10th EAST ASIA DAM CONFERENCEISSUES SEEN IN EXISTING CONCRETE DAMS AS THE RESULT OFCOMPREHENSIVE INSPECTIONS
Katsutoshi Yamagishi 1, Shuji Takasu 2

1

Senior Researcher,& 2 Director-General, Dam Engineering Research Institute, Japan Dam Engineering
Center,Tokyo,110-0008,Japan
Email yamagishi@jdec.or.jp ,takasu@jdec.or.jp

ABSTRACT: As an effort to extend dam service life, comprehensive dam inspections were systematizedin Japan in 2013, and comprehensive inspections are being carried out on dams that are roughly 30 years
old. Comprehensive dam inspections is carried out by each dam manager applying technological expertiseto perform a comprehensive fact-finding survey and evaluation of the soundness of the state ofmaintenance and degree of deterioration of civil-engineering structures of the dam, which are theconstituent elements of each dam. Comprehensive soundness of civil-engineering structures of dams inparticular, are evaluated by additionally performing advanced surveys on items which cannot be easily
surveyed in daily and periodical inspections for methodological and cost reasons, and items which could
deteriorate over time. This study organizes the results of comprehensive dam inspections controlled under
the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) implemented for dams in Japan inrecent years, looks at issues to note during comprehensive dam inspections, methods for examining issues,
procedures for evaluating examination results, and summarizes the trend in maintenance issues forconcrete gravity dams.
Keywords: Existing dam, Comprehensive dam inspection, Civil engineering structure, Other dam
facilities, Dam measurement

1.EFFORTS TO EXTEND DAM SERVICE LIFE IN JAPANDams are critical social infrastructure with functions that have an extremely high level of social
impact, including flood control and water use. Dams that have been in use for 50 years or longer
currently account for about 10% of all dams under the control of Japan’s MLIT, while dams thathave been in service for at least 30 years account for roughly 50% of all dams, and the number ofdams where deterioration or damage from the passage of time will be actualized, or where
maintenance costs for facilities upkeep and repairs will climb, is expected to increase in the
future. Because dams are vital facilities that must be maintained and operated for many years, itis critical to understand the state of deterioration and damage at an early stage and make repairs
as necessary at the appropriate time, and these actions are linked to minimizing maintenanceexpenses and maintaining long-term dam safety and functions. With this viewpoint in mind, in2013 MLIT enacted the Comprehensive Dam Inspection Implementation Guidelines andInterpretation (“Implementation Guidelines”) [1], which will newly systematize comprehensive
dam inspections to be implemented on a cycle of about 30 years[2].
We consider that this paper is to organize the results of comprehensive 16 dam inspections
implemented in recent years and summarizes the maintenance issues for concrete gravity dams,
and to contribute to the implementation of rational comprehensive inspections in the future
through a survey of deterioration status awareness and a survey of repair techniques.

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2. CONCRETE GRAVITY DAM MAINTENANCE
The Implementation Guidelines notes that, “For comprehensive dam inspections, verify thestatus of maintenance cycle execution and the check and inspection records including daily
check and regular inspection, and evaluate comprehensively the soundness of dam from a longterm point of view as a matter of course. For dam civil engineering structures, evaluate
comprehensively soundness by performing an additional examination for items that, from theperspective of technique and cost, are highly difficult to inspect during daily checks, and for
those items in daily checks or regular inspections for which a possibility of deformation in the
future is considered.”, and seeks to arrange maintenance schedules based on considerations suchas evaluations of soundness according to the items below.
a) Maintenance of measurement functionb) Continuous measurement to evaluate soundnessc) Continuous understanding of the state of deteriorationd) Action to address individual issuese) Organization of various types of data, etc.
Based on the results from analyzing common issues and the status of additional examinations,and comprehending the points to note and trends in maintenance for the comprehensive
inspections of concrete dams implemented in recent years, we selected and inspected thefollowing each matter as topics that deserve closer scrutiny for items b) and c) in the paper.
ĀContinuous measurement to evaluate soundness”
࣭Measurement of dam body deformation
࣭Measurement of uplift pressure and seepage volume at foundation drainage holes
࣭Measurement of seepage volume at joint drainage holesĀContinuous understanding of the state of deterioration”
࣭Method for evaluation and repair of dam body deterioration progress
࣭Method for evaluation and repair of seepage impact on dam body deterioration
࣭Method for evaluation and repair of foundation drainage hole function࣭Water analysis of rock contact surface seepage and evaluation of seepage path
࣭Analysis and evaluation of ditch sediment of drainage gallery
3. MATTERS TO SCRUTINIZE BY ITEM DURING COMPREHENSIVE DAMINSPECTIONS
3.1 Measurement of dam body deformation
In the Ordinance Concerning Structural Standards for River Administration Facilities, Etc.
(“the Structural Ordinance”), measurement of dam body deformation is required for dams
with a dam height of 50m or more. In many places, dam body deformation measurements ofconcrete gravity dams are made by direct plumb line measurement in a single location
established at the maximum cross section, but in locations where parts of the bedrock
geology are poor, there also are dams for which measurement and monitoring are performed
by inverted plumb line. For dams with low dam height, some dams have no deformation

