04 evaluation of instrumentation data...interpreting instrument data 2 for an existing dam: •...

Interpreting Instrument Data

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David Rees Gillette, PE, PhD

Evaluation of Instrumentation Data

Reasons for Installing Instruments

• Design– Gather information used for design work

• Construction– Verify design assumptions– Safety of construction workers (e.g. potential slope instability)– Control construction (e.g. monitor concrete temperatures)

• First Reservoir Filling• Long-term Performance

– Specific issues (potential failure modes)– General health monitoring– General understanding of dam and damsite

• Dealing With Unexpected Performance• Research• Legal

What are we looking for in the data?Evidence that the dam is performing adequately, or that it is not.

• Are the data consistent with expected behavior from theory and engineering analysis?

• Are they consistent with previous months and years?

• If not, why?

• Whether or not the data are as expected, is there a dam-safety deficiency we must act upon?


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For an existing dam:

• Understand the geology and design!

• Monitor for changes in:– Locations of visible seepage

– Amount of seepage

– Water pressures (piezometric levels)

– Sediment that could be from internal erosion

– Deformation or movement of embankment or concrete structures

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Can you conclude that all Potential Failure Modes are very unlikely?

• Internal Erosion

• Embankment slope instability

• Concrete dam sliding or deformation

• Concrete deterioration

• After extreme loadings (earthquake or flood)

For a new dam:• Understand the geology before you design it!

• Determine expected behavior of instruments

• Closely monitor (instruments and visual) embankment, foundation, abutments, and reservoir during first filling– Visible seepage

– Measured seepage quantities

– Rising piezometers

– Settlement and deflection

– Landslides

Do observations agree with what the designers expected? 6


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Causey Dam Initial Reservoir Filling

Not by instruments alone…

• Instrument data

• Visual observation

• Theory and analysis

We are ENGINEERS, not ROBOTS.

Bright Green Vegetation on Dam Abutment in the Desert


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This inspector found water where we never saw it before, and there is NOT thick green vegetation.

When We Need Data Review

• Immediate Review of Data (as it is received)• Somewhat cursory – much data to handle

• Timely detection of significant anomalies

• Computers can be a big help

• Periodic In-Depth Review• Looking at the bigger picture, seeking deeper

understanding

• Compare data obtained by different instrument types

• Allows immediate reviews to be more effective

• Unusual Loading or Unexpected Behavior – Is a PFM developing?

Limitations on Instruments(Not Just Instrument Failure)• "False positive" – Because of instrument

data, you see a developing failure when there really isn't one.– Increased weir flow due to rain or melting snow

– Windblown soil in weir box, not internal erosion

– Clogged drains / cleaned drains

• "False negative" – Monitoring fails to detect a failure that is actually developing. – The instruments are simply not in the right place –

they can't be everywhere.

– Not exactly sure what to look for


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Immediate Data Review

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Data Review Process• Data check as readings are entered in the

instrumentation computer database


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Is this normal performance?Observation Wells and Piezometers

Expected Performance (R = Reservoir Elevation)

Instrument

MinimumExpected

Piezometric Water Surface Elevation

(in feet)

Maximum Expected

Piezometric Water Surface Elevation

(in feet)

Minimum Expected Depth to Water*(in feet)

MaximumExpected Depth to Water**(in feet)

PTP-1.5-2B (0.15 x R) + 3467.1 (0.15 x R) + 3472.4 51.2 22.1

PTP-3-3-A (0.39 x R) + 2477.6 (0.39 x R) + 2487.0 44.0 65.4

PTP-3-3-B (0.26 x R) + 3005.5 (0.26 x R) + 3012.0 48.7 71.2

PTP-4.5-1A (0.46 x R) + 2191.0 (0.46 x R) + 2198.0 43.5 64.7

PTP-4.5-1B (0.22 x R) + 3170.0 (0.22 x R) + 3175.5 58.6 70.9

PTP-6.5-1A 4112.4 4113.4 30.6 31.6

PTP-6.5-1B 4073.0 4076.0 68.0 71.0

PTP-8-2A 4123.0 4124.0 20.6 21.6

PTP-8-2B (0.60 x R) + 1625.6 (0.60 x R) + 1636.4 27.6 57.0

TITP-1 (0.11 x R) + 3617.7 (0.11 x R) + 3621.8 10.5 17.9

TOW-1 (0.13 x R) + 3534.3 (0.13 x R) + 3538.5 11.8 19.9

TOW-2 (0.12 x R) + 3575.6 (0.12 x R) + 3579.8 15.9 23.8

TOW-3 (0.13 x R) + 3534.5 (0.13 x R) + 3539.0 13.4 21.8

TOW-4 (0.13 x R) + 3535.7 (0.13 x R) + 3539.5 11.3 19.0

* The values are associated with a “high” reservoir elevation of 4136.0 feet. ** The values are associated with a “low reservoir elevation” of 4105.0 feet.# Vibrating-wire frequency readings are obtained for these instruments, rather than depths.

Table of expected piezometer ranges for quick check, based on data thus far and understanding of seepage

Periodic In-Depth Review• Comprehensive Review (CR) - Once each 8

years• Inspection

• Review of design and previous analyses

• Review of all performance data

• Updated flood and earthquake hazard

• Probabilistic risk analysis

• Update requirements for future performance monitoring

• Recommendations for maintenance and dam-safety actions


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• Technical Response Team (TRT) Meeting and Periodic Facility Review (PFR) - Once each 8 years, midway between CRs.• Inspection

• Review of performance data

• Requirements for future monitoring

• Update requirements for future performance monitoring

• Recommendations for maintenance and dam-safety actions

Periodic In-Depth Review

Periodic In-Depth Review• Plots of data throughout the existence of the

dam – look for changes with time.

