9.3 A Note on Liquefaction: A Special Hazard
137
In any case, static liquefaction occurs when rapidly loaded, loose saturated sand
(such as that deposited in an underwater tailings beach) do not have time to drain
(dissipate the water pressure) and hence the soil collapses into a liquid form. The
prediction of the in situ undrained strength for these materials is highly uncertain
due to the intrinsic uncertainties on the initial void ratio and on the fabric (the way
sand grains are packed together) of field scale deposits of these materials (tailings
ponds).
The most readily identified of these rapid loading conditions, at least from a
design perspective, is the transient loading from seismic events. Whether limited
deformation or eventual flowslide development, the effects of transient seismic loads
on mine tailings are well documented in the literature and well recognized by current
engineering standards. However, there are many other rapid or undrained shear loads
that affect mine tailings.
The numerous potential triggers of undrained response can be of equal importance
to seismic loads due to their more common occurrence at mine sites. Included in these
common loads are incremental raise construction and episodic spigotting. The first
can lead to relatively rapid increases in stress levels and undrained conditions in
susceptible materials, while the second can cause temporary changes to the amount
of tailings saturated in a given section of an impoundment. Conversely, traditional
static loads are taken to be those in place for a considerable period. Other mechanisms,
such as a transient saturation of the downstream shell of a tailings structure, can also
trigger liquefaction due to rapid reductions in effective stress.
When any combination of triggers is possible it should be automatic practice
to invoke undrained strength properties (S u ) in loading situations where significant pore pressures could develop, i.e., performing undrained stress analysis (USA).
Additionally, the safety of dams susceptible to static liquefaction is perhaps even
better expressed in terms of the cumulative probability of potential triggering mechanisms. Despite the obvious difficulties in quantifying this cumulative probability,
the approach has the virtue of at least recognizing the potential for static liquefaction.
Here is a list of well documented potential static liquefaction triggers in tailings
impoundments including:
• Increased pore pressures induced by an increase in the piezometric surface, and/or
change of pore pressure conditions from below hydrostatic to hydrostatic, or to
higher than hydrostatic. Thus poor management/monitoring, but climate change
event could also be a trigger.
• Excessive rate of loading due to rapid raising of the impoundment. Also rapid
rate of rise as the trigger for an upstream beach below water leading to static
liquefaction failure. Poor Management is a trigger.
• Static shear stresses in excess of the collapse surface, leading to “spontaneous”
liquefaction. This is for example the result of aggressive slopes, foundations
deformations, etc.
• Removal of toe support from an overtopping event, lateral erosion from
a watercourse encroachment or any other situation when the toe can be
steepened/removed.
137
In any case, static liquefaction occurs when rapidly loaded, loose saturated sand
(such as that deposited in an underwater tailings beach) do not have time to drain
(dissipate the water pressure) and hence the soil collapses into a liquid form. The
prediction of the in situ undrained strength for these materials is highly uncertain
due to the intrinsic uncertainties on the initial void ratio and on the fabric (the way
sand grains are packed together) of field scale deposits of these materials (tailings
ponds).
The most readily identified of these rapid loading conditions, at least from a
design perspective, is the transient loading from seismic events. Whether limited
deformation or eventual flowslide development, the effects of transient seismic loads
on mine tailings are well documented in the literature and well recognized by current
engineering standards. However, there are many other rapid or undrained shear loads
that affect mine tailings.
The numerous potential triggers of undrained response can be of equal importance
to seismic loads due to their more common occurrence at mine sites. Included in these
common loads are incremental raise construction and episodic spigotting. The first
can lead to relatively rapid increases in stress levels and undrained conditions in
susceptible materials, while the second can cause temporary changes to the amount
of tailings saturated in a given section of an impoundment. Conversely, traditional
static loads are taken to be those in place for a considerable period. Other mechanisms,
such as a transient saturation of the downstream shell of a tailings structure, can also
trigger liquefaction due to rapid reductions in effective stress.
When any combination of triggers is possible it should be automatic practice
to invoke undrained strength properties (S u ) in loading situations where significant pore pressures could develop, i.e., performing undrained stress analysis (USA).
Additionally, the safety of dams susceptible to static liquefaction is perhaps even
better expressed in terms of the cumulative probability of potential triggering mechanisms. Despite the obvious difficulties in quantifying this cumulative probability,
the approach has the virtue of at least recognizing the potential for static liquefaction.
Here is a list of well documented potential static liquefaction triggers in tailings
impoundments including:
• Increased pore pressures induced by an increase in the piezometric surface, and/or
change of pore pressure conditions from below hydrostatic to hydrostatic, or to
higher than hydrostatic. Thus poor management/monitoring, but climate change
event could also be a trigger.
• Excessive rate of loading due to rapid raising of the impoundment. Also rapid
rate of rise as the trigger for an upstream beach below water leading to static
liquefaction failure. Poor Management is a trigger.
• Static shear stresses in excess of the collapse surface, leading to “spontaneous”
liquefaction. This is for example the result of aggressive slopes, foundations
deformations, etc.
• Removal of toe support from an overtopping event, lateral erosion from
a watercourse encroachment or any other situation when the toe can be
steepened/removed.