Geoscience Reference
In-Depth Information
isthecostwhichisneededforsoilimprovement. Itisgoingtobemoredifficulttospend
expenditures that exceed benefits gained from the mitigation. With these recent situa-
tions,thepresentpaperaddressestopicsandachievementsinmitigationmeasuresagainst
liquefaction.
2. Sand densification
2.1. GENERAL REMARKS
Densification of sand has been one of the most important mitigation measures for lique-
faction problems. Sand compaction pile in particular has been studied and constructed
intensely. In practice, pressurized air of, for example, 500kPa, is frequently used to help
formation of sand columns in ground. Okamura et al. (2005, 2006) suggested that the
pressurized air remains for a long time in subsoil and reduces the degree of saturation.
Reducedsaturationthusachievedimprovesliquefactionresistanceofsoil.Sincethecon-
ventionaldesignprincipleofsandcompactionpileassumessoiltobefullysaturated,this
new point implies additional safetymargin. Another safetymargin maybethe long-term
increaseofSPT-N;seeFigure15.1.Otherwisecalledageing,thisfeaturefurtherincreases
the liquefaction resistance. The mechanism behind this ageing is not known yet. More-
over, the SAVE Technology (Ohbayashi et al., 2006) achieved static installation of sand
compaction piles and drastically reduced noise as well as ground vibration (Figures15.2
and15.3).Thishasmadeitpossibletoexecutesandcompactionpileswithinafewmeters
from existing structures.
2.2. BLASTING
From the viewpoint of cost, blasting is a very promising technology. Port and Airport
Research Institute conducted blasting study in Tokachi Harbor in 2001 and a young
N
1,60
/
Dr
2
AgeSPT-N.fig
60
Skempton
s suggestion (1986)
on increase of SPT-N with age
50
Natural
deposits
Laboratory tests
40
Recent fills
30
0.01
0.1
1
10
100
Age (years)
Fig. 15.1. Skempton's suggestion on increase in SPT-N with age
(relative density,
Dr
, indecimal)
