Proceedings of the 22nd IAHR-APD Congress 2020, Sapporo, Japan

Topographic and Geological Features Involved in Water Leakage at Levee Foundations
and in Levee Body Deformation by Piping on the Kakehashi River
Yutaka SATO
Research and Development Initiative, Chuo University, Tokyo, Japan, satoh_yt@kitac.co.jp
Shoji FUKUOKA
Research and Development Initiative, Chuo University, Tokyo, Japan, sfuku@tamacc.chuo-u.ac.jp

ABSTRACTThis study aims to estimate the locations of levee collapse resulting from the piping in foundations, and themechanism behind such collapse. First, topographic and geological features were investigated on the Kakehashi
River. Based on the interpretation of aerial photos taken by the US military in 1947, levee leakage occurred
where the present river crosses the former river channel. In the geologic profile along the levee, there was a
gravel layer distributed in the foundation. Field investigations quantitatively demonstrated that the seepage
conditions there were determined by the relationship between the surface soil thickness and the permeability
coefficient of permeable layers. We found that leakage occurred at locations where the surface soil thickness
was 3m or less and the permeability coefficient was greater than 10-4m/s. It was also demonstrated that levee
deformation from piping in the foundation was mainly controlled by the grain size distribution, thickness anddistribution depth of the permeable layers and the embankment load.
Key words: seepage, piping, sand boil, levee failure, abandoned river

1. INTRODUCTION
The authors investigated on the Kakehashi River (Ishikawa Pref., Hokuriku District, Japan) and found waterleakage at the levee foundation were observed near the former river channel. Thus, the water leakage at the levee
foundation was closely related to the location of the former river channel. However, such water leakage does notalways occur at a place where a levee and a former river channel cross, so it is critical to accurately predictvulnerable places in a levee so that monitoring can focus on these places. Tabata et al., (2017) proposed a levee
vulnerability index which is useful for risk management and levee management. However, its accuracy needs tobe improved, and for that purpose, the distribution of former river channels and the relationship between thelongitudinal distribution of foundation ground soils and the water leakage at levee foundations should beanalyzed.
Cui et al., (2017), Saito et al., (2016), Ueno et al., (2017), and Sasaoka et al., (2017) conducted modelexperiments to analyze the relationship between levee body deformation (due to piping and water leakage atlevee foundations) and the soil properties and soil composition of the foundation ground/levee body. The
present study compares the data from these model experiments to data on observed soil properties and soil
compositions of the levee where deformation occurred on the Kakehashi River in order to improve the accuracyof a method for identifying vulnerable locations in a levee.
2. METHOD
Aerial photos taken by the U.S. military (Geospatial Information Authority of Japan, 1946&1947) were chiefly
used for identifying the former river channels of the Kakehashi River by using a stereoscope (magnification x3;
field of view: 70mm). Plan views (MLIT, 1902) of the river improvement work in the Meiji Era (1868-1912)
owned by the Kanazawa River and National Highway Office (Ministry of Land, Infrastructure, Transport andTourism, Japan (MLIT)) were available for identifying the locations of the former river channel, and suchlocations and the aerial photos were used to develop a distribution map of the former river channels. Based on
the map of the former river channels and a longitudinal geological profile, the relationship between the locationsof water leakage (Table 1) and their foundation ground soil properties was summarized. The results of the modelexperiments show that it is important to understand the soil composition of the surface layer at the toe of thelevee landside slope so that levee body deformation due to water leakage can be accurately predicted at the earlystage of water leakage. Thus, for each cross-sectional surface where boring survey was conducted, the soil
properties and the thickness of the surface layer up to 3m thick near the toe were investigated. These and thepermeability coefficient (hereinafter: “k”) of the underlying gravel bed (the permeable layer) are summarized ina schematic. Locations of water leakage were analyzed in relation to the thickness of the surface layer and k ofthe permeable layer. The particle size composition of the surface layer was also analyzed toward understanding
its association with types of water leakage.

