Drained tests in dense sands indicated a difference in localized to global void ratio increase of about 0. Equivalent undrained tests are not yet reported, but it seems reason- able to infer that moisture content redistribution such that the usual procedure in interpre- tation of dilatant data is fundamentally flawed. However, measuring the void ratio of sand to the precision required to make this a reliable exercise is far from straight-forward as noted by McRoberts and Sladen , Been and Jefferies , Jefferies and Davies , and Martin and McRoberts Almost negligible undrained shear loading would be required to ini- tiate liquefaction e.
However, material initially in zones B, and even A, could have a static liquefaction concern if the state of static shear bias were to result in a change in state to Zone C. The small increment of shear for Zone C tailings, or a change in state from A or B tailings, can be termed the static liquefaction trigger, these are discussed sub- sequently.
Issues in Static Liquefaction That loose sands could behave in a puzzling manner is not new, and has been rec- ognized as early as the first use of the term "liquefies" in our context by Hazen re- porting on the failure of the Calaveras Dam during construction. Writing in German, Ter- zaghi defined the essential processes of liquefaction and the subsequent lecture by Casagrande formed the basis for practice at that time.
In this early paper, and re- fined in subsequent work Casagrande, , Casagrande defined the critical void ratio concept. This method used drained direct shear or triaxial tests to define the void ra- tio at which neither drained contraction or dilation occurs at high strain, and observed that there was a unique relation between this so-defined critical void ratio and the log of effec- tive stress.
But, in , the hydraulic fill Fort Peck Dam failed during construction Mid- dlebrooks, As noted by K pper and Morgenstern the collapse of the Fort Peck Dam constituted a watershed in the evolution of hydraulic fill methods for dam con- struction in North America and marked an abrupt decline in the technique [but not for tail- ings dam construction].
The indirect answer was to deduce that for in-situ void ratios dense of critical that shearing would result in dilation, pore pressures would reduce and flow or liquefaction would not occur.
After the failure of the Fort Peck Dam a consulting board reported on extensive investigations as summarized by Middlebrooks While the majority of the board concluded that the failure was not caused by liquefaction based on the fact that in-situ densities were greater than the minimum critical density observed Middlebrooks, , the minority opinion, including Casagrande, was that liquefaction had occurred, but he was not at the time able to fully elucidate the failure.
Subsequently, Casagrande elaborated on his reasons and in noted "even today we have no laboratory tests that can measure reliably the sus- ceptibility of a sand to liquefaction".
This might well be applied to the current state of practice. Reference to standard textbooks from the to 70's makes it clear that the pri- mary basis for consideration of loose sand was the critical void ratio concept. An excep- tion was the work of Bjerrum et al. These tests never gained wide currency and have few citations in the litera- ture of the time. Bjerrum reported on a fine-grained Norwegian sand and the impetus for these test were the large subaqueous flow slides reported in the Norwegian fjords.
These tests reported the surprising observation that the mobilized friction angle at peak deviator o o stress was as low as 11 well below the expected value of 35 or more. It spite of this work and the many developments since them, static liquefaction is a much less well recognized phenomenon than its seismic counterpart. There is limited mention of static liquefaction in regulatory literature and a good portion of the publications that refer to static liquefaction either do not explain the phenomenon being referred to or use the term as an explanation for any non-seismically triggered flow failure with no other common failure mechanism.
Many designers do clearly not recognize the mechanism. The fact that many more tailings dams have not failed due to this mechanism is in part due to the designers taking measures to combat seismic loadings and have also uninten- tionally guarded against static liquefaction. However, designs in low seismic areas may not have this co-incidental safeguard.
Unfortunately, a number of tailings dam failures have been mislabelled with other failure modes only to eventually have static liquefaction correctly noted as the contributing mode of dam failure e.
Fourie et al, However, Martin and McRoberts and Davies et al. Fourie et al. Only drained loading response was examined and peak frictional strengths with hydrostatic pore pressures was the only framework considered.
