The purpose of this design standard is to offer a practicaldesign guide which emphasizes the interrelationship betweenfundamental engineering concepts that are applied inembankment dam analyses and appropriate instrumentation fornew and existing structures. It is stressed thatinstrumentation cannot be properly designed withoutunderstanding the concepts, failure modes, and assumptionsused in the analysis. Instrumentation should be adequate tomonitor the expected and unexpected performance of thestructure. This design standard provides a general overviewof a number of instrumentation-related topics, and includessome guidance concerning appropriate instrumentation types tobe considered in various monitoring situations. More detailedinformation can be found in Reclamation's Embankment DamInstrumentation Manual [7] and other standard references.

When a structure, such as an embankment dam, is disturbed,either by natural events or by the activities of man, itundergoes a redistribution of stress, often accompanied by achange of shape. These changes can be minor, or mayconstitute cost factors or hazards which have to be promptlyrecognized and dealt with. They are commonly reflected indeformations, displacements, loads, pressures, stresses, andstrains which can be identified and measured byinstrumentation.

Instrumentation is used to gather scientific data of aphysical nature in order for decisions to be made with regardto the design, construction, first filling of the reservoir,and long-term performance and safety of the dam. Ideally, aninstrumentation system should provide surveillance of anentire mass or structure so that important decisions are not


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11.2made on the basis of insufficient data. The type ofinstrument selected is based on the engineering concept to beverified. The number and distribution of instruments areusually determined by the need to provide an acceptable sampledistribution at a reasonable cost.

SCOPE

.2 The usual factors or quantities which need to be monitored inembankment dams are structural displacements, deformations,settlement, seepage, piezometric levels, and interstitial(pore) pressures within the structure and its foundation. Thereasons for using instrumentation can be grouped into fourgeneral categories: analytical, predictive, legal, andresearch.

Analytical

A. Analytical Reasons. -

1. Verification of engineering concepts. - In order todesign a safe embankment dam, design engineers mustdevelop conceptual ideas on existing conditions andhow these conditions may change as a consequence ofthe design. The margin of safety incorporated intothe design is usually directly related to thesurety of the engineering concepts. It is thereforeboth economical and prudent to verify theseengineering concepts throughout the life of theproject. Instrumentation can be installed:

a. During the exploration phases of the projectto determine existing conditions

b. During design to observe any changes in theseconditions with time and under no load,


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11.2A

c. During construction to monitor the influenceof construction on the preexisting conditions,and

d. During the subsequent life of the project togather information related to specificproblems and to confirm continued satisfactoryperformance of the project according to thedesign intentions.

2. Verification of suitability of new constructiontechniques. - Most new or modified constructiontechniques are not well accepted by the engineeringor construction professions until provensatisfactory on the basis of actual performance.Data obtained from instrumentation can aid inevaluating the suitability of new or modifiedtechniques. Instrumentation is often installedjust prior to or after remedial work at a site inorder to determine the effectiveness of theimprovements.

3. Diagnosing the specific nature of an adverse event.- When a failure, partial failure, or severedistress has occurred at a damsite, data from theinstrumentation system can be extremely valuable inthe determination of the specific nature of theevent.

4. Verification of continued satisfactory performance.- An instrumentation system that consistentlyyields data which indicate that the dam isperforming in a satisfactory manner may, on firstthought, appear to be unnecessary. However, suchinformation can prove to be valuable should some


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11.2Bperformance is, in itself, valuable to consider infuture design efforts.

Predictive

B. Predictive Reasons. - It is important to be able to useinstrumentation data as they accumulate so thatinformed, valid predictions of future behavior of thedam can be made. Such predictions may vary fromsatisfactory performance to an indication of severefuture distress which may become threatening to life orsafety and necessitate remedial action.

Legal

C. Legal Reasons. - Valid instrumentation data can bevaluable for several reasons ranging from simpledetermination of actual fill placement quantities forconstruction pay estimates to the establishment of aninformation data bank for later possible use inlitigation. Damage claims arising from dam constructionor from adverse events can reach many millions ofdollars. The presence of a dam, reservoir or otherhydraulic structure may cause seepage or ground-waterchanges that could lead to damage of adjacentproperty. Instrumentation data can be an aid in thedetermination of causes of adverse events so that properlegal adjudication can be accomplished.

Research

D. Research Reasons. - In order to better understand thecomplex nature of the multitude of forces acting in ausually interdependent manner on a dam, it is verydesirable to study the performance of existing dams andthe instrumentation data generated thereon, which shouldprovide quantifiable information for use on futuredesigns. Such research has led to advances in


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construction techniques, improved and innovative designconcepts, and a better understanding of failuremechanisms, i.e., advancement of the state-of-the-art.

We are normally interested in measuring variations inthree principal properties during the life of an earthdam. These properties are the resistance to watermovement, resistance to deformation, and strengthcharacteristics. Unfortunately, neither the physicalproperties of the structure nor the external naturalforces acting on a structure vary independently of eachother, making clear cause-and-effect relationshipsdifficult to identify. Such relationships becomeapparent only as the mass of data secured bymeasurements becomes rather large.

Damage to an embankment is commonly attributed to one ormore of the following:

1. Internal erosion caused by excessive seepagethrough the embankment or its foundation,

2. Slope instability,

3. Loss of integrity of the core due to differentialsettlement-induced cracking,

4. Deterioration and clogging of drainage systems,

5. Longitudinal cracking due to differentialsettlement along the interface between embankmentzones,

6. Overtopping of the embankment due to unusually highreservoir inflow and inadequate spillway capacity,

7. Settlement or instability caused by earthquakeloading.


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11.3Instrumentation can become expensive to install andobtaining measurements can be very time consuming. Itis therefore necessary to limit the amount and kind ofinstrumentation used and the frequency with whichmeasurements are obtained. No simple rules exist todetermine the quantity or exact type of instrumentationneeded at a site. The determination of the number,type, and location of instruments at a dam remains amatter of experienced judgment.

There are many reasons for installing instrumentation inboth new and existing dams. The question of number,type, and location of instruments at a dam can only beaddressed effectively by the combination of experience,common sense, and judgement. Most dams represent uniquesituations and require unique solutions to theirinstrumentation requirements. The instrumentationsystem design, therefore, needs to be conceived with agreat deal of care considering the site-specificgeotechnical conditions present at the dam and theengineering concepts used in the analysis and design.In general, it has been found that an adequate, butcost-effective, instrumentation installation at a newdam will constitute approximately 1 percent of the totalconstruction cost of the dam. At more difficult damssites, where there are unusual aspects to the design orwhere there are unusual circumstances, this cost can beas high as 2 to 3 percent of the construction cost.

DEVIATIONS FROM STANDARD

.3 Instrumentation design within the Geotechnical Engineering andGeology Division should be guided by this standard; as well asthe Embankment Dam Instrumentation Manual [7]. Designs not inaccordance with this standard and/or the instrumentationmanual should be documented such that any deviations are fullyapparent during the review process.


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11.4

REVISIONS OF STANDARD

.4 This standard will be revised periodically as its use and thestate-of-the-art indicates the need. Comments or suggestedrevisions should be forwarded to the Chief, GeotechnicalEngineering and Geology Division (D-3600).

MINIMUM INSTRUMENTATION

.5 The problem of determining the minimum instrumentation for thesafety of dams can be divided into two categories, existingdams and new dams. Each category has problems not common tothe other. Some of the important differences are:- New dams will likely have additional monitoring

requirements to demonstrate safety during construction,first filling, and early age.

