Determining silt state from CPTu

Dawn Shuttle,Michael Jefferies

doi:10.1680/jgere.16.00008

Abstract

Tailings, produced when rock is crushed to recover metals, are normally discharged as slurry of predominantly silt-sized particles into storage areas that are created using dams. These dams have a poor safety record with billions of dollars in damages over the past decade alone, making reliable engineering of silt an important economic and safety issue for the mining industry. Equally, engineering of silts is challenging, as understanding of soil behaviour relates mostly to ‘sands’ or ‘clays’. Undisturbed silt samples suffer substantial densification between sampling, transfer to element test and reinstatement of in situ stresses. Hence, silts require a sand-like approach that combines laboratory tests on reconstituted samples with in situ cone penetration test (CPT) soundings. This paper presents calibrated spherical cavity expansion in a general critical-state soil model to simulate the CPT in silt. The developed methodology is numerical, accurately captures calibration data and allows determination of the in situ state parameter in silts from CPT data. A validation is presented for a large tailing impoundment using stacked thickened tailings. Open-source software implementing the methodology is provided on the journal website as supplementary material.

Highlights

Tailings are produced when rock is crushed to recover metals, and are predominantly silt
1, 2 alternative locations for pore pressure sensor on CPTu c critical state lim cavity expansion limit min minimum value of parameter p variable defined on mean stress ref reference stress level; by convention, pref = 100 kPa sph spherical cavity symmetry tc triaxial compression condition (q = p/6)
Tailings within a tailing storage facilities (TSFs) are commonly characterised using the cone penetration test (CPT), and some examples of CPT data are shown in Figure 2 to illustrate their in situ condition

Summary

F G Gmax H friction ratio elastic shear modulus

F/L2 elastic shear modulus measured geophysically: F/L2 plastic hardening modulus. K0 geostatic stress ratio prior to loading; K0 1⁄4 s h0=s v0 k, m dimensionless coefficients in Qp = k exp(−my). M critical friction ratio; equals hc at the critical state; varies with Lode angle, value at triaxial compression (Mtc) taken as the soil property. CPT tip resistance: F/L2 scalar measure of overconsolidation undrained soil strength: F/L2 pore pressure: F/L2 shear wave velocity: L/T specific volume = 1 + e dilation angle (usually in NAMC model): °. State-dilatancy constant; value in triaxial compression taken as the soil property. 1, 2 alternative locations for pore pressure sensor on CPTu c critical state lim cavity expansion limit min minimum value of parameter p variable defined on mean (as opposed to vertical) stress ref reference stress level; by convention, pref = 100 kPa sph spherical cavity symmetry tc triaxial compression condition (q = p/6)

Introduction

A: MPa a b

Background

10 Subaerial sand beaches

Findings

Concluding remarks