In most cases, we talk about dynamic or cyclic loading on deep foundations(piles). It brings me to wonder how we can introduce a cyclic compaction on a soil matrix meant for stabilization by mechanical compaction.
In many respects all compaction is dynamic in the sense that repeated passes are applied leading to low (and variable) frequency loading. However, more straightforward is to use vibrating rollers, polygonal rollers (vibrating or otherwise) or, indeed, as seems to have fallen out of favour compared to a few decades ago dynamic (drop-weight) compaction. These are well-known and widely-used techniques.
The key seems to be deformation cycles. If you can produce and sustain strains larger than 0.1% amplitude - compaction will occur. Ricardo Dobry has phenomenal work showing causal correlation between deformation and volume change. If you are forced to use stress dependent solutions - be aware of the limitations. You will have to predict how much deformation a stress produces, before you can start predicting the consequence of that deformation. Deformation is caused by the stress history, but volumetric change is caused by the deformation....
Recently, we were able to densify specimens in a frictionless triaxial apparatus to a purely dilative soil state. We did so by repeatedly liquefying and draining the specimen. At some point, the specimens became resistant to liquefaction - but some excess pore pressure was still generated during one phase of a loading cycle. Thus, we paused the strain at the point were minute quantities of excess pore pressure are still generated - and squeezed the last few milliliters. Alas, pure dilative state was accomplished (test conducted at low confining pressures, 60kPa, thus no crushing of sand granules)
We notice a lot of sedimentation during cyclic loads applied on foundation prototypes as well. Those are publications in the making, but I attach a video of a data plot showing how sedimentation and cyclic loads work together. Notice how "initial state stiffness path" is recovered repeatedly, as sedimentation is recovered. The test was done on a Mono Bucket prototybe, tested for Universal Foundation A/S.
Conference Paper Triaxial testing beyond yielding
@Tomas, thank you very immensely for your help. The comments and the resource materials were helpful. I wish you well
@Thomas, I am sure that you were not using fully-saturated specimens but it is always worth noting that soils are rarely full-saturated when we compact them. And when we do try to compact fully-saturated soils we end up with a mess, both literally and figuratively, as excess pwps are generated and failure rapidly follows.
@Mike, - We take great care to saturate all our specimens (purely offshore projects). And we take additional steps to preserve maximum pore water stiffness after full saturation, in triaxial tests.
The water is vacuum treated (boiled) to remove all soluble gasses. The specimens are CO2 flushed with 20L of CO2, before washing them with de-aired water. The resultant saturation is always checked by measuring Skemptons B, and the value is never below 0.96 at (200 kPa pore pressure). Note - normally we accomplish B values beyond 0.96. this is the "lowest threshold".
In addition, we use extra stiff nylon drainage tubes to preserve maximum pore water stiffness (water is only stiff as the container) . Due to high preserved water stiffness, our undrained measurements can detect microscopic volumetric strain changes in saturated undrained tests. So much so, that we had to fight molecular permeability of latex (osmosis due to pressure difference) by introducing a second membrane with additional vacuum grease in between the two latex sleeves... In combination with isotropic yielding, provided by frictionless triaxial apparatus, we get superb quality measurements of fully saturated specimens.
Aalborg University has been developing geotechnical testing equipment since the 70's. We have 50 years of experience, and a full brigade of technicians to assist the researchers. We keep making new and upgrading the old equipment to this day. We have pressurized foundation prototype testing rigs, used to research dynamic response of offshore foundations with up to 40m overhead water pressure. We have multiple frictionless triaxial apparatuses - for monotonic testing of clay, and dynamic testing of sand. Also worth mentioning - improved consolidation tests, with thick, low friction walls. All custom built by using top grade materials and sensors certified by by HBM, MGC, MOOG and so forth... We even have a true triaxial apparatus (for fully saturated specimens), which has been collecting dust in the corner for the last decade, but nevertheless - remains functional.
Our long history of geotechnical testing began in the 70's, with M.Jakobsen, His solutions were modernized by Lars Bo Ibsen in the 90's. And I am part of the latest generation of researchers exploring the frontiers of geotechnical testing.
@Thomas, you rather miss my point - when we compact soils they are generally not, and nor should they be, saturated (otherwise you will have problems as outlined in my earlier post). Therefore using analogues that are based upon saturated soil tests, including triaxial ones, are not appropriate. This is one of the main reasons why we have unsaturated soil mechanics.
On the subject of history, we can all point to the past success of others - my own institution can trace its history in this subject area to soon after its creation in 1933 and had several papers presented at and in the proceedings of in the first International Conference on Soil Mechanics in 1936.
Interesting. I have no experience with non-saturated tests. I missed your point because I was too defensive. We look for "new testing plausiblities" and success occasionally brings forth some bizarre resistance. It can be quite intimidating, it's making me excessively defensive.
Only once I tried squeezing an unsaturated specimen. It formed 6 pairs of perpendicular shear ruptures. I took it as a sign of near isotropic stress state, as the "dominant" failure plane kept jumping randomly - top to bottom, left and right. Saturation improves uniformity of the stress field, we never had a rupture during compression. Not once in over 100 tests. We do reach "isotropic yielding" consistently when the specimens is saturated. However, Vardoulakis (1983) discovered "isotropic yielding" on unsaturated sand specimens. Thus, I'm not sure what to think. I'll trust you on this one.
Thank you for clarifying. It is good for me to be reminded that "on shore" geotechnics exist as well, and "off-shore" criterion for a "good test" are not universal. Thank you.
However "as excess pwps are generated ..." - we are able to control the process of pwps accumulating very well. The matter of fact is, me and my student use post-liquefaction cycles to solidify the specimens. Turning the liquefied slurry back into a solid piece - makes cleaning a lot easier and faster. I notice students using "undocumented" soil properties to make cleaning easier quite oftern, actually. It's funny. They avoid moving prototypes of small scale foundations after measuring ultimate strength - because otherwise the things recover their strength. This recovery is poorly if at all documented anywhere. Yet, it makes cleaning that much easier if avoided, thus students learn to avoid the "undocumented phenomenon" intuitively. I find it quite funny.There are no equations describing it, but a student "discovers it" within 2 days of cleaning.
@Thomas, there was nothing defensive about my point it was simply short and to the point. I am sorry that you took in a different way.
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