COMMISSION INTERNATIONALE
DES GRANDS BARRAGES------VINGT-QUATRIÈME CONGRÈS
DES GRANDS BARRAGES

Vienne, Juillet 2018-------SLAKING COUNTERMEASURES RELATED TO ROCK CONTACTEXECUTION AT THE SOFT ROCKS FOUNDATION(*)

Satoshi YOSHIMURA & Singo TAKASUGIApporo Dam Construction Office, HOKKAIDO PREFECTURAL GOVERNMENT

Nario YASUDA
Director of Engineering Department Ⅰ, JAPAN DAM ENGINEERING CENTER
Hidetaka SATOH
Senior engineer, DOCON CORPORATION
JAPAN

1.

INTRODUCTION

Apporo dam was originally planned as a concrete gravity dam, but survey
and investigation aiming at trapezoidal CSG (Cemented Sand and Gravel) dam
started in 2003, and ultimately the type of trapezoidal CSG dam was selectedfrom the view points of the economic and environmental advantages as thesecond following Tobetsu dam in Hokkaido prefecture. Apporo dam used soft rockas the CSG material for dam body, and its dam foundation is also soft rock.
Properties of the soft rock in area of Apporo dam showed slaking caused by cyclic
wetting and drying. Additionally, it is known that its foundation is dominated bycracks which extend at an angle of about 20º on the upstream side of left
abutment parallel to the bedding plane, and that unevenness along the cracks
tends to be formed on its excavated surface.
(*)

Contre-mesures de relâchement liées à l'exécution de contact rocheux sur la

fondation de roches tendres

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2.

OUTLINE OF APPORO DAM

Apporo dam is a multi-purpose dam now under construction at the Azuma
river in Horonai frontage of Atsuma town, Yufutsu district of Hokkaido prefecture
as part of comprehensive development of the Azuma river. The dam is, as shownin Table 1, 47.2 m in height, 47,400,000 m3 in total reservoir capacity, and43,100,000 m3 in effective reservoir capacity. The purpose of dam is to regulate
flooding, maintain the normal functions of the river flow, and to supply water toirrigation systems and to urban area. As shown in Fig. 1, the construction of main
dam body has been completed and initial impoundment started in October 2017.
Table 1Main feature of Apporo dam
Dam type

Trapezoidal CSG

Dam height47.2 mCrest length

516 m

Body volume481,000 m3

Catchment area

105.3 km2

Reservoir area

3.03 km2

Total storage capacity

47,400,000 m3

Effective storage capacity

43,100,000 m3

Fig. 1
Apporo dam under construction
Barrage d’Apporo en construction

3.
3.1.

TOPOGRAPHY AND GEOLOGY AT THE DAM SITEOUTLINE OF THE TOPOGRAPHY

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The dam site is surrounded by medium to low relief-mountains withelevations less than 300 m. The alluvial lowland about 60m wide has formed
along the Azuma river which meanders across this lowland. Both left and rightabutments at the dam site are formed by the slope of gentle mountainside withgradient of about 25º, and three terrace plains.
3.2.

OUTLINE OF THE GEOLOGYAt the dam site, Karumai formations consisting of Neogene-Miocene
sedimentary rock are distributed. The dam foundation is mainly shale strata (shstrata) and alternation strata of shale and sandstone (shal strata), enclosing
thinner stratum of conglomerate (cg strata) or tuff (tf strata) (see Figs. 2 and 3).
These layers of foundation bedrock are covered with fallen volcanic ash andpumice stone in the Quaternary distributed to a depth of about 10 m. The strata of
the foundation bedrock are inclined at an angle of about 20º on the upstream sideof left abutment, but no large-scale fault fracture zones are distributed. The shale
strata (sh strata) and the alternation strata of shale and sandstone (shal strata) of
the dam foundation form massive bedrock with few cracks and with unconfinedcompressive strength from about 20 to 50 N/mm2 in the fresh parts. Additionally,
its concreteness of bedrock permits to occur cracks of its own when struck hard bya hammer, but in the shallow bedrock, cracks to form along the bedding plane has
been confirmed [1].

