If we:
1) define "overconsolidated" soils as soils with current effective stress
smaller than the maximum past effective stress; and
2) define "underconsolidated" soils as soils with excess pore pressure due
to a recent increase in overburden, then,
there is the interesting (albeit confusing) possibility that soil may be
"overconsolidated" and "underconsolidated" at the same time. Even if the
soil has OCR > 1, it may have excess pore pressures due to a recent
increase (or decrease) in overburden. Try explaining that to a Soil
Mechanics student!
I make a motion that the term "underconsolidated" be avoided. Many students
(and engineers?) mistakenly assume that "underconsolidated" must be the
opposite of "overconsolidated". We could replace "underconsolidated" with
more explicit wording like "pore pressures are not at steady state" or "the
soil is still consolidating under existing overburden", or possibly
"beingconsolidated".
Bruce Kutter
At 12:13 PM 7/4/00 +0900, Shiwakoti wrote:
>One more important point on falsely Underconsolidated Soil:
>
>False impression on underconsolidated state
>could also be observed, if the quality of a
>sample under investigation is not good. Its very
>easy for a soft soil sample to get disturbed during
>extraction, handling, transporting etc. Such
>disturbed samples underestimate the preconsolidation
>pressure of the soil. Therefore, for NC or lightly
>overconsolidated soil, one may get preconsolidation
>pressure smaller than the in situ overburden pressure,
>resulting in false impression of underconsolidated
>state of the soil.
>
>The main sources of sample disturbance in soft soil could
>result due to the use of wrong type of sampler or/and careless
>sampling technique. Open type Shelby tube sampling for example, invariably
>results in large degree of sample disturbance of a soft soil.
>The selection of a sampler type is decided by the importance of a
>project and also the extent of fund available for it.
>In soft soil sampling, at least fixed piston type sampler should be used
>to get reasonably good quality sample. Sampling using samplers
>such as Laval and Sherbrooke samplers are very costly and are not used in
>routine works, although they are frequently used for research works.
>
>Sampling technique adopted and the skill of the operator are also equally
>important factors in influencing the quality of an extracted sample.
>Displacement method (in which the sampler is directly pushed into the
>ground to the desired sampling depth without making a borehole) and wash
>boring method of sampling, for example, invariably invite disturbance of a
>soft soil. Use of casing or drilling mud
>also lower the degree of sample disturbance.
>
>Unfortunately, open type Shelby tube is still extensively used in
>recovering soft soil samples in many places. This may not only result in
>underestimation of preconsolidation pressure, but also the strength and
>deformation behaviours of such sample get highly distorted.
>It is well known that strength of a soft soil could easily be
>underestimated by as much as fifty percent or even more, if
>appropriate sampler and sampling method is not adopted.
>It is ironical that, often times, degree of uncertainty of strength
>resulting from bad sampling is just ignored (which could be as high as
>100% or even more); but in its data analysis, sixteen digits figures are
>used along with highly complicated models and elaborate interpretation
>techniques!
>
>With warm regards
>Dinesh Raj Shiwakoti
>
>
>
>
>
>
>At 17:32 00/07/03 -0400, Gilliane Sills wrote:
> >I find the discussion on 'underconsolidated' soils very interesting and
> would like to expand a
> >little on my previous message to the mailbase list.
> >
> >Consolidation is defined in terms of effective stress, and there are
> only two options when
> >considering the current state of effective stress by comparison with
> earlier states. The
> >current state is either equal to the maximum that the soil has
> previously experienced, in
> >which case the soil has a normally consolidated stress history, or it is
> less than the maximum
> >the soil has previously experienced, in which case it is over
> consolidated. The only sensible
> >meaning of the term 'underconsolidated' is to indicate that
> consolidation is not complete.
> >
> >Two examples have been quoted, one of sedimentation and one of artesian
> pressure. It seems to
> >me that it is possible to have a state in which the sedimentation rate
> has been sufficiently
> >fast, and the thickness is sufficiently great, that excess pore
> pressures have not dissipated
> >and therefore consolidation is not complete. In this case, the soil
> itself is normally
> >consolidated, under the effective stresses that have developed so far,
> but the state could be
> >described as one of underconsolidation. Incidentally, it is not
> possible to quote critical
> >sedimentation rates, since the drainage path length is equally
> important. For example, in
> >current experiments here at Oxford using a natural estuarine clay, we
> have achieved a steady
> >deposition rate of 20mm in the first month without any build up of
> excess pore pressure. This
> >is clearly a much higher rate than the 10-15mm a year suggested by Dr
> Shiwakoti. However, we
> >would not expect this dissipation rate to continue as the bed thickness
> increases.