- Additional options for installing instruments areavailable during construction that cannot be consideredfor existing structures.

- Specific problems such as instability, seepage,settlement, or desiccation that were unanticipatedduring design may have developed that requiremonitoring.

- Many old dams were built using design, construction, andperformance standards significantly different from thoseexisting under the present state-of-the-art.

The minimum instrumentation for an embankment dam shouldideally be sufficient to detect, in a timely fashion, anyunusual performance that might eventually lead to dam failureor serious performance problems. Though this ideal is


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11.6

unattainable, studies of incidents and failures of embankmentdams indicate that seepage monitoring, embankment measurementpoints, and structural measurement points should be routinelyprovided at all dams to efficiently and cost effectivelymonitor for the development of unexpected problems.Piezometers have high informational value as well, but theappropriateness of such installations needs to be consideredon a case-by-case basis, considering their significant expenseand the risks associated with drilling in existing dams. Theyshould, however, be included in all new dams during theirconstruction. These minimum instrumentation guidelines are inaddition to the instrumentation necessary to monitor the site-specific concerns unique to each dam.

New dams should be designed to contain highly reliable andsufficiently accurate instruments in the proper quantity andlocated at optimum positions to determine dam behavior andmonitor important parameters over a long period of time.Existing dams should be retrofitted with instrumentation asdictated by their needs on a site-specific basis. Thedesigner should not hesitate to consider additionalinstrumentation for an existing dam if any aspect of itsperformance is uncertain.

INSTRUMENT SELECTION AND LAYOUT

.6 Each damsite is unique and the designer must evaluate anddecide which types of measurements are necessary for theparticular project. Also, it must be decided what type ofinstrument, which manufacturer, and what installation detailsshould be used. These decisions must consider factors such asrisk taken if information is not obtained, costs, neededaccuracy, expected reading schedules, environmental


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11.6

conditions, use of the data, and capabilities of availablepersonnel. Cost should include not only the cost of theinstruments, but installation costs and the cost of carryingout the required observations, data processing, and review.Accuracy and sensitivity requirements for instrumentationsystems are dictated by geotechnical and/or structuralcriteria relevant to the project that is to be instrumented.It is useful to make a rough estimate of the magnitudes of thevarious parameters to be monitored so that the instrumentswill have the proper measuring range and sensitivity.

The selection of instruments must also be based on importantdetails such as robustness, watertightness, corrosionresistance, long-term stability, and preferred type of outputsignal. During the design of systems, key factors such asserviceability, remote reading capability, constructioncompatibility, cable routing, lightning protection, andmechanical protection must be carefully considered.

In order to understand general patterns of embankmentperformance, instrumentation is commonly laid out with respectto cross sections of the dam, the plan view of the dam, and avertical section through the dam, following or paralleling itsaxis. Cross sections of the dam generally can be assumed tobe (nearly) in a state of plain strain, and the results oftwo-dimensional deformation analyses can be compared to actualdata obtained at a cross section. Two-dimensional seepageanalyses can also be compared with pressure contours frompiezometers grouped at a section. Contours of surfacedeformations can be effectively obtained and analyzed withrespect to a plan view orientation. Profiles along the axisof the dam (or paralleling the axis) sometimes are usefulperspective from which to monitor foundation settlements.


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11.7

The extent of the instrumentation program that is appropriatefor a given new dam depends to a limited degree on the heightand length of the dam, but depends more importantly on thesite conditions and the design issues and concerns regardingthe dam.

SAFETY EVALUATION OF EXISTING DAMS

.7 Reclamation's dam safety work consists of two fundamentalcomponents: (1) identifying and addressing existingdeficiencies of dams, and (2) maintaining an on-goingmonitoring program to detect the development of new problemsat dams. Instrumentation plays a key role regarding bothcomponents. Instrumentation is used to help identify anddefine existing problems, and along with dam safetyinspections, is used to monitor for the development of newsituations. Instrumentation is a major tool in Reclamation'songoing commitment to dam safety.

PRESSURE MEASURING DEVICES

VERIFIABLE ENGINEERING CONCEPTS

Ground-water Level

.9 A. Ground-water Levels and Pore Pressure Observations. -

1. Regular conditions. - The free ground-water level,or table, is defined as the elevation that the freewater surface assumes in a hole extending a shortdistance below the capillary zone, where the watersurface is at equilibrium with atmosphericpressure. Ground-water conditions are described


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11.9A

as "regular" when the pore water pressure increaseshydrostatically with depth below the ground-waterlevel [2].

2. Irregular conditions. - When the pore waterpressure does not increase hydrostatically withdepth below the groundwater level, ground-waterconditions are described as "irregular." Theseconditions may result from perched water tablescaused by relatively impermeable strata above themain ground-water level. The presence of morepervious and better drained strata below theground-water level may cause irregular ground-waterconditions [2].

3. Variation in ground-water levels and pressures. -Ground-water levels and pressures are rarelyconstant over an extended period of time. Naturalforces such as seepage, precipitation, andevaporation may cause wide variations in theground-water level. The pore water pressure isconsidered to be under positive excess pressurewhen the pore water pressure at a point is morethan hydrostatic and to be under subhydrostaticpressure when the pore water pressure is less thanhydrostatic [2].

a. Artesian pressure. - Artesian pressures arefound in strata that are confined betweenimpervious strata and are connected to a watersource at a higher elevation. A well drilledto a stratum having a pore water pressureabove the ground surface will flow withoutpumping and is called a free-flowing artesianwell [2].


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11.9Bb. Induced pore water pressure. - Pore water

pressure may be temporarily changed from ahydrostatic condition as a result of stresschanges. Stress changes may be caused by suchactivities as construction loading orunloading, induced vibrations, or naturalforces such as erosion, deposition, and earthtremors [2].

Soil Saturation

B. Partially Saturated Soils. -

1. Below ground-water level. - Soils below theground-water level are generally considered to besaturated. However, ground water is generally inequilibrium with the atmosphere and contains dissolvedgases absorbed from the atmosphere. Ground water mayalso contain dissolved and free gas resulting fromdecomposition of organic matter. When dissolved gasesare partially released by a reduction in pore waterpressure or by an increase in temperature, the gas comesout of the solution and forms bubbles which may escapeinto overlying strata of partially saturated soils orbecome trapped below the ground-water level to produce athree-component system of soil solids, water, and gas[2].

2. Above ground-water level. - Soils above theground-water level contain both gas and water in thepore spaces and are, therefore, three-component systems.When the ground-water level rises or the soil is wettedor inundated, some gas is partially dissolved in thepore water because of the increase in pressure, only tobe released later when the pressure is reduced.


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11.9C

Gas may also be expelled, trapped under pressure, and/orabsorbed in the pore water when stresses on a partiallysaturated soil mass are increased [2].

Pore Pressures

C. Pore Water Pressures Associated With Dams. - Duringconstruction of earth dams, relatively high excess porewater pressures may develop in impervious orsemipervious zones and foundations as the height of anembankment is increased. Under some circumstances, thismay require stability berms, flatter outer slopes, orcontrol of the rates of construction. Cutoff walls anddrainage facilities are often provided in foundationsand embankments to control and/or to reduce excess porewater pressures that develop when water is impounded.Periodic measurement of pore water pressures associatedwith such control measures and drainage facilities isnecessary to assess their effectiveness and properfunctioning throughout the life of the dam [2].