Shale

Lithofacies division

Shale・Sand alternation (Shale priority)
Conglomerate
Sand・Shale alternation (Equal)Fig. 2Geological longitudinal section of dam site
Coupe longitudinale géologique du site du barrage

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ShaleLithofacies division

Shale・Sand alternation (Shale priority)
Conglomerate
Sand・Shale alternation (Equal)Fig. 3
Geological plan of dam site
Plan géologique du site du barrage
However, where the surface of bedrock is exposed, hexagonal cracks
caused by slaking occur and leads to fine fragments of bedrock (see Fig. 4). So,
since the start of the survey, it was understood that this would be a problem to besolved for dam construction. As a result of a past survey, it was known that if
bedrock surface was exposed, hexagonal cracks occurred in about 2 days, butthey did not become mud, and that when the surfaces were protected bysubmersion or mortar, hexagonal cracks did not occur.
After 23-days

Fig. 4
Slaking of shale strata exposed at long period
Relâchement de la couche de schiste exposée à longue période

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4.

SLAKING PROPERTIES OF FOUNDATION BEDROCK

To clarify the slaking properties of sh strata and shal strata distributed at thedam site, in-situ slaking tests were carried out during the preliminary survey and ina laboratory.

4.1.

OUTLINE OF THE SLAKING TEST AT THE PRELIMINARY SURVEY

Heavy machinery was used to remove only the loosened and weatheredparts (about 30 to 50cm in depth) at the trench bottom which was excavated andexposed for several years. On the fresh shal strata of the riverbed trench, anotherdeeper trench was excavated to prepare three test surfaces (each, 1.5m×1.5m):
non-protection, protection by submergence, and protection by mortar (see Fig. 5).
Non-protection
Protection by thesubmergence
Protection by mortar

Thickness ofmortar is 5cm
Submerged stateFig. 5
Schematic view of in-situ slaking test at preliminary survey
Vue schématique du test de relâchement in situ à l’enquête préliminaire
On the submerged surface, a lower surface was excavated to form steps,
and after initial observation of the test surface was completed, it was maintainedin moist condition with a curing sheet and then submerged. The mortar protectedsurface was covered by mortar immediately after initial observation of the test
surface.
Each test surface was confirmed the state of cracks 3 hours, 6 hours, 1 day,
2 days, 3 days, 7 days, 26 days, 45 days and 1 year after the excavation(see Fig.
6), then the slaking depth was measured by cores sampled 26 days, 45 days and1 year after the excavation. Locations of cores are shown in Fig. 7.
In the laboratory tests, the core of 1.0 m long sampled just after the
excavation in Fig. 7 was repeatedly wetted and dried (10 cycles), confirmed thechange of their water absorptions and splintering.

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Cracks

After excavation
2days later
3days later

7days later
26days later
45days later

Fig. 6In-situ test, non-protection surface (45 days after exposure)
Test in situ, surface non-protectrice (45 jours après l’exposition)
Shaping range（1.5m×1.5m）（Mortar protection range）

26daysTest plane (1m×1m)（Plane sketch）

1year
Just after
excavation[Sampling of boring core]・「Just after excavation」「26 days later」「45 days later」「1 year later」
・Sampling of 1 m core

45daysFig. 7
Schematic plan of test surface
Plan schématique de la surface de test
4.2.

TEST RESULTS AND CHARACTERISTICS OF SLAKING

The in-situ test of the unprotected surface indicated almost no change
immediately after, 3 hours after, and 6 hours after the excavation. However, it was
found that cracks appeared 2 days after the start of the test and that the cracks
advanced with the passage in time. Seven days later, hexagonal cracks partially報－68

increased and partial exfoliation occurred. But the depth of cracks was a few
centimeters from the surface. The results up to 45 days after the excavation did
not show any transformation into mud. However, until 1 day after the start of test,
the test surface was maintained wet condition by rainfall. Cores sampled from theunprotected surface after one year confirmed that the cracks caused by slaking
reached to a depth of only 8 cm.
On the submersion protected and mortar protected surfaces, no cracks
occurred 26 days, 45 days, and 1 year after the excavation.
The laboratory test using specimens sampled from the unprotected section
and mortar protected section, showed splintering after wetting and drying for two
cycles and advancing of splintering of fine fragments to several millimeters for 10
cycles. Furthermore, test results indicated the increase of water absorption from
10% to 15%. But the specimen did not dissolve into mud.
Judging from the test results during the preliminary survey, it has been clearlyshown that if the foundation bedrock at this site was left unprotected, cyclic
wetting and drying would advance cracks and cause splinter-shaped exfoliation. It
was also confirmed that, (1) splintering is caused by cyclic wetting and drying but
the material is not dissolved into mud, (2) at the first stage of cyclic wetting anddrying, cracks occur, but the affected depth is less than 10 cm after 1 year, and (3)the protection work by submergence or mortar can restrict the progress of slaking.
4.3 .

