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Stability and movements of open-pit lignite mines in Northern Greece
Stabilité et mouvements de terrain dans les mines de lignite à ciel ouvert en Grèce du Nord
Kavvadas M.
National Technical University of Athens, Greece
Agioutantis Z.
Technical University of Crete, Greece
P. Schilizzi P.
Public Power Cooperation, Greece
Steiakakis C.
General Consulting Ltd “ISTRIA”, Greece
ABSTRACT: This paper presents a case of a lignite mine in Northern Greece with excavated slopes exceeding 100-120m in depth in
which substantial movement is occurring, with an average rate 10-20mm/day. The Mavropigi mine is very important for the power
supply of Greece and uninterrupted operation is often critical, meaning that excavation is taking place on moving soil masses. The
stability of the moving southeast slope is investigated and the information developed from an extensive monitoring campaign, with
survey prisms, inclinometers and piezometers is presented. The use of the investigation data to evaluate the type of movement, the
geometry of sliding surface and the effectiveness of remediation measures are analyzed in detail. The procedure of assessing the
stability and safe slope operation during production, even with high rates of movement and the effect of precipitation are presented. It
is shown that there are situations that mine slopes can move several meters laterally and still be operational without catastrophic
failures.
RÉSUMÉ : On présente dans cet article une étude des mouvements de terrain dans une mine de lignite du nord de la Grèce. Dans cette
mine à ciel ouvert de 100 à 120 m de profondeur, des glissements de pente (lents) sont observés : 10-20 mm/jour. L’exploitation de
cette mine (Mavropigi) de lignite, primordiale pour la production d’énergie en Grèce, s’effectue sans interruption dans des pentes en
mouvement permanent. Les problèmes de stabilité de la pente sud-est de la mine sont étudiés et les mesures effectués in situ à l’aide
de différents appareillages (mesures topographiques, inclinomètres et piézomètres) sont présentés. L’analyse de l’ensemble des
mesures est effectuée afin de caractériser le type de mouvement de terrain et la géométrie de la surface de glissement. Par ailleurs,
l’efficacité des mesures adoptées pour stabiliser les pentes est analysée via les mesures effectuées sur le site. Une procédure
permettant d’évaluer le risque de perte de stabilité pendant les phases d’exploitation (intensive) en tenant compte des effets
défavorables des précipitations est proposée. On met ainsi en évidence des cas pour lesquels les pentes peuvent se déplacer
latéralement de plusieurs mètres sans pour autant engendrer des ruptures majeures.
KEYWORDS: Slope movement, Coal open pit, Slope monitoring, Slope stability, Landslide.
1 INTRODUCTION
The Public Power Cooperation (PPC) operates a number of
large open pit lignite mines in Northern Greece (Amyntaio-
Ptolemais Basin). The Mavropigi mine has been mined since
2003 and at present the excavated slopes have reached depths
exceeding 100-120m. Since 2011, the southeast slopes have
shown persistent large horizontal movements at an average rate
of 10-20mm/day, at times reaching more than 40-50mm/day
affected by increased precipitation. The moving mass was
estimated around 6Mm
3
. This paper presents a case of
significant movements that occurred at the southeast slope (Fig.
1), and details the monitoring, evaluation and mitigation
measures taken to safeguard mining operations which had to be
uninterrupted for production management purposes.
Figure 1. Southeast slopes of Mavropigi lignite mine.
2 GEOLOGICAL AND GEOTECHNICAL CONDITIONS
The Mavropigi mine is in the sedimentary fill of the Ptolemais
basin which includes terrestrial and lacustrine deposits of
Miocene up to Pleistocene age, with abundant lignite horizons
(Diamantopoulos, 2006). Near horizontal intercalations of
Marls, Lignites, Stiff Clays and Sands are the predominant
materials; from a geotechnical point of view they can be
described as “Hard Soils – Soft Rocks”. The main intercalations
are Marl and Lignite. The Marl material is mostly classified as
Elastic SILT or Organic SILT (MH-OH) per USCS (ASTM
D2487). Locally in the Marl – Lignite intercalations, thin (few
centimeters thick) beds of High Plasticity CLAY (CH) are
found. These almost horizontal thin beds have very low residual
shear strength and often act as slip surfaces. The mechanical
properties of the different materials encountered in the area of
Mavropigi are presented in Table 1. These parameters are
assessed from triaxial, direct shear and ring shear tests
performed on selected core samples and are used for the slope
stability calculations of the Mine.
Table 1. Range of Geotechnical Properties of Mavropigi Mine
aterials.
M
Parameters / Materials
Marl
Lignite
Clay (CH)
Unit weight γ (kN/m
3
)
16-18
11-13
16-18
Effective Cohesion c’ (kPa)
50-150
150-200
5-50
Effective friction φ’ (deg)
28-35
34-36
26-30
Residual Friction φ
r
’ (deg)
-
-
5-10
