10th Asian Regional Conference of IAEG (2015)

Effects of Snow Load on Water Infiltration in the Ground Surface Layer of a Landslide
Hikaru OSAWA(1), Sumio MATSUURA(2), Takashi OKAMOTO(3),Yuki MATSUSHI(2), Tomomi TERAJIMA(2) ,Tatsuya SHIBASAKI (1)(1) Graduate School of Science, Kyoto University, Japan
E-mail:osawa@scs.dpri.kyoto-u.ac.jp(2) Disaster Prevention Research Institute, Kyoto University, Japan(3) Forestry and Forest Products Research Institute, Japan
Abstract
We observed the pore-water pressure and infiltration capacities in a re-activatedlandslide with a shallow sliding surface in Japan. Observations on the response ofpore-water pressure to meltwater and/or rainfall (hereinafter referred to as “MR”), show
different characteristics for periods with and without snowfall. In particular, during a periodof melting snow, MR infiltrates the ground by ~20 to 100 mm/day for over a month. We
determine patterns in infiltration capacities based on observations that gradually increased
from the maximum water equivalent of snow to the height of summer, followed by a slow
reduction from summer to early winter. It can be said that one of the reasons for infiltration
capacity changes, was the compression and expansion by the snow load of the undergroundshallow parts composed by overconsolidated clay. The oxidation layer occurs at Ksat=1×10-5
m/s from the ground level (G.L.) to G.L. −0.8 m, and the reduction layer is Ksat=5×10-7 m/sfrom G.L. −0.8 m to G.L. −2 m. In the event of huge amounts of MR in the ground, thewater vertically infiltrates the oxidization and reduction layers. It then constitutes athroughflow from a boundary layer when the reduction layer is saturated. It can be said
hydraulic characteristics of geology prescribe an upper limit of the pore-water pressure.
Keywords: heavy snow districts, field observations, pore-water pressure, infiltration capacity,
compression, expansion
1.

Introduction

heavy snow district.

Landslides in regions subject to snows frequently
occur owing to an increase in pore-water pressurecaused by melting snows. However, Matsuura (2000)
reported that pore-water pressure fluctuations duringseasons with and without snowfall were totallydifferent, with landslides having a gentle slope angle
and shallow sliding surface. Okamoto et al. (2008)
studied by the safety factor increasing by snow load,
which was increased the effective stress of landslide
mass with shallow sliding surface and a gentle slope
angle. Okamoto et al. (2015) reported fieldobservations of a vertical displacement gauge on
shallow ground, reflect compression and expansion inassociation with snow loading and unloading. Thissuggested occurrence of a clayey soil in an
overconsolidated state. We suggest that snow load
affects the process of infiltration of meltwater into theground, and influences the fluctuation of pore-waterpressure characteristics. We describe hydrologicaland soil mechanical characteristics of the surface
layer in a re-activated moving landslide body in a2.

Overview of the study site

The Busuno landslide site, Niigata Prefecture,
Japan, is situated in the mountainous area facing theSea of Japan, where it is underlain by Neogene
sedimentary rocks (Fig.1). This region is affected byheavy snowfall with a maximum snow depth of over
5 m. The study site comprised a landslide area of 300
m long and 50-70 m wide, at an elevation of 550-650
m, with a mean inclination of 10˚-15˚. This site is atypical example of a re-activated landslide and thesliding surface depth was 3-6 m. The surrounding
forest cover ratio was 7%, comprising Cryptomeria
Japonica and broadleaf trees. The density ofvegetation cover is almost 100% in summer, but isabsent in winter, the region being snowbound. The
dominant vegetation species are Miscanthus
sacchariflorus and Phragmites australis.
3. Test and Observation method3.1 In-situ infiltration test1

10th Asian Regional Conference of IAEG (2015)

Fig.2

Depth of sliding surface
piezometers and boring log

and

3.3 Pore-water pressure
Pore-water pressure was observed using adiaphragm-type piezometer of a full scale of 50 kPa,
which was installed in the clayey soil in the middle
block at G.L. −2.85 m. The surrounding space in thesilica sand, and the top and bottom were plugged with
Bentonite (Fig.2).
3.4 Meteorological observation
The snow depth was observed using a laser-typesnow depth gauge at a meteorological station. The
water equivalent of snow was observed using a metal
wafer type sensor. The water that reaches the ground(hereinafter referred to as MR) was observed using
lysimeters with a tipping-bucket discharge gauge.