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measurement instruments and deformation is not measured, but there also are dams where thecrest reference point is being measured and the amount of deformation controlled.
Together with a rise in the reservoir water level during test impoundment, the measurement
values change on the downstream side, but no remarkable accumulation is evident.
While no measurement is required for dams with a dam height of less than 50m, somemeasurement is preferred so that any signs of deformation might be detectedbecause dam
body deformation is an important item for evaluation of safety. There also are cases where acomprehensive inspection provides the opportunity to monitor and record the condition of thehorizontal joint openings or gaps, as one supplement to deformation measurement.
3.2 Measurement of uplift pressure and seepage volume at foundation drainage holes
Under the Structural Ordinance, the measurement of seepage volume and uplift pressure at
concrete dams is required. Foundation drainage hole seepage volume has been seen toincrease temporarily at some dams directly following a large-scale earthquake, but thevolume gradually falls, and over the long term generally exhibits a declining trend.
While dams where the measurement value has partially exceeded the design uplift pressure
have been seen, such uplift pressure is not a problem on a stability analysis, and generally adeclining trend is seen with the passage of time after initial impoundment.
Foundation drainage hole problems such as drainage conduit blockage and hole conduit
corrosion with the passage of time, and the deterioration of Bourdon tube-type pressuregauges, have been confirmed to be common.
The method of uplift pressure measurement is an issue common to each dam. To measure
uplift pressure under the conditions acting continuously on a dam, or to reduce the upliftpressure load on a dam, use of the partition hole measurement method (measured by closing
each other holes) as a measurement standard is recommended for uplift pressure
measurement, but here and there dams where the pressure is being measured by total hole
blockage method (measured by closing all holes) , for reasons such as past experience withuse of the method or to shorten the measurement time, also were seen. With regard to the
uplift pressure measurement method, making measurements with the partition holes blocked
rather than the total holes blocked, and taking measurements after a “stabilization period” haspassed to allow uplift pressure to stabilize, is preferred in order to reduce the load on the dam
body and ensure its stability.
Moreover, examples were seen where the valve blocking time until measurement has been
decided based on experience without verifying the time until the uplift pressure has stabilized,
and for these dams, the time to stability is being confirmed by an additional examination andan evaluation of the validity of the measurement method performed.
For seepage volume, it also is important to check the precision of the measurement
instruments etc. by a comparison of each hole’s measurement results and the measurement
results for total volume based on a triangular weir. In addition, when recording andorganizing the measurement results, it is best to organize the data by matching it with records
from times such as earthquakes, floods, and equipment upgrades.

3.3 Measurement of seepage volume at joint drainage holes

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Although seepage from joint drainage holes rarely becomes a significant problem for dam
body stability, measures are required from the viewpoint of extending dam service life if theseepage volume is substantial.
For many dams, seepage from joint drainage holes frequently increases at initial
impoundment, then decreases and stabilizes with the passage of time.
At many dams, measurements of joint seepage volume are not made at each hole, and thetotal amount of joint seepage volume is calculated by measuring the foundation drainage holeseepage volume when uplift pressure is measured and subtracting this from the total seepage
volume in the drainage gallery area.
Same as foundation drainage holes, it is preferable to measure whether there is seepage fromeach hole, and the status for the joint drainage holes.
Figure 1 shows the adhesion of free lime to the hole discharge port. A preferable approach is
to give drainage hole discharge ports a shape that makes measurement feasible, such as
turning the outlet downward (Figure 2).
Furthermore, depending on the dam, watestops might not be installed on the downstream sideof non-overflow sections, and cases where seepage in the drainage gallery increases during
rainfall were seen.