> >
> >As Dr Shiwakoti points out, the case of artesian pressure can be
> misleading, and give a false
> >impression that consolidation is not complete. In fact, artesian
> pressure implies only that
> >the steady state condition is different from hydrostatic. If the
> existing pore pressures are
> >at these steady state values, then this condition is not associated with
> any sense that
> >further consolidation will occur, and 'underconsolidated' is not a
> useful description. If the
> >artesian pressure has been constant or decreasing throughout the soil
> history, the soil is
> >normally consolidated. If the artesian pressure had, for some reason,
> previously been lower,
> >the soil will be overconsolidated.
> >
> >Gilliane Sills
> >
> >On Mon, 03 Jul 2000 15:32:32 +0900 Shiwakoti <[log in to unmask]> wrote:
> >
> >
> >> I agree with Dr. G. Sill's remark on the state of underconsolidation
> of a soft
> >> soil. I have following
> >> additional remarks on underconsolidated soil:
> >>
> >> 1.
> >> relatively high rate of sedimentation also causes underconsolidation of a
> >> soil,
> >> in such cases, some excess pore pressure remains in the soil. Although
> it is
> >> often difficult to make clear boundary on sedimentation rate which
> results in
> >> underconsolidation, typically the sedimentation rate of higher than 10-15
> >> mm/year could result in underconsolidation of a soil.
> >>
> >> 2.
> >> presence of aquifer could also give false impression on underconsolidated
> >> state
> >> of a soil. Pusan clay deposit around the Yangsan delta, of Korea, is an
> >> example.
> >>
> >> 3.
> >> Prof. Silva of University of Rode Island is dealing with underconsolidated
> >> soils extensively,
> >> perhaps he could be helpful in furnishing further details.
> >> His email address is: [log in to unmask]
> >>
> >> Warm Regards
> >> Dinesh Raj Shiwakoti, D. Eng.
> >> Geotechnical Investigation Laboratory,
> >> Port and Harbour Research Institute, Japan
> >>
> >>
> >>
> >>
> >> At 22:19 00/06/29 -0400, Gilliane Sills wrote:
> >> >'Underconsolidated' is a curious expression, because the soil is really
> >> normally consolidated,
> >> >but to an effective stress level lower than the maximum that could be
> >> achieved
> >> under the
> >> >existing total stress - ie, pore pressures have not yet dissipated to
> their
> >> equilibrium
> >> >values. The term is usually applied to consolidation under self-weight
> >> stresses rather than
> >> >that due to an external load. Reasons why this might be the case
> include
> >> recent deposition or
> >> >sedimentation. The situation can also be mistakenly identified in (soft)
> >> soils containing
> >> >undissolved gas, as gas entering the measurement system (piezometer) can
> >> cause
> >> a higher
> >> >reading of pressure than actually exists in the pore water. I think
> it is
> >> likely that many
> >> >cases of so-called under-consolidation are in fact due to this incorrect
> >> measurement of the
> >> >pore pressure.
> >> >
> >> >Gilliane Sills
> >> >
> >> >On Thu, 29 Jun 2000 07:04:00 -0700 (PDT) shbm msnbsh <[log in to unmask]>
> >> wrote:
> >> >
> >> >
> >> >> Hi every one. I am looking for info on "under
> >> >> consolidate" soft ground. Anybody out there to share
> >> >> info with me.
> >> >> Thanks
> >> >> Shahul.
> >> >>
> >> >> __________________________________________________
> >> >> Do You Yahoo!?
> >> >> Get Yahoo! Mail - Free email you can access from anywhere!
> >> >> http://mail.yahoo.com/
> >> >
> >> >----------------------
> >> >Dr Gilliane Sills
> >> >Department of Engineering Science
> >> >University of Oxford
> >> >Parks Road
> >> >Oxford OX1 3PJ
> >> >
> >> >Tel (44) (0)1865 273164
> >> >Fax (44) (0)1865 273907
> >> >[log in to unmask]
> >> >
> >
> >----------------------
> >Dr Gilliane Sills
> >Department of Engineering Science
> >University of Oxford
> >Parks Road
> >Oxford OX1 3PJ
> >
> >Tel (44) (0)1865 273164
> >Fax (44) (0)1865 273907
> >[log in to unmask]
> >
___________________________________________________________________________
Bruce L. Kutter tel: 530 752-8099 fax: 530 752 7872
Professor,
Civil and Environmental Engineering
University of California, Davis, CA 95616
http://cgm.engr.ucdavis.edu
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