Severe rainfall from mid-November to early December 2004 triggered numerous sediment disasters and flashfloods on the eastern coast of Luzon Island in the Philippines, resulting in extensive damage to life and property. Among the affected areas, the coastal towns of Quezon Province suffered the most damage, with numerous landslides occurring in the town of Real and debris flows burying the towns of Infanta and General Nakar. This paper outlines the observations from reconnaissance works conducted at the affected sites following the disaster, with emphasis on the hydro-geological aspects of the disaster and its impact on civil engineering structures and other infrastructures. Moreover, the results of unsaturated-saturated seepage analyses conducted to investigate the mechanism of the collapse of one of the slopes which resulted in death of more than 100 people is presented.

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During 2004 Niigata-ken Chuetsu Earthquake in Japan, a large number of landslides occurred on natural slopes, especially at the hillsides in the region of Yamakoshi. In many of the large slides, the debris has travelled through a fairly long distance, aggravating the disaster caused by the landslides. In recognizing its importance, case studies were undertaken on the run-out distance of the landslides at several sites at Higashi-Takezawa, Mushigame and Naraki. Case studies are also undertaken for slope failures involving the man-made deposits behind retaining walls surrounding the residential hill at Takamachi-Danchi in Nagaoka. In the first section of the present study, a simple analytical method is introduced based on the energy principle, in which the residual strength is taken up as a sole parameter to determine the run-out distance combined with the geometry of the landslides. The slope failure is herein assumed to consist of two phases, sliding and spreading, and the sliding distance is defined as the length of a slope on which the mass of soils slides down, and the run-out distance is determined as the one on a gentle slope or flat plane on which the phase of spreading occurs. Soil samples were retrieved from the sites of landslide, and laboratory triaxial tests are conducted on unsaturated soil samples with varying water contents. The residual shear strength thus obtained was used as an input parameter in the simple analysis to forecast the run-out distance. The outcome of the present study is presented in a form of simple charts in which the run-out distance is expressed as function of relevant geometrical parameters and the residual shear strength of soils involved in the landslide.

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Slope failures occurring near earthquake centres have attracted much attention since the 2004 Niigata-ken Chuetsu Earthquake. The 1995 Hyogoken-nambu Earthquake affected large modern cities and urban infrastructure, but hilly and mountainous areas suffered heavy damage from the Niigata-ken Chuetsu Earthquake. During that earthquake, numerous landslides occurred in Koshi of Nagaoka city (formerly Yamakoshi village). Social problems developed when many towns became isolated because landslides cut off traffic and public service lifelines. Soil from landslides closed river channels and formed natural dams along the Imo River, which flows north to south in Koshi. The natural dams submerged some towns and emergency measures were promptly undertaken to prevent debris flows caused by natural dam breaks. In addition, many surface slides also occurred at steep slopes. The endogenous and exogenous factors of those landslides must be clarified because of their few precedents. For that purpose, topography and geology in this region are arranged according to each failure type. In addition, triaxial tests of saturated and unsaturated soils obtained from a slope failure site were conducted to examine the soils' strength properties. This research is intended to propose estimation items that can help to judge the risk of natural slope failure.

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Damage to earth structures for national highways by the 2004 Niigata-ken Chuetsu earthquake is reported. After overviewing damage to roads, several case histories on severe damage and rehabilitation work for national highways are documented. Immediately after the mainshock, national highways and prefectural roads were closed at 101 sites. Among them, main national highway routes Nos. 8, 17 and 116 were closed at 17 sites, while all of them could be re-opened within ten days after the mainshock. On the other hand, national highway route No. 291 that runs through mountaneous areas suffered from extensive damage at two sites, where alternative routes had to be constructed. The national highway route No. 291 suffered also from damage to embankments constructed on valley and/or landslide areas and damage to cantilever and gravity type retaining walls constructed in gorge area. The damage to the national highway route No. 17 included collapse of a gravity type retaining wall constructed on uncemented debris or weathered terrace deposits and collapse of a segmental retaining wall supporting a cut slope. The national highway route No. 8 suffered from embankment failures due possibly to liquefaction of a loose sand subsoil layer and/or concentration of longitudinal or transverse ground water flow.

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Numerous natural or artificially embanked slope failures were caused by the 2004 Niigata-ken Chuetsu Earthquake. Characteristics of large scale natural slope failures that occurred at folding hills, river terraces, etc, were classified from a geometric structural view point. It was noted that surface failures occurred at steep cliffs and landslides occurred at gently slanting dip slopes. The cause of a landslide in a dip slope at Yokowatashi, Ojiya city was analytically investigated based on cyclic shear test results of the laminar sand at the bedding plane. This study revealed that the safety factor of the slope stability became momentarily smaller than 1.0 several times during the earthquake due to the small undrained shear strength of the tuff sand seam and the landslide behavior was simulated by an elasto-plastic dynamic finite element analysis where the strength decrease of the sand seam with increasing number of cycles was taken into consideration. The cause of destructive damage of a school building at Nagaoka National College of Technology (NNCT) was next examined by another elasto-plastic dynamic FEM. From the analysis results, it was confirmed that the building was pulled and twisted by the landslide of the bank shotcrete accompanied with pile fracture.

