7. Conclusions
The gigantic 2011 off the Pacific coast of Tohoku
Earthquake caused five slope failures in Taiyo New Town,
a housing site on artificial valley fills in Yamamoto Town,
Miyagi Prefecture. To understand the mechanism of the
slope failures, a field inspection, a strong motion record
analysis, laboratory tests, microtremor measurements and
a slope stability analysis were conducted. The following are
the major observations from the site investigations and the
conclusions from the study presented herein:
1. The residential site of Taiyo New Town was constructed
through the cut and fill method. The original hilly ground
consisted of weakly consolidated tuffaceous sandstone,
which showed a high degree of weathering towards the
surface and manifested itself as sand on the surface. Thus,
the fill material used to fill up the valleys was generally
sandy soil.
2. The maximum surface acceleration recorded at the
Yamamoto seismic station was 850–870 cm/s2 with a
predominant period of 4–6 Hz. The estimated maximum
acceleration on top of the fills, based on the seismic
response analysis, was 1170-1320 cm/s2
.
3. Each of the five slope failures at the housing site was
observed to have occurred either at the edge of the
artificial valley fills or at embankment sections that had
been widened for road construction.
4. The sandy fill material had a fines content of Fc¼20%.
Even when compacted at a 90% degree of compaction, the
fill material showed a very low internal friction angle and
low liquefaction resistance. Moreover, the liquefaction
resistance further decreased with the application of initial
shear stress.
5. Microtremor measurements showed that the natural
periods of the ground near the failed sites were greater
than 0.5 s, indicating the large depth of the soft soil
layers.
6. The pseudo-static slope stability analysis, using conventional
strength parameters, could not explain the slope
failure of Block 1, based on the assumption that Noda’s
method of estimating kh was valid for the high PGA
recorded during this earthquake. However, when the
dynamic strength (or liquefaction resistance) was used
to represent the soil strength at the slip surface, the yield
seismic coefficient required to induce failure was lower
than the estimated seismic coefficient on the fill.
7. Based on the analysis of one block, the slope failure in
Taiyo New Town may be attributed to the liquefaction of
the fill material induced by the intense and prolonged
shaking. It is possible that during the earthquake, the
groundwater level was near the boundary of the original
ground and the fill, making liquefaction possible.