REPORT OF FIELD TRIP TO SHOBARA, HIROSHIMA, OCTOBER 28-29, 2011
Faris Fikri(D1, Interdisciplinary Graduate School of Science and Engineering, Shimane University)

General Description about the Excursion
The trip to Shobara, Hiroshima prefecture was start from Shimane University at 08:30 am on October 28,
2011 by car. It spent about 3 hours from Matsue to the gathering point at Shobara. At 11:00 am, wejoined the excursion that organized by Kansai Branch of Japan Landslide Society around the landslide
prone area surrounding Shobara area. There are 4 (four) points of landslide occurrence which werevisited. The route of excursion and the landslide occurrence sightseeing is shown at Figure 1. There were
two stopping point, first at the location 1 and location 3. In this stopping points, we able to explore andexamined the post-landslide condition and have detail explanation from the guide. The field trip wasend at 15:00, and we continued the travel to Hiroshima city to check in into the hotel.
The next day, October 29, 2011, we attended the Symposium which started at 08:30. The presenters ofthis symposium were:
1. Dr. Masahiro (Graduate School of Science, Hiroshima University). Topic: Overview of Shobara
Landslides, Hiroshima, July 2010
2. Takeshi Tamura (Department Sabo Hiroshima). Topic: Landslides in the Bihoku region due toheavy rainfall in July 1972
3. Kiichiro Ogawa (Asia Air Survey Co., Ltd.). Topic: Laser measuring for profiling post-landslide
morphology
4. Hidemasa Ohta (Ohta Geo Research). Topic: Residents post-disaster compensation
The panel discussion was take place at 13:30 pm to 16:00 pm.
The Shobara Landslide
Shobara is the part of Hiroshima prefecture that has mountainous terrain with 35o to 45o degree slope.
The geological condition of the region is mainly andesitic and rhyolitic rocks. From the scrapped groundof the flow path, the soil layers are revealed. The base layer is weathered andesitic rock that estimatedas the landslide slip base. Above the base layer, there is some pumice layer with 50 cm to 75 cm thick.
The top soil consists of andesitic fragment disrupted with decayed material about 30 cm to 1 m inthickness. The exposed soil layer can be seen in Figure 2.
Many landslides occurred in Shobara, Hiroshima Prefecture on July 16 2010 during 2-3 hours of heavy
rainfall which is considered as triggering factor the landslides events. The type of the landslides iscommonly debris flow which has long path from the top to the base of the mountains (Figure 3). Fromthe examined landslide source area, the landslide can be regarded to translational shallow landslide
which has about 1 m to 2 m depth sliding surface (Figure 4). Generally, the landslides flow path have

narrow width and long flow path, in line with the hill valley. The deposit of the landslide consists ofstripped trees, rock fragment and black soil.
Some physical mitigation actions are already implemented surrounding the landslide prone area. Basedon the purpose, one can concluded that there are two type of physical mitigation, there are: structures
to avoid the impact of debris flow and structures to stabilize slopes. The first type of mitigation on thearea is represented by sabo dam structure. Sabo dam is the massive structure that commonly using
concrete material to retain the debris mass from landslide (Figure 5). The sabo dam is usually built byseries depends of the potential debris mass that could happen in the future. Another type isrepresented by bolt anchored slope which is found the area (Figure 6).

Figure 1 Travel path and landslide location map

Figure 2 Exposed soil layer

Figure 3 Debris flow travel path

Figure 4 Debris flow source area

Figure 5 Sabo dam structure

Figure 6 Bolted Anchor structure