DEALING WITH SLAKING AT THE STAGE OF PRELIMINARY SURVEY

As described above, it has been confirmed by slaking tests during thepreliminary survey that the foundation bedrock of Apporo dam shows slaking
properties. On the other hand, the base of a trapezoidal CSG dam in contact withthe foundation ground is divided into seepage control concrete area, unified CSG
area, and cohesive CSG area [2] (see Fig. 8).
The foundation treatment method that includes rough excavation,
secondary excavation, bedrock cleaning, and placing is categorized for seepage
control concrete area and unified CSG area those require foundation treatment
similar to that of a concrete gravity dam, and for cohesive CSG area where
exfoliated or loosened rock, etc. caused by rough excavation are removed.
Therefore, for each foundation of seepage control concrete area, unified CSG
area and cohesive CSG area, it was necessary to establish the foundation
treatment method considering slaking from rough excavation, secondary
excavation, bedrock cleaning, and placing [3]. In the following chapter, practicaltreatment method during execution will be described.

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Protection concrete

Protection concrete

CSG
1：0.8

1：0.8
Structural concrete
Seepage control concrete

Seepage control
concrete area

Rich-Mix CSG

Cohesive CSG area
Unified CSG area

Water-tightness security section
Slide resistance security section

Fig. 8Rock contact division at the base of CSG dam
Division du contact rocheux à la base du barrage de type CSG

5.

SLAKING CONFIRMATION TEST BEFORE CONSTRUCTION(INCLUDING LOOSENESS CONFIRMATION TEST OF
FOUNDATION BEDROCK)

Regarding the suitability of foundation treatment method at Apporo dam,as shown in Fig. 9, the duration and construction cycles required to spray mortar
to protect the exposed surface after the rough excavation were investigated byconfirmation test. As the next step, the duration and construction cycles required
to maintain moisture condition using a curing mat after removal of the sprayedmortar (and secondary excavation), were also studied by the same test. Fig. 9shows the foundation treatment for cohesive CSG area. Just before theconstruction of CSG part in rock contact, the confirmation test to verify theprocess of foundation treatment for slaking (including test to confirm looseness offoundation bedrock) was performed.
The geology of the foundation bedrock of the dam site is Karumai
formation consisting mainly of shale enclosing shallow strata of sandstone, hard
shale, tuff, tuffaceous mudstone, and conglomerate. Of these, enclosed parts withsparse sandstone and those with dense sandstone were categorized as shale
strata (Sh strata) and alternation strata of shale and sandstone (Shal strata)
respectively. It was confirmed that in shale, cyclic wetting and drying causeslaking very rapidly. This test confirmed the slaking rate of alternating shale and報－70sandstone strata (Shal strata) and of shale strata (Sh strata), and verified thenumber of days allowed until the foundation bedrock loosens before specified
slaking countermeasures have been taken.
Construction flow of rock contact at cohesive CSG area
Foundation treatment atcohesive CSG area

Slaking countermeasure

Non slaking

SlakingRough excavation（T = +50cm）

Rough excavation（T = +50cm）

Confirmation of bedrock
Bedrock surface protection by
Mortar spraying

Mortar removal,
Secondary excavation

Secondary excavation（Excavation line）
Upper：Small breaker
Lower：Manpower (pickhammer, bar)YES

CSG placing justafter removal ?

Bedrock cleaning（loose stone removal）
Bedrock cleaning（loose stone
removal）
Bedrock sketch
and inspection

NO

Less than 1 hr

Protection by curing mat(Wet condition）
Bedrock cleaning（Curing mat removal,
water-washing ）Less than 6 days

・Wet condition
・Spread mortar
CSG placing

CSG placing
打設

Fig. 9Construction flow of rock contact at the dam foundation (cohesive CSG area)
Flux de construction du contact rocheux à la fondation du barrage (zone cohésive
CSG)
5.1.

OUTLINE OF THE TESTThe tests were executed as follows.
(1) Secondary (finishing) excavation by manpower is performed.

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(2) The objects of the test are for four geological properties (rock-massclassification BⅢb and BⅢc of Shal strata, and rock-mass classification BⅢband BⅢc of Sh strata). After the bedrocks with prescribed four rock-massclassifications are exposed by the excavation, loosened and weathered parts
are manually removed. Here, the criteria of rock-mass classification are shownin Table 2.
Table 2
Parameters of rock-mass classification of Apporo dam
Item

SymbolA

Hardness

Trench(Bottom)

Very hard, does not break easily by the strike by hammer.

