Geoscience Reference
In-Depth Information
sand (hygroscopic water content of 0.5%) and partially saturated sand (water contents of
3.1% and 3.4%).
A 100-mm-diameter pipeline with 4.1-mm wall thickness and a 0.9-m depth of cover
was used in the tests. It was composed of two straight pipes welded to a 90
◦
elbow.
Both ends of the pipeline were bolted to reaction walls. The elbows were composed
of STPT 370 steel (Japanese Industrial Standard, JIS-G3456) with a specified minimum
yieldstressof215MPaandaminimumultimatetensilestrengthof370MPa.Thestraight
pipewascomposedofSGPsteel(JIS-G3452)withaminimumultimatetensilestrengthof
294MPa. About 150 strain gages were installed on the pipe to measure strain during the
tests. Extensometers, load cells, and soil pressure meters were also deployed throughout
the testsetup.
The large-scale experiments had three principal results. First, they were used to improve
and validate a hybrid finite element model, which combines beam and shell elements
for the pipeline with nonlinear p-y formulations to simulate soil-structure interaction
(Yoshisaki et al., 2001). This model is now used by Tokyo Gas to plan and design
pipelinesforextremeloadingconditions.Second,theanalyticalmodelwasusedtoshow
thatincreasingthewallthicknessofpipe,whichisweldedtotheelbow,by1.5mmresults
instrainreductionofapproximately 200%for abrupt groundrupture of 2m. Simple,rel-
atively inexpensive adjustments in pipeline fabrication, therefore, can lead to substantial
improvements in performance. Third, the strains induced in the experimental pipeline
weremarkedlyhigherfortestsinpartiallysaturatedsandthanforthoseindrysand,even
though most other variables wereheld constant.
5. Lateral soil-structure interaction during ground failure
To explore the effects of partially saturated sand on the lateral force conveyed to buried
conduits due to relative soil-pipe displacement, a second series of tests were performed
on pipe similar in size and composition as that investigated by Yoshisaki et al. (2001).
The tests were designed to be similar to those performed by Trautmann and coworkers
(TrautmannandO'Rourke,1985;Trautmannetal.,1985),whoestablisheddesigncharts
fromwhichp-yandq-zrelationshipscanbedevelopedforanalyzingsoil-structureinter-
action in response to lateral and vertical PGD. A detailed description of the tests and
resulting data isprovided byTurner (2004).
These design charts were developed on the basis of experiments in dry sand. However,
thegreatmajorityofpipelinesinthefieldareembeddedinpartiallysaturatedsoils.Shear
deformation of partially saturated sand mobilizes surface tension, or negative pore water
pressure, which increases shear resistance relative to that in dry sand under comparable
conditions of soil composition, in situ density, and loading. Moreover, the geometry of
thefailedsoilmassforpartiallysaturatedsandissignificantlydifferentthantheflowand
displacement pattern of drysand around buried pipelines.
The experimental facility was constructed to model the effects of relative horizontal
displacement between soil and pipe under conditions that duplicate the actual scale,