Time Lag

D. Time Lag in Ground-water Observations. - When pore waterpressures change, the time required for water to flow toor from the piezometer to effect equalization is calledthe hydrodynamic time lag. The hydrodynamic static timelag is dependent primarily on the permeability of thesoil type and dimensions of the device, and change inpore water pressures. The volume of flow required forpressure equalization in sensitive piezometers, such asthose of the diaphragm type, is extremely small, and the


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11.9E

hydrodynamic time lag is very short. In less sensitivepiezometers such as open-standpipe piezometers, the timelag may be reduced somewhat by providing a large intakearea and increasing the diameter of the drill hole theinstrument is installed in. Hydrodynamic time lag isusually not significant when piezometric devices areinstalled in materials other than impervious zone 1 corematerial.

Disturbance of the soil structure may occur wheninstalling a piezometric device. Changes in void ratioand water content of the in situ soil usually occur inadvancing a borehole or driving a well point. Fillingwith water and flushing air from hydraulic piezometerscause changes in the pore water pressure in the soilmass surrounding the piezometer tip, and flow of waterto or from the affected soil mass must take place toreestablish equilibrium conditions.

Earth Pressures

E. Total Earth Pressures. -

1. General. - This section is concerned with themeasurement of total earth pressures. Two generaltypes of measurement arise in foundationengineering: stresses in a soil mass and stresseson a boundary at a soil-structure orsoil-foundation interface. For convenience ofmeasurement, the stress vector at a "point" on aboundary or in a soil mass is resolved into normaland shear average components.


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11.9E

Many problems arise when making earth pressuremeasurements. The main problem is to ensure thatthe earth pressure measuring device (theearth-pressure cell) measures the average stressacting. If the design is faulty, an erroneous,misleading reading results. When an earth pressurecell is introduced into a mass of soil, the stressfield in the vicinity of the cell is modifiedbecause of strain incompatibility and specialcompaction of material against the instrumentresulting in variation in soil densities. It isthis modified stress field which is recorded by thecell. The basic problem in earth pressuremonitoring is to obtain a measure of the stresswhich would have existed in the ground had thepressure cell not been inserted. In mostinstances, this cannot be successfully achieved.Data will be obtained, but will not berepresentative of actual stresses.

2. Measurement of dynamic pressures. - The civilengineer on occasions has to measure dynamicpressures on structures or in a soil mass.Comprehensive information on the problemsassociated with the measurement of earth pressuredue to dynamic loading is given in the Proceedingsof the International Symposium on Wave Propagationand Dynamic Properties of Earth Materials (1967).

3. Measurement of normal and shear stresses. - Fordirections other than those of the principal stressdirections. Shear stresses occur in addition tonormal stresses and these can be of particularsignificance in embankments and on soil/structureinterfaces.


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11.9F

4. Principal stresses. - In many foundation problemsthe size of the cell is small compared with themass of soil under study and hence the cell isassumed to measure that stress at a "point." Ko isthe coefficient of earth pressure at rest and isfound from Poisson's ratio. The value of Ko forsaturated clays in undrained loading or quickloading is sometimes also expressed in totalstresses that include the neutral stress. Inembankment dam studies, it is possible, due to thetwo-dimensional nature of the problem, to arrangethe earth pressure cells in one plane with one ormore cells at right angles to this plane. Byarrangement of four cells to form an equivalent 45rosette, the stress state in the soil can bemeasured. However, considering the difficulty ingetting one cell to work accurately in a soil mass,it may be practically impossible to get four towork. When cells are oriented in directions whichare not principal directions the cell action factormay depend on stress direction. Care andexperience are required in the use of earthpressure cells for the determination of stressstate in a soil mass [3].

Rigid Boundary Earth Pressure

F. Earth Pressure Against Rigid Boundary. -

1. General. - In the design of an appurtenantstructure (e.g., spillway or outlet works) for anearthfill or rockfill dam, assumptions are made


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11.9F

concerning the magnitude and distribution ofpressures at the soil-structure interface. Forcomplex or unique structures, it may be desirableto measure the earth pressure to determine thevalidity of the design assumption and to developdata for prototype analysis [4].

2. Earth pressure on a retaining wall. - The pressurecells should be installed at a typical section ofthe wall. The number of cells required will dependon the height of the backfill. However, asufficient number should be installed so that thedistribution of pressure along the entire height ofthe wall may be determined even if a few cellsbecome inoperable. The cells should not be locatedwithin 2 feet (600 mm) of a construction joint. Ifthe wall section is not constant, it may bedesirable to instrument more than one section [4].

3. Pressures acting on and around a conduit. -Ordinarily, pressures around a conduit are notmeasured. However, if these pressures are to bemeasured, the segment of a conduit that would beinstrumented is that section beneath the maximumheight of fill. In order to determine thedistribution of pressure along the length of theconduit, pressures at other sections may also bemeasured. Installation may also consist ofpressure cells installed around the entire outerperimeter of the conduit [4].

In some instances, it may be desirable for researchpurposes to obtain data concerning earth pressuresacting on a conduit. Generally, lateral earthpressures and arching effects are being studied in


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an effort to reduce overdesign in future work dueto excessively conservative pressure assumptions.It may be desirable to monitor water pressures inthese areas as well, so as to determine effectiveas well as total pressures.

TYPES

.10 A multitude of styles and types of pressure measuring devicesare available. Table 1 lists some of the advantages anddisadvantages of various devices.

The closed-system devices used to measure pore water pressuresinclude HPI (hydrostatic pressure indicators), hydraulic TTP(twin-tube piezometers), pneumatic piezometers, VWP(vibrating-wire piezometers), and other electrically operatingpiezometers while the open-system devices include PTP(porous-tube piezometers), SPP (slotted-pipe piezometers), andOW (observation wells).

There are two basic types of earth-pressure measuring devices,commonly referred to as earth-pressure cells. One type isdesigned for measuring earth pressure against structures suchas retaining walls and conduits and is relatively accurate andreliable. The other type is designed for measuring stressesin an earth mass. An earth- pressure cell generally consistsof a flexible diaphragm backed by a fluid-filled chamber and asensing device.


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11.11

SEEPAGE MEASUREMENT DEVICES


General

.11 A. General. - Seepage through, around, or under anembankment dam is an extremely valuable indicator of thecondition and continuing level of performance of a dam.The quantity of seepage entering a seepage collectionsystem is normally directly related to the level of thewater in the reservoir. Any sudden change in thequantity of seepage collected without a correspondinglyobvious cause such as change in the reservoir level orheavy rainfall or snowmelt could signal a seepageproblem. Similarly, should the seepage water becomecloudy, discolored, contain increasing quantities ofsediment, or change radically in chemical content, aserious seepage problem could be indicated. Wet spotsor seepage appearing at new locations downstream from anembankment could also indicate a seepage problem.

Failures

B. Failure Modes and Criteria. -

1. Excessive exit gradient will cause soil particlesat the toe of a dam to become buoyant. Themanifestation of this loss of gravitationalstability depends on the composition of the soil inwhich it is occurring.


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11.11B

In a foundation soil with a high percentage oflarge particle sizes, the fine particles may beremoved and deposited on the surface as "sandboils" while the structure of the large particlesremains stable, resulting in an increase inpermeability and seepage flow [5].

