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Geotechnical News • March 2017
WASTE GEOTECHNICS
This tailings has the consistency of chocolate pudding:
a formal comparison of the geotechnical vane shear strength
of food and soft tailings
Gord McKenna, Vanessa Mann, Bereket Fisseha, Nick Beier, Nicolas Olmedo
Introduction
“This tailings pours like chocolate
milk, but when you add flocculant,
it becomes more like blueberry pie
filling.”
We often compare the unusual geo-
technical properties of soft tailings
to common foods. Soft tailings shear
strengths are much lower than most
geotechnical people are accustomed
– indeed many tailings are better
described using fluid mechanics than
soil mechanics. So people describe
the consistency (mainly strength, but
also sensitivity and density) of soft
tailings using informal comparisons to
foods such as chocolate milk, yogurt,
porridge, cottage cheese, and peanut
butter.
Lacking is an accessible reference
that provides a more direct, more
authoritative comparison of tailings
and foods. So with two hours at the
grocery story, two days of hard work
in the University of Alberta laboratory,
and two weeks of crunching the data
and writing it up for a recent oil sands
tailings publication (see McKenna
et al 2016a) we’re pleased to share
the results with a wider geotechnical
audience.
To allow a direct comparison of tail-
ings and food, we measured the peak
and remolded strength of 75 samples
of soft tailings, soft foods, and house-
hold products using the geotechnical
laboratory vane test, specifically the
ASTM D4648 test method for vane
strength of saturated fine-grained soils.
We also performed some other infor-
mal tests on each sample as described
below.
While our comparison of tailings to
foods is meant to be perhaps a bit
lighthearted, it also has point. Many
mines (most notably the oil sands)
have major issues with the technical
and operational challenges of manag-
ing soft tailings – stabilizing or repro-
cessing or the tailings to allow mine
reclamation to produce useful post-
mining landscapes. Soft tailings by
definition have such low strengths and
densities that they cannot be trafficked
by typical earthmoving equipment,
which severely limits reclamation.
The presence of soft tailings, and the
cost to reclaim them, usually comes
as a surprise to mines. That one can
directly compare soft tailings to com-
mon foods highlights the challenge of
converting these soft or fluid tailings
to solid landscapes. Oil sands min-
ing operations in northeastern Alberta
have produced over one billion cubic
metres of soft tailings which are chal-
lenging to stabilize and reclaim to
boreal forest landscapes (CCA 2015)
and the subject of billions of dollars
of research and development and
commercialization (see CTMC 2012).
Many metals mines worldwide have
similar issues, but at smaller scales.
Background
Soil consistency descriptors, avail-
able in most soil mechanics textbooks,
have been used for over a century.
Very soft soils can be extruded
between the fingers when squeezed
(shear strength <12 kPa). Soft soils
can be molded with light finger pres-
Figure 1. Three oil sands soft tailings. A - fluid fine tailings; B - centrifuged
fluid fine tailings; C - thickened tailings (TT) + fly ash.