Figure 1˖
˖Free lime adhesion to joint drainage hole   Figure 2˖Joint drainage hole(example of shape that enables measurement at each hole)

Another problem for the formulation of maintenance policies is poor recordkeeping, such as
when the records of countermeasure works for large seepage from joint drainage holes
cannot be stored, or when the structure of waterstops or joint drainage holes cannot be
confirmed from drawings, or when onsite confirmation isn’t possible even though drawing
descriptions are available. Organizing and retaining records such as facility schematic
diagrams is helpful for maintenance.

3.4 Method for evaluation and repair of dam body deterioration progress
Concrete dam body deformations are classified as either initial flaws or deterioration that
advances with time. Initial flaws are mainly defects such as cracks that occur during
construction or immediately thereafter because of construction-based problems; major flaws
are addressed when they are confirmed. Minor flaws that are not addressed frequently will

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have to be re-evaluated during a comprehensive inspection, because up until the inspection
nothing has been left behind as a record.
Much of the deterioration seen in the dam bodies of concrete dams typically is freeze
damage; damage also is caused by deterioration from alkali aggregate reaction attributable toaggregate, and by expansive harmful mineral substances. In addition, potential cracks
forming during construction will sometimes expand over time and cause concrete splinters toflake off. It is vital to understand appropriately such detrioration and predict the progress ofdeterioration, but the evaluation is difficult if time series data are not obtained, making it
important to show accurately a maintenance plan that makes it possible to monitor changes in
dam condition over an extended period by means of comprehensive dam inspections.
Although cracks in construction joints and parts of spillway channel walls and the occurrenceof seepage and efflorescence on the upstream and downstream sides of dam bodies have been
noted frequently in cases up to now, damage at a level that would directly affect the dam
body has not been seen.
Moreover, examples can be noted where cracks have entered concrete structures such as thespillway channel walls or footing. Because reinforcing bar corrosion is a concern with such
reinforced concrete structures, examining the effects on rebar also is important.
As additional examinations, efforts such as deterioration diagnosis using infrared cameras,
drone photography of components that cannot be viewed directly, surveys of aggregate
laumontite content, strength tests based on cores, and tests of chloride ion content are being
implemented.
Because confirmation of strength with a test hammer and neutralization tests can be
implemented simply to verify the status of dam body concrete deterioration, theirimplementation in basic inspections is well established, and it was confirmed there was noremarkable deterioration. In addition, there also have been many cases recently where theextent of concrete surface flaking has been checked by means of infrared camera
photography; an example of inspection using an infrared camera is shown in Photo 3 andPhoto 4.
Because comprehending the extent to which dam body concrete surface deterioration has
progressed is so important for future maintenance, we are working to organize a list of cracks
and other damage for each dam and understand present conditions.
Infrared heat image

Visual image

Float, flaking

Figure 3˖˖ Image of visible concrete deterioration area Figure 4˖Example of heat image
taken with an infrared camera

3.5 Method for evaluation and repair of seepage impact on dam body deterioration

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Because of the possibility seepage from the horizontal construction joints or transverse joints
on the downstream side of a dam will hasten dam body concrete deterioration, a proper
evaluation is necessary when performing a comprehensive dam inspection. While seepage
from horizontal construction joints or efflorescence precipitation can be seen at many dams,
cases where seepage has caused noticeable advanced deterioration of the dam body are rare.
Because the amount of seepage and locations where it occurs will vary depending on theseason and water level behind the dam, it is important to understand the differences inexternal conditions.

3.6 Evaluation of effectiveness foundation drainage hole function
Function of foundation drainage holes installed a dam’s foundation must be properly
maintained, because these are both a facility for measuring seepage volume and upliftpressure and a critical facility used to reduce uplift pressure in the foundation rock andimprove dam stability. There are many cases, however, where the loss of this uplift pressure
alleviation function is thought to be a real possibility when the foundation drainage holes
become blocked with free lime or other materials over the course of dam operation.
Verification inside the holes (insertion of an inspection rod, verification by camera inside theholes, survey of the change in water level inside the holes, etc.) is effective as a means toconfirm drainage hole function, and for blocked holes confirmed, a study of countermeasures
such as re-boring are required.
For holes without drainage water, a survey of the change in water level inside the holes(pumping test) is conducted by lowering the water level inside the holes using a bailer (inhole water sampler device) and confirming the change in water level (has the water levelrecovered) after passage of a fixed time (typically after 24 hours). A pumping test makes it
possible to perform effectively a function evaluation even if drainage hole conditions are
difficult to understand sufficiently through direct observation.
The majority of dams are stable because dam seepage volume from the foundation drainage
holes exhibits a downward trend over time after initial impoundment, and no upward
tendency in uplift pressure is seen as well.
Although these are not a problem if impermeability has increased because of foundation rockblockage, a blockage confirmation inspection of foundation drainage holes is implemented
for dams where impermeability is thought to be the result of foundation drainage holedeterioration and blocking is feared.

