Field Reconnaissance: 18 November 2004

Embankment Fill Failures in Takamachi Residential Area

Ellen Rathje and Keith Kelson

GEER Beyond Reconnaissance Team

The reconnaissance team developed initial documentation of the pattern of embankment fill failures within the Takamachi residential area east of Nagaoka City. The residential area is on a linear ridgetop bordering the eastern margin of the Shinano River valley. The ridge may be a remnant alluvial terrace, originally underlain by a thin alluvial gravel deposit on semi-consolidated early Pleistocene sand and silt. Construction of the residential area presumably involved cutting the top of the hillside and using the native material as fill along the edge slopes. In most places the fill was confined by a 4-m high rigid, concrete retaining wall. Four large sections of retaining wall failed catastrophically, with the soil sliding a significant distance downslope. In addition to the catastrophic failures, severe cracking due to slope and retaining wall deformations was observed in many areas along the edge of the slopes. As much as 1 m of non-catastrophic downslope movement was observed in the retaining walls, and the deformation of the fill material caused significant damage to structures. No failures or ground cracking were observed in areas that did not have a retaining structure along the margins of the ridge. The yellow and red-tagged structures within the Takamachi area were concentrated exclusively in areas with adjacent slope failures and ground cracking associated with the ridge-margin fills. Within the interior of the hillside area, only green-tagged structures were encountered. Thus, it appears that all of the structural damage in Takamachi can be attributed to the deformations and failures of the hillside fills placed on the margin of the ridge.

Fig. 1. Large west-facing headscarp of embankment failure that destroyed roadway and retaining wall, southern end of Takamachi residential area.

Fig. 2. Large west-facing headscarp of failure that destroyed roadway and retaining wall, south-western edge of Takamachi residential area.

Fig. 3. Panoramic shot (with photographs in Figs. 4 through 6) showing large embankment failure that destroyed roadway and retaining wall, western side of Takamachi residential area

Fig. 4. Panoramic shot (with photographs in Figs. 4 through 6) showing large embankment failure that destroyed roadway and retaining wall, western side of Takamachi residential area

Fig. 5. Panoramic shot (with photographs in Figs. 4 through 6) showing large embankment failure that destroyed roadway and retaining wall, western side of Takamachi residential area

Fig. 6. Panoramic shot (with photographs in Figs. 4 through 6) showing large embankment failure that destroyed roadway and retaining wall, western side of Takamachi residential area

Fig. 7. Large embankment failure that destroyed roadway and retaining wall, southern end of Takamachi residential area. Close-up view shown in Fig. 8 taken beneath in-place roadway and car in upper left corner of this photograph.

Fig. 8 . Close-up view of artificial fill beneath roadway, as also shown in Fig. 7. Most, if not all, failures in the Takamachi residential area are associated with the locations of this artificial fill.

Fig. 9. Large embankment failure that destroyed roadway and retaining wall, northeastern end of Takamachi residential area.

Fig. 10. East-facing headscarp of embankment failure traversing beneath, eastern side of Takamachi residential area.

Fig. 11. Small east-facing headscarp of embankment failure traversing wall and adjacent house in background, eastern side of Takamachi residential area.

Fig. 12. Small north-facing headscarp of embankment failure that traverses wall and adjacent house, northern end of Takamachi residential area.

Fig. 13. Small east-facing headscarp of embankment failure that involved tilting of retaining wall but skirted around eastern edge of house in middle ground, eastern side of Takamachi residential area.