In a granular foundation soil with a narrowdistribution of relatively small grain sizes, amass of soil can become fluidized as the reservoirreaches the hydraulic head necessary to produce thecritical gradient of the soil mass. Rapidcatastrophic failure can then result through lossof shear resistance [5].

A common occurrence in a heterogeneous foundationsoil mass that may have some cohesion in thesurficial layer is the phenomenon called "PIPING."Piping occurs when the velocity of the seepagewater is high enough to carry soil particles awayfrom the fill, forming a conduit in the embankment.It will usually start at points of discontinuity orflow concentration such as along a poorly compactedcontact between backfill and structures that extendfrom upstream to downstream, open drill holes, postholes, root holes, and ditches [5]. These arelocations where the hydraulic gradient is higher,due either to less head loss or to a shorterseepage path.

2. Excessive pore pressure can contribute to failuremodes in the embankment and the foundation.

Embankment slope instability and deformation arecommon concerns in embankment dams, to which porepressures and seepage forces are contributingfactors.


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11.11B

Foundation pore pressure can create unstableuplift force on hydraulic structures associatedwith an embankment.

Foundation pore pressure can exert significantuplift force on a confining layer of soilimmediately downstream of a dam. This occurs whenthere is a more permeable layer, capable oftransmitting a large percentage of the reservoirhead to the downstream side, under the confininglayer. Failure begins to occur when the porepressure on the bottom of the confining layerexceeds the overburden pressure created by theweight of the confining layer. The resultinguplift eventually breaches the confining layerproducing an instantaneous high exit gradient inthe lower transmitting layer.

3. Unfiltered high internal gradients may cause soilparticles to migrate from one zone to another in anembankment dam, or from the embankment into voidsin the foundation. Because the water barrier zonesin most embankment dams possess some cohesion, thedeterioration can be progressive, sometimesresulting in lowered density or the formation ofpipes.

4. The chemical content of reservoir water is oftenquite different from the water within a foundationor even an embankment. Cementation within afoundation may offer a great deal of shear strengthtoward the embankment's stability and resistance todeformation. Relatively pure reservoir water maytend to dissolve this cementation and carry it awayfrom the foundation or embankment material, leaving


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a much weaker structure. The total dissolvedsolids of seepage water should be compared to thatmeasured in the reservoir water to verify thisdissolutioning if it is suspected.

5. Excess seepage flow without removal of materials isusually not a structural failure mode but could beconsidered in terms of project failure if itresults in inadequate reservoir storage to meetproject requirements. Downstream flooding ordestructively high ground-water levels could alsobe considered as project failure due to excessiveseepage [5].

6. Desiccation cracking of the embankment corematerial results from moisture content in the coreof a dam being reduced far below the constructed ordesign intended moisture content. Desiccation to asignificant depth is necessarily a long-termprocess that usually occurs during extended periodsof low reservoir and there is little or no moisturereplenishment to the embankment above reservoirlevel. The resulting reduction of moisture contentin the core material can cause shrinkage crackingin the water barrier core of the dam which resultsin serious leakage, erosion, and possible failureduring subsequent occurrence of high reservoir [5].

TYPES

.12 Commonly used seepage monitoring devices include quantitativedevices for measuring seepage including weirs, flumes, andcalibrated catch containers. Flowmeters, velocity meters, andturbidity meters also are sometimes used. Geophysical methodsused for qualitative seepage analysis include thermotic


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surveys and self-potential measurements. Table 2 lists someof the advantages and disadvantages of various seepagemonitoring approaches.

Weirs are one of the oldest, simplest, and most reliableapproaches that can be used to measure quantity of flow ofwater. The critical parts of weirs can be easily inspected,and any improper operations can be easily detected and quicklycorrected. Weirs normally used are of the 90 V-notch,rectangular, and trapezoidal (Cipolletti) types. The quantitydischarge rates are determined by measuring the verticaldistance from the crest of the overflow portion of the weir tothe water surface in the pool upstream from the crest. Thedischarge may then be computed by the appropriate formula orby reference to tables prepared for that purpose.

Flumes are a specially shaped open-channel flow section. Thedischarge may be computed or determined by reference to tablesand charts prepared with throat width of flume, upstream head,downstream head, and inclination of sidewalls as variables.

Containers with a known volume may be used to measure low flowquantities from a pipe outlet or other freefalling,concentrated flow location. The time required to fill thecontainer of known volume is measured and the flow computed.The water may be saved in plastic or glass containers tomonitor or test its quality.

Flowmeters are sometimes used to determine quantity of flow ina pipe or open channel. Thermotic survey techniques may inspecial instances aid in identifying zones of highpermeability and ground-water flow concentrations withinfractured rock and alluvial deposits. Although thesetechniques may not replace the need for borings or the need toinstall conventional instrumentation, they may be valuable in


DS-13(11)-8 - 7/1/90 24

11.13

directing the location of more quantitative investigationmethods such as drill holes and pumping tests. Self-potentialor streaming potential surveys may also be useful in thedetection of discrete seepage paths.

MOVEMENT

GENERAL

.13 Movements occur when the shear strength of the soil isexceeded by the shear stresses over a relatively continuoussurface. Instability results when shear failure has occurredat enough points to define a surface along which the movementtakes place. Anything that results in a decrease in soilstrength or an increase in soil stress contributes toinstability and should be considered in the design of earthstructures. Causes of decreased strength are as follows:

Swelling of clays by adsorption of water.

Pore water pressure.

Pore air pressure in fine-grained, very dry soils.

Breakdown of loose or honeycombed soil structurewith shock, vibration, or seismic activity.

Hairline cracking from alternate swelling andshrinking.

Strain and progressive failure in sensitive soils.

Mechanical cracking associated with large-scaledifferential settlements or deformations.


DS-13(11)-8 - 7/1/90 25

11.14

Thawing or frozen soil.

Deterioration of cementing material.

Loss of capillary tension when soil becomes verydry or saturated.

Among the most common of earth mass failures are thoseresulting from unstable slopes. Gravity, in the form of theweight of the soil mass and of any water above it, is themajor force tending to produce failure, while the shearingresistance of the soil is the major resisting force.

INTERNAL MOVEMENT MEASURING DEVICES


.14 Internal movement readings should be watched for excessivevertical or horizontal movement and for heave. Rate ofmovement and changes in rate of movement may be even moreimportant than total movement. Changes in pore-waterpressures and internal or external movement will reflectchanges in stress conditions within the embankment that willinfluence the stability of the structure. Because behaviorconditions originate during construction and vary continuouslythroughout operation, data from the instruments may be used byboth construction and operating personnel, as well as by thedesigning engineers [6].

Vertical

A. Vertical Movement. - Settlement observations duringconstruction often can give warning of trouble fromother sources. Landslides, underground subsidences, and


DS-13(11)-8 - 7/1/90 26

11.14A

bearing-capacity failures usually begin with slow butgradually increasing settlement rates. Some conditionsthat necessitate the installation of measuring devicesfor particular movements are:

1. Foundations containing compressible clays or loosedeposits of sands, silts, or silty sand mixtures

2. Foundations containing heterogeneous or lenticularsoil deposits that include soft compressible soils

3. Zoned embankments containing soils of differentcompaction and consolidation characteristics

4. Embankments adjacent to concrete structures,

5. Deep excavations, especially in fine-grained soils

Horizontal

B. Horizontal Movement. - Low shear strength of foundationsand/or embankments could lead to significant shearingmovements of the embankment or appurtenant structureswithin or adjacent to the embankment [4].

