"Liquefiable Materials and their Treatment by Vibro-Draining"

This small volume was written by Hodge some time after the Phoenix™ Machine field trials in the tailings slimes at Myra Fall. What brought on this need to write was the fact that I couldn't understand what had happened at Myra Falls – we just shouldn't have been as successful as we were in densifying such fine materials. There was something missing in my conceptual model and I thought by going through the logical rigour of "preparing" a paper-style explanation for "others", then it would become clear to me (too). Well, the effort half-worked.

The Monograph has been fully reproduced as PDFs through the links below. In order to keep each file to a reasonable size for uploading the book was subdivided into seven Sections, A to G.

Section A Section B Section C Section D Section E Section F Section G

If you read these sections you will see that there are three parts to this little book.

In the first I presented the good results achieved inside the offshore Molikpaq drilling platform for the record, and to show how uncomplicated is the task of making clean sands very dense. How different is the case of the confusing results obtained in the slimes of Myra Falls.

The second part is where I made an honest effort to come to grips with the fundamentals of soil strength and how deformation of the soil-structure could affect inter-particle friction in an environment where the solid phase was submerged under changing hydrostatic pressures.

Then in the third I applied this thinking to Myra Falls in the hope of resolving that dilemma and made some progress. But the truth is that I was left with what I considered somewhat of a paradox. So the dilemma remained a dilemma for me until several years later when the penny dropped and I wrote the six articles published by Geotechnical News.

Nevertheless, during the course of battling with submerged frictional strength, I managed to gain a clear picture of the hydrodynamic rules governing water-soil behaviour, and visualize how depositional hydraulic conditions can make soils vulnerable to liquefaction.

Hydraulic Gradient Rules

*   The soil-structure is entirely indifferent to ambient hydrostatic pore pressure changes, no matter how great the magnitude. But, the soil-structure is affected by any hydraulic gradient to which it is exposed.

*   If the soil-structure is exposed to a hydraulic gradient there will be pore water flow. If there is no flow there is no gradient.

*   Hydraulic gradients can move grains, but then, only unencumbered surface particles.

Immediately following liquefaction, sand "volcanoes" can erupt at ground surface because of the high velocities involved in venting the supernatant water concentrated at isolated weak spots in the overlying layer. Similarly, such venting can occur between the inter-bedded layers found in deltaic deposits, when a hydrostatic pressure difference (gradient) between two layers finds a weakness to exploit. This condition comes about, for example, when two sand layers are separated by a thin silt layer; the sand layers having their pressure fixed by the elevation their upstream side daylights in the streambed. Liquefaction is not involved in this latter case.

Deposits Vulnerable to Liquefaction

A good appreciation of the geologic forces at work when the deposit was formed is the priority requirement in site assessment.

Earthquake liquefaction is limited to uniformly graded water saturated masses of non-plastic materials which are in the sand-size range and which were deposited in a manner which promoted the formation of a very loose soil-structure.

Loose natural deposits can form when sands are deposited in the presence of upward water flow vectors (river bar), and also where fine grained deposits are formed in a moist environment (periglacial) when they can accumulate into very loosely aggregated structures. Earthfills, such as dredgate and tailings, can end up as loose materials when either the slurry concentration is too thick to permit particles to settle out independently, or if the discharge velocity creates upward currents.

Particles deposited from streams and rivers are naturally predisposed towards forming a dense soil-structure. Whether natural deposit or earthfill, well graded materials cannot liquefy because the amount of relative movement possible between the solid and water phases is obstructed to an extent that falling particles cannot find room to reach their Terminal Velocity.