3.7 Rock contact surface seepage
Although seepage sometimes occurs in the rock contact surface with downstream toe of thedam, if the volume is significant it is necessary to comprehend the origin and path andinvestigate countermeasures. Dams where seepage from downstream rock contact surface has
been confirmed at the footing region or areas abutting the surrounding earth have been noted,
and in all instances, the water is being exuded slowly and no remarkable seepage has been
seen. While the bulk of this seepage was at the level of a slow trickle, and the precise volume
was not grasped during daily observations, should the seepage volume be substantial, daily

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monitoring will be necessary to understand the change over time and change because offactors such as weather conditions.
As an additional examination, we are implementing water analyses and inferring percolationroutes by comparing the water quality characteristics of the seepage and samples such as
water from the reservoir, water from the surrounding mountain streams, and groundwater.

3.8 Ditch sediment
Precipitate sediment originating in the seepage can be found in the drainage gallery ditch,
along stairs, and in the triangular weir for seepage measurementIf the volume is substantial
its source and path must be understood to evaluate the foundation rock’s stability. It is
important to understand where the ditch deposits are originating and the quantity flowing out,
which requires regularly cleaning the ditch so the conditions over time can be understood.
The impurities contained in seepage from the foundation drainage holes also must be
examined carefully and the knowledge gleaned reflected in daily management.
Furthermore, because the presence or absence of outflows of foundation rock constituent
materials that serve as an indicator of foundation rock seepage failure is a critical viewpoint,
it is important to evaluate an investigation of ditch deposits based on the conditions of thefoundation rock, such as the extent of the foundation rock consolidation or weakness of thefault or seams. If an outflow of foundation rock constituent materials from a specificfoundation drainage hole has been confirmed, it becomes necessary to investigate carefully
whether there is any concern of foundation rock seepage failure.
In any investigation concerning ditch deposits it becomes important to first confirm thecircumstances and volume of the deposits based on a general survey inside the drainage
gallery and verify the properties of the deposits visually, and understand the accumulation
characteristics for the dam in question.
If the amount of deposit material is determined as the result of a survey to be comparatively
large, its origin should be surmised by gathering samples from locations that show the
representative characters, conducting tests such as a grain size analysis, fluorescent X-ray
diffraction, and X-ray diffraction, and understanding the percentage of amorphous material,
the involvement of iron bacteria, and the amounts of substances such as quartz, plagioclase,
calcite, and mica contained in the samples.
Furthermore, if there is a high probability the foundation rock is the origin of the deposit
material, a monitoring survey to identify the effluent pathway and study carefully the depthof concern of a foundation rock seepage failure must be conducted, and steps to address thesituation undertaken if necessary.
Precipitate has been confirmed in the drainage galleries of several dams in recent years, andthe causes of its occurrence have been surmised based on additional examinations and theissues that require careful attention for maintenance have been organized and reflected inmaintenance policy.
While “rock fragments, etc. associated with foundation rock deterioration and failure”,“slime during foundation drainage hole excavation”, “precipitation of substances ingroundwater”, and “materials originating in human activity (adhered substances duringdrainage gallery entry and egress)” have been considered as possibilities for the origin of thematerials deposited (precipitated) in drainage gallery ditch, once the characteristics of each of報－152

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these potential sources have been understood it might be feasible to surmise the origin of
ditch deposits by comparing their components (mineral composition and granularity).
Using the samples gathered, we performed a componential analysis to understand thepercentage of amorphous material, the involvement of iron bacteria, and the amounts ofsubstances such as quartz, plagioclase, calcite, and mica contained in the samples, and used
this as base data to surmise the origin.
Here the methods are shown in Table 1 as componential analyses, and while each possessed
the characteristics shown in the table, further improving the precision for specific mineralsubstances will be possible by implementing these methods in combination.
When we analyzed the dams in this study mainly by fluorescent X-ray diffraction andpowder X-ray diffraction, and also included observations by polarizing microscope for somedams, the deposits were judged to have been generated by a chemical reaction attributable to
the water quality of the infiltrating water and the dam body concrete and not to haveoriginated in the foundation rock, and we evaluated the substances were not exuded fromdamage to the foundation rock. For deposit control, priority monitoring will be important in
the future as well as a crucial control factor for understanding whether there is expansion of
the percolation path from the foundation rock.