Table 1. Records of levee water leakage on the Kakehashi River
Month and yearof flooding

District

Left/rightbank

Location(closest KP)

Water leakage type

Jul. 2006

KawabeRight bank

KP5.0

Leakage at levee body

Sand boil

Jul. 2006

ShiraeLeft bank

KP6.6

Leakage at levee foundation

Sand boil

Jul. 2006

KanayaLeft bank

KP6.8

Leakage at levee body

Sand boil

Jul. 2006

SendaiRight bank

KP7.6

Leakage at levee foundation

Leakage only

Jul. 2013

Kofu

Right bank

KP8.6

Leakage at levee foundation

Sand boil, sliding failure

Sept. 2013

ArakidaLeft bank

KP9.5

Leakage at levee foundation

Leakage only

Water leakage situations

3. RESULTS
3.1 The relationship between water leakage at the levee foundation and the topographic/geological features ofthe levees on the Kakehashi River
The Kakehashi River is a gently sloping river running through the City of Komatsu, Ishikawa Prefecture. The
bed slope gradient of the river section under the control of the national government is 1/690～1/4,500. On thisriver section, water leakage occurred in the districts shown in Table 1 due to floods in August 2006, July 2013,
and September 2013 (Photos 1 and 2).

Photo 1. Water leakage (with sand boil) in the Shirae
district (KP6.6 on the left bank) (MLIT, 2007)

Photo 2. Water leakage in the Arakida district (KP9.5
on the left bank) (MLIT, 2014)

In the Shirae district (KP6.6 on the left bank) shown in Photo 1, sand boils took place near the toe of the leveelandside slope and in crop fields protected by the levee. Flood-fighting activities were conducted in that districtby “hooping.” In this method, sandbags are piled up in a semi-circular arrangement to prevent the spread ofwater leakage when water begins to leak at the back of the levee. In the Arakida district (KP9.5 on the left bank)shown in Photo 2, water leakage occurred in paddy fields protected by the levee. There were no sand boils, as
shown in the enlarged photo. Figure 1 shows a distribution map of the former river channels which wereidentified by using U.S. military aerial photos taken after the Pacific War and survey maps of 1902 created by
the Kanazawa Construction Office of the former Ministry of Construction. In the Shirae, Sendai, Kofu, andArakida districts, where water leakage at the levee foundation occurred, the water leaked near where the levee
crossed the former river channel and in areas bordered by the former river channel and the levee.
The basin at the lower reaches of the Kakehashi River has long been a damp area, and it has frequently flooded.
In 1869, the section of the Kakehashi River between the Haccho River and the Nabetani River was straightenedby the cutting off of large meander river channels. From 1911 through 1923, the Kakehashi River was improvedby the cutting off of a large southward meander river channel downstream of Komatsu Castle and the shorteningof the river channel there into a 1.1km-long section. The aerial photos taken by the U.S. military after the war
show that most of the former river channels had been converted to paddy fields. The bed slope gradient of theformer river channel between the river mouth and the Kakehashi River’s confluence with the Nabetani River is
1/4,500. The former river channel was serpentine, and the meander belt had a constant amplitude. The former
river channel was joined by the Haccho River, largely bent southward near Komatsu Temmangu Shrine, and wasjoined by the Mae River before flowing into the Sea of Japan. In the former river section upstream of theNabetani River, the meander amplitude was smaller than at the lower reaches, and the meander width was
restricted by the surrounding hills. Water leakage at the levee foundation occurred immediately upstream anddownstream of the Kakehashi River’s confluence with the Nabetani River. The bed slope gradient changes at theconfluence.
Legend凡例
Formerriver channel
旧河道(空中写真判読)(aerial photo interpretation)
Current river channel

The Sea of Japan

現 河 道White broken line: Mender width of
白色点線
旧河道の蛇行幅(範囲)
former river…
channelHaccho R.
Meander width

KomatsuTemmangu Shrine

Nabetani R.
SendaiOrange broken line: Boundary bet. hilly
area and flat land … 丘陵と平地の境界
オレンジ色点線
Kofu

KomatsuCastle Remains

Hilly area

Downtown
Komatsu

ShiraeArakidaMae R.

