3045
Characterisation of landfill steel mill sludge waste in terms of shear strength, pore
water pressure dissipation and liquefaction potential
Caractérisation de la résistance au cisaillement, de l'évolution des pressions d'eau interstitielle et
du potentiel de liquéfaction des boues d’aciérie dans un centre de stockage
.
Lavoie J.L.N., Sinclair T.J.E.
Tonkin &Taylor Ltd, 105 Carlton Gore Road, Newmarket, Auckland, 1026; PH (+64) 9-355 0703; FAX (+64) 9-307 0265
ABSTRACT: The unique method of iron sands mining undertaken at the New Zealand (NZ) Steel Mill, produces an inert waste
sludge comprised primarily of clay and iron-sands/grit. This wet sludge waste is landfilled in cells to heights up to 25m. The purpose
of this paper is to characterise the sludge waste and to investigate its potential for liquefaction. This paper presents an investigation of
the sludge in the existing landfill based on in situ and laboratory testing. Design parameters such as shear strength, and pore water
pressure are developed and confirmed for the sludge material. Liquefaction potential of the sludge due to earthquake shaking is
investigated using a CPT-based assessment and Atterberg limits test results. The paper concludes with a discussion of liquefaction
potential and recommended total stress and effective stress parameters for detailed design of a new landfill development.
RÉSUMÉ : La méthode unique pour l’extraction de minerai de fer utilisée par New Zealand (NZ) Steel Mill produit une boue inerte
résiduelle composée principalement d’argile et de sable ferreux. Cette boue liquide est stockée dans des casiers sur une hauteur
pouvant atteindre 25m. L’objectif de cet article est de caractériser la boue résiduelle et d’évaluer son potentiel de liquéfaction. Cet
article présente les résultats des essais in-situ et en laboratoire, réalisés sur la boue présente dans le centre de stockage existant. Les
caractéristiques telles que la résistance au cisaillement et l'évolution des pressions d'eau interstitielle sont développées et confirmées
pour cette boue. Le potentiel de liquéfaction de la boue lors d’un tremblement de terre est évalué par des essais réalisés au moyen de
pénétromètres coniques ainsi que par la détermination des limites d'Atterberg. L’article conclut par une discussion sur le potentiel de
liquéfaction et sur les valeurs de contrainte totale et contrainte efficace recommandées pour la conception et le développement d’un
nouveau centre de stockage.
KEYWORDS: sludge, landfill, liquefaction, shear strength, pore water pressure
1 INTRODUCTION
The objectives of this paper are to characterise landfilled sludge
waste in situ and to investigate the potential for liquefaction of
the sludge. Various in situ test results are presented and
parameters for detailed design of a proposed landfill are
recommended. The potential for liquefaction of the sludge due
to earthquake shaking is investigated.
2.1
New Zealand Steel Mill
The NZ Steel Mill is located approximately 60 km Southeast of
Auckland and is the only mill in the world to manufacture iron
and steel from titanomagnetite iron sands. The iron sands are
found along the western coast of New Zealand’s North Island
and are the remains of rocks which once formed the flanking
volcanoes of Mount Taranaki (located about 250 km south of
the mill). They are the largest reserves of metal ore in New
Zealand.
2.2
Waste streams
As a result of the unique nature of the NZ Steel sand mining and
iron/steel making processes, a number of waste streams are
produced and landfilled onsite. These wastes include: wet
sludge, slag, reduced primary concentrate and char (RPCC),
ironbearing dusts and general works debris.
The sludge waste is a mixture of clay slimes that result from
the slurry pumping operation and the byproducts of pollution
control operations. It is generated at a rate of approximately
80,000m
3
/year and forms approximately half of the waste
deposited in the landfill. The sludge waste is a mud-like fine
grained material with a solids content of 15 to 20% by weight
when it is carted to the landfill. It is comprised of clay, coal ash,
ironbearing dusts and carbon.
2.3
Landfill history
For over 20 years, the sludge (and other wastes) have been
deposited into the existing West Landfill facility in a series of
cells. A new East Landfill facility has recently been designed to
replace the existing West Landfill which is nearing its capacity.
The East Landfill has an expected life of 30 years and a final fill
volume of 4.7 million m
3
. It will accept approximately
160,000m
3
/year of waste.
The materials to be stored in the East Landfill are the same
materials that have been landfilled in the West Landfill. As this
existing facility has been in operation since 1992, it offers an
ideal means for testing and characterising the sludge materials
in situ for the design of the new East Landfill.
3 SLUDGE CHARACTERISTICS
3.1
Prior to landfilling
Prior to landfilling, the clay slimes and black waste are
deposited into separate settling ponds to reduce the moisture
content to approximately 80% to 85%. The clay slimes and
black waste are then landfilled into the cells, forming a sludge
waste. The sludge typically exhibits no free water, but some
decant water is produced during excavation from the settling