TYPES

.15 A number of devices for measuring internal movements areavailable. Among those devices currently in use or beingconsidered for use are IVM (internal vertical movement)devices, baseplates, PSS (pneumatic settlement sensors), VWSS(vibrating-wire settlement sensors), inclinometers,tiltmeters, MPBX (multipoint borehole extensometers), shearstrips, and radiosonde devices. Table 3 lists some of theadvantages and disadvantages of these devices.


DS-13(11)-8 - 7/1/90 27

11.16

SURFACE MOVEMENT MEASURING DEVICES


.16 It is important to observe surface evidence of stabilityproblems such as slope bulging, sagging crests, foundationheave at or beyond the toe, and/or lateral spreading of thefoundation and embankment, as well as surface manifestationsof material removal by seepage flows.

TYPES

.17 External vertical and horizontal movements are measured on thesurface of embankments by use of level and position surveys ofmeasurement points. Measurement points may be monuments, ordesignated points on the crest, slopes, or toe of theembankment or on appurtenant structures. Measurements ofthese points should be made relative to one or more absolutelystable points located away from the influence of thestructure, preferably on rock. The human eye is another, muchless sensitive, instrument for recording surface movement. Inthis connection, it is emphasized that the surface of anembankment has to be kept free of high vegetation so thatmovement can be easily observed. Short grasses or mowedgrasses are acceptable to help resist surface erosion, butshould be kept trimmed to a uniform height above theembankment surface. Photographs are often used to record whatthe human eye has observed.

Commonly used devices for measuring external movement consistprincipally of reference points and targets set in concreteoff the structure and steel bars embedded in the embankment(embankment measuring points) or reference marks or points onstructures. Measurements are made using surveying equipmentincluding levels, theodolites, rods, calibrated survey tapes,


DS-13(11)-8 - 7/1/90 28

11.18

and EDM (electronic distance measuring) devices. Otherapproaches to monitoring surface movements include usingtiltmeters and crack measuring devices. Table 4 lists some ofthe advantages and disadvantages of these various approaches.

VIBRATION MEASURING DEVICES


.18 Installations of measurement instruments include vibrationrecording devices to indicate the results of transientstresses on the embankment. The vibrations could be due tonaturally occurring earthquakes or the construction-relatedvibrations from blasting or construction equipment. Problemsthat can result from major vibrations at a damsite aremovements of the dam and/or foundation, or increases in porepressures. Increases in pore pressures can lead to excessiveseepage, settlement, shear failure, and liquefaction of thefoundation soils or the embankment resulting in stabilityproblems.

The physical properties of the soil that are factors inresisting the effects of vibrations are:

Material type,

Material density,

Water content, and

The natural frequency of soil deposits.

The natural frequency of the structure is also a significantfactor.


DS-13(11)-8 - 7/1/90 29

11.19

TYPES

.19 Vibration or seismic instrumentation is used to recordresponses of the structure, foundation, and abutments toseismic events. The general term, seismograph, refers to alltypes of seismic instruments that write a permanent,continuous record or time history of earth motion(seismogram). The basic components of a seismograph include aframe anchored to the ground or embankment, one or moretransducers, timing devices, and a recorder. As the framemoves with the ground, the transducers respond according toprinciples of dynamic equilibrium. Signals of horizontalmotion in two planes and vertical motion may be sensed eitherelectrically, optically, or mechanically, and the motionsensing may be proportional to acceleration, velocity, orground displacement.

Various commercially available instruments include severalmodels of digital and analog strong-motion accelerographs,peak recording accelerograph, and other seismographs. Table 5lists some of the advantages and disadvantages of thestrong-motion accelerographs which are used by Reclamation.

VISUAL INSPECTIONS

GENERAL

.20 The visual inspection of the condition of the dam and thesurrounding area on a regular basis by a person familiar withthat specific dam is one of the most important factors in thecontinuing safety of the dam. Reclamation's SEED (SafetyEvaluation of Existing Dams) Program provides for regularformal inspections including visual inspections. The requiredvisual inspections are detailed in appendix B of the SEEDpublication. An extremely valuable supplement to that


DS-13(11)-8 - 7/1/90 30

11.20program is an awareness and alertness to apparent changes bydam tenders and the instrument readers at the regularinstrument reading intervals. Suspicions of such visiblechanges should promptly be reported.

None of the instrumentation described in this design standardwill detect an impending failure if the instrumentation is notappropriately located. Visual awareness, therefore, becomes amost vital factor. In many instances, a visual observationappropriately reported on the field data forms, may serve toexplain reasons for anomalies in the instrument readings.Principal items to watch for include:

- Cracking of the embankment in any plane or direction- Cracking or landslide-type movements of the upstream and

downstream valley walls- Bulging of the lower portion of the embankment slopes,

the abutments, or valley walls- Subsidence of any portion of the crest- Sinkholes in the reservoir bottom or the upstream face

of the dam- New boils or springs or an increase in volume from a

spring in the downstream face of the dam, the abutments,or the downstream valley walls or floor

- A persistent vortex (whirlpool) in the reservoir that isunrelated to any outlet works operation

- Seepage water that is discolored or carrying soilsediments

- Unusual cracking in any concrete appurtenant structuresIf any of these items or any other physical occurrence that isnot readily explainable is noted, the Structural Behavior andInstrumentation Section should be contacted by telephone assoon as possible. It may also be desirable to obtainphotographs at regular intervals of any distress noted. Ingeneral, all maintenance and operations personnel at a damshould be constantly alert for signs of distress.


DS-13(11)-8 - 7/1/90 31

11.21

DATA ACQUISITION, PROCESSING, AND REVIEW PROCEDURE

GENERAL

.21 Acquisition and processing of a vast amount of data arenecessary to maintain effective monitoring of Reclamation'sembankment dams. The specific goals of the processing andreview procedure are to provide for accurate and timelyevaluation of data relating to the safety of the facility [7].

DATA ACQUISITION

.22 Instrumentation data are normally obtained from instrumentreadings and observations by water district or Reclamationfield personnel. These data are acquired on a specificmonitoring schedule which has been established for eachinstrument type at each dam and is indicated on the dam'scurrent "Schedule for Periodic Readings" (L-23). In mostcases, instrumentation data are recorded manually on a fielddata form and promptly transmitted to the Structural Behaviorand Instrumentation Section. A number of examples of fielddata forms are given in reference [7]. Mail is the mostcommon transmittal method and is appropriate for non-urgentsituations involving lesser data qualities. Many dams in theprocess of first reservoir filling or having extensiveinstrumentation programs have their data transmitted usingterminals linked to the Denver Office CYBER computer.Interactive use of the "GO" program allows data entry,reduction, checking, and finally transmittal.

Other less common data acquisition methods include telephonereporting of data, reporting of data by facsimile, use ofdataloggers for taking and storing readings, and fullyautomated systems that take readings and transmit the data viasatellite or telephone.