Hilly area

Figure 1. The former river channels of the Kakehashi River identified in U.S. military aerial photos, and places of waterleakage (MLIT, 2008)

Figure 2 shows the longitudinal geological profile along the levee on the right bank of the Kakehashi River. The
surface-layer soil distribution characteristics reflect the topographic features. Sandy soil is distributed from near
the ground surface in the area downstream of the place near Kokatsu Temmangu Shrine where the Kakehashi
River bends southward. In the area upstream of the shrine toward the Haccho River, there is a thick layer ofcohesive soil near the ground surface, and the cultivated land in that area is used for paddy fields. In the river
section between the Haccho River and the Nabetani River, the former channel of the Kakehashi River meanders
widely, and natural levees were developed along the former channel. On that river section, sandy soil is chiefly
distributed in the surface layer. In the river section upstream of the confluence with the Nabetani River, where
the bed slope gradient changes, the meander amplitude of the former channel is smaller than at the lower reaches
and gravel is distributed in the surface layer. Water leakage took place on the river section upstream of the
Haccho River. On that section, a bed of highly permeable gravel is distributed in the foundation layer. The
geology of the foundation ground in the Kakehashi River can be summarized as follows: Gravel is chiefly
distributed at the upper reaches, with the particle size decreasing toward the lower reaches, where sand is chiefly
distributed. The transition of soil layers follows the sediment deposition of rivers.
Regarding the permeable layer (sand or gravel), which is a causal factor in water leakage at the levee foundation,
Figure 3 shows the relationship between the distance from the river mouth (the x-axis) and k of the permeable
layer (the y-axis). The value of k tends to decrease toward the lower reaches and to increase toward the upper
reaches, consistent with the geological layers shown in the longitudinal geological profile.

KP3.0 (Komatsu
Temmangu Shrine)

KP5.0 (Confluence with
the Haccho R.)

KP7.5 (Confluence with the Nabetani R.)

ShiraeSendai

Kofu

ArakidaLegendSoil
Symbol of soil
classification classification
Embankment
B
Cohesive soil
Ac
Alluvium
Sandy soilAsperiodDune sand
Asd
GravelAg
Pleistocene Gravel
Dg
periodSandy soil
Ds
Cohesive soil
Dc
Tertiary Foundationcperiod (tuff breccia, mudstone)

Sandy soil
layer

Quaternary period

Gravelbed

Cenozoic Era

Geological age

Figure 2. Longitudinal geological profile along the levee on the right bank of the Kakehashi River (MLIT, 2014)

The datum in the black square in Figure 3 is the value for homogeneous dune sand, and the order of k is 10-4m/s.
The data in the blue square in Figure 3 are k of the gravel layer deposited in the valley bottom plain, and theorders of the values of k are 10-4~10-5m/s. The sediment in the valley bottom plain consists of soil from thesurrounding mountains, and the matrix of the gravel layer has a high content of fine-grained fraction. Thus, thevalues of k are low.
By using the cross-sectional geological profile (MLIT, 2014) created on the basis of detailed surveys of thelevee, the diagrams shown in Figure 4 were developed. These diagrams show the thickness of the surface layer
near the toe of the levee landside slope on the right bank and k of the permeable layers below the surface layer.
The thickness of the surface layer and k are factors that directly affect the occurrence of piping. The permeable
layers are ‘As’ layers and ‘Ag’ layers, which are continuously distributed from the river mouth to KP10.5 in
Figure 2. Based on these diagrams, Figure 5 shows the relationship between k of the permeable layer (the x-axis)
and the thickness of the surface layer (the y-axis). Figure 5 indicates that water leakage occurred at places where
cohesive soil is distributed near the ground surface, the surface layer is 1~3m thick, and the k of the permeable
layer is at least 10-4m/s. Water leakage did not occur in the region within the green square in Figure 5. The region
contains a sand dune and a valley bottom plain. Regarding the places where water leakage occurred, causal
relationships between water leakage and factors such as a change in the bed slope gradient and topographic
features (such as the former river channel) need to be analyzed.
1.E‐02

0.0

Arakida1.E‐03

ShiraeSendai1.E‐04

1.E‐05Right bank, sandy soil layer (As)(m/s)