Fig.1 Locations of installed sensors andhorizontal distribution of hydraulicconductivities of top soil in the Busuno
landslide

4.

Infiltration capacities were measured by aportable single ring infiltrometer that consumed asmall amount of water (4.5 l). We measured 6 points(1. Upper block, 2. Middle block-up, 3. Middle
block-down, 4. Lower block-up, 5. Lower
block-down, 6. Meteorological station), across thelandslide in order to investigate patterns of spatial
distribution. Cylindrical pipes were placed in the soiland were then studied. Water temperature for tests
was adjusted to approximately 0˚C to unify viscosity.

Results and discussion

4.1 Observation of the pore-water pressure
Figure 4 shows the fluctuation of pore-waterpressure, snow depth, snow load and MR. The
pore-water pressure fluctuation during times with andwithout snowfall displayed totally different
characteristics. The pore-water pressure reacts
sharply on MR in seasons lacking snowfall. However,pore-water pressure reacts little in the season ofsnowfall, especially during the snowmelt period. The
snowmelt period was usually marked by infiltrationon the ground surface by melting-water of 20-100
mm/day for over a month. Figure 4 shows that theupper limit of the pore-water pressure fluctuation wasapproximately 16 kPa (=164 cmH2O) over the whole
period. We took the piezometer depth at G.L. −2.85
m into consideration with the water tableapproximately at G.L. −1.2 m. Figure 2 shows thepiezometer depth and boring log. A slightly brown
oxidized color zone, termed the oxidization layer,
occurred from ground surface to G.L. −80 cm. A
blue-gray reduction color zone, was termed thereduction zone, from G.L. −80 cm. Figure 3 shows
the vertical profile of hydraulic conductivities and astratigraphic profile. The hydraulic conductivity of

3.2 Saturated permeability test
We carried out constant head and falling head
permeability tests on 3 samples collected from one
spot. We dug a pit to G.L.−2 m for sampling soil inthe middle block that revealed a vertical profile ofpermeability. The vertical section was divided into 2layers comprising oxidization and reduction zones.
Soil specimens in the oxidization layer of organic
clayey soil from the ground level to −0.8 m, weresampled vertically to assess vertical infiltration. Soil
specimens in the reduction layer of tuffaceous clayey
soil, were sampled parallel to the slope, to assesslateral flow.

2

10th Asian Regional Conference of IAEG (2015)

the infiltration capacity. Figure 5 shows changes ininfiltration capacities, MR, snow depths, and airtemperatures. We conducted infiltration tests at lower
block-up and down on March 5, 2013. Results of afield snow survey at the maximum water equivalentof the snowfall period, show a snow depth of 470 cm,
mean density of snow being 0.44 g/cm3, and a snow
load of 19.4 kN/m2 acting on the earth’s surface.
Results showed that infiltration capacities gradually
increased from the maximum water equivalent of thesnow season to the height of summer, and slowly
reduced from summer to early winter.
4.2.1 Upper block(×), Middle block-down(△)
The infiltration capacity of the upper block andmiddle block-down were increased by over 3 times
from each initial observation on May 12th and 25thand July 5th. At these points, the inclination angles
were 3˚, these being water catchment topographies.
The ground surface was in a saturated state, like amarsh, after the snowmelt period, as noted in field
investigations. Moreover, the hydraulic conductivity
at the spot of Middle block-up and infiltration
capacity were closed result on May 12. Following
this, the water table gradually lowered with increasedpermeability.

Fig.3 Vertical profiles of the hydraulicconductivity
the oxidization layer is of the order of 10-5 m/s.
Further, the mean hydraulic conductivity of thereduction layer is of the order of 5*10-7 m/s. This
result suggests that there is a difference of 500 times
between the former and the latter, which is theboundary of permeability. The water tablecorresponded to the boundary between theoxidization and reduction layers. Water infiltrates
vertically into the oxidization layer, and then forms athroughflow from under the boundary layer, in thecase of a saturated reduction layer.