DS-13(11)-8 - 7/1/90 32

11.23

DATA REVIEW

.23 It is advisable to perform engineering analyses for variousexpected conditions before the conditions arise. Instrument"alarm" levels can then be established based upon specifiedsafety criteria. The engineering analysis is accomplished byfirst establishing a safety criteria and then determining whatreading of the instrument would have to be observed to reflectthe specified level of safety. Examples of safety criteriafor slope stability instrumentation may be:

Condition Safety criteria

Design stability Factor of safety = 1.2

Impending failure Factor of safety = 1.0

Unacceptable Total movement > 1.0 inch (25 mm)movement Movement velocity > 0.25 in/d (6 mm/d)

Movement acceleration > 0.01 in/d/d(2.5 m/d2)

Instrument readings can then be compared to alarm levels setto represent specified safety criteria to quickly understandthe performance and margin of safety existing in an embankmentdam.

During construction it is important that piezometer data beevaluated as soon as they are received from the field. Arapid buildup of pore pressure in either the embankment orfoundation during construction may indicate a potentiallydangerous condition, and remedial measures such as decreasingrate of fill placement, temporarily discontinuing fill


DS-13(11)-8 - 7/1/90 33

11.23

placement or providing berms or flatter slopes may be requiredif the pore pressures become too high. To assess thestability of an embankment during construction in a generalway, piezometer pressures may be compared with predictedpressure [2].

Piezometer data collected after construction should be used tocheck the long-term stability of the embankment. Datacollected during the reservoir filling should be used to checkthe embankment stability for the partial pool and steadyseepage conditions. Piezometer data collected following arapid lowering from maximum reservoir level should be used tocheck stability under drawdown conditions. Piezometer data cansometimes be analyzed to look for indications of piping, butbecause piezometer data is localized data, it is anunconservative approach to monitor for potential piping.

Data from earth pressure cells should be reduced and timeplots should be maintained for each pressure cell during andafter construction. When the pressure cell is below the watertable, both water and earth pressure act on the cell. Thewater pressure on the cell should be determined from apiezometer in the vicinity of the cell, and the effectiveearth pressure should be computed for each measurement ofpressure. Analysis of data should include a comparison ofobserved earth pressures with earth pressure assumed for thedesign of the structure. For the comparison, observed loadingconditions should be selected that are similar to loadingsused in design. By such a comparison, the validity of thedesign assumption and procedures can be checked. Also, thesedata will be useful in the future design of similar structures[4].

Embankment and structural measurement point data should bethoroughly analyzed for indications of unusual or unexpected


DS-13(11)-8 - 7/1/90 34

11.24

behavior. Large total movements, large differentialmovements, and anomalous settlements that might be associatedwith transportation of materials with seepage flows deservespecial attention and analysis.

POSSIBLE ACTIONS

.24 If no unusual circumstances or potential problems are detectedin the data itself or in the potential consequences of thedata, the information is plotted and stored for futurereference. These data are used for periodic preparation of"Structural Behavior Reports" on each dam.

Should the potential for a serious problem be detected at adam, the regional and ACER (Assistant Commissioner -Engineering and Research) management personnel are promptlynotified of the situation and appropriate actions aredetermined and implemented. These actions might include suchmeasures as emergency lowering of the reservoir. Regardlessof the management decisions made, it is normal to continuemonitoring at a much increased monitoring frequency until suchtime as the potential problems are considered to have beenappropriately resolved.

All phases of the process of data management, review, etc.,are obviously of great importance in dam safety. However, itmust be stressed that none of the phases are of muchsignificance if the initial data acquisition phase is notconducted in an accurate and timely manner by personnel whounderstand the value and use of the data and possess a stronginterest in the future safety of the facilities. Thepersonnel collecting the data play a key role in the processand should be appropriately trained because they are in theideal position to raise immediate concerns when significantanomalies occur.


DS-13(11)-8 - 7/1/90 35

APPENDIX

REFERENCES

TABLES

EXAMPLE


DS-13(11)-8 - 7/1/90 36


DS-13(11)-8 - 7/1/90 37

Appendix

REFERENCES

1. Proceedings of the Conference on Construction Practices andInstrumentation in Geotechnical Engineering, "Instrumentationfor Embankments and Embankment Dams," by Stanley D. Wilson andP. Erik Mikkelsen, December 20-23, 1982.

2. Corps of Engineers Manual EM 1110-2-1908, Part 1 of 2, titled"Instrumentation of Earth- and Rock-Fill Dams (Groundwater andPore Pressure Observations)," August 31, 1971.

3. "Foundation Instrumentation" by T. H. Hanna, Published byTrans Tech Publications, First Edition, 1973.

4. Corps of Engineers Manual EM 110-2-1908, Part 2 of 2, titled"Instrumentation of Earth- and Rock-Fill Dams (Earth-Movementand Pressure Measuring Devices),"November 19, 1976.

5. Design Standards No. 13 - Embankment Dams, Chapter 8 -"Seepage Analysis and Control," U.S. Department of theInterior, Bureau of Reclamation, Denver, Colorado

6. "Earth Manual," Second Edition, U.S. Department of theInterior, Bureau of Reclamation, Denver, Colorado

7. "Embankment Dam Instrumentation Manual," U.S. Department ofthe Interior, Bureau of Reclamation, Denver, Colorado, January1987.


DS-13(11)-8 - 7/1/90 38


DS-13(11)-8 - 7/1/90 39

AppendixTABLES

Table 1. - Pressure measuring devices

____________________________________________________________________________________________

Type What is Advantages Disadvantagesmeasured

____________________________________________________________________________________________

Standpipe Pore pressure Simple. Reliable. Occasionally slowpiezometers Long experience response time. Pipeor well record. No and tubing must bepoint elaborate terminal raised nearly

panel needed. vertical, causingInexpensive. them to be subject to

damage byconstructionequipment. Freezingproblems. Couldinvolve costlydrilling and relatedproblems.

Closed Pore pressure Long experience Location of terminalhydraulic or total record. Rapid well. Freezing andsystem pressure response and less corrosion problems.

prone to damage by Periodic de-airingconstruction equipment required. Maintenancebecause readout tubes problems. Oftencan be installed substantial losses ofhorizontally to instruments with time.centralized monitoring Complex flushingstations. procedure.

Pneumatic Pore pressure Level of terminal Must prevent humid airsystem or total independent of tip from entering tubing.

pressure level. Rapid response. Shorter experienceNo freezing problems. record than hydraulic

system. Complexreading procedure.Tubing vulnerableduring construction todamage; not effectivelyrepairable.


DS-13(11)-8 - 7/1/90 40

Table 1Table 1. - Pressure measuring devices - Continued

____________________________________________________________________________________________


____________________________________________________________________________________________

Vibrating- Pore pressure Simple to read and Sensitive towire system or total maintain. Level of temperature. Risk

pressure terminal independent of zero drift.of tip level. Rapid Sensitive toresponse. Potential barometric changes.high sensitivity. Vulnerable toSuitable for automatic lighting damage.readout or data logger.Frequency signalpermits datatransmission over longdistances. No freezingproblems. Can be usedto read negative porepressures. Less vulnerableto construction damage.

Resistance Pore pressure Level of terminal Risk of zero drift.strain gauge or total independent of tip Sensitive tosystem pressure level. Rapid temperature,

response. Potential moisture, splices, cablehigh sensitivity. Suit- length, and changesable for automatic readout. in connections thatNo freezing problems. would affect circuitCan be used to measure resistance. Not normallynegative pore recommended for long-pressures. term installations.