Right bank, gravel bed (Ag)

1.E‐06

Arakida0.5

Left bank, sandy soil layer (As)Left bank, gravel bed (Ag)Thickness of surface soil layer (m)Permeability coefficient

Kofu

KP0.6, left bank

1.0

KP0.3, left bank

1.5

Shirae246Distance (k)

8

10

12

Kofu

2.0
Sendai2.5
3.0

No damage (cohesive soil)

3.5

No damage (sandy soil)

4.0

Water leakage, etc.
(cohesive soil)

1.E‐070

KP10.4, left bank

4.5
1.00.E‐07

1.00.E‐06

KP9.6, right bank

1.00.E‐05

1.00.E‐04

1.00.E‐03

1.00.E‐02

Permeability coefficient of the permeable layer (m/s)

Figure 3. Permeability coefficient of the permeable layer inthe foundation ground in the river longitudinal
direction

Left bank
KP6.4
ShiraeLeft bank
KP6.8
KanayaLeft bank KP8.0

Left bank
KP9.6
ArakidaLeft bank KP10.4

Levee body soil
classification

Silty sandThicknessof layer

Permeability
coefficient

Surface layer soil
classification

Sand

2m

3.57.E-05

Permeable layer

Sand

-

5.64.E-04

Levee body soil
classification

Silty sandThicknessof layer

Permeability
coefficient

Surface layer soil
classification

Cohesive soil

3m-

Permeable layer

Sand

-

9.61.E-05

Levee body soil
classification

Silty sandThicknessof layer

Permeability
coefficient

Surface layer soil
classification

Cohesive soil

3m-

Permeable layer

Gravel

-5.13.E-05

Cohesive soil

Thicknessof layer

Permeability
coefficient

1m-

Levee body soil
classification
Surface layer soil
classification

Cohesive soil

Permeable layer

GravelLevee body soil
classification

GravelPermeable layer

GravelThicknessof layer

Permeability
coefficient-4.02.E-04

Figure 5. Relationship between the permeability coefficientof the permeable layer and the thickness of thesurface layer
Levee body soil
classification

Sand
boil

Leakage
at levee
body

Right
bank

Right
bank
Sendai

Right
bank
Kofu

Leakage
only

Right
bank

Right
bank

KP6.6

KP7.7

KP8.4

KP9.6

KP11.0Surface layer soil
classification

Silty sandThicknessof layer1m

Cohesive soil

1m-Permeable layer

Sand

-

9.45.E-06

Levee body soil
classification

Cohesive soil

Surface layer soil
classification

Silty sand3m

3.95.E-05

Permeable layer

Gravel

-1.50.E-04

Levee body soil
classification

Silty sandSurface layer soil
classification

Cohesive soil
Sand

0.5m
2m

1.84.E-06

Permeable layer

Gravel

-1.60.E-03

Levee body soil
classification
Surface layer soil
classification

Silty sandCohesive soil

3.53.E-05

Thickness Permeabilityof layercoefficient

Thickness Permeabilityof layercoefficient

Leakage
only

Sand
boil,
sliding
failureThickness Permeabilityof layercoefficient

Cohesive soil

3m-

Permeable layer

Gravel

-5.57.E-04

Levee body soil
classification

GravelPermeable layer

GravelThickness Permeabilityof layercoefficient-

1.34.E-05

Unit of permeability coefficient: m/s

Figure 4. Soil property composition diagram near the toe of the levee landside slope on the right bank3.2 Mechanism of levee body deformation due to piping
Figure 6 shows the locations of water leakage due to the flood in the Kofu district in July 2013 and the
distribution of the former river channels. Photo 3 shows KP8.4, where sliding failure occurred at the levee due towater leakage at the levee foundation. Sand boils also occurred in the area protected by the levee. At KP8.6,
sand boils occurred and the levee crown cracked (Photo 4). Sand boils also occurred at KP8.2. The soil
properties and the levee conditions at these sites are summarized in Tables 2 and 3. Table 2 shows the following:
levee height (H); gradient of the levee landside slope (gradient of the line connecting the shoulder and the toe ofthe slope); piping value, G/W, calculated on the basis of non-stationary saturated and unsaturated seepage
analysis that utilizes external forces of rainfall and the record of river water level (MLIT, 2014) and on the basisof stability calculation (the total stress method); safety factor against circular failure on the landside slope(Fs)(Japan Institute of Country-ology and Engineering, 2012); and disaster type.

