Geotechnical News • June 2017
31
WASTE GEOTECHNICS
The impact of shear strength, density, and settlement
on capping and reclaiming soft tailings deposits
Gord McKenna, Brent Mooder, Bill Burton, Andy Jamieson, Derrill Shuttleworth
Soft tailings are operationally chal-
lenging and expensive to stabilize,
cap, and reclaim to agreed land uses.
This article highlights recent work
(see McKenna et al 2016) on the role
of shear strength, density, and settle-
ment on capping soft tailings, with
examples from Athabasca oil sands
mining.
To extend this work beyond oil sands,
the authors are collecting additional
data from the literature on capping
tailings at metal and coal mines,
dredge spoils, sewage lagoons, soft
industrial wastes, and embankments
on soft ground. If you have strength
and density data, and especially cap-
ping experience you can share, please
contact us.
What are soft tailings?
Soft tailings (Figure 1) are the residues
separated during ore processing that,
due to their low strength and bearing
capacity, are difficult to stabilize, cap,
and reclaim (Jakubick et al 2003).
Many metal and coal mines have a
few hectares of soft tailings that are
capped using soft-ground techniques
(Figure 2). Other mines, especially oil
sands mines, have many square kilo-
metres of deep soft tailings deposits,
necessitating a more holistic approach
to tailings management.
Oil sands soft tailings are generally
underconsolidated clay-rich tailings.
They may be deposits formed by fines
segregated from tailings sand slurry
during hydraulic placement, or they
may be deposits of reprocessed fine
tailings. Typical geotechnical mois-
ture contents are 40 to 250%, which
corresponds to solids contents of 30 to
70%. They have peak undrained shear
strengths that are less than
25 kPa with sensitivities of 2 to 13;
many have strengths comparable to
soft foods (McKenna et al 2017). Soft
tailings with shear strengths less than
5 kPa are sometimes referred to as
fluid tailings.
For soft tailings with low permeabili-
ties, deep deposits are expected to take
decades to centuries to consolidate. In
the oil sands, post-reclamation settle-
ments of 10 to 50% of the original tail-
ings thickness may be expected, due to
the combination of self-weight and the
weight of a cap.
Successful reclamation
Successful tailings reclamation
requires clear, achievable goals and
performance objectives. The goal is
clear in oil sands: regulatory approvals
require oil sands operators to construct
reclaimed landforms that are capable
of supporting a self-sustaining, locally
common boreal forest (which includes
Figure 1. Two oil sands soft tailings.
A: Untreated fluid fine tailings;
B: Fly ash amended thickened
tailings.
Figure 2. Soft-ground techniques employed at Wismut Uranium Mines,
Germany.
