136
9 Hazard Identification
9.2.3 Conclusion
Integrating big data and thick data brings value and should be fostered. Using big data
in isolation can be problematic. Thus it is crucial to explore how big data and thick data
can supplement each other. This demands the integration of qualitative evaluation
and expert-based judgments with “hard” quantitative data. While we recognize that
melding big and thick data together isn’t easy, we do it on a daily basis.
9.3 A Note on Liquefaction: A Special Hazard
There is an important body of literature on static and dynamic liquefaction of fine
sands and silt. Interested readers should probably start with an article entitled “Static
Liquefaction Of Tailings – Fundamentals And Case Histories” (Davies et al. 1998).
Static liquefaction, and the resulting flowslide of liquefied tailings materials, is a
relatively common phenomenon among tailings impoundment failure case histories.
Static liquefaction can be a result of slope instability issues alone, or can be triggered
as a result of other mechanisms. It is a special hazard as it can be both cause and
consequence of failures. Here are a few examples from the 1990s:
• Sullivan Mine, Canada, 1991. The dam had been built on a foundation of older tailings that were placed as beach below water material. The failure of the upstreamconstructed facility was triggered by shear stresses in excess of the shear strength
in the foundation tailings. As the material strained, the pore pressures rose and
drainage was impeded, leading to liquefaction event. The downstream slopes of
the dam averaged roughly 3H:1V. The failure was very brittle and sand boils,
water expressed from standpipes, and other “classic” liquefaction expressions were
evident. The only trigger to the liquefaction failure was the slope geometry: a prefailure dyke slope of about 2.5H to 3.5H:1V with a maximum dam height of about
25 m.
• Merriespruit Harmony Mine, South Africa, 1994. A relatively minor rainstorm
caused an over-topping event, and runout caused toe erosion, which in turn initiated
the flow failure. The tailings were quite fine-graded, with more than 60% finer than
74 µm. However, these fines were also essentially cohesion-less 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.
• Los Frailes Mine, Spain, 1997. The initial movement of the rock-fill dam (due to
a foundation failure in a weak layer of marls) was the triggering mechanism that
allowed the tailings to liquefy. This liquefaction exerted a thrust against the dam
that contributed to the relatively rapid progression and large lateral displacement
of the overall failure event.
As it can be seen from these three examples, liquefaction can either be the trigger
of a failure (Sullivan Mine) or the consequence of another mechanism (Merriespruit,
Los Frailes).
9 Hazard Identification
9.2.3 Conclusion
Integrating big data and thick data brings value and should be fostered. Using big data
in isolation can be problematic. Thus it is crucial to explore how big data and thick data
can supplement each other. This demands the integration of qualitative evaluation
and expert-based judgments with “hard” quantitative data. While we recognize that
melding big and thick data together isn’t easy, we do it on a daily basis.
9.3 A Note on Liquefaction: A Special Hazard
There is an important body of literature on static and dynamic liquefaction of fine
sands and silt. Interested readers should probably start with an article entitled “Static
Liquefaction Of Tailings – Fundamentals And Case Histories” (Davies et al. 1998).
Static liquefaction, and the resulting flowslide of liquefied tailings materials, is a
relatively common phenomenon among tailings impoundment failure case histories.
Static liquefaction can be a result of slope instability issues alone, or can be triggered
as a result of other mechanisms. It is a special hazard as it can be both cause and
consequence of failures. Here are a few examples from the 1990s:
• Sullivan Mine, Canada, 1991. The dam had been built on a foundation of older tailings that were placed as beach below water material. The failure of the upstreamconstructed facility was triggered by shear stresses in excess of the shear strength
in the foundation tailings. As the material strained, the pore pressures rose and
drainage was impeded, leading to liquefaction event. The downstream slopes of
the dam averaged roughly 3H:1V. The failure was very brittle and sand boils,
water expressed from standpipes, and other “classic” liquefaction expressions were
evident. The only trigger to the liquefaction failure was the slope geometry: a prefailure dyke slope of about 2.5H to 3.5H:1V with a maximum dam height of about
25 m.
• Merriespruit Harmony Mine, South Africa, 1994. A relatively minor rainstorm
caused an over-topping event, and runout caused toe erosion, which in turn initiated
the flow failure. The tailings were quite fine-graded, with more than 60% finer than
74 µm. However, these fines were also essentially cohesion-less 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.
• Los Frailes Mine, Spain, 1997. The initial movement of the rock-fill dam (due to
a foundation failure in a weak layer of marls) was the triggering mechanism that
allowed the tailings to liquefy. This liquefaction exerted a thrust against the dam
that contributed to the relatively rapid progression and large lateral displacement
of the overall failure event.
As it can be seen from these three examples, liquefaction can either be the trigger
of a failure (Sullivan Mine) or the consequence of another mechanism (Merriespruit,
Los Frailes).