Photo 3. Site of sliding failure near KP8.4 on the rightbank (MLIT, 2014)

Photo 4. Sand boils near KP8.6 on the right bank (MLIT,
2007)

Table 2. Data on the sites where piping occurred, and the consequences of piping
Pipingjudgement

Back slope
sliding failure

G/W

Min. factor of safety, Fs

Analysedcross-sectionLeveeheightH [m]Slopegradient

KP8.2

3.8

1：1.40

3.178

1.469

KP8.4

4.3

1：1.17

0.233

1.046

KP8.6

4.0

1：1.35

0.438

1.311

Soil classification andlocation of circularsliding surface

Damage situations

Cohesive soil layer(Levee body)
Sandy soil layer(Foundation ground)
Sandy soil layer(Levee body)

Sand boil
Sand boil
Sliding failure
Large amount ofboiling sandTable 3. Soil parameters used for calculating the data in Table 2
Use

Levee bodyFoundation
ground(kN/m3)
14.9

Internal
frictionangle(degree)
9.5

Cohesivestrength(kN/m3)
20.7

Permeability
coefficient(m/s)
5.3E-06

16.9

16.3

9.9

12.5

2.9E-06

20.0
17.3

19.0
16.7

30.0
0.0

1.0
29.4

9.1E-06
1.0E-07

Saturated unit
weightUnsaturated
unit weight

Sandy soil

(kN/m3)
16.8

Bs2

Sandy soil

Bs3
Ac1-1

Sandy soil
Cohesive soil

As1-1

Sandy soil

-

17.0

27.2

0.0

1.8E-06

Ag1-1

Gravel

-

19.0

34.6

0.0

1.6E-03

Symbol ofsoil layer

Soilclassification

Bs1

Because KP8.2 on the right bank is close to where the bank merges with the former river channel, the soil
properties are the same as those of the former river channel. Near the toe of the levee landside slope, a cohesive
soil layer with a thickness of up to 3.5m is distributed. At the same place, the piping value (G/W=3.178) is
higher than 1. Water leakage did not occur at the toe of the landside slope. The water leakage and sand boils that
occurred in the paddy fields protected by the levee were probably a result of the cohesive soil layer there being
thin. The minimum safety factor against circular failure (Fs) is 1.469, a value higher than at other sites, and levee
body deformation did not occur at this site (Figure 7.)
At KP8.4 on the right bank, sand boils occurred in a footpath between rice paddies. The cause is likely to havebeen the reduction in the inland water levels near the inside of the levee. At this site, the cohesive soil layer inthe surface layer is 0.5m thick, which is thinner than at other sites, and the piping value is 0.233, which is much

lower than 1. The sand layer beneath the cohesive soil layer is up to 2m thick. The circular sliding surface givenby the calculation of the minimum safety factor extends through the sand layer. The minimum safety factor is
1.046, which is higher than 1; thus, sliding failure did not occur (Figure 8.)
In the levee body deformation at KP8.4 on the right bank, the canal at the foot of the levee near the toe of thelandside slope moved horizontally and upward. The soil of the foundation ground seems to have moved, too.
Because water was retained in the canal at the foot of the levee upstream of where the levee collapsed, it is likelythat the collapse occurred when the inland water level decreased to ground level around the canal. Sand boils
occurred in a footpath between paddies, followed by sliding failure. A portable cone penetration test conducted
after the levee failure (Figure9.) indicates that part of the sand layer in the foundation ground is relatively weak.
Because the location, shape and the dimensions of the weak part are consistent with those of the calculated
circular sliding surface, it is possible that the sliding failure was due to a decrease in the effective stress causedby an increase in hydrostatic pressure. At the same time, because the gabion walls sank significantly, the levee
body deformation might have been caused by subsidence that followed the hollowing of the foundation grounddue to piping and the movement of soil particles, as were simulated in model experiments by Ueno et al., (2017)
and Sasaoka et al., (2017).