4.2.2 Lower block up • down (○•■)
Infiltration capacities of the lower block-up •down were at 1.3, 1.9 times with each other fromMarch 5 to the period following snowmelt. Moreover,these were at over 2 times with each other from aftersnowmelt to summer. Okamoto et al. (2015) reportsoil expansion by decrease in the snow load frommid-April to the beginning of May, during thesnowmelt period. The ground surface wascompressed by a snow load reaching 20 kN/m2,
which with a decreased void ratio. According to thestudy by Tavenas et al. (1983), the void ratio of clays
is positively correlated with its permeability. It is
possible that the pore-water pressure response was

4.2 Seasonal changes of infiltration capacity
Figure 1 shows the horizontal distribution ofsaturated hydraulic conductivities that were partial atspecific points; the mean value is 5*10-5 m/s, themaximum value is 1*10-4 m/s, and the minimum
value is 1*10-8 m/s. Consequently, we infer that asaturation overland flow does not occur even after
heavy rains (>180 mm/h).
Infiltration capacities were tested twelve times in
the field, and the results suggest seasonal variation of

Fig.4 Fluctuation of the pore-water pressure and its related factors

310th Asian Regional Conference of IAEG (2015)

Infiltration capacities changed with seasons. It is
possible that infiltration capacities were affected bysnow loads on the ground surface. Result from thesetests indicates that permeability changes by changing
void ratios in soils, which is a result of compression
and expansion of the underground shallow parts
composed of overconsolidated clay by the snow load.
According to the vertical profile of hydraulicconductivity, the oxidation layer existed from
Ksat=1×10-5 m/s from ground surface to G.L. −0.8 m,
the reduction layer from Ksat=5×10-7 m/s from G.L.
−0.8 m to G.L. −2 m. As a result of the pore-waterpressure observations and the vertical profile ofhydraulic conductivity, the boundary between thehydraulic characteristics of the geology prescribedsaturated and un-saturated states. In addition, theboundary regulates the upper limit of the pore-waterpressure fluctuation.
Response characteristics of the pore-waterpressure have been sparsely investigated. We plan toconduct further observations and to run numericalsimulations.
Acknowledgements
The present study was funded by a Grant-in-Aidfor Challenging Exploratory Research, Disaster
Prevention Research Institute, Kyoto University (No.
H26-02).

Fig.5 Changes of Infiltration capacities andits related factors
delayed by lowering the infiltration capacity. It seems
reasonable to assume that the earth’s surface
gradually expanded by unloading of snow, thereby
increasing the void ratio and permeability. Someauthors suggest that the control of seasonal changes
in infiltration capacities is due to plant growth andanimal activities within the soil (e.g., Gifford, 1972;
Cerda, 1996; Bormann and Klaassen, 2008). Roots ofreed grass and Amur silver-grass in all thesurrounding top soils in the landslide area were active
during seasons with a high temperature. This suggests
the influence of pipes and macropores on water
infiltration.

References
Matsuura, S. (2000): Fluctuations of pore-waterpressure in a landslide of heavy snow districts, J. ofthe Jpn. landslide Soc. Vol. 37, No. 2, pp. 10-19.
Okamoto, T., Matsuura, S., Asano, S. (2008):
Deformation mechanism of a shallow lamdslide ina snow-covered area, J. of the Jpn. landslide Soc.
Vol. 44, No. 6, pp. 358-368 (in Japanese withEnglish abstract).
Okamoto, T., Matsuura, S., Abe, K. (2015):
Observation of vertical displacement in a landslide
block during snow season, J. of the Jpn. landslide
Soc. Vol. 52, No. 1, pp. 21-28 (in Japanese).
Tavenas, F., Jean, P., Leblond, P., Leroueil, S. (1983):
The permeability of natural soft clays. Part 2:
Permeability characteristics, Canadian geotech. J.
Vol. 20, pp. 645-660.
Gifford, G.F. (1972): Infiltration rate and sedimentproduction trends on a plowed big sagebrush site, J.
of Range management. Vol. 25, pp. 53-55.
Cerda, A. (1996): Seasonal variability of infiltration
rates under contrasting slope conditions in southeast
Spain, Geoderma, Vol. 69, pp. 217-232.
Bormann, H., Klaassen, K (2008): Seasonal and landuse dependent variability of soil hydraulic and soilhydrological properties of two Northern Germansoils, Geoderma, Vol. 145, pp. 295-302.

4.2.3 Meteorological station (★)
A meteorological station was established, and it
consists of an artificial ground of sand. We conducted
tests with the assumption of it remaining unchanged
the void ratio over a whole year. Results indicate thatthe first observation value was high, that explained
problems of cylindrical pipe driving. There were few
changes in infiltration capacities.
5.

Summary

We observed infiltration capacities and pore-waterpressure of a landslide with a shallow sliding surface
in a heavy snowfall district of Japan.

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