Same

Ⅰ
Ⅱ

Stick-type core of 30 cm to 50 cm

30～50cm

Ⅲ

Stick-type core of 5 cm to 30 cm
Disc-type or stick-type core of less than 5 cm. Disc-type
core of 1 cm.
Cohesive, unweathering along the cracks.

15～30cm

Crack surface is weathering. Clast is unweathering.
Weathering along cracks. Softening along cracks. Cracks
catch the clay.

Same

Ⅳ
a
State ofcrack

Boring coreHard, break by the strike of hammer. Sound of strike isscream or impure.
Medium hard, break easily by the strike of hammer. Soundof strike is impure.
Stick-type core of more than 50 cm

B
C

Crack
interval

Subdivision criteriabcYard 4; shale (CLh grade)

Yard 1; shale + sand (CM grade)

Same
Same
Same

5～15cmSame
Same

Yard 3; shale (CM grade)

Yard 2; shale (CLh grade)Fig. 10
Locations of slaking confirmation test (left side of the dam base)
Emplacements du test de validation de relâchement (côté gauche de la base dubarrage)Fig. 10 shows the locations of the four test yards selected.
(3) The test was, as shown in Fig. 11, done for three cases: “continuous water
spraying”, “moisture mat curing”, and “non-protection” where any curing of

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exposed surfaces was not performed. The exposed surface with non-protection
was observed at every one hour to confirm the advance of slaking.
(4) Sketches of the surface observed during each test were made. When slaking
was seen, the depth of crack was measured, and at the same time as therepulsion degree was also measured with a Schmidt hammer.
TEST CONDITIONS

5.2.

Table 3 shows the curing conditions and verification frequency of the slaking
confirmation test. At the test locations, because the confirmation test was
reluctantly performed in the winter month of March, the test field was enclosed
and heated to adjust weather conditions to match the dam body placing period(see Fig. 12).
5.3.

TEST RESULTS

5.3.1.

Slaking Confirmation Test Results of Foundation Bedrock

Test results are shown in Table 4.
・On Shal strata with BⅢb; when the exposed surface was left uncured, slaking
caused hexagonal cracks 2.5 hours later, and drying was accompanied byadvance of cracks (see Figs. 13 and 14 (1) ).
・On Shal strata with BⅢc; when the exposed surface was left uncured, slaking
caused hexagonal cracks 1 hour and 40 minutes later, and drying was
accompanied by advance of cracks (see Fig. 14 (2) ).
・On Sh strata with BⅢc; when the exposed surface was left uncured, slaking
caused hexagonal cracking 1 hour after the start of drying, but this depended
on the weather.
Table 3
Slaking confirmation test, curing condition and validation frequency
Test cases
Continuous
water spraying
Moisture mat
curing

No curing
while leaving
surface
exposedCuring conditions

Verification
frequency• Sprinkler + supplementary continuous water spraying,
immediately after finishing excavation,• Moisture mat curing, immediately after finishing excavation(spray curing until the mat is placed)• Moisture mat used is for curing concrete with 3mm of thickness,and spraying is done before the mat dries.
• The rock surface immediately after finishing excavation is left in
exposed condition without curing by moisture mat or spraying.
• Observation is done 1/hour, and the time until cracks by slaking
occur is recorded.

After 6 hours
After 12 hours
After 24 hours
After 3 days
After 7 days

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3hrs
later
Moisture
Mat curing

1hr
later

7hrs
later
5hrs
later

15hrs
later
9hrs
later

21hrs
later
Rough excavation
plane

Excavation
Continuous
water spraying
2hrs
later

12hrs
later

6hrs
later
4hrs
later

8hrs
later

24hrs
later
18hrs
later

Fig. 11The shape of the trench and division on test surface
La forme de la tranchée et la division de la surface de test

Fig. 12
Slaking confirmation test in field
Test de validation de relâchement sur le terrain
It was confirmed that on both the Shal and Sh strata, hexagonal cracks
caused by slaking occurred between 1 hour and 2.5 hours from the start of drying
after the excavation. It was also confirmed that as a result of performing
continuous water spraying or moisture mat curing for 7 days, the hexagonal
cracks by slaking and a decline of the repulsion degree of a Schmidt hammer did
not occur, confirming that they restrict the advance of cracks. But on bedrock with
BⅢc, in addition to the advance of slaking, conspicuous looseness caused by thestress release after the excavation was seen [4].