Figure 6. Site of water leakage in the Kofu district (MLIT,
2014)

Figure 7. Geological profile at KP8.2 on the right bank
(MLIT, 2014)

Sand boil

N-value

The sliding surface confirmed by thecone penetration test conduced after
sliding failure

Figure 8. Geological profile at KP8.4 on the right bank
(MLIT, 2014)

Ground profile
after failure

Figure 9. Portable cone test results for KP8.4 on the rightbank (MLIT, 2014)100
Sendai, KP7.7, right bank(sandy silt)

90
80

Sendai, KP7.7, right bank(sandy silt)

70
Shirae, KP6.4, left bank(fine sand; sand boil)

60

Kofu, KP8.6-1, right bank(fine sand; sand boil)

50
40

Kofu, KP8.6-2, right bank(fine sand; sand boil)

FC = 35%

30
Arakida KP9.6, left bank(gravel)20

Experimental foundation
ground (silica sand #6 (0.33
sand)
0.001

0.01

0.75mm0.1

1

10

10
0

100

Grain size (mm)Figure 10. Geological profile at KP8.6 on the right bank
(MLIT, 2014)

Figure 11. Grading curves for the soil of sand boil sites

Weight content (%)

Area within broken line:
susceptible to liquefaction, as
indicated in the TechnicalStandards and Commentaries forPort and Harbour Facilities inAt KP8.4 on the right bank where sliding failure occurred, the levee was higher and the landside slope gradient
was steeper than at other sites. Consequently, the safety factor against circular failure was lower than at the othersites, being close to 1.
At KP8.6 on the right bank, large-scale sand boils occurred in the canal at the foot of the levee near the toe of thelandside slope, but sliding failure of the levee did not occur. At this site, the cohesive soil layer in the surface
layer was as thin as 1.0m, and the piping value was 0.438, a value higher than at KP8.4. There was no
relationship between the sand boils and the piping value. The minimum safety factor against circular failure is ashigh as 1.311. The circular sliding path given by the calculation of the minimum safety factor extended throughthe levee body (Figure 10.) Sand particles but no gravel particles spurted to the ground surface; thus, it is likelythat the soil particles moved in the sand layer on the gravel bed. Even when the strength of the sand layer
decreases because of failure due to piping (on the assumption that the angle of shear resistance φ=0 ), the safety
factor is 1.149. A safety factor higher than 1 means that sliding failure does not occur.
Figure 11 shows the particle size composition of the sand layer where sand boils occurred (or where soil
particles moved). The particle size composition is based on the soil test data from Results of Soil Tests at River
Levees (River Division, National Institute for Land and Infrastructure Management, 2012). According to theJapanese Geotechnical Society, the coefficient of uniformity is D60/D10, but because D10 cannot always be
obtained in soil that is high in fine fraction content, D60/D20 was used in this study.
Judging from the particle size composition, the boiling sand in the Shirae district was sand distributed in thesurface layer (Photo 1). The coefficient of uniformity of that sand is 10 or less, and the fine fraction content is
35% or lower. Values greater than these indicate a high likelihood of liquefaction, according to theSpecifications for Highway Bridges (Japan Road Association, 2012). Water leakage alone took place in the
Sendai district. The surface layer consists of silt and sand, and the fine fraction content is 50%. The boiling sandin the Kofu district (Photos 3 and 4) is sand distributed in the surface layer, as in the case of the Shirae district.
The surface layer in the Arakida district consists of cohesive soil. Consequently, there were no sand boils, andonly water leakage occurred (Photo 2). A grading curve of the permeable layer that makes up a gravel bed isshown in Figure 11. This curve is on the right of the grading curve of boiling sand. The coefficient of uniformity
is 44, which is quite high, and the curve is gentle.
In Figure 11, the area between the two broken lines corresponds to a range of particle sizes that are especiallysusceptible to liquefaction, as indicated in the Technical Standards and Commentaries for Port and Harbour
Facilities in Japan (The Overseas Coastal Area Development Institute of Japan, 2008). This suggests that thetype of soil that is prone to sand boils due to piping has the same range of particle sizes as the type of sand thatis likely to undergo liquefaction.
Figure 11 also shows a grading curve for the type of soil used by Ueno et al., (2017) in a model experiment. The
grading of the silica sand #6 (0.33mm) used in the upper layer of the foundation ground of the model experimentis similar to the grading of sand ejected at the sand boils on the Kakehashi River. From the model experiment, it
was reported that when the upper layer and lower layer of the foundation ground were made the silica sand #6
and silica sand #3, respectively, and a levee was built with bentonite-mixed soil (an impermeable material with
k=4.1×10-9), not only was the sand in the foundation ground discharged, but so were portions of the soil in the
bentonite-mixed levee body, and that rapid subsidence caused the levee body to collapse.
Even when the levee body consists of an impermeable material, soil particles move at the boundary between thebottom of the levee body and the top layer of the foundation ground if the top layer is sand that is susceptible topiping and whose particle size distribution is within a certain range. The movement of soil particles causes
erosion at the boundary, which leads to the settlement and deformation of the levee body. The soil of the levee
body in the Kofu district had a higher fine fraction content and had a different particle size composition than theimpermeable material used in the experiment. In the sliding failure of the levee in the Kofu district, the effective
stress decreased due to the boiling of sand in the foundation ground. Consequently, the balance between thefoundation ground and the load of the levee body was lost, which resulted in deformation similar to the circular
failure of the levee body.
4. DISCUSSION
Water leakage at the levee foundation occurred where the levee crossed the former river channel. To identify
sites susceptible to water leakage, it is important to develop a map of former river channels on the basis of oldmaps and aerial photos. Not all water leakage sites were at places where the levee crossed the former river
channel, but many were. It is critical to understand the soil properties of the surface layer to the depth of 3m aswell as the k of the underlying permeable layer. Based on the relationship between the thickness of the surface
layer and the k of the permeable layer, it was confirmed that water leakage occurred at places where thethickness of the surface layer was less than 3m and the k of the permeable layer was at least 10-4m/s. It is likely

