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4. Dissipation of excess pore water pressure
Quick dissipation of pore pressure by means of gravel drain has been one of the useful
measurestomitigateliquefactionproblems.Oneofthegoodpointsofthismethodisthat
graveldraindoesnotmakenoise,groundvibration,orgrounddeformationuponinstalla-
tion. Hence, it is possible to employ this method near existing structures (Figure15.39).
There are, however, situations in which a tall installation machine (Figure15.39) can-
not be employed. One of them is the liquefaction problem of pile foundation of elevated
railways and flyovers (Figure15.40).
Harada et al. (2004, 2006) developed metal pipe drains which can be installed in loose
sand by mechanical blows. Since the size of a required blow machine is limited, it is
possible to execute it under elevated structures. This drain pipe is called a screen pipe
(Figure15.41) which is made of metal rods in the axial direction with metal rings sur-
roundingthem.Verysmallspacingbetweenringsallowsporewatertoflowintothepipe,
thereby dissipating the excess pore water pressure, while the field shaking tests showed
that clogging is negligible after liquefaction. Since the pipe is hollow, its permeability is
significantly higher than that of gravel drain.
Verification of the effects of a screen pipe was conducted in a laminar box of a 1-G
model scale (Figure15.42) as well as in the field. In a laminar box, a model ground
(Figure15.43) measured 99cm in depth and therein a group pile of 2
2insizewas
installed. The pile heads were connected to each other by a pile cap and screen pipes
were placed both inside and outside the group pile. The horizontal size of the group pile
was26cm
×
×
26cm.Porefluidwasasolutionofcellulosewhichmadetheviscosityofpore
Fig. 15.39. Construction of gravel drain (Fudo-Tetra Inc.)
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