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MECHANISM OF LIQUEFACTION-INDUCED
SETTLEMENT OF WOODEN HOUSES BASED
ON THE DEPTH DISTRIBUTION OF LIQUEFACTION STRENGH OF RECLAIMED LAND
Keisuke Ishikawa & Susumu Yasuda
Department of Civil and Environmental Engineering – Tokyo Denki University, Saitama, Japan
ABSTRACT
In the 2011 Great East Japan Earthquake, liquefaction occurred in the reclaimed land in the Urayasu area of the Tokyo
Bay coast, causing serious damage to wooden houses. The reclaimed land that exhibited liquefaction was new ground,
whose development started in 1965. According to an engineer who supervised the reclamation work in Urayasu, the
reclamation work involved dredging soil from the sea bed and then using it to build up the land mass to a height equal to
about sea level. Thus, the reclaimed land that liquefied can be thought of as having a very heterogeneous layer
structure, given the reclamation method, such that when studying the mechanism leading to liquefaction-induced
settlement damage to wooden houses, many problems arise.
This study set out to address two problems affecting the mechanism of liquefaction-induced settlement damage to
wooden houses. First, a detailed soil investigation was carried out to determine the depth at which liquefaction of the
reclaimed land actually occurred, leading to our understanding of the depth distribution of the liquefaction strength
property. Second, a one-dimensional effective-stress analysis based on the results of an investigation was conducted,
the status in the event of an earthquake striking reclaimed land was evaluated, and the mechanism leading to damage to
wooden houses was considered.
The results of a soil investigation revealed that the shear wave propagation velocity in reclaimed land ranges between 80
to 110 m/s, that the soil itself is loose, and that the soil is essentially silty sand containing many non-plastic fine-grained
particles. Furthermore, there was no regularity in the silt content in the reclaimed land for any given depth, with the
ground being very heterogenous. The results of a cyclic triaxial test on an undisturbed sample revealed that the
liquefaction strength of the reclaimed land varies from 0.22 to 0.43. That depth at which the liquefaction strength was the
lowest was near the boundary between the reclaimed soil and the alluvial sand.
The results of our analysis first confirmed the elevation of the excess pore water pressure near the boundary between
the reclaimed soil and the alluvial sand, which gives rise to a low liquefaction strength. Then, the excess pore water
pressure was spread throughout the upper layer, eventually eliminating the effective stress near the ground-water level.
As mentioned above, in addition to the propagation of the excess pore water pressure in the layer with poor resistance to
liquefaction, when the first aftershock occurred 30 min after the initial tremor, settlement damage worsened as a result of
the loss of the bearing capacity under the foundations of wooden houses.
a) In-situ tests
b) Laboratory tests
c) Example of the one-dimensional
effective-stress analysis result
Figure 1 Content discussed by this study