Predicting Changes in Static Liquefaction Susceptibility Using the Cone Penetration Test 

Stability of tailings dams and tailings run-out

Mark Styler

ConeTec Investigations, Ltd

Flow liquefaction is a geotechnical hazard in mine tailings facilities.  Soil states that are contractive, or loose of the critical state line, are susceptible to flow liquefaction.  Tailings facilities are dynamic structures, typically growing in height each year. Dilative tailings may become contractive later in the mine life due to increases in the effective stress.  In this paper we show how to use Cone Penetration Test data along with tailings construction planning to estimate the current and future state of in-place tailings.

Susceptibility to flow liquefaction is typically evaluated by calculating a clean-sand equivalent normalized cone tip resistance (Qtn,cs) or by estimating the state parameter (y).  It is challenging to estimate how these metrics will change due to planned construction.  An alternative approach is to evaluate the soil-state using the yield-stress-ratio (YSR = s’y/s’v0).  If the YSR equals one it is on the normal consolidation line and the soil-state is contractive.  YSR values greater than three are typically dense of the critical state line (CSL) and the soil-state is dilative.  In this paper we review how to estimate s’y and how the YSR can be used to screen for contractive soil behaviour.

Unlike Qtn,cs and y, it is very simple to estimate how the YSR will change due to an increase in overburden stress.  This YSR approach was evaluated at three tailings facilities that saw increases in overburden of 10 m, 20 m, and 25 m.  We show that the s’y calculations are consistent and the increase in effective stress moves the soil state towards the NCL.  Moving towards the NCL can turn a dilative soil into a contractive soil that is susceptible to flow liquefaction.  This work has implications in developing better QA criteria for tailings dam construction.

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