Dear Prof Bala,
I understand the various schools of thought involved in this field and some
conflicting as you say.
To make matters worse, I am also looking into the use of soft materials like
energy dissipating characteristics
in geotechnical engineering if they can respond better than rigid materials
especially in the wake of dynamic
forces such as waves, quakes etc. To my knowledge this is relatively an
unknown area and I don't know the
extent current theories can help me.
Best Regards,
Parminder Singh
-----Original Message-----
From: A Balasubramaniam [mailto:[log in to unmask]]
Sent: Thursday, June 02, 2005 11:05 AM
To: Parminder Singh
Cc: [log in to unmask]; [log in to unmask];
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Subject: Re: FW: Use of net bearing pressure for ultimate limit state
design ofspread foundations
Dear Parminder,
I like the thoughts of John McKinley. I generally tune the sophistication
according to the situation. Roscoe was interested way back in 1961 in Shear
Bands.
There are lot of things going on in Geotechnics, with very great
sophistication, which at times I tend to overlook
Recently, I sent a paper to Geotechnique, which simply said, theories were
developed from experimental data, after that people forget they come from
experiments. One guy here showed me the work of Roscoe in 1958 and he can
understand the plasticity version of the same to understand the data. MY
paper was not accepted as I did not worship the Gurus.
I second a second one Lime and Cement Stabilization in Soft Clays for deep
and shallow stabilization. Two guys had gone to Carnical writing all sort of
thing. Whether I measured the strain locally etc. In 1965 ,I tortured myself
to do local measurements with X rays in Triaxial tests anbd found there is
not such a big issue on local measurements or boundary measurements, unless
of course we are dealing with very small strains in excavations. THese
reviewers were writing all type of junk and sais i need another guy to teach
me Intrinsic line, this line and that line. Now for everything we have
atleast half a dozen lines. Structured clays we dont find them in Southeast
Asia in Singapore , BKK etc. Then everytime we write some thing, it goes to
some guy for review who works on Structured clay.
To make things worse now we have one theory for sand and clay even though
the permeability of the two are miles apart.
Bala
"Parminder Singh" <[log in to unmask]>
02/06/2005 12:44 PM
To: <[log in to unmask]>
cc:
Subject: FW: Use of net bearing pressure for ultimate
limit state design of spread foundations
Dear Prof Bala,
I am studying about shear bands and know little about numerical modelling
such as nonlinear modelling with strain softening or strain dependent of
modified Cam-clay (Dasari & Brito), these are some info I picked up on
the internet. Basically I am examining shear bands and if it could be a
suitable strain/stress state to consider in developing factors of safety.
Recently there has been some discussion on servicibility and ultimate
states in UK & Euro Codes of practice. Some say this senseless factoring
can cause brittle failure. As I understand shear bands occurs in localised
zones and there maybe some elasticity left behind elsewhere.
So I think it is not necessary to go to a complete plastic state at
failure.
i.e. somewhere between lower and upper boundaries. Shear bands might tell
us something on displacement characteristics like in servicibility state.
I don't know if this approach is suitable to deal with soils of varying
stiffness and how constitutive models treat conditions like this and
offcourse making sense on the FoS issue.
If you like to know more, I have a pdf file on a presentation concerning
shear bands by Tatsuoka recently.
Like to hear from you.
Best Regards
Parminder
-----Original Message-----
From: Geotechnical Engineering Email List
[mailto:[log in to unmask]]On Behalf Of Parminder Singh
Sent: Thursday, May 12, 2005 9:27 AM
To: [log in to unmask]
Subject: Re: Use of net bearing pressure for ultimate limit state design
of spread foundations
Dear Dr McKinley,
Off late (after the tsunami), there are Qs on the inter-dependence of
geotechnical and structural designs. The performance criteria of the
overall system including superstructure and foundation such as in
earthquakes often requires a complicated understanding of the two
responses. In general it is difficult to determine whether a stiffer
or softer foundation would result in greater displacements and there
are some cases where the relative stiffness of different foundation
components exist within the structure. Applying different safety
approaches could lead to uneconomical designs and possibly fatal
ones also, hence, I agree there should be some overall consistency
in the approaches.
Best regards,
Parminder Singh
-----Original Message-----
From: Geotechnical Engineering Email List
[mailto:[log in to unmask]]On Behalf Of John McKinley
Sent: Wednesday, May 11, 2005 11:52 PM
To: [log in to unmask]
Subject: Use of net bearing pressure for ultimate limit state design of
spread foundations
Dear colleagues,
To assist in curriculum development, I'm curious to find out whether there
is a general preference within the geotechnical community to define
factors
of safety for spread foundations in terms of net pressures:
FOS = (q_ult - q_0) / (q_design - q_0)
or in terms of gross pressures:
FOS = q_ult / q_design
I'm aware that practice varies between organisations and companies.
However,
I wonder whether there is a general preference for one or the other.
Philosophically, I think that it makes more sense to define an overall
factor of safety as the ratio of the load / pressure / action at the
ultimate limit state to that at the design state, rather than as the ratio
of the increase in load / pressure / action at the ultimate limit state to
the increase at the design state. It seems to me that using the net
pressures leads to odd results for special cases, such as buoyant
structures. Leaving aside the question of partial factors (as in, for
example, Eurocode 7), I also think that the gross pressure approach is
more
consistent with what would usually be done for design of non-geotechnical
structures in civil engineering.
Yours sincerely,
John D. McKinley
------------------------------------------------------------------------
Dr. John D. McKinley +44 (0) 28 9097 4690
Lecturer in Environmental Engineering
School of Civil Engineering, Queen's University Belfast
www.prb-net.qub.ac.uk/eerg/People/Academic_staff/jmckinley/jmckinley.htm
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