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The peak ground acceleration (PGA) of 13.08 m/s² recorded at K-NET station in Ojiya city during the 2004 Mid Niigata Prefecture earthquake was 1.5-2 times greater than either of the other nearby stations on a similar soft soil (JMA) and on the outcrop rock (SSI). Those at K-NET during earthquakes with less intensity, however, tended to be smaller than those at JMA. The variation in relative magnitude of PGA with earthquake intensity is probably due to the difference in site conditions between K-NET and JMA. Field investigation including boring, PS logging, and spectral analysis of surface wave methods as well as laboratory tests on undisturbed samples obtained from the sites from this and previous studies reveal near-surface soil profiles and their nonlinear dynamic soil properties at both stations. Equivalent linear dynamic analysis is then performed using the detected soil profiles and properties for the two sites with the motions recorded at SSI as input outcrop motions. It is shown that, despite its very low shear wave velocity of about 50 m/s, the near-surface peat at K-NET exhibits weaker nonlinearity with larger damping at small strains but with smaller damping at large strains than does the near-surface silt at JMA. This might have had strong effects on the extremely large peak acceleration at K-NET during the main shock as well as on the variation in relative magnitude of PGA between K-NET and JMA during earthquakes of different intensity.

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The slope disaster and damage in Genkaijima, induced by the 2005 West Off Fukuoka Earthquake, have been investigated and studied by the investigating commission organized by the Japanese Geotechnical Society. The slope disasters in this island are classified into three areas: (1) damages of the housing site foundations and the retaining walls of steep slopes in the resident area, (2) landslides over the circumference road of the island, (3) small-scale collapses and cracks on the eastern slope from the Genkai elementary school to the Genkai junior high school. For each area, the results from on-site inspections, filed tests, indoor tests, measurements and analysis are reported together with the suggestion on restoration countermeasure work in this paper.

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The 2005 Fukuoka-ken Seiho-oki earthquake caused heavy damage to residential areas at the Genkai-jima Island. A field survey was carried out to investigate the damage of retaining walls in the residential areas. From the survey of 218 retaining walls, it appeared that 83% of the retaining walls were damaged and 61% of the whole lost their functions (collapsed or distorted/cracked). Especially in masonry retaining walls using natural rocks and boulders, the damage was serious. The damage features of dry masonry retaining walls (collapsed: 62%) and wet masonry retaining walls (collapsed: 25%) were different, and the difference in the earthquake-resistant capacity between the two retaining walls became obvious. Moreover, it appeared that most of the residential houses are built on transitions of cut slopes and fill embankments. It seems that this condition is one of a great factor which causes the heavy damage. Only 17% of the retaining walls escaped from any damage. Furthermore, to evaluate the collapse mechanism of masonry retaining walls during earthquake, a theoretical model based on the earth pressure theory was proposed. Through the theoretical considerations, it became possible to quantitatively evaluate the stability of the masonry retaining walls.

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On March 20, 2005, an earthquake of magnitude 7.0, which was named as the 2005 Fukuoka-ken Seiho-oki Earthquake, occurred in the northwest part of Fukuoka City. During the earthquake, liquefaction occurred mainly in the reclaimed lands of the Hakata Bay area, resulting in structural damage. In this paper, the distribution of the sites where liquefaction occurred and the characteristics of the gradation of sand boils collected at the sites were clarified. Further, the occurrence of liquefaction in the reclaimed land was discussed with reference to some instances, and structural damages, mainly to port and harbor facilities, and a flow failure of sloping ground due to liquefaction were summarized. Furthermore, liquefaction analysis was performed using a program for one-dimensional seismic response analysis and several soil profiles of the reclaimed land and alluvial ground in order to investigate the relationship between the occurrence of liquefaction and the distribution of acceleration; this was estimated from the analyses and the mechanical and physical properties of the soils.

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This report summarizes the results of the investigations on the sediment disasters that occurred in Yamaguchi Prefecture during the storm of Typhoon No. 14 on September 5 to 7, 2005. This typhoon caused serious damage to Kyushu, Shikoku and Chugoku areas. A number of sediment related disasters occurred in Yamaguchi Prefecture, especially in its eastern area, Iwakuni City. Most serious damages included flooding of the Nishikigawa River and a slope failure of the Sanyo Expressway bank, which caused three deaths. This report describes the morphology and scale surveyed for disasters at 59 sites, which consisted of 47 slope failures, landslides and debris flows, and 12 floods and inundation, and discusses their causes. The generation mechanisms involved in typical failures were also investigated by analyzing precipitation, topography, geology, and strength properties of collected soils. The analysis showed that the cases could be classified into six patterns of 1) failure of artificial slope (46%), 2) failure of natural slope (7%), 3) destruction of revetment (22%), 4) debris flow (15%), 5) landslide (8%), and 6) others (2%). These disasters were most likely attributable to the record-making storm, which resulted in accumulated rainfall of over 500 mm in this region. The disasters occurred in districts that were not affected by the Geiyo Earthquake in 2001 (M6.7) and the former storm on July 3.

