FIELD TRIP TO SHOBARA HILLS AND ENVIRONS
Organized by: Japan Landslide Society (Kansai Branch)Date: 28th September, 2011

Departure point: Faculty of Science and Engineering car parking lot.

Number of personnel: Seven (7)FIELD GEARS USED: Protective head-gear (Helmet).
 Potable water can(s).
 Foot-gears.
 Safety eye wears.
 Rain coats/ Umbrellas.
 Geologic compass (Prof. F. Wang].
 Field knap-sacks.
 Personal computers.
 Measuring tapes.
 Stationeries.
 Digital cameras.
 Topographic base maps (1: 50000).
 Landslide pictures.

Field trip to Shobara Hills and environs was organized by Japan Landslide Society (JLS), Kansai
branch which Geoscience department of Shimane University has members in attendance.
We left the faculty packing lot in company of our Professor, (Prof. F. Wang), on the wheels of
the 4×4 SUV owned by Geoscience department of Shimane University.

After a brief stop for lunch in a restaurant in Hiroshima Prefecture, we joined other colleagues
from other Universities. At about 1230hrs, we left our meeting point in a convoy of two (2)vehicles: a 40-seater bus and our car.

Observations on Route 183
Two distinctive landslides were observed on route 183. From the topographic base map, thelandslides are located at Site #1 and Site #1C respectively.

Their characteristic features are described as follows:Site #1C:
Site 1C is located at the eastern side of route 183 near Shinjocho Town. The distance of thelandslide site to the major road was estimated to be about 2km. The site is located farther awayfrom houses and settlements therefore, no visible hazard was seen.
Dip direction of slope: West
Material composition: Colluvium.

Site #1:
Site #1 is located in the southern part of Hirako Town, along route 183. Based on estimation, thedistance between the landslide site and the major road was taken to be about 1.8km. No visible
damage was seen because the location of the landslide is not very close to houses.
Dip direction of slope: East.
Material composition: Colluvium

However, corrective measures should be taken because high magnitude debris and mud flow
should not be neglected considering the elevation of hills and steepness of slopes surrounding theinhabited areas.
Geologic Setting of Shobara Hills
Shobara Hills is located in the northern part of Hiroshima Prefecture. The altitude of the hills
range between 200m and 1200m above sea level (asl) with steep slopes measuring less than 400.
Shobara Hills and environs are underlain by rhyolite of Mesozoic Age. Mean annual temperature
is about 12.80C while mean annual rainfall is about 1610mm.

General Characteristics of Landslides Studied Along Route 445, Shobara
The initial movement of unstable materials was by sliding especially near the head scarpof the landslides. Liquefaction of the sliding material by intense rainfall brought about
debris and mud flows.

Majority of the landslide sites visited has length averaging 12km with width of about
5km (special reference to Site #5). Landslide dimension of 5km × 3km has been recorded
by Igwe and Fukuoka (2010) in Shobara. Where the hills are close to the highway, theoriginal dimensions of the landslide could not easily be known due to remediation works
that has been done on the highway to make it accessible to vehicles again.

Generally, a close observation from a distance of about 50m on Shobara Hills show acharacteristic side-by-side feature of the Hills. The landslide surfaces (failure plane),were seen to be reddish to brownish coloured while the stable vegetation cover lies
adjacent to the landslide surface. This characteristic feature is similar to an unripe (green)orange that is peeled in a like fashion where its skin is removed in spaces of 3cm. Prof. F.
Wang termed the phenomenon ‘Skin Failure.’

The energy of the mud and debris flow was adjudged to be of high magnitude. The result
was observable in the field as big boulders, rocks and uprooted trees were deposited at
the toes of the landslides.

The characteristic shape of landslides observed on route 445 is trapezoidal.

Pumice, (an extrusive volcanic rock formed by sudden cooling of pressurized magma)
was adjudged to be probably, the main liquefying material within the landslide. Pumice isknown for its ability to retain water due to ‘holes’ created in it during the escape ofpressurized gases. This statement is yet to be supported with experimental results.

Geotechnical Remediation Works on Route 445
Almost all the sites visited on route 445 are undergoing remediation works. This could be
attributed to the steepness of slopes, material composition of the landslides and the elevation of
the hills within the area. The various remediation techniques employed are described as follows:Concrete Check Dam A:
This was seen at Site #4. The landslide occurred in July, 2010 after an intense rainfall
causing heavy debris flows which crossed the highway (route 445) and attacked a housesituated just opposite the landslide and in front of a drainage channel. One fatality wasrecorded with total destruction of the property (Igwe and Fukuoka, 2010).

Concrete Check Dam B:
This was seen at Site #5. The landslide in this site has the longest length. The associateddebris flow was said to cross the major road and continued downstream. No fatality ordamage was recorded because the area was covered by vegetation. Length of the Checkdam was measured to be 85.5m while height is 10m. Total volume of the dam is3957.4m3.
Drain Pipes:
To monitor and control groundwater level within the subsurface which may initiate slope
failure, drainpipes were seen installed at distinct areas of the slope to direct groundwaterout of the subsurface.

Reinforced Earth Walls:
At places where slopes are near-vertical, reinforced earth walls are constructed to holdthe unstable material from collapsing.

Gabion Walls:
Down south, along route 445, between landslide site #2 and #4, gabion walls were seen
erected on the western side of the highway to hold the near-vertical slopes and preventslope failure.

Other Remediation Techniques used are: Rock curtain, wire mesh, anchors and bolts.

Off we go……Climb the hill!
We rounded off the field trip with an adventurous climbing of one of the hills located at Site #3
to see the main head scarp of the landslide. I was amazed to see the strength and vigor of almost
all the senior professors as they made their way up the thick forest of the hilly slope. I almost
gave up but was astonished when I looked up front and saw my professors slowly but
meticulously ascending the hill. They will pause sometimes to catch their breath at the same timecollecting samples from the slope surfaces and analyzing it with intense but lovely argument. Itwas really fun.

Alas! We made it to the head scarp. I remember sticking my hands on thorny weeds at the sametime hearing my professor beckoning on me to keep it steady! From the head scarp down-slope,the length of the slope surface was estimated to be more than 4km. I also saw my professor using
his compass to take some readings which I presume might be slope angle, distance, and elevation.

At about 1700hrs, we left Shobara Hills and went back to Hiroshima. We had party in a poshhotel where we were lodged and went to bed around 1130hrs.
The next day (29th, September, 2011) was mainly technical paper presentation by professors andresearchers. The paper presentation ended around 1500hrs. We left Hiroshima after exchanging
pleasantries with our friends and colleagues and drove back to Shimane University.