• Check that ranges of expected readings make sense with respect to design, foundation conditions, and previous data.

• Verify that the instruments work properly

• Decide which instruments should continue to be used, and whether any new ones or replacements are needed.

Portraying Data for Review and Analysis – Time and Space


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Piezometers Plotted With Time

What's happening here?

Data Plotted Against Reservoir Level

Rapid piezometer response incoarse soil or fractured bedrock

Slow piezometer response in material with low permeability.

Slow Response

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1

3

2

2

3


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Rapid Filling of Reservoir, Sustained Full Reservoir, Then Rapid Lowering

When the reservoir reaches approximately elevation 4325, more pervious seepage paths are apparently available

Rapid Piezometer Response, Not Linear


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Different Seepage Paths for Different Reservoir Elevations?

Relatively Impervious

Relatively Pervious

El. 4325

Slope Stability and Embankment Deformation

Monitoring

• Visual monitoring

• Surveyed monuments

• Water pressures

• Inclinometer

• Shear strip

Inclinometeror Shear Strip

Slope StabilityLongitudinal Cracking at the Crest


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Potential for Transverse Cracking of an Embankment Dam

“Mechanically” Induced Transverse Cracks (due to differential settlement of the foundation)

Low compressibility Foundation material

High compressibilityFoundation material

Transverse Cracking(may not be visible at crest if surface is gravel)

Embankment Dam Settlement Points

~2.5 m


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Individual settlement points plotted with time

Settlement profiles for different dates

Is there influence from the reservoir?


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Deflection• Inclinometers

45 cm movement on shear plane in shale bedrock before inclinometer became unusable

Piezometers and Inclinometers Shown on Section View of Embankment and Abutment

Inclinometer Casing Broken Here

Piezometer Readings

45 cm movement on shear plane in shale bedrock before inclinometer became unusable

Inclinometer -Profile of Movements with Depth


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Replacement Inclinometer Plotted with Depth and Specific Depths with Time

Or, as Vectors in Plan View

Need picture of a concrete dam

Concrete Dams

Stresses to compare to model resultsMeasure strains compute stressesTemperatures important input


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Visual Evidenceof Structural Distress

Evidence of Structural Distress

Evidence of Structural Distress

Plumbline Data


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Concrete Gravity Dams

Collimation System for Deflection

Concrete Gravity DamDeflection with Time and Reservoir Load

Uplift Pressure Data With Time

AB C D E


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Uplift Pressure Data With Location

Pressure Diagram Showing That Grout and Drains Are Effective

AB C D E

100%

0%

Percentage of Full Reservoir Pressure

Grout Curtain

Drilled Foundation

Drains

Formed Drainsin Body of Dam

AB C D E

AB C D E

Reservoir Level

Drain FlowGallons/Minute

Unusual Loading or Unexpected Behavior

• Earthquake (typically, 0.05 g or more)

• Extreme Flood

• A New Seep

• Observed Crack, Void, or Depression


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Upstream Face of Rudramata Dam2001 Bhuj Earthquake, IndiaRepublic Day, 2001

Post-Earthquake Inspection

Post-Earthquake Inspection

• Visible / Measurable Deformation

• Cracking

• Changes in Seepage - Quantity, Location, Turbidity!!!

• Changes in Piezometric Levels

• Damaged Appurtenant Structures

• Failed Instruments

• Landslides

Downstream of Rudramata Dam

Data Plotted Against Reservoir LevelVery Useful for Rapid Assessment After Unusual Loading!

Wednesday?!?

Tuesday


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During/After Flood• During flood

– Visible new or changed seepage

– Piezometers

– Weirs – read and check for sediment

– Spillway performance with large flow

• After flood– Condition of spillway

– Return of seepage and piezometers to normal

– Problematic erosion

During/After Extreme Flood

Previously "Untested" Embankment Contact

Normal Pool

Flood Pool

Toe Drains

A New Sinkhole?

Piezometers

The inspector found water where we never saw it before, and there is NOT thick green vegetation.

• Piezometers• Drain flows• Construction records• Geology drawings• Older inspection reports• Weather!


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Where is all this water from? Is there a PFM in progress?

The wet area is very large, but the rate of seepage is small. Why?

Because the ground surface is flat, and the water does not drain.

Analyze the Right ProblemFuse Plug Embankment and Base Slab

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Grout Curtain

*


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Flow Net of Seepage Through Thin Core

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• Indicates Q = 0.05 m3/day per meter of dam

Transformed Section(Dimensions in ft.)

Very little water from seepage collector on base slab

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Is there a PFM in progress?

No. The water is because of a leaky grout curtain. This seepage in bedrock is benign because core is protected by concrete.

Geology and Material Properties

Analysis

Observations and Instrument Data

Understanding!

Sometimes, the process does not go in a straight line from Step A to Step B to Step C!

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Failure Due to Embankment Cracking?Rogers Dam, Nevada, 1952

Discussion?

04 evaluation of instrumentation data...interpreting instrument data 2 for an existing dam: •...

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