that the difference between the river water level and the level of the ground at the toe of the levee landside slope
was among the causal factors in water leakage. A causal relationship between the difference of these two levels
and water leakage needs to be investigated.
Only water leakage occurred when the surface layer consisted of cohesive soil, and both water leakage and sandboils took place when sand of a uniform particle size was distributed in the surface layer. Thus, the type of waterleakage varies depending on the soil properties of the surface layer.
The thickness of the sand layer and the length of a circular sliding path affect the occurrence of sliding failure.
Thus, it is important to determine the geometry and dimensions of the circular sliding surface according to theload exerted by the levee. From laboratory model experiments, it is known that when seepage failure progresses,
soil masses in the foundation ground begin to move at some point in time and sliding failure occurs. The
relationship between the duration of seepage and sliding failure needs to be analyzed for different soil propertiesof the surface layer.
5. CONCLUSIONS
The relationship between water leakage at the levee foundation and levee body and the topographic/geologicalfeatures was analyzed for the sites where water leakage occurred at levees on the Kakehashi River due toflooding. The analysis results are as follows.
1. Judging from the configuration of the former river channels identified in aerial photos, water leakage at thelevee foundation took place at the boundaries between meander channels on floodplains and braided
channels on alluvial fans or valley bottom plains.
2. An analysis of longitudinal geological profiles along levees indicated the following: Water leakage occurredon river sections where the values of k were high (10-4m/s or higher for the Kakehashi River) in sand or
gravel layers that were continuously distributed in the foundation ground along the river, and water leakage
occurred where the surface layer thickness was between roughly 1m and 3m.
3. The particle size composition of soil susceptible to sand boils is in the range of particle sizes associated withliquefaction. Water leakage alone took place in soil consisting of particle sizes outside that range.
4. Regarding the levee body deformation at KP8.4, because the soil particle size for the levee was outside therange of particle sizes associated with sand boils, it is likely that the circular failure occurred due to adecrease in shear resistance caused by soil particle movement in the sand layer of the foundation ground.
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