____________________________________________________________________________________________


DS-13(11)-8 - 7/1/90 41

Table 2. - Seepage measuring approaches____________________________________________________________________________________________


____________________________________________________________________________________________

Weir Flow quantity Simple. Reliable. Significant head loss.Long experience Requires arecord. Little fairly accurate estimatemaintenance. of flow before theSome materials appropriate weir istransported with selected. Non-standardflow may settle out in installations canfront of the weir produce misleading data.allowing recognition ofthe materials transport.

Parshall Flow quantity Simple. Reliable. Cannot be used influme Long experience close-coupled

record. Little combinationmaintenance. Small structures consistinghead loss. of a turnout, controlInsensitive to and measuring devicevelocity of approach, because of length ofdegree of flume. Installationsubmergence. requires high quality

workmanship involvinggreater costs. Mustbe built on a solid,watertightfoundation.

Trapezoidal Flow quantity Particularly useful in Must be constructedflume measuring a potentially within watertight

large range of flows. materials.Commercially availablein precast units ofmolded plastic.

Velocity Flow velocity Useful in measuring To derive flowmeter flows in unusual quantity, the cross-

situations. sectional area of theUsually avoid flow needs to beinstallation and determined. Lessermaintenance costs of accuracy andpermanent structures. sensitivity thanEquipment easily other approaches.portable.


DS-13(11)-8 - 7/1/90 42

Table 2

Table 2. - Seepage measuring approaches - Continued____________________________________________________________________________________________


____________________________________________________________________________________________

Calibrated Flow quantity Simple. Reliable for Requires that the watercontainer fairly low flows. be in freefall.devices Inexpensive. Difficult and inaccurate

for all but verylow flows.

Thermotic Ground Inexpensive. Requires asurveys temperature Equipment is easily significant period

changes due portable. The system of monitoring beforeto ground- can be automated. conclusions can bewater flow Minimal maintenance drawn. Significantconcentrations and environmental analysis required.

impact. Data are foran area rather than apoint.

Self Electrical Inexpensive. Monitors Requires noncomplexpotential current a large area in detail. geologic setting.surveys anomalies due Effective up to a depth Influenced by water

to ground- of about 1,000 feet. chemistry. No long-water flow May be used to map the term monitoringconcentrations origins of seepage in experience.

reservoir bottoms or Significantdelineate seepage paths interpretationoutside of the reservoir. required, with different

interpretations possible.____________________________________________________________________________________________


DS-13(11)-8 - 7/1/90 43

Table 3Table 3. - Internal movement measuring devices

____________________________________________________________________________________________


____________________________________________________________________________________________

Internal Vertical Movement can be Cannot be installedvertical movement in related to a specific through a drill hole.movement the embankment. interval. Long Must be installeddevice experience record. during construction.

Susceptible tocorrosion problems.Relatively expensiveto install.

Foundation Vertical Readily site Yield a limited amountbaseplate movement due adaptable. Rugged. of information. May

to foundation Easy to install and rust or develop othersettlement. read. Some types may be maintenance problems.

installed through a drillhole after construction.

Pneumatic Vertical Construction interference Relatively long readingsettlement movement in associated with time. Needs thesensor the embankment vertical standpipes elevation of the

or foundation. is eliminated. terminal reservoirfor each set ofreadings. Limitedexperience record.Liquid reservoirs maybecome frozen. Datagenerally are not

sufficientlyaccurate and sensitivefor monitoring smallmovements.

Vibrating- Vertical Easy reading. Needs the elevation ofwire movement in Construction the terminal reservoirsensor interference for each set of

associated with readings. Limitedvertical standpipes experience record.is eliminated. Liquid reservoirs may

become frozen. Accuracyand sensitivity may notbe suitable for monitoringsmall movements.


DS-13(11)-8 - 7/1/90 44

Table 3

Table 3. - Internal movement measuring devices - Continued____________________________________________________________________________________________


____________________________________________________________________________________________

Inclinometer Lateral May be installed during Significant degree of(normal movement in the or after construction. specialized traininginstallation) embankment, Reliable and accurate. required for

abutment, or Adequate experience monitoring personnel.foundation. record. Settlement data canVertical movement only be obtained ifin the embankment installed duringor foundation. construction.Location of shearplane.

Inclinometer Lateral Can be installed in Only selected points(fixed movement in locations that later along a drill holeposition) the embankment, become inaccessible. are measured. High

abutment, or Automatic readings purchase andfoundation. possible. Easy installation costs.

to read.

Extensometers Axial Very accurate and Careful reset of thedisplacement precise. Highly reference head maybetween points sensitive. Can be be required ifwithin an remotely read range is exceededembankment, electrically if and reference head isabutment, or reference head is accessible.foundation. inaccessible.

Shear strip Differential Identify when and approx. Does not measure amount(shear) where a failure has or rate of movementmovements in occurred. Can be to indicate impendingsoil, rock, wired into an automated failure.or concrete. system. Easy to read.

Relatively inexpensive.

Radiosonde Deflection in Difficult and expensivesystem the embankment installation and

maintenance. Timeconsuming to read.Accuracy is uncertain.

____________________________________________________________________________________________


DS-13(11)-8 - 7/1/90 45

Table 4

Table 4. - Surface movement measuring devices____________________________________________________________________________________________


____________________________________________________________________________________________

Tiltmeters Rotational Lightweight. Compact. Translational movementsmovement of Can be portable or not monitored.the embankment, permanentlysoil, or rock attached.masses.

Embankment Total vertical Inexpensive to install. Measurements, though infrequent,measuring and horizontal Simple. Reliable. can be labor intensive,points movement of the Can be installed at thus, a continuing(monuments) embankment any time. significant expense

surface. is involved insecuring long-termdata. Expensive surveyingequipment may be required.

Structural Total vertical Inexpensive. Simple. Measurements, though infrequent,measuring and horizontal Reliable. are very labor intensive.points structural Expensive surveying

movement. may be required.

Crack Relative or Variety of approachesmeasuring total movement available which candevices of intact masses be simple, inexpensive,

on either side and reliable. Remoteof a crack. readout possible.

____________________________________________________________________________________________


DS-13(11)-8 - 7/1/90 46

Table 5

Table 5. - Vibration measuring devices____________________________________________________________________________________________


____________________________________________________________________________________________

Strong- Acceleration time Can be placed in Requires power source (a.c.motion histories; strategic locations or solar) for chargingearthquake velocities, away from, near, or batteries, hardwireaccelerographs displacements, on an embankment interconnect

and response to measure the effects between units forspectra can be and response of the simultaneous triggering,derived. dam. Automatically time code generator

triggered when a preset for common time base.level is exceeded. Periodic site visitsHigh reliability. for operationalSimple operation. checks and forLow maintenance. retrieval of

acceleration timehistories.

____________________________________________________________________________________________


DS-13(11)-8 - 7/1/90 47

AppendixEXAMPLE

I. The Problem

An existing 100-foot (30-m) high, semipermeable embankment dam islocated in a wide but steep-sided river valley. Its purpose ishydroelectric power generation. The community using the power willexpand within the next 5 years and will need to accommodate threetimes as many users of electricity. The powerplant associated withthe embankment dam is being modified to meet the increased demand.Peaking operations in the future will result in a rapid drawdown ofthe reservoir that would lower the upstream stability of theembankment below generally accepted factors of safety. However,slope stability analyses show that the upstream slope should notfail if the water level and pore pressures within the dam decreaseas expected during rapid drawdown due to the semipervious nature ofthe fill. Therefore, it has been decided not to design anyadditional stabilization features at the dam, but to monitor itsperformance during peaking operations. Instrumentation needs to beselected and laid out for this purpose.