When tailings materials are monotonically sheared, they can respond in two very distinct manners: 1. The loading rate is slow enough so that irrespective of how contractant the tail- ings skeleton, any shear-induced pore pressure changes have no effect on strength as they drain as quickly as they are induced. If the tailings are con- tractant, then shearing under drained conditions results in a decrease in the void ratio, increasing their undrained shear strength. The loading rate is quick enough, or the tailings of sufficiently low relative hy- draulic conductivity, that shear-induced pore pressures are generated e.
When undrained monotonic loading occurs, it should be automatic practice to invoke undrained strength properties. Drained strength, e. Mohr-Coulomb relation- ships, could be used but accurately estimating the complex pore pressure regime at failure is often very difficult. Consequently, to avoid this difficulty, it is more sensible to use undrained strengths su in loading situations where significant pore pressures could de- velop. Undrained strength of contractant materials is typically characterized by an undrained strength ratio with the ratio being the undrained strength to the effective over- burden stress.
The range is dependent upon a number of factors; the most im- portant of which is material density. Tailings are usually cohesionless except for the fine fraction of ore bodies with sub- stantial mineral clay content. Plewes et al. This is described further below.
Initiating a spontaneous liquefaction event does not require very much additional shear stress beyond that in place from the at-rest soil condition. If there is a slope condition, i. Kramer and Seed demonstrated in the laboratory that there is a marked in- crease in static liquefaction susceptibility with increase in principal effective stress ratio. This type of soil behaviour has been observed by many other researchers and described in literature at least as far back as Bjerrum et al.
Unfortunately, the literature on seismic liquefaction i. This conclusion was based on cyclic laboratory testing in which cyclic mobility behavior could only be induced by cycling through a state of zero shear stress, which in turn could only be induced in isotropically consolidated samples. However, in a critique of this ap- proach, McRoberts and Sladen pointed out that in situ stress conditions are rarely, if ever, isotropic, and summarized the numerous misgivings of Arthur Casagrande to this concept.
Considering this question in the context of Figure 3, it is graphically and intui- tively obvious that the lower the static shear bias, the greater the distance from the col- lapse surface and therefore the lower the susceptibility to liquefaction, certainly under static conditions. While this paper is focused on static liquefaction it is necessary for completeness to further discuss the inference that liquefaction resistance increases with principal stress ra- tio i.
This element of the SLAC approach was eventually corrected by Rollins and Seed by recognizing that for loose sands liquefaction susceptibility in- creased with higher static shear bias. Paddock systems are relatively common in South Africa and are essentially upstream constructed tailings impoundments with little freeboard and relatively saturated BBW dam shells.
The mine was located near the town of Merriespruit. The Merriespruit failure occurred on February 22, in the evening. A massive failure of the north wall 3 3 occurred following a heavy rainstorm. Over , m of tailings and 90, m of water 2 were released.
The slurry traveled about 2 km covering nearly , m. A view of the aftermath of the failure is shown on Figure 5. Figure 5 Merriespruit Tailings Dam Failure A relatively minor rainstorm caused the limited freeboard to be overcome and water leaving the impoundment caused toe erosion, which, in turn, initiated the flow failure.
However, these fines were also essentially cohesionless and once an area of the dam toe was eroded and local slopes were increased to the range of 2H:1V, static liquefaction and the massive flowslide was initiated soon after. Much of the post-failure laboratory testing exhibited dilatant behavior, leading a number of well-published engineers to suggest that the failure mode was uncertain. The fact that contractant behavior could not be easily coaxed from the tailings in a laboratory setting yielded the flawed conclusion that they must then be dilatant in both the laboratory and field settings.
This conclusion was reached even in light of in-situ cone data that clearly indicated the potential for an in-situ contractant response to rapid transient loading. The authors have encountered too many geotechnical projects in general, and tailings dam projects in particular, in which sci- ence was revered as king, and his loyal subjects viewed with distrust and skepticism any- thing that could not be repeatedly demonstrated in laboratory testing. In the giant stress- controlled test represented by the dam itself, contractant, undrained behavior clearly re- sulted, and Figure 5 is unambiguous in this respect.