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Test
yard
1

Foundation bedrock
geology
Shal layer,
Rock-massclassification BⅢbTable 4
Slaking confirmation test results
Test results
(1)

(2)

2

(1)

Shal layer,
Rock-massclassification BⅢc

(2)

(3)

3

(1)

Sh layer,Rock-massclassification BⅢc(2)

The surfaces not cured remained moist and did not show
cracks until 1.5 hours after start of the test because of cloudy.
But about 2.5 hours later (4 hours after start of test), when thesky had cleared drying the test surfaces, hexagonal cracking
occurred and the cracks extended as drying continued.
If continuous spraying or moisture mat curing was done, nocracking and no looseness of the bedrock surface occurred
until about 1 week after test start.
It was confirmed that continuous spraying or moisture mat
curing restrained cracks until about 1 week after test start.
The surfaces not cured (Nos. 3 to 7) were dry since the test
start because of clear sky, then 1 hour and 40 minutes after thetest start, cracks appeared at right angles to the bedding plane.
Even if continuous spraying or moisture mat curing was done,looseness occurred on the entire bedrock surface because of
the stress release, and it was judged that if this situation
continued, placing would be impossible.
On surfaces not cured (Nos. 3 to 14), from the test start until
9:00 the next day, the surfaces remained wet and no cracking
occurred because of cloudy. The sky cleared at about 9:00 the
day following the test start, so about 1.0 hour later when thetest surfaces had dried, hexagonal cracking occurred.
Even if continuous spraying or moisture mat curing was done,looseness occurred on the entire bedrock surface because of
the stress release, and it was judged that if this situation
continued, placing would be impossible.

Moisture mat
curing

1hr
later

3hrs
later

5hrs
later

7hrs
later

9hrs
later

15hrs
later

Continuous
water spraying

2hrs
later

4hrs
later

6hrs
later

8hrs
later

12hrs
later

18hrs
later

21hrs
later

No-curing, Observation; every 1hr (0-9hr), every 3hr (9-24hr)

Fig. 13
Test results at Yard 1 (alternation strata of shale and sand stone, BⅢb grade）
Résultats du test au chantier 1 (alternance couche de schiste et sable, classe
BIIIb)

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(1) BⅢb grade(2) BⅢc gradeFig 14Crack occurrence at alternation strata of shale and sand stone
Apparition de fissure sur l’alternance couche de schiste et de sable

5.3.2.

Looseness Confirmation Test Results Of Foundation Bedrock

On bedrock of Shal and Sh strata with BⅢc, in addition to the advance ofslaking, conspicuous looseness caused by the stress release was found, so
supplementary excavation of about 50 to 80cm was executed. Therefore thebedrock of BⅢb without looseness was finally exposed and continuous water
spraying or moisture mat curing was performed, confirming the advancement of
bedrock looseness. As a result, the Shal and Sh strata were excavated to the
bedrock with BⅢb and the excavation surface was continually cured by water
spraying and moisture mat curing, confirming that neither slaking nor loosenessoccurred for 7 days (see Table 5).

5.3.3.

Correspondence To Slaking At The Stage Before Construction

It is possible to decrease the advance of hexagonal cracks by slaking byperforming continuous water spraying or moisture mat curing. But, in a casewhere the bedrock surface is rock-mass classification B Ⅲ c, the bedrocklooseness will advance according to the passage in time without treatment. On theother hand, in a case where rock-mass classification BⅢb has also been exposed,
performing continuous water spraying or moisture mat curing prevents loosenessof the bedrock surface up to 7 days after the test start.

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For this reason, secondary (finishing) excavation exposes the bedrock withclassification BⅢb. Then after excavation, moisture mat curing starts within 1 hour,
and the mortar is laid and placing starts within 6 days (see Fig. 15).
Table 5Looseness confirmation test results of foundation bedrock
Test
yard

Foundation bedrock geology

Test results(1)

2

Shal layer
Supplementary excavation to
Rock mass classification BⅢb(2)

3

Sh layer
Supplementary excavation torock mass classification BⅢb(3)

It was confirmed that curing by spraying +moisture mat restrained the occurrence of
cracking and looseness of the excavation surface
for 7 days.
When the excavation of test yard was temporarily
stopped, clear skies dried the test surface, andhexagonal cracking occurred one hour later.
It was confirmed that the test surface lowered by
10cm from the cracked surface (exposing CM
grade) and cured by spraying+ moisture mat,showed neither cracking nor looseness for 7 days.