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Table 1˖˖Summary of typical element analysis methods and points to note for ditch sediment
Method ofanalysis

Summary

Points to note

Stereoscopically viewed aggregate andcrystalline morphology of minute mineralsubstances in spaces between rock and ore,and the rock and ore composition, at about
20-40 times magnification. Although identical
Stereomicroscope to identification by the naked eye in the senseof identifying mineral substances by external
appearance, magnification is higher than byloupe (usually about 10 times) and could beviewed stereoscopically.

Mineralcolor
andthreedimensional form are judged by
the naked eye through a
microscope, and judgments ofgeneral rock-forming mineralssuch as quartz and feldspar are
possible to some degree, but
judgments with a high degree ofprecision and judgments ofdistinctive mineral substances are
difficult.

Rock (flake) slides are prepared and the types
and composition of minerals contained in therocks examined. Magnification is about 50400 times. Because slide preparation isdifficult for rocks containing powdery or
fragile mineral substances, such rocks are notexamined in many cases. For loose sediment,
flakes are produced after fixing the sedimentonce with an adhesive, etc.

Using
the
various
opticalproperties unique to minerals,
accuracy is high for specificminerals but requires a highdegree of expertise.

Non-crystalline materials such asorganic substances, and substanceswith poor crystalline structuresthat do not display a diffraction
pattern, cannot be detected.

X-ray powder
diffraction

Identification of types of mineral substances
is made from data based on the atomic
arrangement (crystallographic structure) of
the minerals. X rays are incident on the
crystalline material, and when each atom
scatters the X rays and the scattering intensity
is recorded for each scattering angle, theunique scattering spectrum of the material is
obtained. The diffraction angle positions andstrength are unique to each crystallographic
structure, and mainly inorganic compounds
can be identified from the diffraction patterns.
This is especially effective for identificationof mineral substances having the samechemical composition but different atomic
arrangements.

Fluorescent Xray analysis

Samples are irradiated with X rays, causing
the fluorescent X-rays of the various
wavelengths generated to be incident, and bycontinuously changing the angle of thecrystals, the strength of the diffraction X rays
is measured sequentially. From the X-ray
strength obtained, the types of elementscomposing the sample and their rough content
are examined.

This is a method for examining thecomposition and content of theelements in the rocks; it cannot
specify the mineral composition
directly.

Granularity
analysis

The origin (genesis) of deposits is surmisedby analyzing grain size and comparing thiswith the results from studying the genesis of
the original materials.

Can be applied only if there arecharacteristics in the grain size
composition of the original
materials.

Polarizing
microscope

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When used together with X-ray
diffraction, can also deal withvarious mineral compositions.

When specifying substances suchas clay minerals a fixed orientation
analysis involving several separate
preprocessing steps is required.

The method is effective when theaccuracy of the material’s origin is
increasedthroughuse
incombination with other methods.

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4. CONCLUSION
In this study we have selected and organized issues from the viewpoint of extending the lifetimeof existing dams, based on examples of comprehensive dam inspections implemented after
formulation of the Implementation Guidelines. From the results we selected issues common toexisting dams and items that need to be measured and analyzed to diagnose deterioration, andsummarized the investigation procedures and evaluation methods.
In the future, comprehensive inspections of dams will be used to extend dam service life byrepeating the PDCA management process based on a 30-year cycle. We will continue observing
closely to see whether maintenance based on plans to extend service life will demonstrate resultsin the future.

REFERENCES[1] Ministry of Land, Infrastructure, Transport and Tourism, Water Management and National Land
Preservation Bureau, River Environment Division (2013): Comprehensive Dam Inspection
Implementation Guidelines and Interpretation (in Japanese)[2] Ministry of Land, Infrastructure, Transport and Tourism (2015): River Erosion Control Technology
Standards: Maintenance (Dam Edition) (in Japanese)

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