With Fourie et al. The Merriespruit case record provides a good example of field evidence being mis- interpreted due to its apparent non-conformity with laboratory data. Blight , pro- duced figures illustrating dilatant behavior of gold tailings.
It is probably reasonable to assume that those tests were carried out under drained to partially drained conditions. It is thus difficult to explain why the remoulded strengths are so much lower. Piezo- cone data was equally compelling in terms of the contractant nature of the tailings. Sullivan Mine, Canada, Davies et al. Figure 6. The dam had been built on a foundation of older tailings that were placed as BBW material. The failure of the upstream constructed facility was triggered by the initiation of shear stresses in the foundation tailings in excess of their shear strength.
As the material strained, the pore pressures rose and drainage was impeded leading to liquefaction event. The down- stream slopes of the dyke average roughly 3H:1V, imposing stresses in excess of the col- lapse surface for the foundation tailings.
Figure 6 shows a ground view of the post-liquefaction appearance of the Sullivan tailings dyke. The only trigger to the liquefaction failure was the slope geometry; a pre-failure dyke slope of about 2.
Davies et al. The fail- ure bore a striking resemblance to a failure that occurred at the facility in , during up- stream raising Figure 7. That event, unlike its descendant, involved significant off- site release from a flow failure, an obvious undrained event.
However, the design analy- ses prior to the failure were based on peak-drained strengths with no regard for shear-induced pore pressures. Here the site-specific lesson from 43 years before was forgotten — history did indeed repeat itself. As another example of TDA, the authors are aware of a tailings impoundment in the United States where no fewer than three failures involving static liquefaction have occurred in the same general area of the impoundment.
Stava Mine, Italy, Perhaps the most tragic tailings dam failure to date occurred on July 19, A fluorite mine, located near Stava in northern Italy, had both of its tailings dams fail sud- 3 denly and release approximately , m of liquefied tailings.
The flowslide destroyed the village of Stava and also caused considerable damage at Tesero, at the junction of Stava Creek and the Avisio River at the 4 km point from the mine. The tailings dams were both nearly 25 m high with one directly upstream of the other.
Figure 8 shows views of the impoundments pre and post-failure, while Figure 9 shows a schematic section of the two impoundments. The failure mechanism began with failure of the upper dam that in turn overtopped and failed the lower dam as well.
The dams were upstream constructed with outer slopes from 1. Based upon the likely state of the in-situ tailings and the aggressively steep slopes, the soil mechanics curiosity with this failure is that the dams could have attained such a height prior to failure achieving states of in-situ stress that were likely far in excess of the collapse surface irre- spective of stress path. Clearly drained, or at least largely drained, loading was sustained for the life of the facility prior to the failure.
There is no question that the design of these dams was not consistent with even the most elementary of engineering principals available at the time. There are a number of "rules" for upstream tailings dam engineering Davies and Martin, that were understood on both a theoretical and empirical basis for many years prior to the Stava failure. As a … Expand. View 2 excerpts, cites background and methods. A mixture model for the compaction of saturated sand.
Abstract Free draining water saturated sands and dry sands compact progressively under cyclic shear loading, and the rate of compaction increases as the shear strain amplitude increases, independent … Expand.
Liquefaction is one of the critical problems in the field of Geotechnical engineering. It is the phenomena when there is loss of shear strength in saturated and cohesion-less soils because of … Expand. View 1 excerpt, cites results. Rayleigh Waves Transformation in Liquefying. The behaviour of a water-saturated sand deposit subjected to dynamic loads induced by the propagation of Rayleigh surface waves is analysed.
Cyclic shearing of the saturated sand matrix due to ground … Expand. Drained and undrained cyclic torsional simple shear tests are conducted for saturated Ottawa sand with and without initial static shear applications.
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