Fig. 15
Moisture mat curing of dam foundation
Maturation par l’humidité de la fondation du barrage

6.

CONCLUSION

Apporo dam was constructed on the soft rock foundation. Properties of thesoft rock in area of Apporo dam showed slaking caused by cyclic wetting anddrying. Additionally, it is known that its foundation is dominated by cracks which
extend at an angle of about 20º on the upstream side of left abutment parallel tothe bedding plane, and that unevenness along the cracks tends to be formed on

報－77its excavated surface. So, the in-situ tests were performed to investigate theslaking characteristic of soft rocks at the preliminary survey and just before
construction work. Finally, the proper construction flow of rock contact at thefoundation was obtained.
The construction work of dam body began in October 2014, concrete
placing started on April 30, 2015, and CSG placing started on June 12, 2015. The
rock contact of Apporo dam was executed by overcoming the geological problems- deterioration of bedrock by slaking - then completing CSG placing at dam bodyon July 4, 2016. Concrete placing was completed on September 20, 2016. After
construction works of appurtenant structures such as downstream energy
dissipation, water intake facility and discharge systems, and the crest bridge etc.
have been completed, the initial impoundment started in October 2017

REFERENCES[1] OKABE, Y., TAKASUGI, S. and KONNO, M. Design and construction ofApporo trapezoidal CSG dam (part 1), Engineering for dams, Japan dam
Engineering Center, No.351, pp.45-77, 2015 (in Japanese).
[2] JAPAN DAM ENGINEERING CENTER. Engineering manual for design,
Construction and Quality control of Trapezoidal CSG Dam, Japan Dam
Engineering Center, 2012.
[3] FUJISAWA, T. (2014): CSG note (Part 4) －Design and Construction at therock contact, Engineering for dams, No.330, pp.3-16, 2014 (in Japanese).
[4] YOSHIMURA, S., TAKASUGI, S., KONNO, M., FUJISAWA, T., YOSHIDA H.
and YASUDA, N. Construction of Apporo Trapezoidal CSG Dam, 4th APG
Symposium and 9th EADC on Innovation technologies for dams andreservoirs toward the future generations, 2016.

SUMMARY
Apporo dam is a multi-purpose dam now under construction at the Azuma
river in Horonai frontage of Atsuma town, Yufutsu district of Hokkaido prefecture
as part of comprehensive development of the Azuma river. The dam is a
trapezoidal CSG dam with of 47.2m in height, 47,400,000m3 in total reservoir
capacity, and 43,100,000m3.in effective reservoir capacity. Apporo dam uses softrock as the CSG material of dam body, and its dam foundation is also soft rock.
Properties of the soft rock surrounding Apporo dam include slaking caused bycyclic wetting and drying, so various technical studies were conducted in order touse soft rock as the foundation bedrock. The dam foundation is extremely unevenso the excavation procedure was also studied. Slaking tests during the survey and報－78

slaking confirmation tests before construction works showed clearly the slaking
characteristic of this bedrock and leads to the effective countermeasures. The
construction works were completed and the first impoundment started in October
2017.

RÉSUMÉ
Le barrage d’Apporo est un barrage polyvalent actuellement en construction
surà la rivière d’Azuma à Horonai, en front de la ville d’Atsuma, préfecture de
Yufutsu, district de Hokkaido, dans le cadre du développement global de la rivière
d’Azuma. Le barrage est un barrage trapézoïdal de type CSG d’une hauteur de47.2 m, d’une capacité totale de réservoir de 47,400,000 m3, et d’une capacité deréservoir efficace de 43,100,000m3. Le barrage d’Apporo utilise des roches
tendres comme matériau CSG pour le corps du barrage, et sa fondation estégalement en roche tendre. Les spécificités de la roche tendre autour du barraged’Apporo incluent le relâchement causé par mouillage et séchage cyclique, ainside diverses études techniques ont été menées afin d’utiliser la roche tendre
comme fondation. La fondation du barrage est extrêmement inégale donc laprocédure d’excavation a également été étudiée. Les tests de relâchement aucours de l’enquête et les tests de validation de relâchement avant le travail deconstruction ont clairement montré des caractéristiques de relâchement de cettefondation et ont conduit à des contre-mesures efficaces. Les travaux deconstruction ont été achevés et la première mise en eau a débuté en octobre
2017.

Key words; Foundation treatment, Geological investigation

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