The upstream slope of the Blackdome gold mine tailings pond is built from fine tailings with more than 95% passing #200.
A rock-waste pad was needed to allow equipment access onto that flat slope.
Craters appeared around each Phoenix™ Machine treatment axis, thereby giving clear evidence of the success of this attempt to compact the fine material under the pad. Light-weight penetration tests confirmed this fact.
The simultaneous action of the Phoenix™ Machine's two modules help make it possible to achieve this response. The filter/drain module, by introducing a low pressure zone around the vibrator module, creates a hydraulic gradient field which attracts the loose particles.
This apparently negates the otherwise hopeless condition in very fine materials where the excess water available from the densifying soil-structure keeps the silt in suspension, and the vibrator ends up trying to compact a slurry.
Myra Falls is an underground copper mining operation in the heart of Vancouver Island.
The slimes residue in the tailings pond forms an inter-layered deposit containing seams of very fine particulates.
Samples from boreholes tested in the laboratory for grain size range show these very loose wastes to be essentially a silt with from 10% to 30% clay sizes, see gradation curves to right.
Such very fine materials are invariably considered to be "untreatable" by vibration. The fact is that had Hodge been shown this material, rather than bags of medium sand, he would never have sent Phoenix there for a field trial. As it turned out it was a real eye opener.
Phoenix™ Machine treatments were performed at Myra Falls on a 2m triangular test pattern. The hardware was activated from 2.5m to 18.2m below the surface. A 2m thick sand blanket incorporating a Nilex geogrid mat was necessary to permit equipment onto the tailings. Improvement was obvious from the general ground depression caused around the test site; furthermore, craters developed under the geogrid and consequently were not apparent at ground level. Standard Penetration Test N-values taken before and after treatment showed them increasing by a factor of 2.73.
The most significant, and surprising, change in behaviour was in the dynamic pore pressure response measured by the piezo-electric CPT.
In the accompanying plot: The red shaded area to the right of the zero-pressure ordinate is the pre-treatment data; the black dots are the post-treatment record. Before treatment the entire column of tailings displays very high water pressure increases as the cone tip penetrates the deposit, indicating highly contractive behaviour. The post-treatment response, especially below 11m, shows a remarkable switch to strongly dilative behaviour, with negative pore pressures tending towards a total vacuum (minus 10.3m = cavitation).
The fascinating theoretical point is that the hydrodynamic response of the rapidly deforming saturated mass had been changed from one extreme to the other: from liquefaction, to strain-hardening.
The practical point is that the post-treatment tailings responded to distortion by trying as hard as physically possible to suck in more water. Such extreme water shortage, indicative of pore water cavitation, clearly demonstrates that the post-treatment tailing slimes were in no condition to turn into a fluid. They just simply could not liquefy.