II. Analysis

Further stability analysis has identified a critical circular shearsurface encompassing the dam's full height (see fig. E-1). Becausethere are no distinctively different boundary conditions that wouldcontrol the shape of this failure in three dimensions, one canassume that the failure mass would be about as wide as it is long.Outlines of possible locations of this failure mass are shown ascircles on the face of the embankment on figure E-2.

The slope stability analysis assumed that the water pressureswithin the embankment would be hydrostatic and respond quickly tothe reservoir drawdown because the embankment is semipervious.


DS-13(11)-8 - 7/1/90 48

Example

This assumption is shown on figure E-1. If the response is slower,such that high excess pore pressures remain in the vicinity of thecritical failure surface when the reservoir reaches its lowestpoint, failure may result. Therefore, it is important to verifythat the pore pressures drop during drawdown to at least the levelassumed and shown on figure E-1.

The instrumentation plan for this problem will serve two purposes.The first purpose is to verify the pore water pressure assumptionsmade for the analyses. Both the static water level within theembankment and the effect that rapid drawdown has on it should beverified. The second purpose of the instrumentation would be toprovide a warning if the critical failure mass is about to move.This warning would allow for corrective measures (such as raisingthe reservoir to buttress the slope, or decreasing the rate ofdrawdown, or increasing the embankment's ability to resistshearing) to be taken.

III. Instrumentation Selection and Layout

Vibrating-wire-type piezometers will be used on two sectionsthrough the dam to verify the pore water pressure assumptions madein the slope stability analysis. Two of the piezometers will beplaced within the upstream portion of the embankment beneath thereservoir. The vibrating-wire type are selected because they canbe read without having access to the top of the hole in which theywere placed and they should be free of lag time difficulties whichcan be associated with open-standpipe piezometer installations.Lag time effects may cause one to believe that high pore pressuresexist in a dam during drawdown when high water levels in astandpipe piezometer are actually due to the slow drainage of waterout of the instrument. Because it is important to know theembankment pore pressures as the reservoir is drawn down,vibrating-wire piezometers are used to obtain accurate, real-time


DS-13(11)-8 - 7/1/90 49

Example

data. The piezometers will be located in the zone of theembankment where drawdown is expected to occur. Pore waterpressure under the upstream edge of the crest should be quiteindicative of the assumed rapid response to the reservoir levelchange. Pore water pressures further upstream will also record thepressures in the vicinity of where shearing would most likelyoccur. Piezometers will be separated by bentonite seals in theholes so that pressures at one instrument will not influence theother. Two sections of piezometers provide the redundancy neededto allow for cross checking. The location of the sections will becontrolled by the location of inclinometers as described in thefollowing paragraph.

Two holes will be drilled through the upstream portion of the damto install fixed-position inclinometers such that at least onelocation will be within the potential slide mass. Because theembankment dam is longer than the failure mass is wide, the exactlateral location of the potential failure mass is not certain. Thetwo outside circles on figure E-2 show the limits within which thefailure masses may occur. The circles indicate possible locationsof the likely failure mass. In order to ensure that theinclinometers would be located within the failure mass, wherever itdecided to fail, the least amount of drilling required would be oneinclinometer hole for each area of overlap of possible failure masslocations. No failure at least as wide as what is expected couldavoid detection if inclinometers were located as shown on figureE-2. Fixed position inclinometers were chosen because it isnecessary to continuously monitor for the possibility of rotationalmovements deep within the embankment. Continuous monitoring isrequired so that immediate action can be taken if movement isobserved. Rotational movements are expected because the materialis homogeneous, favoring no particular failure geometry.Horizontal movements are not anticipated and would not be readilyobserved by this system. These instruments are also able tooperate and be read even when submerged beneath the reservoir.Measurements for the magnitude and direction of internal


DS-13(11)-8 - 7/1/90 50

Example

displacements can be obtained from these instruments. Oneinstrument should be fixed in each hole below and two or threeshould be placed within the anticipated failure mass. The numberof instruments placed in each hole should be minimized because oftheir high cost. Horizontal and vertical positioning of theinstruments is controlled by a balance of strategic and economicconcerns.

Embankment surface measuring points will be spaced 100 feet aparton four lines parallel to the dam centerline. One line will bealong the dam centerline. Another line will be in line with theinclinometers and piezometers out on the upstream face. Anotherline will be placed between the two lines just mentioned and afourth line will be placed on the downstream face as shown onfigure E-2. Consideration may be given for smaller spacing wherethe slide mass is most likely to occur and measurements can easilybe taken. Fixed reference points should be installed at the end ofeach line off of the dam in the rock of the abutments.Measurements for surface deflections and settlements should beobtained from these monuments.

IV. Monitoring and Evaluation

After installation and before peaking operations begin, thepiezometers should be read monthly and all other instruments shouldbe read semiannually. The existing outlet works are capable ofdrawing down the reservoir fast enough to simulate future peakingoperations. A phased test peaking program should be performed tostudy the response of the embankment and check it with the responsethat was assumed. This test should be conducted as soon aspossible after the instrumentation is installed so thatcorrections, if necessary, can be accomplished without delaying theactual peaking operations. One month before peaking operationsbegin, all instruments should be read weekly. This should providean adequate set of base data. As peaking operations begin,


DS-13(11)-8 - 7/1/90 51

Example

readings on all instruments should be obtained at both the highwater level and low water level until the maximum drawdown has beenachieved twice. Thereafter, for the first year, readings on allinstruments should be obtained biweekly. Visual observationsshould also be recorded during this entire time period.

If pore water pressures are higher than those that were assumed inthe slope stability analysis, further analysis is required.Decisions regarding the safety of the dam and the actions to takedue to higher than expected pore pressures will be based on thestability analysis and the readings of the other instruments.

If any point within the embankment that is monitored by a movementdevice shows total movement greater than 1.0 inch, or movementvelocity exceeds 0.25 inch per day (6 mm/d), or movementacceleration exceeds 0.01 inch per day per day (2.5 m/d2), thenpeaking operations should be curtailed and the possibility ofstabilization treatment should be considered. If curtailingreservoir drawdown does not stop the embankment movement, then thereservoir should be raised and stabilization treatment recommended.

This example is a simple demonstration of a minimal type ofinstrumentation design. It is not complicated by geologic,seismic, embankment material or any other uncertainties. It doesnot have a budget. It is included in this standard as an exampleof the relationship between engineering concepts and the selectionand placement of instruments. It emphasizes the fact that a validunderstanding of the engineering concepts involved is essentialbefore instrumentation to verify these concepts is selected.


Example Fig. E-l

LEGEND

I - Piezometer

hriticol Failure Surface

Section A-A and B-B

Figure E-l. - Stability section for example problem showing pore pressure assumptions, critical shear

surfaces, and piezometer locations.

DS-13(11)-8 - 7/l/90 52


Example Fig. E-2

LEGEND

1 -Piezometer l -Inclinometer A -Surface Manument

-+A 48 Cwnstrem Toe

Outline of Embonhmnt Dam

Mline of Wble failure

Figure E-2. - Plan of example embankment dam showing possible locations of slope failure and locations

of instrumentation.

DS-13 (11) -8 - 7/l/90 53

DS13-11

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Transcript of DS13-11