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In recent years, major developments in remote sensing have made it possible to use these technologies to document the effects of earthquakes. Specifically, high-resolution satellite imagery and three-dimensional laser scanning (LIDAR) can provide important observations of earthquake damage that supplement traditional observations from field reconnaissance. The 2004 Niigata Ken Chuetsu earthquake provided an opportunity to use remote sensing to document the distribution of landslides in the epicentral region through the use of high-resolution satellite imagery and to document the detailed three-dimensional geometries of several failures using LIDAR. The satellite imagery was acquired the day after the earthquake, but at very large acquisition angles that resulted in image distortion. Nonetheless, the satellite imagery accurately identified the landslide distribution in the epicentral region, although the total area of landslides was underestimated by about 25% as compared with traditional aerial reconnaissance because of the large acquisition angle for the satellite imagery. Terrestrial LIDAR was used to collect three-dimensional data at several failure sites, including two large rock slides and a railroad tunnel portal affected by ground deformation. The LIDAR data allowed for precise measurement of failure deformations and geometries, and provided digital terrain models that could be archived and used in future analyses. In the future, satellite imagery and LIDAR, as well as other remote sensing technologies, will play an increasing role in documenting and understanding the effects of earthquakes.

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The years of 2004 and 2005 had many slope failures in Japan. Those failures were caused by heavy rainfalls and the 2004 Niigata-Chuetsu earthquake. It was thought to be important in such a situation to examine the validity of existing hazard assessment methodologies concerning slope failures. Site visits were therefore made at those failed slopes, and soil samples were collected. With a special attention focused on the effects of water on mechanical properties of soils, plasticity index and swelling tests were conducted on those soils as well as triaxial compression tests on dry and water-submerged remoulded specimens. The water effects on mechanical properties of soils were accelerated to occur for experimental purposes by grinding soil particles and increasing their surface area. It was concluded that the existing hazard assessments are basically reasonable. It was shown, however, that they can be improved so that the extent of damage such as the run-out distance of sliding soil mass may be assessed by further taking into account the physical properties of soils.

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Typhoon No. 14 moved along the west coast of Kyushu from 4th to 7th September 2005 and caused severe damages at many locations mainly in Miyazaki-prefecture, Kagoshima prefecture and Oita prefecture. This typhoon moved at a very slow speed of travel varying from 25 km/h to 30 km/h in Kyushu area and this caused successive formation of strong clouds for a long time in the east side of the Kyushu Mountain Region which exists on the right side to the direction of this typhoon. According to AMeDAS data, the rainfall at Osumi in Kagoshima prefecture exceeded 900 mm and at Mikado in Miyazaki prefecture exceeded 1300 mm since the beginning of the rainfall. This typhoon is characterized by heavy rains and many places in the south and east of Kyushu have exceeded the maximum record of rainfall. Flood disaster hit many places along the Oyodo River and Gokase River including their tributaries and more than 6000 houses suffered from inundation above floor. Geodisasters also occurred at many places due to this typhoon and 22 lives were lost and some villages had been isolated for a long time. In this paper, the mechanisms of slope failure observed in the geodisasters in Kyushu area caused by Typhoon No. 14 in September 2005 are mainly outlined in relation to the rainfall characteristics and geological and soil mechanical aspects.

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Torrential rains in Fukui, July 2004 resulted in heavy damage to ground in the Asuwa river basin. This paper reports an outline of the matters and causes of the following damage: 1) The dike in the lower reaches of Asuwa river was breached in city area. A detailed survey was undertaken to determine why the dike was breached at that location. 2) Many cases of damage to revetments were observed in the upper reaches of Asuwa river and its tributary streams, and the damage was worsened by inundation resulting from timber debris and sedimentation. There were many cases of slope failure above revetments, which were thought to be due to scouring. 3) Many slope failures occurred at locations distant from rivers. In many cases they were relatively shallow collapses. 4) Many mud and debris flows occurred along small rivers or in mountain valleys, and large quantities of mud and timber were displaced. 5) At five steel bridges on the JR Line, bridge girders were washed away and bridge supports collapsed or were damaged. Considerable damage was caused to road bridges and to road and rail embankments. Our survey team carried out investigations into these 5 types of damage, to grasp the full extent of the damage and establish issues for future consideration.

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About 1,400 sewage manholes were uplifted during the 2004 Niigataken-chuetsu earthquake in Japan. Many buried sewage pipes were also uplifted. Before the restoration work, detailed soil investigations were carried out to reveal the mechanism of the uplift. Based on the investigations, it was clarified that uplift occurred in mainly clayey grounds. No sand boils were observed on the clayey ground. However, boiled sands were observed just beside the uplifted manholes and above the buried pipes. During the construction of buried pipes and manholes, the ground was excavated first, the pipes and manholes were placed in the ditches, then the ditches were filled with sand. The soil investigation of the sand fill after the earthquake revealed that the sands were very loose and easy to liquefy. It was concluded that the uplift of the manholes and pipes occurred due to the liquefaction of the sand fill. In preparation for the restoration work, appropriate countermeasures for future earthquakes were discussed, and finally, the cement mixing method was selected.

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