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December 2005 |
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In this issue:
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Merry Christmas!
Establishing Insitu Stresses with
SIGMA/W
Generally, the best that we can do is to simulate what essentially becomes equivalent
to a centrifuge test.
We build the numerical model and then we "turn on" gravity so that the stresses develop due to the weight of the existing soil. It's like
starting up the centrifuge after the scale model has been built. In finite element
terminology this is called a "gravity turn-on" analysis. Assigning a unit weight
to the elements simulates the effect of gravity.
Turning on gravity after the fact brings with it some complications
and limitations. To compute stresses we have to compute strains, but the computed strains
are meaningless. The relationship between vertical and horizontal stresses is related
to Poisson’s ratio which cannot be greater than 0.5 and therefore Ko cannot be greater
than 1.0. Also, Ko can only be applied to the effective stresses,
and not to the water
in the soil. The soil stiffness (E) needs to be a constant value throughout the profile
to avoid the development of unrealistic stress concentrations. This means that the stiffness
properties required for computing the insitu stresses are not necessarily representative
of the actual field conditions.
All of this necessitates great care and understanding when we are doing analyses
to establish the insitu stress conditions. If we do not get this part of the analysis
correct, then there is little value in using a sophisticated, nonlinear, elastic-plastic
constitutive model for the subsequent load-deformation part of the analysis. The
soil properties are, after all, dependent on the initial stress state in the ground.
SIGMA/W attempts to make this part of an analysis as straightforward as possible
with special types of analyses, but it is always necessary to do spot checks by comparing
the finite element computed results with simple hand calculations based on the overburden
(weight), the pore-pressures, and the assumed Ko conditions. The SIGMA/W
results will
not always be the same as the hand calculations, but they must always be
reasonable in light of the hand calculations. For example, are the pore-pressures
or the computed horizontal effective stresses consistent with your hand-calculated
expectations?
In the end, remember that your load-deformation results are only
as good as your initial or starting stress conditions in the ground. So the next time you do an analysis to establish insitu stresses, think about the analogy of a centrifuge test.
License Update Feature
No matter which product you are using, you can find all your current FLEXnet licenses
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John Krahn Receives R.F. Legget Medal
John, our President and CEO, was awarded the R.F. Legget Medal this fall at the Canadian Geotechnical Society's annual conference. The medal is the
most senior and prestigious award of the CGS, and is
presented annually to an individual who has made significant personal contributions and achievements
in the field of geotechnical engineering
in Canada.
A transcript of the R.F. Legget Medal presentation ceremony is published in the December 2005 issue of
Geotechnical News.
We are fast approaching the end of another year, and we at GEO-SLOPE would
like to take the time to thank you for your feedback and your continued interest
in our software. We would also like to take this opportunity to wish you a Merry
Christmas and a Happy New Year!
One of the more important aspects of any stress-deformation analysis in
geotechnical engineering is establishing the insitu stresses. The insitu stresses describe the
stress state in the ground before they are altered during the course of engineering
projects where structures are built and earth is either added or removed. Ideally
we should numerically model the geologic processes that have led to the insitu stress
conditions. However this is seldom, if ever, possible or warranted. So what approach
should we use?
New with GeoStudio version 6.17, your license files will be updated automatically.
Every time you run GeoStudio, it will check for new license files, then download
and install them for you. Never worry about expired licenses again!
