Topographic and Geological Features Involved in Water Leakage at Levee Foundations
and in Levee Body Deformation by Piping on the Kakehashi River
Yutaka SATO, Shoji FUKUOKA (Research and Development Initiative, Chuo University, Tokyo, Japan)
INTRODUCTION
The authors investigated on the Kakehashi River (Ishikawa Pref., Hokuriku District, Japan) and found water leakage at the levee foundation were observed
near the former river channel. However, such water leakage does not always occur at a place where a levee and a former river channel cross, so it is critical toaccurately predict vulnerable places in a levee so that monitoring can focus on these places. The present study compares the data from these modelexperiments to data on observed soil properties and soil compositions of the levee where deformation occurred on the Kakehashi River in order to improve
the accuracy of a method for identifying vulnerable locations in a levee.

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).
Based on the map of the former river channels and a longitudinal geological profile, the relationship between the locations of water leakage (Figure 2) andtheir foundation ground soil properties was summarized.

RESULTS
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(Figure 1,2.)
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 atleast 10‐4m/s(Figure 5.)
At KP8.4 on the right bank, the minimum safety factor is 1.046, which is higher than 1; thus, sliding failure did not occur (Figure 8.) A portable cone penetration
test conducted after the levee failure. indicates that part of the sand layer in the foundation ground is relatively weak. Because the location, shape and thedimensions 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 theeffective stress caused by an increase in hydrostatic pressure(Figure 9.)
At KP8.6 on the right bank, 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(Figure10.)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 thetoe were investigated(Figure 5.)
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 thetoe were investigated. These and the permeability coefficient (hereinafter: “k”) of the underlying gravel bed (the permeable layer) are summarized in aschematic. (Figure 5.) Locations of water leakage were analyzed in relation to the thickness of the surface layer and k of the permeable layer. The particle size
composition of the surface layer was also analyzed toward understanding its association with types of water leakage(Figure 11.)

Leakage onlySand boil,
sliding failure

Leakage only

CONCLUSIONS
1.
2.
3.

Judging from the configuration of the former river channels identified in aerial photos, water leakage at the levee foundation took place at the boundaries
between meander channels on floodplains and braided channels on alluvial fans or valley bottom plains(Figure1,2.)
The particle size composition of soil susceptible to sand boils is in the range of particle sizes associated with liquefaction. Water leakage alone took place in soil
consisting of particle sizes outside that range(Figuer11.)
Regarding the levee body deformation at KP8.4, it is likely that the circular failure occurred due to a decrease in shear resistance caused by soil particle movementin the sand layer of the foundation ground(Figure8,10.)